WO2018181486A1

WO2018181486A1 - Resin foam sheet, method for producing resin foam sheet, and adhesive tape

Info

Publication number: WO2018181486A1
Application number: PCT/JP2018/012790
Authority: WO
Inventors: 麻美永井; 秀明矢野; 哲史濱田
Original assignee: 積水化学工業株式会社
Priority date: 2017-03-30
Filing date: 2018-03-28
Publication date: 2018-10-04
Also published as: CN110475812A; JPWO2018181486A1; JP7000316B2; KR102557823B1; KR20190129885A

Abstract

A resin foam sheet having a breaking compression parameter of 700 or more and less than 1500 and a degree of crosslinking of 35% by mass or more, wherein the breaking compression parameter is defined by formula A: (strength at breakage in MD direction (MPa))×(strength at breakage in TD direction (MPa))/(25% compression strength (kPa))×1000.

Description

Resin foam sheet, method for producing resin foam sheet, and adhesive tape

The present invention relates to a resin foam sheet, a method for producing a resin foam sheet, and an adhesive tape.

Conventionally, in electronic devices such as mobile phones, cameras, game devices, electronic notebooks, tablet terminals, and notebook personal computers, sealing materials or shock absorbing materials made of foamed sheets have been used. These sealing materials or impact absorbing materials may be used as an adhesive tape using a foam sheet as a base material. For example, the display device in the electronic apparatus generally has a structure in which a protective panel is installed on a display panel such as an LCD. However, in order to bond the protective panel to a frame portion outside the display panel, foaming is performed. An adhesive tape having a sheet as a base material is used.
As a foam sheet used inside an electronic device, a crosslinked polyolefin resin foam sheet obtained by foaming and crosslinking a foamable polyolefin resin sheet containing a pyrolytic foaming agent is known (for example, Patent Document 1). reference).

International Publication No. 2005/007731

By the way, in recent years, electronic devices have been reduced in size, and the functions of various components have also been improved, so that not only the restrictions on the space inside the electronic devices have increased, but also the internal structure of the devices has become complicated. As the internal structure becomes more complex, it is expected that the number of cases with various steps will increase. At that time, if there is a gap between the tape and the casing, the waterproof property is lost, and the tape is easily peeled off from that point.
Accordingly, when a tape is attached to an electronic device or the like, a step following property is required from the viewpoint of waterproofness and adhesive strength. At the same time, from the viewpoint of reusing parts, reworkability that allows easy peeling without breaking the tape is required.

The present invention has been made in view of the above circumstances, and provides a resin foam sheet having good adhesiveness following a step, and a pressure-sensitive adhesive tape including the resin foam sheet. Let it be an issue.

As a result of intensive studies, the inventors have found that a resin foam sheet that satisfies a specific parameter (breaking compression parameter) composed of a breaking strength in the MD direction, a breaking strength in the TD direction, and a 25% compressive strength follows the steps. The present invention was completed by finding that it exhibited good adhesiveness and good reworkability. That is, the present invention is as follows.

[1] A resin foam sheet, wherein the fracture compression parameter represented by the following formula A is 700 or more and less than 1500, and the degree of crosslinking is 35% by mass or more.
Formula A: Strength at break in MD direction (MPa) × Strength at break in TD direction (MPa) / 25% compressive strength (kPa) × 1000
[2] The resin foam sheet according to [1], wherein each of the average cell diameters in the MD direction and the TD direction is 100 μm or less.
[3] The resin foam sheet according to [1] or [2], which has a thickness of 0.01 to 0.3 mm.
[4] The resin foam sheet according to any one of [1] to [3], wherein the breaking strength in the MD direction is 7 to 30 MPa, and the breaking strength in the TD direction is 10 to 25 MPa.
[5] The resin foam sheet according to any one of [1] to [4], wherein the 25% compressive strength is 130 to 500 kPa.
[6] The resin foam sheet according to any one of [1] to [5], wherein the expansion ratio is 1.2 to 4 cm ³ / g.
[7] The resin foam sheet according to any one of [1] to [6], wherein the resin foam sheet includes a polyolefin resin.
[8] The resin foam sheet according to [7], wherein the polyolefin resin is a polyethylene resin.
[9] The resin foam sheet according to [7] or [8], wherein the polyolefin resin is linear low-density polyethylene polymerized with a metallocene compound polymerization catalyst.
[10] The method for producing a resin foam sheet according to any one of [1] to [9], wherein the foamable composition containing a resin and a pyrolytic foaming agent is crosslinked and heated to A method for producing a resin foam sheet, in which a decomposable foaming agent is foamed and stretched in at least one of a TD direction and an MD direction at a stretch ratio of 1.1 times or more.
[11] An adhesive tape comprising the resin foam sheet according to any one of [1] to [9] and an adhesive layer provided on at least one surface of the resin foam sheet.

