JP2020139075A - Foam sheet and adhesive tape - Google Patents

Abstract

To provide a foam sheet excellent in waterproofness, and an adhesive tape including the foam sheet.SOLUTION: There are provided a foam sheet having an average bubble diameter of 40-200 μm and 50% compressive strength B after the lapse of 3 hours after the start of compression of 25-800 kPa, and an adhesive tape including the foam sheet.SELECTED DRAWING: None

Description

The present invention relates to a foam sheet and an adhesive tape.

Conventionally, foam sheets have excellent cushioning, heat insulating properties, waterproof properties and moisture proof properties, so that they are used as packing materials for articles, sealing materials for sealing peripheral parts such as parts that need to be protected from gas or liquid, vibration and impact. It is used for various purposes such as a cushioning material for cushioning gas and a base material for an adhesive sheet. As a specific foamed sheet, Patent Document 1 discloses a crosslinked polyolefin-based resin foamed sheet obtained by foaming and cross-linking a foamable polyolefin-based resin sheet containing a pyrolysis-type foaming agent (see Patent Document 1). ).

International Publication No. 2005/007731

By the way, in recent years, high waterproof performance is required for small electronic devices such as smartphones and wearable terminals. For this reason, a gasket material is arranged around the gap generated between the display panel and the housing and the opening such as the SIM card slot from the viewpoint of improving waterproofness. The gasket material needs to be flexible enough to fit into a minute step on the surface of the adherend, and also needs to have adhesiveness for temporarily fixing to the arrangement location.
As a material satisfying such physical properties, an adhesive tape including a foamed sheet and an adhesive layer provided on at least one surface of the foamed sheet is known. Among them, the single-sided adhesive tape in which the adhesive is provided on one side of the foam sheet is easier to peel off from the adherend than the double-sided adhesive tape. Therefore, after using the single-sided adhesive tape as a sealing material inside the electronic device, the electronic device When disassembling and repairing, it can be easily peeled off and has excellent operability. However, although the single-sided adhesive tape has an advantage of being excellent in operability, it has been confirmed that the adhesiveness to the adherend is inferior and the waterproof property is lowered.
Therefore, an object of the present invention is to provide a foamed sheet having excellent waterproofness and an adhesive tape provided with the foamed sheet.

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

According to the present invention, it is possible to provide a foamed sheet exhibiting good waterproofness and an adhesive tape provided with the foamed sheet.

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

<50% compression strength B>
The 50% compression strength B (hereinafter, also simply referred to as 50% compression strength B) 3 hours after the start of compression of the foamed sheet of the present invention is 25 to 800 kPa. In this way, the foamed sheet of the present invention has a 50% compression strength B maintained in a certain range. This means that the foamed sheet of the present invention has a certain repulsive force even after a certain period of time has passed from the start of compression, whereby the good adhesion of the foamed sheet to the adherend is maintained. Dripping and waterproofing are improved. If the 50% compression strength B is less than 25 kPa, the repulsive force of the foam is not maintained for a long period of time, and the waterproof property deteriorates. If the 50% compression strength B exceeds 800 kPa, the flexibility of the foamed sheet deteriorates, and if there is a fine step on the surface of the adherend, the followability of the foamed sheet to the surface of the adherend deteriorates. Waterproofness deteriorates.
The 50% compression strength B of the foamed sheet is preferably 25 to 500 kPa, more preferably 30 to 200 kPa.
The 50% compression strength B can be adjusted to a desired range by adjusting the average cell diameter, bubble flatness, foaming ratio, etc. of the foamed sheet described later.
The 50% compression strength B is a value of the 50% compression strength when the foamed sheet is compressed to a thickness of 50% and held in that state for 3 hours. The compression strength is measured according to JIS K6767.

<50% compression strength A>
The 50% compression strength A of the foamed sheet of the present invention is preferably 50 to 800 kPa, more preferably 60 to 600 kPa, and even more preferably 70 to 500 kPa. When the 50% compression strength A of the foamed sheet is at least these lower limit values, the repulsive force of the foamed sheet increases the adhesion to the adherend and improves the waterproofness. When the 50% compression strength A of the foamed sheet is not more than these upper limit values, the flexibility of the foamed sheet is increased, the followability of the foamed sheet to the surface shape of the adherend is improved, and the waterproof property is improved. Further, when it is not more than the upper limit value, the ratio of the compression strength B to the compression strength A (compression strength B / compression strength A) becomes high, and the flexibility of the foamed sheet and the change in the physical properties of the repulsive force over time are suppressed. Improves stability.

The ratio of the compression strength B to the compression strength A (compression strength B / compression strength A) is 0 from the viewpoint of suppressing changes in physical properties over time such as the flexibility and repulsive force of the foamed sheet and maintaining the waterproof property for a long time. It is preferably 0.05 or more, more preferably 0.2 or more, and usually 1 or less.

