JP2018172643A - Foam sheet, and adhesive tape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a foam sheet that has excellent reworkability even when the foam sheet is made thin.SOLUTION: A foam sheet has a thickness of 0.05-1.5 mm, where, at least one surface layer has an average cell diameter smaller than that of the inner layer, and the average cell diameter of the at least one surface layer is 10 μm or more and less than 160 μm.SELECTED DRAWING: None

Description

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

  In electronic devices such as mobile phones, cameras, game devices, electronic notebooks, and personal computers, foam sheets are widely used as sealing materials or shock absorbing materials. The foam sheet may be used as an adhesive tape by applying an adhesive on at least one surface, for example, inside an electronic device. Conventionally, as a foam sheet used in these applications, a cross-linked polyolefin resin foam sheet obtained by foaming and cross-linking a foamable polyolefin resin sheet containing a pyrolytic foaming agent is known (for example, Patent Documents). 1).

JP 2014-28925 A

  In recent years, foam sheets used in electronic devices have been required to be thin as electronic devices become smaller and thinner, and may be, for example, 1.5 mm or less. However, when the foamed sheet is thin, mechanical strength such as tensile strength tends to be low. For example, when the foamed sheet is used as an adhesive tape, the foamed sheet is easily damaged when reworked, that is, the reworkability is poor. Tend to be.

  This invention is made | formed in view of the above situation, and makes it a subject to provide a foam sheet with favorable rework property, even if it is a case where a foam sheet is made thin.

As a result of intensive studies, the present inventors solved the above problem by making the average cell diameter of at least one surface layer part smaller than the average cell diameter of the inner layer part in the foam sheet having a sheet thickness within a certain range. The present invention has been found out and the following invention has been completed.
That is, the present invention provides the following [1] to [7].
[1] A foam sheet having a thickness of 0.05 to 1.5 mm, wherein the average cell diameter of at least one surface layer portion is smaller than the average cell diameter of the inner layer portion, and the average cell diameter of at least one surface layer portion is A foamed sheet characterized by being 10 μm or more and less than 160 μm.
[2] The foam sheet according to [1], wherein the average cell diameter of the at least one surface layer part is 0.7 times or less than the average cell diameter of the inner layer part.
[3] The foam sheet according to the above [1] or [2], wherein the average cell diameter of at least one surface layer portion is 10 to 120 μm.
[4] The foam sheet according to any one of [1] to [3], wherein the thickness is 0.05 mm or more and less than 1.0 mm.
[5] The foam sheet according to any one of the above [1] to [4], wherein the foaming ratio is 1.5 to 15 cm ³ / g.
[6] The foam sheet according to any one of [1] to [5], which is used inside an electronic device.
[7] An adhesive tape comprising an adhesive on at least one surface of the foamed sheet according to any one of [1] to [6].

  According to the present invention, it is possible to provide a foam sheet having good reworkability even when the sheet thickness is reduced.

  The foam sheet of the present invention is a foam sheet having a thickness of 0.05 to 1.5 mm, wherein the average cell diameter of at least one surface layer part is smaller than the average cell diameter of the inner layer part, The average bubble diameter is 10 μm or more and less than 160 μm. In this invention, a surface layer part means the area | region to 20% of the whole thickness of a foam sheet in the thickness direction from each surface of a foam sheet. Moreover, in this invention, an inner layer part means area | regions other than the surface layer part in both surfaces. Hereinafter, the foamed sheet of the present invention will be described in detail.

(thickness)
The foamed sheet of the present invention has a thickness of 0.05 to 1.5 mm. When the thickness of the foamed sheet is less than 0.05 mm, it is difficult to improve various functions such as sealability and impact absorption. On the other hand, if it is larger than 1.5 mm, it is difficult to apply it to various thinned electronic devices.
The thickness of the foamed sheet is preferably 0.05 mm or more and less than 1.0 mm in order to make it easy to use in a thin electronic device while improving various performances. The thickness of the foam sheet is more preferably 0.08 to 0.50 mm, and still more preferably 0.1 to 0.30 mm.