According to the present invention, it is possible to provide a resin foam sheet having good adhesiveness following a step and having good reworkability, and a pressure-sensitive adhesive tape including the resin foam sheet.

It is explanatory drawing explaining the outline of a push test.

Hereinafter, the present invention will be described in detail using embodiments.
[Resin foam sheet]
In the resin foam sheet according to the embodiment of the present invention, the fracture compression parameter represented by the following formula A is 700 or more and less than 1500, and the degree of crosslinking is 35% by mass or more.
Formula A: Strength at break in MD direction (MPa) × Strength at break in TD direction (MPa) / 25% compressive strength (kPa) × 1000
Here, the unit of strength at break in Formula A is MPa, and the unit of 25% compressive strength is calculated as kPa.
That is, when each of the breaking strength in the MD direction and the breaking strength in the TD direction is 12 MPa and the 25% compressive strength is 147 kPa, the above formula A is calculated as follows.
Formula A: 12 × 12/147 × 1000 = 980

The step following property is required to be soft in the surface direction and the thickness direction of the sheet. In addition, the reworkability is required to be difficult to stretch and cut in a direction horizontal to the surface of the sheet. From these viewpoints, in the present invention, the ratio between the product of the breaking strength in the MD direction and the breaking strength in the TD direction and the 25% compressive strength (= formula A) is set within a predetermined range, thereby following the step. Both workability and reworkability.
Therefore, when the fracture compression parameter is outside the range of 700 or more and less than 1500, at least one of the step following property and the rework property is inferior. The fracture compression parameter is preferably 740 to 1400, more preferably 750 to 1380, even more preferably 750 to 1200, and more preferably 800 to 1000.
In order to set the fracture compression parameter within the above range, for example, the stretch ratio during the production of the resin foam sheet may be increased. Alternatively, the degree of crosslinking may be increased, or a resin having high mechanical strength may be used. Moreover, it can adjust by setting an average bubble diameter, an expansion ratio, etc. in the range etc. which are mentioned later suitably.

Here, the breaking strength in the MD direction is preferably 7 to 30 MPa, and more preferably 10 to 25 MPa. The breaking strength in the TD direction is preferably 7 to 30 MPa, and more preferably 10 to 20 MPa. Since the breaking strength in the MD direction and the TD direction is 7 to 30 MPa, when peeling a tape that is thinly punched in both directions, such as a frame mold, it can be easily peeled off without being broken in either direction. .
Further, the MD elongation until breakage is preferably 100 to 600%, more preferably 120 to 500%. The TD elongation is preferably 100 to 600%, more preferably 120 to 300%. When the elongation rate in the MD direction and the TD direction is 600% or less, when the tape end is pulled and peeled off, it can be easily peeled off easily.

Further, the 25% compressive strength is preferably 130 to 500 kPa, and more preferably 140 to 500 kPa. When the compressive strength is 130 to 500 kPa, it is possible to adhere to an adherend with a step without any gap, and to ensure waterproofness.
In addition, 25% compressive strength means what measured the resin foam sheet based on JISK6767.