<Average cell diameter>
The average cell diameter of the foamed sheet of the present invention is 40 to 200 μm. If the average cell diameter exceeds 200 μm, it becomes difficult to adjust the above-mentioned compression strength B to a desired range, and the surface of the foamed sheet tends to become rough, so that the waterproof property of the foamed sheet deteriorates. When the average cell diameter is less than 40 μm, it becomes difficult to manufacture the foamed sheet, and the effect obtained by reducing the average cell diameter becomes low. The average cell diameter of the foamed sheet is preferably 50 to 170 μm, more preferably 50 to 130 μm. By adjusting the average cell diameter to be relatively small in this way, it is considered that the number of cell walls per unit length is increased, and therefore the waterproof property is improved. The average cell diameter of the foamed sheet can be adjusted by, for example, the gel fraction described later.
The average cell diameter of the foamed sheet is an average value of the average cell diameter in the MD direction and the average cell diameter in the TD direction.

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

The average cell diameter in the MD direction, the average cell diameter in the TD direction, and the average cell diameter in the ZD direction can be obtained by the following methods.
A foam sheet cut into 50 mm squares is prepared as a foam sheet sample for measurement. After immersing this in liquid nitrogen for 1 minute, it is cut in the thickness direction along the MD direction and the TD direction with a razor blade. A 200x magnified photograph of this cross section was taken using a digital microscope (“VHX-900” manufactured by KEYENCE CORPORATION), and all existing on the cut surface having a length of 2 mm in each of the MD direction, the TD direction and the ZD direction. The bubble diameter is measured for the bubbles in the above, and the operation is repeated 5 times. Then, the average value of all the bubbles was taken as the average cell diameter in the MD direction, the TD direction, and the ZD direction.
The MD direction means a machine direction and is a direction that coincides with the extrusion direction and the like, and the TD direction is a direction that is orthogonal to the MD direction and means a Transverse direction. The ZD direction is the thickness direction of the foamed 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, and further preferably 38 to 60% by mass. When the gel fraction is at least these lower limit values, sufficient crosslinks are formed, and by foaming the crosslinks, a foamed sheet having a small average cell diameter can be obtained. Further, when the gel fraction is not more than these upper limit values, it becomes easy to secure the flexibility of the foamed sheet.
The gel fraction can be measured according to the method described in Examples described below.

<Expansion ratio>
The foaming ratio of the foamed sheet of the present invention is preferably 2 to 20 cc / g, more preferably 3 to 15 cc / g, and further preferably 4 to 10 cc / g. When the foaming ratio is in such a range, appropriate flexibility is ensured, it becomes easy to follow the surface shape of the adherend, and it becomes easy to improve the waterproof property. The foaming ratio is expressed as the reciprocal of the apparent density.

<Closed cell ratio>
The foamed sheet of the present invention preferably has closed cells. Having closed cells means that the ratio of closed cells to total cells (referred to as "closed cell ratio") is 70% or more. If the foamed sheet used in the present invention has closed cells, it becomes easy 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, further preferably 90% or more, and 100% or less.
The closed cell ratio can be determined according to ASTM D2856 (1998). Examples of commercially available measuring instruments include the dry automatic densitometer Accupic 1330.

The closed cell ratio is more specifically measured as follows. A test piece having a flat 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 was measured, to measure the weight W ₁ of the specimen to calculate the apparent volume V ₁ of the test piece. Next, the apparent volume V ₂ occupied by the bubbles is calculated based on the following formula. The density of the resin constituting 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, the test piece is taken out from the water to remove the water adhering to the surface of the test piece, the weight W _{2 of the} test piece is measured, and the open cell ratio F ₁ and the closed cell ratio F ₂ are calculated based on the following formulas. To do.
Continuous bubble ratio F ₁ (%) = {(W _2- W ₁ ) / V ₂ } × 100
Closed cell ratio 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 in such a range, the surface is relatively smooth, the minute gaps at the interface between the foam sheet and the adherend are reduced, and the waterproof property is likely to be improved. The glossiness of the foamed sheet is more preferably 25 to 80%, still more preferably 30 to 80%. By adjusting the compression strength B to the above range while setting the glossiness to such a value, a foamed sheet having better waterproofness can be obtained. The glossiness can be adjusted by, for example, adjusting the average cell diameter of the foamed sheet, the cell flatness, and the like.
The glossiness of the foamed sheet is measured based on JIS Z 8741 with a glossiness meter TC-108DP / A manufactured by Tokyo Denshoku Co., Ltd. using a foamed sheet of TD5 cm * MD10 cm square. In consideration of the surface roughness of the test piece, the measurement method is the method 2 (incident light angle 75 °).

<Contact angle>
The contact angle of the foamed sheet of the present invention is preferably 80 to 120 °. When the contact angle is in such a range, the water repellency of the foamed sheet is enhanced, and it becomes easy to prevent water from entering due to the capillary phenomenon from the minute gap at the interface between the foamed sheet and the adherend, and as a result, the foamed sheet is waterproofed. The sex becomes high. The contact angle of the foamed sheet is more preferably 85 to 120 °, still more preferably 95 to 120 °. The contact angle can be adjusted by the composition of the foamed sheet or the like, and for example, as will be described later, it can be adjusted by including a lubricant in the foamed sheet.
The contact angle is a value obtained by the sessile drop 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 to 70, more preferably 30 to 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 pressure-bonded to the adherend, and as a result, the waterproof property 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 to 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 a miniaturized electronic device. Further, when the thickness is 0.03 mm or more, it becomes easy to secure the waterproofness and flexibility of the foam sheet. From such a viewpoint, the thickness is more preferably 0.05 to 1.5 mm, and further preferably 0.1 to 1.0 mm.