(Average bubble diameter)
In the foamed sheet of the present invention, the average cell diameter of at least one surface layer part is smaller than the average cell diameter of the inner layer part. In the present invention, the average cell diameter of the surface layer means the average value of the average cell diameter of MD of the surface layer and the average cell diameter of TD. The same applies to the average cell diameter of the inner layer portion.
In addition, MD means Machine direction and is a direction that coincides with the extrusion direction and the like, and TD means Transverse direction and is a direction orthogonal to MD and a direction parallel to the sheet surface of the foam sheet. .
When the average cell diameter of the surface layer part is smaller than the average cell diameter of the inner layer part, the reworkability (hereinafter also simply referred to as reworkability) when the adhesive material layer is laminated on the foamed sheet to form an adhesive tape is improved. The reason for this is not clear, but because the average cell diameter of the surface layer portion is small, the bubbles in the inner layer portion are difficult to break starting from the origin, or the surface layer portion is small due to the small average cell diameter of the surface layer portion. It is conceivable that the resin ratio increases, and the tensile strength in the peeling direction of the outermost layer, which can be the starting point of breakage when peeling the adhesive tape, increases.
In the foam sheet of the present invention, it is preferable that the average cell diameter of both surface layer parts is smaller than the average cell diameter of the inner layer part. Thereby, even if an adhesive material is provided on any surface of the foam sheet, the reworkability is improved.
The average cell diameter of at least one surface layer part, which is smaller than the average cell diameter of the inner layer part, is preferably 0.7 times or less, more preferably 0.6 times or less with respect to the average cell diameter of the inner layer part. More preferably, it is preferably 0.5 times or less, more preferably 0.2 times or more, more preferably 0.25 times or more, and further more preferably 0.35 times or more. preferable. Moreover, the average bubble diameter of both surface layer parts should be in these ranges. By making the average cell diameter in such a range, the reworkability is improved.

The average cell diameter of at least one surface layer portion of the foamed sheet of the present invention is 10 μm or more and less than 160 μm. With such an average cell diameter of the surface layer portion, reworkability is good. The average cell diameter of the surface layer part is preferably 10 to 120 μm, more preferably 20 to 100 μm, still more preferably 30 to 80 μm, still more preferably 40 to 65 μm. By setting it as such an average bubble diameter, rework property becomes more favorable. Moreover, the average bubble diameter of both surface layer parts should be in these ranges.
Moreover, it is preferable that the average bubble diameter of an inner layer part is 50-400 micrometers, It is more preferable that it is 60-300 micrometers, It is further more preferable that it is 100-200 micrometers, It is further more preferable that it is 100-190 micrometers. By setting the average cell diameter of the inner layer portion as described above, the flexibility of the foam sheet can be easily increased.
The average cell diameter of MD in the surface layer is preferably 10 μm or more and less than 160 μm, more preferably 10 to 120 μm, further preferably 20 to 100 μm, and further preferably 30 to 80 μm. preferable. Further, from the same viewpoint, the average cell diameter of TD in the surface layer portion is preferably 10 μm or more and less than 160 μm, more preferably 10 to 120 μm, still more preferably 20 to 100 μm, and further preferably 30 to 80 μm. More preferably.
The average cell diameter of MD in the inner layer portion is preferably 50 to 400 μm, more preferably 60 to 300 μm, still more preferably 100 to 220 μm, and still more preferably 100 to 190 μm.
Moreover, it is preferable that the average cell diameter of TD of an inner layer part is 50-400 micrometers, It is more preferable that it is 60-300 micrometers, It is further more preferable that it is 100-220 micrometers, It is further more preferable that it is 100-190 micrometers. .
The average cell diameter can be measured based on the description in the examples.

(Closed cell rate)
The foamed resin layer has closed cells, and the closed cell rate is 70% or more. As described above, the bubbles included in the foam sheet are generally closed cells, and it is easy to improve the sealing performance, the shock absorption performance, and the like. The closed cell ratio is preferably 80% or more, more preferably 90 to 100%. The closed cell ratio can be determined according to ASTM D2856 (1998).