The degree of crosslinking of the foamed sheet according to the present invention is 35% by mass or more. If the degree of crosslinking is less than 35% by mass, at least one of the step following property and the rework property will be inferior. By setting it as 35 mass% or more, it becomes easy to make the bubble of a resin sheet fine, and also it becomes easy to reduce the dispersion | variation in the magnitude | size of each bubble, and it can improve mechanical strength. The degree of crosslinking is preferably 40 to 65% by mass, more preferably 43 to 60% by mass. By setting it to these upper limit values or less, the foam can be easily foamed appropriately, and the foaming ratio can be easily increased. By increasing the expansion ratio of the foam sheet, it becomes easy to increase the flexibility, and the compressive strength is easily set to an appropriate value.

<Average bubble diameter>
The foamed sheet has an average cell diameter in the MD and TD directions of preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 70 μm or less. Bubbles having such an average bubble diameter are generally called fine bubbles. If the 25% compressive strength is small, the interlaminar strength may be reduced and the tape strength may be impaired, but the strength can be compensated for when the average cell diameter is 100 μm or less. In addition, since the foam sheet has fine bubbles, even when the sheet width is narrowed, many closed cells exist between the narrow widths. Therefore, even when the width is narrowed, appropriate compressive strength and breaking strength can be ensured.

Further, each of the average bubble diameters of MD and TD is preferably 10 μm or more, more preferably 20 μm or more, and further preferably 30 μm or more from the viewpoint of ease of production.

Moreover, it is preferable that the bubbles spread flatly along the surface direction of the resin foam sheet. That is, the ratio of the average bubble diameter of MD to the average bubble diameter of the bubbles ZD (MD / ZD) and the ratio of the average bubble diameter of TD to the average bubble diameter of ZD (TD / ZD) are both larger than 1. More preferably, both are 2 to 7.

In addition, an average bubble diameter means what was measured in the following way.
A resin foam sheet cut into a 50 mm square was prepared as a foam sample for measurement. This was immersed in liquid nitrogen for 1 minute and then cut with a razor blade in the thickness direction along the MD and TD directions. This cross-section is taken with a digital microscope (Keyence Co., Ltd. “VHX-900”), and a 200 times magnified photograph is taken. All of the cross sections present in a length of 2 mm in each of the MD, TD, and ZD directions. The bubble diameter of each bubble was measured, and the operation was repeated 5 times. And the average value of all the bubbles was made into the average bubble diameter of MD direction, TD direction, and ZD direction.
The MD direction means Machine direction and coincides with the extrusion direction, and the TD direction means Transverse direction, which is a direction orthogonal to the MD direction and parallel to the sheet surface of the resin foam sheet. Direction. The ZD direction is the thickness direction of the foam and is a direction perpendicular to both the MD direction and the TD direction.

<Closed cell ratio>
The resin foam sheet of the present invention preferably has closed cells. Having closed cells means that the ratio of closed cells to all bubbles (referred to as “closed cell rate”) is 70% or more. The closed cell ratio is preferably 75% or more, more preferably 90% or more.
The closed cell ratio can be determined according to ASTM D2856 (1998). Commercially available measuring instruments include a dry automatic densitometer Accupic 1330 and the like.

More specifically, the closed cell ratio is measured in the following manner. 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 is measured, the apparent volume V ₁ of the test piece is calculated, and the weight W _{1 of the} test piece is measured. 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 over 3 minutes. Thereafter, the test piece is taken out of 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 rate F ₁ and the closed cell rate F ₂ are calculated based on the following formulas. To do.
Open cell ratio F ₁ (%) = 100 × (W ₂ −W ₁ ) / V ₂
Closed cell ratio F ₂ (%) = 100−F ₁

<Dimension of resin foam sheet>
The thickness of the resin foam sheet is preferably 0.01 to 0.3 mm. When the thickness is 0.01 m or more, it becomes easy to ensure the impact resistance and flexibility of the resin foam sheet. Further, when the thickness is 0.3 mm or less, it is possible to reduce the thickness, and it can be suitably used for a miniaturized electronic device. From these viewpoints, the thickness of the resin foam sheet is more preferably 0.03 to 0.25 mm, and further preferably 0.05 to 0.2 mm.
The resin foam sheet preferably has a narrow width, and specifically, is preferably processed into a thin line shape. For example, the foam sheet may be used with a width of 5 mm or less, preferably 3 mm or less, more preferably 1 mm or less. If the width of the resin foam sheet is narrowed, it can be suitably used inside a miniaturized electronic device.
Although the lower limit of the width | variety of a resin foam sheet is not specifically limited, For example, a 0.1 mm or more thing may be sufficient and a 0.2 mm or more thing may be sufficient. The planar shape of the foam sheet is not particularly limited, but may be an elongated rectangular shape, a frame shape, an L shape, a U shape, or the like. However, in addition to these shapes, any other shapes such as a normal quadrangle and a circle may be used.