<Polyolefin resin>
The foamed sheet of the present invention contains a resin, preferably a polyolefin-based resin. By using the polyolefin resin, it becomes easy to adjust the average cell diameter within the above range while ensuring an appropriate flexibility of the foamed sheet.
Examples of the polyolefin resin include polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer and the like, and among these, polyethylene resin is preferable.

[Polyethylene resin]
Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Cheegler-Natta compound, a metallocene compound, and a chromium oxide compound, and preferably, a polyethylene resin polymerized with a polymerization catalyst of a metallocene compound is used.

Further, as the polyethylene resin, linear low density polyethylene is preferable. By using the linear low-density polyethylene, the foamed sheet can be made flexible and the foamed sheet can be made thinner. The linear low-density polyethylene is more preferably obtained by using a polymerization catalyst such as a metallocene compound. Further, the 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, if necessary, a small amount of α-olefin. The linear low density polyethylene to be obtained is more preferable.
Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Of these, α-olefins having 4 to 10 carbon atoms are preferable.
Polyethylene resin, for example the density of the linear low density polyethylene as described above is preferably 0.870~0.910g / cm ^3, more preferably 0.875~0.907g / cm ^3, 0.880~0.905g / Cm ³ is more preferred. As the polyethylene resin, a plurality of polyethylene resins may be used, and polyethylene resins other than the above-mentioned density range may be added.

(Metallocene compound)
Examples of the metallocene compound include compounds such as a bis (cyclopentadienyl) metal complex having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, one or more cyclopentadienyl rings or their analogs are present as ligands in tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum. Can be mentioned.
In such a metallocene compound, the properties of active sites are uniform, and each active site has the same activity. A polymer synthesized using a metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc. Therefore, when a sheet containing a polymer synthesized using a metallocene compound is crosslinked, the cross-linking is uniform. Proceed to. As a result, since it can be stretched uniformly, it becomes easy to make the thickness uniform even if the foamed sheet is thinned.

Examples of the ligand include a cyclopentadienyl ring, an indenyl ring and the like. These cyclic compounds may be substituted with a hydrocarbon group, a substituted hydrocarbon group or a hydrocarbon-substituted metalloid group. Examples of the hydrocarbon group include methyl group, ethyl group, various propyl group, various butyl group, various amyl group, various hexyl group, 2-ethylhexyl group, various heptyl group, various octyl group, various nonyl group and various decyl group. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a product obtained by polymerizing a cyclic compound as an oligomer may be used as a ligand.
Furthermore, in addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine, divalent anion chelating ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, and aryls. You may use phosphide or the like.

Examples of metallocene compounds containing tetravalent transition metals and ligands include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl. Examples thereof include silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
The metallocene compound exerts an action as a catalyst in the polymerization of various olefins by combining with a specific cocatalyst (cocatalyst). Specific examples of the co-catalyst include methylaluminoxane (MAO) and boron-based compounds. The ratio of the cocatalyst used to the metallocene compound is preferably 10 to 1 million mol times, more preferably 50 to 5,000 mol times.
When the above-mentioned linear low-density polyethylene is used as the polyolefin resin contained in the foam sheet, the above-mentioned 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 the other polyolefin resin is contained, the ratio of the other polyolefin resin to the 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 the polypropylene resin include polypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. One of these may be used alone, or two or more thereof may be used in combination.
Specific examples of the α-olefin constituting the propylene-α-olefin copolymer include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, and 1-. Examples thereof include octene, and among these, α-olefins having 6 to 12 carbon atoms are preferable.

[Ethylene-vinyl acetate copolymer]
The ethylene-vinyl acetate copolymer used as the polyolefin resin contains, for example, vinyl acetate in an amount of preferably 6 to 40% by mass, more preferably 12 to 35% by mass, still more preferably 20 to 33% by mass. Examples include vinyl copolymers.
The ethylene-vinyl acetate copolymer used in the present invention may contain, in addition to ethylene and vinyl acetate, vinyl alcohol produced by hydrolyzing a part of vinyl acetate.
Examples of such ethylene-vinyl acetate copolymers include "Ultrasen" manufactured by Tosoh Corporation, "Evaflex" manufactured by Mitsui-Dupont Polychemical Co., Ltd., "UBE Polyethylene" manufactured by Ube Industries, Ltd., and Asahi Kasei Chemicals Co., Ltd. Company-made "Suntech" etc. can be mentioned.

[Content of each resin]
When a polyethylene resin is used as the polyolefin resin, the amount of the polyethylene resin with respect to the total amount of the resin is preferably 30% by mass or more, more preferably 50% by mass or more, further preferably 60% by mass or more, still more preferably 70% by mass or more. It may consist substantially only of polyethylene resin. When the content of the polyethylene resin is 30% by mass or more, the waterproof property is improved.