The closed cell ratio can be measured in more detail as follows.
First, a flat square test piece having a side of 5 cm is cut out from the foam sheet. Then, the thickness of the test piece is measured to calculate the apparent volume V ₁ of the test piece, and the weight W _{1 of the} test piece is measured.
Next, the volume V ₂ occupied by the bubbles is calculated based on the following formula. The density of the matrix resin constituting the test piece is ρ (g / cm ³ ).
Volume occupied by bubbles V ₂ = V ₁ −W ₁ / ρ
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 over 3 minutes. Then, after releasing from pressurization in water and allowing to stand for 1 minute, the test piece is taken out of the water, the water adhering to the surface of the test piece is removed, and the weight W _{2 of the} test piece is measured. The open cell rate F ₁ and the closed cell rate F ₂ are calculated.
Open cell ratio F ₁ (%) = 100 × (W ₂ −W ₁ ) / V ₂
Closed cell ratio F ₂ (%) = 100−F ₁

(Foaming ratio)
Further, the expansion ratio of the foamed sheet is not particularly limited, is preferably 1.5~15.0cm ³ / g, more preferably 1.8~12.0cm ³ / g, 2.0 More preferably, it is ˜10.0 cm ³ / g.
The expansion ratio is obtained by measuring the apparent density and calculating the reciprocal thereof. The apparent density can be measured according to JIS K7222.

(25% compressive strength)
The 25% compressive strength of the foamed sheet is preferably 20 to 1000 kPa. By setting it as 20 kPa or more, mechanical strength becomes favorable, and by setting it as 1000 kPa or less, the softness | flexibility of a foamed sheet, sealing performance, impact absorption property, etc. become favorable. The 25% compressive strength of the foamed sheet is more preferably 30 to 800 kPa, further preferably 60 to 500 kPa, and further preferably 100 to 200 kPa.

(Tensile strength)
The tensile strength of the foamed sheet is preferably 3 to 30 MPa in MD and 2 to 25 MPa in TD, more preferably 10 to 25 MPa in MD, and 8 to 20 MPa in TD. By setting the tensile strength within these ranges, it becomes easy to improve the mechanical strength of the foamed sheet while maintaining flexibility. The 25% compressive strength and tensile strength of the foamed sheet were measured according to the method of JIS K6767.

[resin]
As the resin constituting each of the foam sheets, various resins may be used. For example, in addition to polyolefin resin, polyamide resin, polycarbonate resin, polyester resin, olefin elastomer such as ethylene propylene diene rubber (EPDM), An elastomer resin such as a hydrogenated styrene-based thermoplastic elastomer (SEBS) can also be used. In these, polyolefin resin is preferable.

(Polyolefin resin)
Examples of the polyolefin resin include a polyethylene resin, a polypropylene resin, and an ethylene-vinyl acetate copolymer. Among these, a polyethylene resin is preferable. Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, and a chromium oxide compound.

The polyethylene resin is preferably linear low density polyethylene. By using linear low-density polyethylene, the foamed sheet is given high flexibility and the foamed sheet can be made thinner. 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 a small amount of α-olefin as required. More preferred is linear low density polyethylene.
Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Especially, a C4-C10 alpha olefin is preferable.

(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, tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum have one or more cyclopentadienyl rings or their analogs as ligands (ligands). Can be mentioned.
Such metallocene compounds have uniform active site properties 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., so when a sheet containing a polymer synthesized using a metallocene compound is crosslinked, the crosslinking is uniform. Proceed to. Since the uniformly crosslinked sheet is uniformly foamed, the physical properties are easily stabilized. Moreover, since it can extend | stretch uniformly, the thickness of a foamed sheet can be made uniform.

Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. 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 a methyl group, an 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, phenyl groups and the like. The “various” means various isomers including n-, sec-, tert-, and iso-.
Moreover, what polymerized the cyclic compound as an oligomer may be used as a ligand.
In addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine or divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, aryls Phosphide or the like may be used.