<Foaming ratio>
The expansion ratio of the resin foam sheet is preferably 1.2 to 4 cm ³ / g. By setting the expansion ratio within these ranges, the compression strength can be easily adjusted within the above range. Moreover, by setting the expansion ratio to 1.2 cm ³ / g or more, the compressive strength and flexibility are good, and the shock absorption property and sealing property of the foamed sheet are likely to be good. On the other hand, by setting it as 4 cm < ³ > / g or less, mechanical strength becomes high and it becomes easy to improve impact resistance etc.
In view of the foregoing, expansion ratio is more preferably 1.3 ~ 3.5cm ³ / g, more preferably 1.5 ~ 3.0cm ³ / g. In the present invention, the density of the resin foam sheet is determined in accordance with JIS K7222, and the reciprocal thereof is defined as the expansion ratio.

<Polyolefin resin>
Various resins may be used as the resin used for the resin foam sheet, and among them, it is preferable to use a polyolefin resin. By using the polyolefin resin, it is possible to reduce the average cell diameter while ensuring appropriate flexibility of the resin foam sheet.
Examples of the polyolefin resin include polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer. Among these, polyethylene resin is preferable.
Examples of the polyethylene resin include a polyethylene resin polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, and a chromium oxide compound. Preferably, a polyethylene resin polymerized with a polymerization catalyst of a metallocene compound is used.

The polyethylene resin is preferably linear low density polyethylene. By using linear low density polyethylene, flexibility can be imparted to the foamed sheet and the resin foamed sheet can be made thinner. The linear low-density polyethylene is more preferably obtained using a polymerization catalyst such as a metallocene compound. 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. 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. Of these, α-olefins having 4 to 10 carbon atoms are preferred.
Polyethylene resin, for example the density of the above-mentioned linear low density polyethylene is preferably 0.870 ~ 0.910g / cm ^3, more preferably ^{0.875 ~ 0.907g / cm 3, 0.880} ~ 0.905g / Cm ³ is more preferable. As the polyethylene resin, a plurality of polyethylene resins can be used, and a polyethylene resin outside the above-described 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, 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. As a result, since the film can be uniformly stretched, it is easy to make the thickness uniform even if the foam sheet is thinned.

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. The proportion of the cocatalyst used with respect to the metallocene compound is preferably 100,000 to 1,000,000 mole times, more preferably 50 to 5,000 mole times.
When the above-mentioned linear low density polyethylene is used as the polyolefin resin contained in the resin 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, for example. When the other polyolefin resin is contained, 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.

Examples of the ethylene-vinyl acetate copolymer used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.
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- Among these, α-olefins having 6 to 12 carbon atoms are preferable.

In the case where the resin foam sheet uses a polyolefin resin as the resin, the resin contained in the resin foam sheet may include the polyolefin resin alone, but may include a resin other than the polyolefin resin. 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 for the resin foam sheet include various elastomers such as styrene thermoplastic elastomer, ethylene propylene thermoplastic elastomer such as EPDM, and rubber components.

(Pyrolytic foaming agent)
The resin foam sheet of the present invention is preferably formed by foaming a foamable composition containing the above resin and a pyrolytic foaming agent. As the pyrolytic foaming agent, an organic foaming agent or an inorganic foaming agent can be used. Organic foaming agents include azodicarbonamide, azodicarboxylic acid metal salts (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N′-dinitrosopentamethylenetetramine, And hydrazine derivatives such as hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium carbonate, 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 pyrolytic foaming agent in the foamable composition is preferably 1 to 10 parts by weight, more preferably 1.5 to 8 parts by weight, and further preferably 2 to 6 parts by weight with respect to 100 parts by weight of the resin. It is.