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

<Glidant>
The foamed sheet of the present invention preferably contains a lubricant. By containing the lubricant, it becomes easy to adjust the contact angle of the foamed sheet to the above-mentioned desired range, and it becomes easy to obtain a highly waterproof foamed sheet. The content of the lubricant in the foamed sheet is preferably 0.7 to 3 parts by mass, and more preferably 0.8 to 2 parts by mass with respect to 100 parts by mass of the resin contained in the foamed sheet.
Examples of the lubricant include higher fatty acids such as stearic acid and palmitic acid, higher alcohols such as palmitic acid and stearyl alcohol, calcium stearate, zinc stearate, barium stearate, aluminum stearate, magnesium stearate, sodium palmitate and the like. Examples thereof include metal salts of higher fatty acids, higher fatty acid esters such as butyl stearate and glyceryl monostearate, higher fatty acid amides such as oleic acid amide, stearic acid amide and erucic acid amide. Here, "high grade" has 9 or more carbon atoms, preferably 9 or more and 30 or less carbon atoms.
Among these, at least one selected from higher fatty acids and metal salts of higher fatty acids is preferable as the lubricant from the viewpoint of easily adjusting the contact angle to a desired range and enhancing the effect of improving waterproofness. Among them, since the metal salt of the higher fatty acid can easily control the average cell diameter within a desired range, the lubricant preferably contains at least the metal salt of the higher fatty acid, and the metal salt of the higher fatty acid and the higher fatty acid. It is more preferable to use in combination.
Among the above-mentioned higher fatty acids, stearic acid is particularly preferable, and among the metal salts of higher fatty acids, zinc stearate and calcium stearate are particularly preferable.

<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.
As the pyrolytic foaming agent, an organic foaming agent and an inorganic foaming agent can be used. Examples of the organic foaming agent include azodicarboxylic amides, azodicarboxylic acid metal salts (azodicarboxylic acid barium, etc.), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N'-dinitrosopentamethylenetetramine, and hydrazine. Examples thereof include zodicarboxylic amides, hydrazine derivatives such as 4,4'-oxybis (benzenesulfonyl hydrazide) and toluenesulfonyl hydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium borohydride, monosoda anhydrous citrate and the like.
Among these, azo compounds are preferable, and azodicarbonamides are more preferable, from the viewpoint of obtaining fine bubbles, economy, and safety. One of these may be used alone, or two or more thereof may be used in combination.
The blending amount of the thermally decomposable foaming agent in the effervescent composition is preferably 1 to 20 parts by mass, more preferably 2 to 18 parts by mass, still 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-mentioned lubricant in addition to the resin and the pyrolytic foaming agent, and in addition to this, an antioxidant, a heat stabilizer, a colorant, a flame retardant, an antistatic agent, and the like. An additive generally used for a foamed sheet such as a filler may be contained.

[Manufacturing method of foam sheet]
The method for producing the foamed sheet of the present invention is not particularly limited, but for example, a foamable composition containing a resin and a pyrolytic foaming agent is crosslinked and heated to foam the pyrolytic foaming agent in the TD direction and in the TD direction. 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 heat-decomposable foaming agent, a lubricant to be blended if necessary, and other additives to form a sheet-like foamable composition. Step (2): Sheet Step of irradiating the foamable composition with ionizing radiation to crosslink the foamable composition Step (3): The crosslinked foamable composition is heated to foam a heat-decomposable foaming agent to foam a foam sheet. Step (4): Step of stretching the foamed sheet in either one or both of the MD direction and the TD direction.

In the step (1), the method for forming the foamable composition into a sheet is not particularly limited, but for example, a resin, a pyrolysis foaming agent, a lubricant to be blended as needed, and other additives are used. It may be formed 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 effervescent composition in the step (2), a method of irradiating the sheet-shaped effervescent composition with ionizing radiation such as electron beam, α ray, β ray, and γ ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of cross-linking of the obtained foamed sheet is within the above-mentioned desired range, but it is preferably 5 to 15 Mrad, and more preferably 6 to 13 Mrad. The accelerating voltage of the ionization irradiation line is preferably 200 to 1000 kV, more preferably 300 to 800 kV.
In the step (3), the heating temperature at which the foamable composition is heated to foam the pyrolytic foaming agent may be equal to or higher than the foaming temperature of the pyrolytic foaming agent, but is preferably 200 to 300 ° C. It is preferably 220 to 280 ° C.

The stretching of the foamed sheet in the step (4) may be carried out in both the MD and TD directions, or may be carried out only in one direction, but is preferably carried out in both directions. Further, the stretching of the foamed sheet may be carried out after foaming the sheet-shaped foamable composition to obtain a foamed sheet, or may be carried out while foaming the sheet-shaped foamable composition. When the foamed sheet is stretched after foaming the sheet-shaped foamable composition to obtain the foamed sheet, the foamed sheet is continuously formed without cooling the foamed sheet while maintaining the molten state at the time of foaming. The foamed sheet may be stretched, or the foamed sheet may be stretched after the foamed sheet is cooled and then heated again to be in a molten or softened state. The foamed sheet can be easily made thin by stretching.
In the step (4), the draw ratio of the foamed sheet in one or both of the MD direction and the TD direction is preferably 1.1 to 5.0 times, more preferably 1.2 to 4.0 times. It is more preferably 1.5 to 3.3 times. Further, when the draw ratio is at least the above lower limit value, the flexibility of the foamed sheet tends to be improved. On the other hand, when it is set to the upper limit or less, it is prevented that the foamed sheet is broken during stretching, or the foamed gas is released from the foamed sheet during stretching and the foaming ratio is significantly lowered, and the flexibility of the foamed sheet is good. Therefore, it becomes easy to make the quality uniform.
Further, the foamed sheet may be heated to, for example, 100 to 280 ° C., preferably 150 to 260 ° C. during stretching.
The foamed sheet obtained as described above may be cut into a desired shape by cutting by a well-known method such as punching.