Examples of metallocene compounds containing tetravalent transition metals and ligands include, for example, cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, dimethyl And silyltetramethylcyclopentadienyl-t-butylamidozirconium dichloride.
The metallocene compound exhibits an action as a catalyst in the polymerization of various olefins by combining with a specific cocatalyst (co-catalyst). Specific examples of the cocatalyst include methylaluminoxane (MAO) and boron compounds. In addition, the usage-amount of the cocatalyst with respect to a metallocene compound has preferable 10-1 million mol times, and 50-5,000 mol times is more preferable.

When a polyethylene resin is used as the polyolefin resin contained in the foamed sheet, the content of the polyethylene resin is preferably more than 70% by mass based on the total amount of the polyolefin resin. The content of the polyethylene resin is more preferably 75% by mass or more based on the total amount of the polyolefin resin, more preferably 85% by mass or more, further preferably 95% by mass or more, and 100% by mass. More preferably it is.
When the above-mentioned linear low density polyethylene is used as the polyolefin resin contained in the foam sheet, the above linear low density polyethylene may be used alone, but may be used in combination with other polyolefin resins. For example, you may use together with the other polyolefin resin described below. When other polyolefin resin is used in combination, the ratio of the other polyolefin resin to the linear low density polyethylene (100% by mass) is preferably 40% by mass or less, more preferably 30% by mass or less, and further preferably 20% by mass or less. preferable.

As for the ethylene-vinyl acetate copolymer used as polyolefin resin, the ethylene-vinyl acetate copolymer containing 50 mass% or more of ethylene is mentioned, for example.
Examples of the polypropylene resin include polypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. These may be used alone or in combination of two or more.
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-heptene, 1- Octene etc. can be mentioned, Among these, C6-C12 alpha olefin is preferable.

  Moreover, as resin contained in a foam sheet, although polyolefin resin may be used independently, resin other than polyolefin resin may also be included. In the foamed sheet, 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, and still more preferably 80% by mass or more. Examples of the resin other than the polyolefin resin used in combination with the polyolefin resin include the various resins described above.

[Foaming agent]
The foam sheet of the present invention is preferably a foam formed by foaming a foamable composition containing the resin and a foaming agent.
Examples of the foaming agent include a pyrolytic foaming agent. As the pyrolytic foaming agent, an organic foaming agent and an inorganic foaming agent can be used. As the thermally decomposable foaming agent, one having a decomposition temperature higher than the melting temperature of the resin is usually used. For example, one having a decomposition temperature of 140 to 270 ° C. may be used.
Specific examples of organic foaming agents include azodicarbonamide, metal salts of azodicarboxylic acid (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, N, N′-dinitrosopentamethylenetetramine, and the like. Examples thereof include nitroso compounds, hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide), hydrazine derivatives such as toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, anhydrous monosodium citrate, and the like.
Among these, an azo compound is preferable and azodicarbonamide is particularly preferable from the viewpoint of obtaining fine bubbles and from the viewpoints of economy and safety. These pyrolytic foaming agents can be used alone or in combination of two or more.
The blending amount of the thermally decomposable foaming agent in the foamable composition is preferably 0.5 to 20 parts by weight, more preferably 1 to 15 parts by weight, and even more preferably 1 to 10 parts by weight with respect to 100 parts by weight of the resin. It is.

[Other additives]
Additives commonly used for foams such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, fillers, decomposition temperature regulators, etc., may be added to the foamable composition. You may mix | blend. Among these, it is preferable to use an antioxidant and a decomposition temperature adjusting agent.

[Method for producing foam sheet]
Although there is no restriction | limiting in particular in the manufacturing method of the rubber-type resin closed cell foam sheet of this invention, It is preferable that the process of the following (1)-(3) is included.
(1) A process of producing a foamable sheet by processing a foamable composition containing a polyolefin resin and a thermally decomposable foaming agent into a sheet,
(2) A step of producing a crosslinked foamable sheet by irradiating at least one surface of the foamable sheet with ionizing radiation,
(3) Step of producing a foamed sheet by foaming a cross-linked foamable sheet

[Process (1)]
Step (1) is a step of producing a foamable sheet by processing a foamable composition containing a polyolefin resin and a pyrolytic foaming agent into a sheet shape. After the foamable resin composition is kneaded using a kneader such as a Banbury mixer or a pressure kneader, the foamable sheet can be produced by continuously extruding with an extruder, calendar, conveyor belt casting or the like. .