Moreover, it is preferable that a foamable composition contains a cell nucleus regulator in addition to the said resin and a thermal decomposition type foaming agent. Examples of the cell nucleus adjusting agent include zinc compounds such as zinc oxide and zinc stearate, and organic compounds such as citric acid and urea. Among these, zinc oxide is more preferable. By using a cell nucleus modifier in addition to the above-mentioned small particle size foaming agent, it becomes easy to reduce the average cell diameter and the variation in the cell diameter. The blending amount of the cell nucleus adjusting agent is preferably 0.4 to 8 parts by mass, more preferably 0.5 to 5 parts by mass, and further preferably 0.8 to 2.5 parts by mass with respect to 100 parts by mass of the resin. It is.
In addition to the above, the foamable composition contains additives generally used for foams such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, fillers, etc. You may do it.

[Method for producing resin foam sheet]
The method for producing the resin foam sheet is not particularly limited. For example, a foamable composition containing a resin and a pyrolyzable foaming agent is crosslinked, heated to foam the pyrolyzable foaming agent, and a draw ratio of 1.1. It manufactures by extending | stretching to at least one of TD direction and MD direction more than double. More specifically, the manufacturing method includes the following steps (1) to (4).
Step (1): Mixing a resin and an additive containing a thermally decomposable foaming agent to form a sheet-like foamable composition (resin sheet) Step (2): Converting the sheet-like foamable composition Step (3) of irradiating ionizing radiation to crosslink the foamable composition: Step (4) of heating the crosslinked foamable composition to foam the pyrolytic foaming agent to obtain a foamed sheet : Stretching the foam sheet in the MD direction or the TD direction or in both directions at a draw ratio of 1.1 times or more

In the step (1), the method of forming the resin sheet is not particularly limited. For example, the resin and additives are supplied to an extruder and melt-kneaded, and the foamable composition is extruded into a sheet form from the extruder. The resin sheet may be formed by
As a method for crosslinking the foamable composition in the step (2), a method of irradiating the resin sheet with ionizing radiation such as electron beam, α ray, β ray, γ ray and the like is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of cross-linking of the foamed sheet to be obtained falls within the desired range described above, but is preferably 5 to 15 Mrad, and more preferably 6 to 13 Mrad.
In the step (3), the heating temperature when the foamable composition is heated to foam the pyrolyzable foaming agent may be not less than the foaming temperature of the pyrolyzable foaming agent, preferably 200 to 300 ° C. The temperature is preferably 220 to 280 ° C.

Stretching of the foam sheet in the step (4) may be performed in both the MD and TD directions, or may be performed only in one direction, but is preferably performed in both. The stretching of the foam sheet may be performed after foaming the resin sheet to obtain the foam sheet, or may be performed while foaming the resin sheet. In addition, when the foamed sheet is stretched after foaming the resin sheet, the foamed sheet may be stretched continuously while maintaining the molten state at the time of foaming without cooling the foamed sheet. After the foam sheet is cooled, the foam sheet may be stretched again by heating it to a molten or softened state. The foam sheet is easily thinned by stretching.
In step (4), the draw ratio of the foamed sheet in one or both of the MD direction and the TD direction is preferably 1.2 to 4.0 times, more preferably 1.5 to 3.3 times. Especially, it is especially preferable to make the draw ratio to both into these ranges. By setting it as such a range, it becomes easy to make a fracture compression parameter into a desired range.
Moreover, when a draw ratio shall be more than the said lower limit, the softness | flexibility and tensile strength of a foam sheet will become favorable easily. On the other hand, when the upper limit value is not reached, the foamed sheet is prevented from breaking during stretching, or the foaming gas escapes from the foamed sheet being foamed and the foaming ratio is significantly reduced. And the quality is easy to be 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 known method such as punching.

However, the 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, an organic peroxide may be blended in advance in the foamable composition, and crosslinking may be performed by a method in which the foamable composition is heated to decompose the organic peroxide. Good.