However, the present production 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 with ionizing radiation, the effervescent composition may be preliminarily blended with an organic peroxide, and the effervescent composition may be heated to decompose the organic peroxide for cross-linking. Good.

[Adhesive tape]
The adhesive tape of the present invention includes the above-mentioned foamed sheet and an adhesive layer provided on at least one surface of the foamed sheet. The adhesive tape of the present invention is preferably a foamed sheet and a single-sided pressure-sensitive adhesive tape in which an adhesive layer is provided on one surface of the foamed sheet. Single-sided adhesive tape is easier to peel off from the adherend than double-sided adhesive tape, so when disassembling and repairing the electronic device after using it as a sealing material inside the electronic device, the adherend It can be easily peeled off from the surface and has excellent operability. In general, the single-sided adhesive tape can be easily peeled off and has excellent operability, but the adhesion to the adherend is not sufficient and the waterproof property is inferior. However, as described above, since the foamed sheet of the present invention is excellent in waterproofness, the single-sided adhesive tape provided with the foamed sheet of the present invention can be easily peeled off from the adherend and is also excellent in waterproofness.

<Adhesive>
The pressure-sensitive adhesive layer is preferably formed by 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, and an acrylic adhesive is preferable from the viewpoint of reworkability and waterproofness.
In the present invention, when the pressure-sensitive adhesive tape has pressure-sensitive adhesive layers on both surfaces of the foamed sheet, the pressure-sensitive adhesives used for the two pressure-sensitive adhesive layers may be the same or different.

(Acrylic adhesive)
Hereinafter, an embodiment of the acrylic pressure-sensitive adhesive used for 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 the present specification, the term "(meth) acrylic acid alkyl ester" refers to a concept including both an acrylic acid alkyl ester and a methacrylic acid alkyl ester, and the same applies to other similar terms. .. Further, the term "polymerizable monomer" is not limited to a compound having no repeating unit, but if it is a compound copolymerizing with the (meth) acrylic acid alkyl ester-based monomer (A), the other monomer itself described later is the repeating unit. Refers to a concept that can include those having.

≪ (Meta) acrylic acid alkyl ester monomer (A) ≫
The (meth) acrylic acid alkyl ester-based monomer (A) is an ester of (meth) acrylic acid and an aliphatic alcohol, and the alkyl group of the aliphatic alcohol preferably has 2 to 14 carbon atoms, more preferably. Alkyl esters derived from fatty alcohols of 4 to 10 are preferred.

Specific examples of the (meth) acrylic acid alkyl ester-based monomer (A) include ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, and isobutyl ( Meta) acrylate, t-butyl (meth) acrylate, pentyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth) acrylate, isooctyl (meth) Examples thereof include acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, undecyl (meth) acrylate, dodecyl (meth) acrylate, tridecyl (meth) acrylate, and tetradecyl (meth) acrylate.
Among these, n-butyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and n-octyl (meth) acrylate are preferable, and n-butyl (meth) acrylate, 2-Ethylhexyl (meth) acrylate or a combination thereof is more preferable.
The (meth) acrylic acid alkyl ester-based monomer may be used alone or in combination of two or more.

The structural unit derived from the (meth) acrylic acid alkyl ester-based monomer (A) constitutes the main component of the pressure-sensitive 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 pressure-sensitive adhesive. Is 40% by mass or more, more preferably 45% by mass or more. As described above, when the content of the (meth) acrylic acid alkyl ester-based monomer (A) is increased, it becomes possible to impart a desired adhesive force to the pressure-sensitive 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. Since they are the same, they can be replaced and expressed. The same applies to the 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-based 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, a vinyl group-containing carboxylic acid such as itaconic acid, an anhydride thereof, a vinyl monomer having a hydroxyl group, and a nitrogen-containing vinyl monomer. ..
Examples of the vinyl monomer having a hydroxyl group include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, caprolactone-modified (meth) acrylate, and polyoxyethylene (meth) acrylate. And polyoxypropylene (meth) acrylate and the like.
Examples of the nitrogen-containing vinyl monomer include (meth) acrylonitrile, N-vinylpyrrolidone, N-vinylcaprolactam, N-vinyllaurilolactam, (meth) acryloylmorpholin, (meth) acrylamide, dimethyl (meth) acrylamide, and N-methylol ( Examples thereof include meta) acrylamide, N-butoxymethyl (meth) acrylamide, and dimethylaminomethyl (meth) acrylate.
Among the polar group-containing vinyl monomer (B), (meth) acrylic acid, a vinyl group-containing carboxylic acid such as itaconic acid, and an anhydride thereof are preferable, (meth) acrylic acid is more preferable, and acrylic acid is further preferable. 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 pressure-sensitive adhesive is the structural unit 100 derived from the (meth) acrylic acid alkyl ester-based monomer (A). It is preferably 1 to 15 parts by mass, more preferably 2 to 12 parts by mass, and further preferably 3 to 10 parts by mass with respect to parts by mass. By setting the content of the polar group-containing vinyl monomer (B) within such a range, it becomes easy to adjust the Tg, cohesive force, adhesive force and the like of the pressure-sensitive adhesive layer within an appropriate range.

