WO2018181982A1

WO2018181982A1 - Cross-linked resin foam sheet, production method therefor, and adhesive tape

Info

Publication number: WO2018181982A1
Application number: PCT/JP2018/013834
Authority: WO
Inventors: 麻美永井; 哲史濱田
Original assignee: 積水化学工業株式会社
Priority date: 2017-03-31
Filing date: 2018-03-30
Publication date: 2018-10-04
Also published as: JPWO2018181982A1; CN110446747A; KR20190129886A; CN115819827A; CN110446747B; JP6901476B2

Abstract

This cross-linked resin foam sheet has closed cells, and the decrease in compressive strength (C1) measured with a 1 mm sample width is at most 60% of the compressive strength (C20) measured with a 20 mm sample width. The present invention can provide a resin foam sheet that is prevented from becoming excessively flexible even when the width is narrowed, and has excellent impact resistance.

Description

Cross-linked resin foam sheet, method for producing the same, and adhesive tape

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

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, recently, electronic devices have been reduced in size, and advanced functions of various parts have been advanced. As a result, restrictions on the space inside electric devices have increased, and the width of foam sheets used inside electronic devices has become narrower. is there. For example, the width of the frame portion outside the display panel is reduced due to the downsizing of electronic devices and the increase in size of the display device, and the width of the adhesive tape attached to the frame portion is also reduced.
However, when the width of the foamed sheet is narrowed, the force applied per unit area is increased and the material is easily broken. Therefore, the foamed sheet may be damaged by an impact such as dropping an electric device. In addition, since the bubbles in the foam sheet are opened at the end face of the sheet and exhibit the behavior of open cells, if the width of the sheet is narrow, the foam becomes too flexible and may cause a sticking failure or the like.
Therefore, the foamed sheet is required to prevent excessive flexibility even when the width is reduced, and to improve durability such as impact resistance.

The present invention has been made in view of the above circumstances, for example, to provide a resin foam sheet that can be prevented from becoming too flexible even when the width is narrowed and that is excellent in impact resistance. To do.

As a result of intensive studies, the inventors have found that when the sample width of the crosslinked resin foam sheet is changed from a thick width to a narrow width, the above problem can be solved by suppressing the rate of decrease in compressive strength. Completed. That is, the present invention provides the following [1] to [12].
[1] A cross-linked resin foam sheet having closed cells, and the reduction rate of the compressive strength (C ₁ ) measured at a sample width of 1 mm with respect to the compressive strength (C ₂₀ ) measured at a sample width of 20 mm is 60% or less. There is a crosslinked resin foam sheet.
[2] The cross-linked resin foam sheet according to [1], wherein both the average cell diameters in the MD direction and the TD direction are 100 μm or less.
[3] The crosslinked resin foamed sheet according to [1] or [2], wherein the thickness is 0.03 to 0.50 mm.
[4] The crosslinked resin foamed sheet according to any one of the above [1] to [3], which contains a polyolefin resin.
[5] The crosslinked resin foam sheet according to [4], wherein the polyolefin resin is a polyethylene resin.
[6] The crosslinked resin foamed sheet according to the above [4], wherein the polyolefin resin is a linear low-density polyethylene polymerized with a metallocene compound polymerization catalyst.
[7] The crosslinked resin foam sheet according to any one of [1] to [6], wherein the degree of crosslinking is 30% by mass or more.
[8] The crosslinked resin foamed sheet according to any one of [1] to [7], wherein the expansion ratio is 1.2 to 4.0 cm ³ / g.
[9] The crosslinked resin foamed sheet according to any one of [1] to [8], wherein the width is 5 mm or less.
[10] The crosslinked resin foamed sheet according to any one of the above [1] to [9], which is obtained by foaming a foamable composition containing a resin and a thermally decomposable foaming agent.
[11] The method for producing a crosslinked resin foam sheet according to any one of [1] to [10] above,
A method for producing a crosslinked resin foamed sheet, wherein a foamable composition containing a resin and a thermally decomposable foaming agent is crosslinked and heated to foam the thermally decomposable foaming agent.
[12] An adhesive tape comprising the crosslinked resin foam sheet according to any one of the above [1] to [10] and an adhesive layer provided on at least one surface of the crosslinked resin foam sheet.