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 has good reworkability even if it is thin, it can be suitably used inside various portable electronic devices in which the space for placing the foamed sheet is small. Examples of portable electronic devices include mobile phones, cameras, game machines, electronic notebooks, tablet terminals, and notebook personal computers. The foam sheet can be used as an impact absorbing material and a sealing material inside the electronic device.
Moreover, you may use for the adhesive tape which uses a foam sheet as a base material. An adhesive tape becomes difficult to produce a sticking defect etc. by using the foam sheet of this invention with a favorable level | step difference followability and rework property as a base material.

[Adhesive tape]
The pressure-sensitive adhesive tape includes, for example, the resin foam sheet according to the present invention and a pressure-sensitive adhesive layer provided on at least one surface of the resin foam sheet. Tape is preferred.
The thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is preferably 5 to 200 μm. The thickness of the pressure-sensitive adhesive layer is more preferably 7 to 150 μm, still more preferably 10 to 100 μm. When the thickness of the pressure-sensitive adhesive layer is in the range of 5 to 200 μm, the thickness of the structure fixed using the pressure-sensitive adhesive tape can be reduced.
There is no restriction | limiting in particular as an adhesive used for 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 on the pressure-sensitive adhesive layer.
The method of forming the pressure-sensitive adhesive layer on at least one surface of the foamed sheet is not particularly limited, and examples thereof include a method of applying the pressure-sensitive adhesive to at least one surface of the foamed sheet using a coating machine such as a coater. Also, spraying and applying adhesive on at least one surface of the resin foam sheet, applying adhesive using brush on at least one surface of the foam sheet, and foaming the adhesive layer formed on the release sheet Examples thereof include a method for transferring to at least one surface of the sheet.

The present invention will be described in more detail with reference to 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.
<Apparent density and expansion ratio>
The apparent density of the resin foam sheet was measured according to JIS K7222, and the reciprocal thereof was taken as the expansion ratio.
<Degree of crosslinking>
About 100 mg of a test piece is taken from the resin 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 allowed to stand for 24 hours, then 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 value, the degree of crosslinking (% by mass) was calculated by the following formula.
Crosslinking degree (% by mass) = 100 × (B / A)

<Average bubble diameter, closed cell ratio>
The average cell diameter and the closed cell rate were measured by the methods described in the specification.

<25% compressive strength>
The resin foam sheet was measured for 25% compressive strength according to JIS K6767.

<Strength at break and elongation>
The resin foam sheet was cut into a dumbbell shape No. 1 defined in JIS K6251 4.1. Using this as a sample, the breaking strength in the MD and TD directions and the elongation at that time were measured in accordance with JIS K6767 at a measurement temperature of 23 ° C.
<Parameter>
The fracture compression parameters were determined from the formula A with the breaking strength (MPa) in the MD direction and TD direction of the resin foam sheet and 25% compressive strength (kPa).

<PUSH adhesive strength evaluation test>
(Production of double-sided adhesive tape)
The adhesive layer obtained by the following method was laminated | stacked on both surfaces of the resin foam sheet, and the double-sided adhesive tape which uses a resin foam sheet as a base material was produced in the following ways.