≪Other monomers≫
The polymerizable monomer may contain other monomers other than those (A) and (B) described above. Examples of other monomers include styrene-based monomers, polyfunctional monomers, and carboxylic acid vinyl esters such as vinyl acetate. Examples of the styrene-based monomer include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene and the like.
In addition, examples of the polyfunctional monomer include a monomer having two or more vinyl groups, and preferably a polyfunctional (meth) acrylate having two or more (meth) acryloyl groups. The use of polyfunctional monomers makes it possible to form a network structure in the acrylic polymer.
Specific polyfunctional monomers include hexanediol di (meth) acrylate, polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, ethicylated bisphenol A di (meth) acrylate, and tris (2-hydroxyethyl). Examples thereof include isocyanurate triacrylate, ethosylated trimeryl propantriacrylate, proxyed trimethylol propantriacrylate, proxyed glyceryl triacrylate, neopentyl glycol adipate diacrylate and the like.
When other monomers are used, the content of the structural unit derived from the other monomer in the pressure-sensitive adhesive is 0.5 with respect to 100 parts by mass of the structural unit derived from the (meth) acrylic acid alkyl ester-based monomer (A). It is ~ 15 parts by mass, more preferably 1 to 7 parts by mass, and further 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.

<Adhesive resin>
The acrylic pressure-sensitive adhesive may contain a tack-imparting resin from the viewpoint of improving the adhesive strength. As the tackifier resin, a tackifier resin having low polymerization inhibitory properties such as hydrogenated terpene resin, hydrogenated rosin, disproportionate rosin resin, rosin ester polymer, and petroleum resin is preferable. Among these, a rosin-based polymer is preferable, and a rosin ester-based polymer is particularly preferable, because a tackifier resin having many double bonds inhibits a polymerization reaction.

The softening point of the tackifier resin may be about 95 ° C. or higher from the viewpoint of improving the cohesive force and adhesive strength of the pressure-sensitive adhesive, but preferably contains 120 ° C. or higher, for example, 95 ° C. or higher and 120 ° C. Those below 120 ° C. and those at 120 ° C. or higher and 150 ° C. or lower may be used in combination. The softening point may be measured by the ring-and-ball method defined in JIS K2207.
The content of the tackifier resin in the acrylic pressure-sensitive adhesive is preferably 2 to 40 parts by mass, more preferably 4 to 35 parts by mass, and further preferably 5 to 25 parts by mass with respect to 100 parts by mass of the acrylic polymer. is there.

<Crosslinking agent>
When the resin constituting the acrylic pressure-sensitive adhesive has a hydroxyl group or a carboxy group, a cross-linked structure may be formed between the main chains by using a cross-linking agent from the viewpoint of improving the adhesiveness.
Examples of the cross-linking agent 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 preferable.

Examples of the isocyanate-based cross-linking agent include polyisocyanate compounds. Specific examples of the polyisocyanate compound include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate, aliphatic polyisocyanates such as hexamethylene diisocyanate, isophorone diisocyanate, and alicyclic polyisocyanates such as hydrogenated diphenylmethane diisocyanate. And so on. Moreover, these biuret form, isocyanurate form, and adduct form which is a reaction product with a low molecular weight active hydrogen-containing compound such as ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane, castor oil and the like can be mentioned.
These may be used individually by 1 type, or may be used in combination of 2 or more type.

Examples of the epoxy-based cross-linking agent 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-xylene diamine, 1,3-bis (N, N-diglycidylaminomethyl) cyclohexane, 1,6-diglycidyl n-hexane And so on.

The peel strength of the pressure-sensitive adhesive layer used in the present invention may be adjusted by adjusting the amount of the cross-linking agent.
The amount of the cross-linking agent to be added to the pressure-sensitive 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, based on 100 parts by mass of the acrylic polymer. More preferably, it is 5 parts by mass or less.

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

(Rubber adhesive)
The rubber-based 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 mass, more preferably 30 to 65% by mass, and even more preferably 45 to 60% by weight. Here, the diblock refers to a diblock composed of styrene and isoprene. When the diblock ratio of the styrene-isoprene block copolymer is 25% or more, sufficient adhesive strength is exhibited, and when the diblock ratio is 70% by mass or less, the shear strength can be easily increased. In addition to diblock, the styrene-isoprene block copolymer also contains those having three or more blocks such as triblock composed of styrene, isoprene, and styrene block.

The amount of styrene in the styrene-isoprene block copolymer is not particularly limited, but is preferably 14 to 24% by mass, more preferably 15 to 18% by mass. When the amount of styrene is 14% by mass or more, the adhesive has high cohesiveness, and the shear strength can be easily increased. Further, when it is 24% by mass or less, the cohesive force becomes an appropriate size and the adhesive force is easily exhibited.
The molecular weight of the styrene-isoprene block copolymer is not particularly limited, but the mass average molecular weight is preferably 100,000 to 400,000, more preferably 150,000 to 250,000. The mass average molecular weight referred to here means that is measured as a polystyrene-equivalent molecular weight by a GPC (gel permeation chromatography) method.