[Process (2)]
Step (2) is a step of producing a crosslinked foamable sheet by irradiating at least one surface of the foamable sheet with ionizing radiation. Examples of ionizing radiation include light, γ-rays, and electron beams. The acceleration voltage of ionizing radiation is preferably 20 to 500 kV, more preferably 60 to 400 kV, and still more preferably 100 to 300 kV. By setting the accelerating voltage of the ionizing radiation within such a range, the degree of cross-linking of the surface layer part tends to be higher than the degree of cross-linking of the inner layer part, and as a result, the average bubble diameter tends to be smaller in the surface layer part. The irradiation dose of ionizing radiation is preferably 2.0 to 13.0 Mrad, more preferably 3.0 to 12.0 Mrad, still more preferably 4.0 to 11.0 Mrad, still more preferably 5.0 to 11.0 Mrad. is there. By setting such an irradiation dose range under the above acceleration voltage conditions, it becomes easier to obtain a foam sheet in which the average cell diameter of the surface layer portion is smaller than the average cell diameter of the inner layer portion, and the crosslinking of the inner layer portion also proceeds to a certain extent. Various physical properties as a foamed sheet such as foaming property are also improved. Irradiation with ionizing radiation may be carried out only on one side of the foamable sheet, but in terms of making the average cell diameter of the inner layer portion and the surface layer portion within a desired range, and an adhesive tape provided with an adhesive layer on both surfaces In view of improving reworkability, it is preferable to irradiate the other surface after irradiating one surface. When irradiating both surfaces, irradiation conditions such as acceleration voltage and irradiation amount may be the same or different within the above-mentioned range. In addition, when it is difficult to control the dose with one irradiation, the irradiation may be performed a plurality of times.
Since the thickness of the foam sheet of the present invention is as thin as 0.05 to 1.5 mm, the bubble diameter distribution in the surface layer part and the inner layer part is easily affected by the irradiation conditions of ionizing radiation, and the irradiation conditions are strictly It is important to control.

[Process (3)]
Step (3) is a step of producing a foamed sheet by foaming the cross-linked foamable sheet. Examples of the method for foaming the foamed sheet include a batch system such as an oven and a continuous foaming system in which a cross-linked foamable sheet is formed into a long sheet shape and continuously passed through a heating furnace.
It is preferable to stretch the foam sheet during the step (3) or in a subsequent step. That is, the cross-linked foamable sheet may be expanded to obtain a foamed sheet, and then stretched, or the cross-linked foamable sheet may be stretched. In this manufacturing method, it becomes easy to obtain the average cell diameter of the surface layer portion and the inner layer portion by stretching the foam sheet, and it becomes easy to adjust the thickness of the foam sheet to a desired range. When the foamed sheet is stretched after foaming the cross-linked foamable sheet, the foamed sheet may be stretched without maintaining the melted state during foaming without cooling the foamed sheet. After cooling, the foamed sheet may be stretched again by heating it to a molten or softened state.

The expansion of the foam sheet is preferably performed so that the thickness of the foam sheet is 0.1 to 0.9 times by stretching, more preferably 0.15 to 0.75 times, and still more preferably 0.25 to 0.25. It is performed so as to be 0.65 times. By stretching the foamed sheet so as to be within these ranges, the thickness of the foamed sheet can be adjusted, and the average cell diameter of the surface layer part and the inner layer part can be adjusted, and the reworkability tends to be good. On the other hand, when the value is not less than the lower limit, the foamed sheet is prevented from breaking during stretching, or the foaming gas escapes from the foamed resin layer during foaming to significantly reduce the foaming ratio.
Moreover, what is necessary is just to heat a foam sheet at 100-280 degreeC at the time of extending | stretching, for example, Preferably it is 150-260 degreeC.