In a reactor equipped with a thermometer, a stirrer, and a condenser, 75 parts by weight of butyl acrylate, 22 parts by weight of 2-ethylhexyl acrylate, 3 parts by weight of acrylic acid, 0.2 parts by weight of 2-hydroxyethyl acrylate, and 80 parts by weight of ethyl acetate Was added, and the atmosphere was replaced with nitrogen, and then the reactor was heated to start refluxing. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator in the reactor. The solution was refluxed for 5 hours to obtain a solution of the acrylic copolymer (z). With respect to the obtained acrylic copolymer (z), the weight average molecular weight was measured by a GPC method using “2690 Separations Model” manufactured by Water Co. as a column, which was 600,000.
15 parts by mass of polymerized rosin ester having a softening point of 135 ° C., ethyl acetate (Fuji Kagaku) with respect to 100 parts by mass of the solid content of the acrylic copolymer (z) contained in the solution of the obtained acrylic copolymer (z) 125 parts by mass of Yakuhin Co., Ltd. and 2 parts by mass of an isocyanate-based cross-linking agent (manufactured by Tosoh Corporation, Coronate L45) were added and stirred to obtain a pressure-sensitive adhesive (Z). The degree of crosslinking of the acrylic pressure-sensitive adhesive was 33% by mass.
A release paper having a thickness of 150 μm was prepared, an adhesive (Z) was applied to the release-treated surface of the release paper, and dried at 100 ° C. for 5 minutes to form an acrylic adhesive layer having a thickness of 50 μm. . This acrylic pressure-sensitive adhesive layer was bonded to the surface of a base material made of a foam sheet. Next, in the same manner, the same acrylic pressure-sensitive adhesive layer as above was bonded to the opposite surface of the substrate. As a result, a double-sided pressure-sensitive adhesive tape whose both surfaces were covered with a release paper having a thickness of 150 μm was obtained.

(Production of test equipment)
(1) Test device A: PC / Glass
In FIG. 1, the schematic diagram of the push test of a double-sided adhesive tape is shown. The obtained double-sided pressure-sensitive adhesive tape was punched into an outer diameter of 46 mm, a length of 61 mm, an inner diameter of 44 mm, and a length of 59 mm to produce a frame-shaped test piece 1 having a width of 1 mm. Next, as shown in FIG. 1 (a), the square hole is approximately in the center of the test piece 1 from which the release paper has been peeled off from the polycarbonate plate 3 having a thickness of 38 mm and a square hole having a width of 50 mm and a thickness of 2 mm. Paste so that it is located at. Thereafter, a glass plate 5 having a width of 55 mm, a length of 65 mm, and a thickness of 2 mm was pasted from the upper surface of the test piece 1 so that the test piece 1 was positioned substantially at the center, and the test apparatus A was assembled.
Thereafter, the glass plate 5 and the polycarbonate plate positioned at the top and bottom are pressure-bonded by applying a pressure of 30 kgf at 70 ° C. for 10 seconds from the glass plate 5 side located on the upper surface of the test apparatus A, and left at room temperature for 24 hours. .

(2) Test device B: SUS / Glass
A test apparatus B was assembled in the same manner as the test apparatus A except that the polycarbonate plate 3 was a stainless steel plate (SUS304, thickness: 2 mm). Thereafter, in the same manner as in the case of the test apparatus A, thermocompression bonding was performed and the mixture was left at room temperature for 24 hours.

(Push test)
As shown in FIG. 1 (b), the produced test apparatus A or test apparatus B is turned upside down (the glass plate 5 is directed downward) and fixed to the support base, and the lower surface of the lower surface is fixed at a speed of 10 mm / min from the opening side. The glass plate 5 was pushed and the load (N) when the glass plate 5 was peeled was measured. The measurement was performed at 23 ° C.
In addition, if the PUSH adhesive strength is 70 to 90N in PC / Glass, and if the PUSH adhesive strength is 150 to 190N in SUS / Glass, it can be said that the ease of sticking due to softness is good, and follows the step. Property can be improved.

<Reworkability evaluation test>
First, the adhesive tape which provided the acrylic adhesive layer on the single side | surface of the resin foam sheet in the same way as the "PUSH adhesive strength evaluation test" was prepared. In an environment of room temperature 23 ° C. and relative humidity 50%, an adhesive tape cut to a size of 2 mm × 100 mm was attached to a stainless steel plate and left for 24 hours. Thereafter, the adhesive tape was peeled off, and the peeled state was subjected to sensory evaluation. If it was peeled off in the same state as before pasting, it was evaluated as “A” as good reworkability, and if the sheet was torn or stretched and glue remained on the stainless steel plate, it was evaluated as “B” as poor reworkability.