The rubber-based pressure-sensitive adhesive preferably contains a tack-imparting resin in addition to the above-mentioned rubber component. As the tackifier resin used for the rubber-based pressure-sensitive adhesive, various tackifier resins can be used, but petroleum-based resin, terpene resin, and kumaron resin are preferably used. The tackifier resin may be used alone or in combination of two or more, but a petroleum-based resin and at least one selected from a terpene resin and a kumaron resin may be used in combination. preferable. The combination of such tackifier resins makes it easier to adjust the peel strength within the above range.
Examples of petroleum-based resins include aliphatic petroleum resins (C5 petroleum resins), alicyclic petroleum resins, aromatic petroleum resins, etc., and fats from the viewpoint of compatibility with styrene-isoprene block copolymers. Family petroleum resins are preferred. Further, it is preferable to use a petroleum-based resin having a softening point of about 90 to 120 ° C.
Further, as the terpene resin, a resin having a softening point of about 80 to 120 ° C. can be used, but a resin having a softening point of less than 100 ° C. is preferable from the viewpoint of ensuring adhesive strength. Further, as the kumaron resin, a resin having a softening point of preferably 110 to 130 ° C., more preferably 115 to 125 ° C. is used in order to secure the cohesive force.

The tackifier resin is preferably 60 to 250 parts by mass, more preferably 100 to 200 parts by mass, and even more preferably 110 to 180 parts by mass with respect to 100 parts by mass of the rubber component. By setting the blending amount of the tackifying resin within the above range, the cohesive force can be improved and an appropriate peel adhesive force can be imparted.
When the petroleum resin and at least one selected from the terpene resin and the kumaron resin are used in combination, the petroleum resin is preferably 50 to 200 parts by mass and 60 to 150 parts by mass with respect to 100 parts by mass of the rubber component. By mass is preferable, and 60 to 110 parts by mass is 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. Further, the kumaron resin is preferably 10 to 60 parts by mass, more preferably 15 to 50 parts by mass, still 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 a softening agent, an antioxidant, a filler and the like, if necessary.

(Urethane adhesive)
The urethane-based pressure-sensitive 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 polyol include polyether polyol, polyester polyol, polycarbonate polyol, polycaprolactone polyol and the like. Examples of the polyisocyanate compound include diphenylmethane diisocyanate, tolylene diisocyanate, and hexamethylene diisocyanate. These urethane-based pressure-sensitive adhesives may be used alone or in combination of two or more.
Further, as the urethane-based pressure-sensitive adhesive, a urethane resin obtained by reacting a polyurethane polyol with a polyfunctional isocyanate-based curing agent may be used. Examples of the polyurethane polyol include those obtained by reacting the above-mentioned polyol with a polyisocyanate compound, or those obtained by reacting a polyol with a polyisocyanate compound and a chain extender such as diamine. The polyfunctional isocyanate-based curing agent may be any compound having two or more isocyanate groups, and the above-mentioned isocyanate compounds can be used.
The urethane-based pressure-sensitive adhesive may contain the above-mentioned fine particles in addition to the urethane resin, and the urethane-based pressure-sensitive adhesive may contain a tackifier resin, a softener, an antioxidant, a filler, etc., if necessary. It may be contained.

[Manufacturing method of adhesive layer]
When an acrylic pressure-sensitive adhesive is used, the pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer heats and refluxes the pressure-sensitive adhesive composition containing the above-mentioned polymerizable monomer and polymerization initiator, and then crosslinks the polymer. Can be obtained by It can also be obtained by irradiating the pressure-sensitive adhesive composition containing the above-mentioned polymerizable monomer with light to polymerize the polymerizable monomer.
The pressure-sensitive adhesive composition may contain at least one of the above-mentioned pressure-imparting 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, but for example, a method of applying the adhesive using a coating machine such as a coater, or a method of spraying and applying the adhesive using a spray. A method of applying an adhesive using a brush, a method of transferring an adhesive layer formed on a release sheet to a foamed sheet, and the like.

<Use of adhesive tape>
The use of the adhesive tape is not particularly limited, but it is preferable to use it as a sealing material inside an electronic device because it is excellent in waterproofness. Further, it can be suitably used inside various portable electronic devices in which the space for arranging the foam sheet is small. Examples of electronic devices include liquid crystal displays, organic EL displays, mobile phones, cameras, game devices, electronic organizers, personal computers, and the like.

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

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

<50% compression strength B>
The foamed sheet was compressed to a thickness of 50% and held in that state for 3 hours, and the 50% compression strength was measured. The compression strength was measured according to JIS K6767.

<50% compression strength A>
Measured according to JIS K6767.

<Expansion ratio>
The apparent density of the foamed sheet was measured in accordance with JIS K7222, and the reciprocal of the measured density was taken as the foaming ratio.