  Although the use of a foam sheet is not specifically limited, For example, it is preferable to use it inside an electronic device. Since the foamed sheet of the present invention is thin, it can be suitably used in thin electronic devices such as various portable electronic devices. Examples of portable electronic devices include notebook personal computers, mobile phones, smartphones, tablets, and portable music devices. The foamed sheet can be used as an impact absorbing material for absorbing an impact, a sealing material for filling a gap between members, and the like inside the electronic device.

[Adhesive tape]
Moreover, you may use a foam sheet for the adhesive tape which uses a foam sheet as a base material. The adhesive tape includes, for example, a foam sheet and an adhesive material provided on at least one surface of the foam sheet. The adhesive tape can be bonded to another member via an adhesive material. The pressure-sensitive adhesive tape may be one in which an adhesive material is provided on both surfaces of the foam sheet, or one in which an adhesive material is provided on one surface. Adhesive tapes can also be used as shock absorbers and sealants.
Moreover, it is preferable that an adhesive material is provided on a surface layer part in a foamed sheet whose average cell diameter is smaller than the average cell diameter of an inner layer part. With such a configuration, the foam sheet is less likely to be damaged during rework.

Further, the pressure-sensitive adhesive material only needs to have at least a pressure-sensitive adhesive layer, and may be a single pressure-sensitive adhesive layer laminated on the surface of the foamed sheet, or a double-sided pressure-sensitive adhesive sheet attached to the surface of the foamed sheet. However, the pressure-sensitive adhesive layer is preferably a single layer. In addition, a double-sided adhesive sheet is provided with a base material and the adhesive layer provided in both surfaces of the base material. The double-sided pressure-sensitive adhesive sheet is used for bonding one pressure-sensitive adhesive layer to the foamed sheet and bonding the other pressure-sensitive adhesive layer to another member.
There is no restriction | limiting in particular as an adhesive which comprises an adhesive layer, For example, an acrylic adhesive, a urethane type adhesive, a rubber-type adhesive etc. can be used. Further, a release sheet such as a release paper may be further bonded onto the adhesive material.
The thickness of the adhesive is preferably 5 to 200 μm, more preferably 7 to 150 μm, and still more preferably 10 to 100 μm.

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

The measurement method and evaluation method of each physical property are as follows.
<Average bubble diameter>
The foamed sheet is cut into 50 mm square, immersed in liquid nitrogen for 1 minute, then cut in the thickness direction along each of MD and TD, and using a digital microscope (manufactured by Keyence Corporation, product name VHX-900). A 200x magnified photograph was taken. In the foamed sheet of the photographed image, the bubble diameter of MD and the bubble diameter of TD were measured for all the bubbles present on the cut surface of 2 mm in length in each of MD and TD, and the operation was repeated 5 times. The average value of the bubble diameters of the bubbles MD and TD existing in the region of 20% of the thickness of the foam sheet from the surface is defined as the average cell diameter of the MD and TD of the surface layer portion, and 20% of the thickness of the foam sheet from the surface. The average value of the bubble diameters of the bubbles MD and TD in the region other than the region was defined as the average bubble diameter of the inner layer MD and TD. In addition, the measurement of the average cell diameter of a surface layer part was performed about both surfaces of the foam sheet.

<Apparent density and expansion ratio>
The apparent density was measured according to JIS K7222. The reciprocal of the apparent density was taken as the expansion ratio.
<25% compressive strength>
The 25% compressive strength was measured according to JIS K6767.
<Tensile strength>
The foam sheet was cut into dumbbell-shaped No. 1 as defined in JIS K6251 4.1. Using this as a sample, the tensile strength of MD and TD was measured with a tensile tester (product name: Tensilon RTF235, manufactured by A & D) at a measurement temperature of 23 ° C. according to JIS K6767.