[Polyolefin resin]
The polyolefin resin used in this example is shown below.
Resin A: Linear low-density polyethylene resin (manufactured by Dow Chemical Company, trade name “Affinity PL1850”, density 0.902 g / cm ³ )
Resin B: Linear low-density polyethylene resin (manufactured by Dow Chemical Co., Ltd., trade name “Affinity KC8852”, density 0.875 g / cm ³ )
Resin C: Hydrogenated styrene / isoprene / butadiene block copolymer resin (manufactured by Kuraray Co., Ltd., trade name “HIBLER 7311”, density 0.890 g / cm ³ )

[Example 1]
100 parts by weight of resin A, 5 parts by weight of azodicarbonamide having a particle size of 13 μm as a pyrolytic foaming agent, and zinc oxide as a cell nucleus regulator (trade name “OW-212F” manufactured by Sakai Chemical Industry Co., Ltd.) 1 0.0 parts by mass and 0.5 parts by mass of an antioxidant (acid prevention) were supplied to an extruder, melted and kneaded at 130 ° C., and extruded into a long resin sheet having a thickness of 300 μm.
Next, after the resin sheet was crosslinked by irradiating an electron beam with an acceleration voltage of 500 kV on both surfaces of the long resin sheet for 7 Mrad, the crosslinked resin sheet was maintained at 250 ° C. by hot air and an infrared heater. And foamed by heating to obtain a foamed sheet having a thickness of 500 μm.
Next, after the obtained foamed sheet was continuously fed out from the foaming furnace, the foamed sheet was maintained in the TD direction with 2.5% in the TD direction in a state where the temperature of the both surfaces was maintained at 200 to 250 ° C. The foamed sheet is wound at a winding speed higher than the speed at which the foamed sheet is fed into the foaming furnace (supply speed). Thus, the foamed sheet was stretched 2.0 times in the MD direction to obtain a resin foamed sheet (thickness: 0.1 mm). In addition, the winding speed | rate of the said resin foam sheet was adjusted considering the expansion | swelling part to MD direction by foaming of the resin sheet itself. The obtained resin foam sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.

[Examples 2 to 6 and Comparative Examples 1 to 5]
A resin foam sheet was obtained in the same manner as in Example 1 except that the thicknesses of the resin, additive, and resin foam sheet were changed as shown in Tables 1 and 2 below. The draw ratio of MD and TD was adjusted within the range of 1.5 to 3.5. The obtained resin foam sheet was evaluated according to the above evaluation method, and the results are shown in Tables 1 and 2.

1 Test piece 3 Polycarbonate plate 5 Glass plate

Claims

A resin foam sheet having a fracture compression parameter represented by the following formula A of 700 or more and less than 1500 and a degree of crosslinking of 35% by mass or more.
Formula A: Strength at break in MD direction (MPa) × Strength at break in TD direction (MPa) / 25% compressive strength (kPa) × 1000
The resin foam sheet according to claim 1, wherein the average cell diameter in each of the MD direction and the TD direction is 100 μm or less.
The resin foam sheet according to claim 1 or 2, wherein the thickness is 0.01 to 0.3 mm.
The resin foam sheet according to any one of claims 1 to 3, wherein the breaking strength in the MD direction is 7 to 30 MPa, and the breaking strength in the TD direction is 10 to 25 MPa.
The resin foam sheet according to any one of claims 1 to 4, wherein the 25% compressive strength is 130 to 500 kPa.
The resin foam sheet according to any one of claims 1 to 5, wherein an expansion ratio is 1.2 to 4 cm 3 / g.
The resin foam sheet according to any one of claims 1 to 6, wherein the resin foam sheet contains a polyolefin resin.
The resin foam sheet according to claim 7, wherein the polyolefin resin is a polyethylene resin.
The resin foam sheet according to claim 7 or 8, wherein the polyolefin resin is a linear low density polyethylene polymerized with a polymerization catalyst of a metallocene compound.
A method for producing a resin foam sheet according to any one of claims 1 to 9,
A foamable composition containing a resin and a thermally decomposable foaming agent is crosslinked, heated to foam the thermally decomposable foaming agent, and stretched in at least one of the TD direction and MD direction at a stretch ratio of 1.1 times or more. A method for producing a resin foam sheet.
10. An adhesive tape comprising the resin foam sheet according to any one of claims 1 to 9 and an adhesive layer provided on at least one surface of the resin foam sheet.