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

<Gel fraction>
Approximately 100 mg of a test piece is collected from the foam 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 left for 24 hours, filtered through a 200 mesh wire mesh to collect the insoluble matter on the wire mesh, vacuum dried, and the weight of the insoluble matter. Weigh B (mg) precisely. From the obtained values, the gel fraction (mass%) was calculated by the following formula.
Gel fraction (mass%) = (B / A) x 100

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

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

<Glossiness>
The glossiness of the foamed sheet was measured using a TD5 cm * MD10 cm square foamed sheet with a glossiness meter TC-108DP / A manufactured by Tokyo Denshoku Co., Ltd. based on JIS Z 8741. In consideration of the surface roughness of the test piece, Method 2 (incident light angle 75 °) was adopted as the measurement method.

<Contact angle>
Water was collected in a syringe to make a 3.0 μL droplet on the needle tip. The prepared droplets were brought into contact with the foamed sheet to form droplets on the foamed 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. The foamed sheet was held for 24 hours in a measurement environment (23 ° C., relative humidity 50%) before measurement.

<Waterproof>
The waterproofness evaluation measures the time (waterproofable time, unit: minutes) from submerging the test sample prepared by the following method to a water depth of 10 cm under the conditions of a temperature of 23 ° C. and a relative humidity of 50% until the start of water immersion. I went by doing. The presence or absence of inundation was visually confirmed.
This waterproofness evaluation was carried out after preparing a test sample and curing it under the conditions of a temperature of 23 ° C. and a relative humidity of 50% for 6 hours.

(Creation of test sample)
(1) Production of Adhesive (A) 70 parts by mass of butyl acrylate, 25 parts by mass of 2-ethylhexyl acrylate and 5 parts by mass of acrylic acid were put into a reactor equipped with a thermometer, a stirrer and a cooling tube, and ethyl acetate 80 was further added. Parts by weight were added and substituted with nitrogen. Then, the reactor was heated to start reflux. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator into the above reactor, and the mixture was refluxed at 70 ° C. for 5 hours to obtain a solution of an acrylic copolymer.
The weight average molecular weight of the obtained acrylic copolymer was measured by the GPC method using "2690 Separations Model" manufactured by Water Co., Ltd. as a column and found to be 500,000.
An isocyanate-based cross-linking agent (1 part by mass) and a rosin ester-based polymer (10 parts by mass) were mixed with a solution of the obtained acrylic copolymer (100 parts by mass) and stirred to obtain a pressure-sensitive adhesive (A).
(2) Preparation of single-sided adhesive tape By preparing a release paper with a thickness of 150 μm, applying the adhesive (A) that forms an adhesive layer to the release-treated surface of the release paper, and drying 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 bonded to one surface of the foam sheets obtained in each Example and Comparative Example, and the release paper was peeled off to obtain a single-sided pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer having a thickness of 20 μm.
(3) Preparation of test sample A single-sided adhesive tape was punched into a frame shape having an outer circumference of 75 mm in length × 50 mm in width and a width of 2.0 mm. Next, the obtained frame-shaped adhesive tape was sandwiched between two acrylic plates having an arithmetic average roughness Ra of 1.0 μm and a maximum height roughness Rz of 5.0 μm. Then, a test sample was prepared by tightening with bolts until the clearance between the two acrylic plates was halved.

The components used in the production of the foamed sheet in each Example and Comparative Example are as follows.
[resin]
-Resin A: Linear low density polyethylene resin (LLDPE) (manufactured by Exxon Chemical Co., Ltd., trade name "EXACT3027", density: 0.900)
[Pyrolytic foaming agent]
・ Azodicarbonamide Average particle size 15 μm
[Glidant]
・ Stearic acid, zinc stearate, calcium stearate [antioxidant]
-Phenolic antioxidant: 2,6-di-t-butyl-p-cresol

[Example 1]
(Manufacturing of foam sheet)
Each component constituting the effervescent composition shown in Table 1 is supplied to an extruder in the number of parts by mass shown in Table 1 and melt-kneaded at 130 ° C. to obtain an effervescent composition, and then the effervescent composition is obtained. The composition was extruded into a sheet. Next, both sides of the sheet-shaped effervescent composition are irradiated with an electron beam having an accelerating voltage of 500 kV for 5.5 mad to crosslink the effervescent composition, and then the crosslinked effervescent composition is subjected to hot air and an infrared heater at 250 ° C. A foamed sheet was obtained by continuously feeding the wine into a sparkling furnace held in the water and heating it to foam it. Then, the obtained foam sheet was continuously sent out from the foam 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 stretching ratio of 1.1 times, and the foam sheet is fed into the foam furnace. The foamed sheet was also stretched in the MD direction at a stretching ratio of 1.1 times by winding the foamed sheet at a winding speed faster than the speed (supply rate). As a result, a foamed sheet (thickness: 0.5 mm) of Example 1 was obtained. The obtained foam sheet was evaluated according to the above evaluation method, 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, production conditions, etc. of the foamable composition were changed as shown in Table 1. The obtained foam sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.
The foamed sheet of Comparative Example 4 had too strong a repulsive force, and could not measure the 50% compression strength B and evaluate the waterproofable time.

The adhesive tape provided with the foamed sheet of the present invention, in which the average cell diameter and the 50% compression strength B after 3 hours from the start of compression were within a certain range, showed excellent waterproofness. On the other hand, the adhesive tape provided with the foamed sheet whose compression strength B was out of the predetermined range was inferior in waterproofness.

Claims (12)

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