[Example 1]
100 parts by mass of a linear low-density polyethylene resin (trade name “Kernel KF283”, manufactured by Nippon Polyethylene Co., Ltd., density: 0.921 g / cm ³ ) obtained by a polymerization catalyst of a metallocene compound as a polyolefin resin, and pyrolytic foaming 2.5 parts by mass of azodicarbonamide as an agent, 1 part by mass of zinc oxide (trade name “OW-212F” manufactured by Sakai Chemical Industry Co., Ltd.) as a decomposition temperature adjusting agent, and 2 as a phenolic antioxidant , 6-Di-t-butyl-p-cresol 0.5 parts by mass was supplied to an extruder and melt-kneaded at 130 ° C. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.21 mm.
Next, the irradiation distance is set to 30 cm on both surfaces of the foamable sheet, and an electron beam with an acceleration voltage of 150 kV is irradiated to 8.8 Mrad to crosslink the foamable sheet to obtain a crosslinkable foamable sheet. The foamed sheet was obtained by continuously feeding into a foaming furnace maintained at 250 ° C. with hot air and an infrared heater and heating to foam. Next, the foamed sheet was obtained by stretching in the MD and TD at 110 ° C. so that the total thickness was as shown in Table 1. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in Table 1.

[Examples 2-6, Comparative Examples 1-2]
A foamed sheet was obtained in the same manner as in Example 1 except that the composition of the foamable composition, the acceleration voltage and dose when irradiating the electron beam, and the thickness of the foamed sheet were as shown in Table 1. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in Table 1.

  In addition, when the expansion ratio of the foam sheet is relatively low, it is considered that the strength is increased, but the reworkability tends to be good. Therefore, in this example, samples having the same expansion ratio were compared to evaluate the superiority or inferiority of reworkability. Evaluation of reworkability is whether reworkability when the foamed sheet is used as an adhesive tape is better than when a foamed sheet whose average cell diameter is the same at the surface layer part and the inner layer part is used as an adhesive tape. Is an index indicating three levels, "3" indicates that it is excellent, "2" indicates that it is good, and "1" indicates that it is comparable. In addition, the adhesive tape used for this evaluation laminates the adhesive layer single layer on the foam sheet surface.

  The foamed sheet of the present invention in which the average cell diameter in the surface layer part was smaller than the average cell diameter in the inner layer part had good reworkability evaluation. Moreover, it turned out that the foamed sheet of this invention can be obtained by adjusting appropriately the acceleration voltage and dose of an electron beam. On the other hand, the reworkability of the foam sheet of the comparative example that does not satisfy the requirements of the present invention was inferior to that of the example.

[Example 7, Comparative Example 3]
A foamed sheet was obtained in the same manner as in Example 1 except that the composition of the foamable composition, the acceleration voltage and dose when irradiating the electron beam, and the thickness of the foamed sheet were as shown in Table 2. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in Table 2.

[Example 8, Comparative Example 4]
A foamed sheet was obtained in the same manner as in Example 1 except that the composition of the foamable composition, the acceleration voltage and dose when irradiating the electron beam, and the thickness of the foamed sheet were as shown in Table 3. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in Table 3.

  From Table 2 and Table 3, the reworkability evaluation of the foamed sheet of the present invention in which the average cell diameter of the surface layer portion was smaller than the average cell size of the inner layer portion was good. On the other hand, the reworkability of the foam sheet of the comparative example that does not satisfy the requirements of the present invention was inferior to that of the example.

Claims (7)

  A foam sheet having a thickness of 0.05 to 1.5 mm, wherein the average cell diameter of at least one surface layer portion is smaller than the average cell diameter of the inner layer portion, and the average cell diameter of at least one surface layer portion is 10 μm or more and 160 μm. A foam sheet, characterized by being less than.   The foam sheet according to claim 1, wherein the average cell diameter of the at least one surface layer part is 0.7 times or less than the average cell diameter of the inner layer part.   The foam sheet according to claim 1 or 2, wherein an average cell diameter of at least one surface layer portion is 10 to 120 µm.   The foam sheet in any one of Claims 1-3 whose thickness is 0.05 mm or more and less than 1.0 mm. Expansion ratio is 1.5~15cm ³ / g, the foamed sheet according to claim 1.   The foam sheet according to any one of claims 1 to 5, which is used inside an electronic device.   An adhesive tape comprising an adhesive material on at least one surface of the foamed sheet according to claim 1.