According to the present invention, even when the width is narrowed, it is possible to provide a resin foam sheet that prevents excessive flexibility and is excellent in impact resistance.

It is a schematic diagram of an impact resistance test apparatus.

Hereinafter, the present invention will be described in detail using embodiments.
[Crosslinked resin foam sheet]
The cross-linked resin foam sheet according to the present invention (hereinafter also simply referred to as “foam sheet”) is a foam sheet having closed cells, and measured at a sample width of 1 mm with respect to the compressive strength (C ₂₀ ) measured at a sample width of 20 mm. The decrease rate of the compressed strength (C ₁ ) is 60% or less. The rate of decrease in compressive strength is calculated from (C ₂₀ -C ₁ ) / C ₂₀ .
The foamed sheet of the present invention can maintain a high compressive strength even if the sample width is narrowed by reducing the rate of decrease in compressive strength. Therefore, even when the sheet width is narrowed, the foamed sheet is prevented from becoming too flexible, and sticking failure or the like that occurs when sticking an adhesive tape using the foamed sheet as a base material is less likely to occur. Further, even when the foamed sheet has a narrow width, the mechanical strength such as impact resistance is maintained high, and the foamed sheet can be suitably used as an impact absorbing material in a miniaturized electronic device.

In order to make the foamed sheet flexible and have good impact resistance, the rate of decrease in the compression strength is preferably 55% or less, more preferably 40% or less, and even more preferably 30% or less. preferable. Further, the lowering rate of the compressive strength is preferably as low as possible, but is practically 5% or more.

In the present invention, either the reduction rate when measuring the 10% compressive strength or the reduction rate when measuring the 25% compressive strength may be within the above range, but both are within the above range. It is preferable that When any reduction rate is within the above range, various performances such as sticking property and impact resistance are improved under various use conditions. In addition, the measuring method of 10% and 25% compressive strength is as showing in the Example mentioned later.

[Closed cell ratio]
The foam sheet of the present invention 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

[Average bubble diameter]
The foamed sheet preferably has an average cell diameter in the MD and TD directions of preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less. Bubbles having such an average bubble diameter are generally called fine bubbles. Since the foam sheet has fine bubbles, even when the sheet width is narrowed, a large number of closed cells exist between the narrow widths.
At the end face of the foam sheet, bubbles are cut and behave like open cells, causing a decrease in compressive strength. However, the bubbles to be cut are fine bubbles and there are many closed cells in a narrow width. By doing so, it is possible to minimize a decrease in compressive strength due to air bubbles at the sheet end face. Therefore, it is possible to reduce the rate of decrease in compressive strength by setting the average bubble diameter of the bubbles within the above range.
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.

In addition, an average bubble diameter means what was measured in the following way.
A 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, and all the bubbles present on the cut surface for a length of 2 mm in each of the MD direction and the TD direction are observed. The bubble diameter 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 and each TD 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 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.

In the present invention, it is preferable that the fine bubbles have a small variation in bubble diameter in the MD and TD directions. For this reason, the standard deviation of the bubble diameters in the MD and TD directions is preferably 60 μm or less, and more preferably 40 μm or less. The standard deviation of the bubble diameter is preferably as low as possible and is preferably 0 μm, but is preferably 1 μm or more from a practical viewpoint. In addition, the standard deviation of the bubble diameter of MD and TD direction is calculated based on each bubble diameter measured in order to obtain | require the average bubble diameter of above-mentioned MD and TD direction.
When the variation in the bubble diameter is small, the size of the bubble wall between the bubbles tends to be uniform, and the bubble wall with low mechanical strength is reduced. Moreover, it becomes difficult for a large bubble to exist in the end surface of a foam sheet. Therefore, even if a width | variety is narrowed, a foaming sheet will become stable in compressive strength, and will become easy to make the above-mentioned reduction rate of compressive strength low.

[Crosslinking degree]
The foam sheet is a cross-linked foam, and the degree of cross-linking is preferably 30% by mass or more. The degree of cross-linking is more preferably 35 to 65% by mass, and further preferably 40 to 49% by mass. By setting the degree of cross-linking to the lower limit value or more, it is easy to make fine bubbles in the cross-linked foamed resin sheet, and it becomes easy to reduce variation in the size of each air bubble. Moreover, it becomes easy to foam a foam appropriately by setting it as these upper limit values or less, and it becomes easy to raise a foaming ratio. 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.

[Dimensions of resin foam sheet]
The thickness of the foam sheet is preferably 0.03 to 0.5 mm. When the thickness is 0.03 mm or more, it becomes easy to ensure the impact resistance and flexibility of the foam sheet. Further, when the thickness is 0.5 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 0.08 to 0.40 mm, and further preferably 0.10 to 0.25 mm.
The foamed 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. Moreover, even if the width | variety of the foamed sheet of this invention narrows, impact resistance and a softness | flexibility are maintained favorable.
Although the lower limit of the width | variety of a foamed 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 foaming ratio of the foamed sheet is preferably 1.2 to 4.0 cm ³ / g. By setting the expansion ratio to 1.2 cm ³ / g or more, the compressive strength and flexibility are improved, and the impact absorption and sealing properties of the foamed sheet are likely to be improved. On the other hand, by setting it as 4.0 cm < ³ > / g or less, mechanical strength becomes high and it becomes easy to improve impact resistance etc. Moreover, it becomes easy to reduce the variation of the average bubble diameter and the bubble diameter.
In view of the foregoing, expansion ratio is more preferably 1.3 ~ 3.5cm ³ / g, more preferably 2.0 ~ 3.0cm ³ / g. In the present invention, the density of the foamed sheet is obtained according to JIS K7222, and the reciprocal thereof is taken as the foaming ratio.

[Compressive strength]
The 10% compressive strength (C ₁ ) of the foamed sheet with a sample width of 1 mm is preferably 30 to 600 kPa, more preferably 40 to 350 kPa, and even more preferably 100 to 200 kPa.
The 25% compressive strength (C ₁ ) of the foamed sheet with a sample width of 1 mm is preferably 150 to 1500 kPa, more preferably 250 to 1400 kPa, and further preferably 270 to 600 kPa.
By setting the compressive strength (C ₁ ) within the above range, the foamed sheet has appropriate flexibility, and the impact resistance tends to be good. Further, it is possible to prevent the sticking property from being lowered due to being too flexible.

[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 and the variation in the cell diameter while ensuring appropriate flexibility of the resin foam sheet.
Examples of the polyolefin resin include a polyethylene resin, a polypropylene resin, an ethylene-vinyl acetate copolymer, or a mixture thereof. Among these, a 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. Since the uniformly cross-linked sheet is uniformly foamed, it is easy to reduce the variation in the bubble diameter as described above. Moreover, since it can extend | stretch uniformly, it becomes easy to make the thickness of a foam sheet 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. 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 foam sheet, the above linear low density polyethylene may be used alone, or may be used in combination with other polyolefin resins. For example, you may use together with the other polyolefin resin described below. 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 (EVA) used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene. When an ethylene-vinyl acetate copolymer (EVA) is used, it is preferably used in combination with the polyethylene resin (PE) described above, and the mass ratio (EVA / PE) is preferably 10/90 to 90/10, 80 to 80/20 is more preferable, and 50/50 to 80/20 is even more preferable.
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 polyolefin sheet is used as the resin for the foam sheet, the resin contained in the foam sheet may be a polyolefin resin alone or may contain 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 foamed sheet include various elastomers such as a styrene thermoplastic elastomer, an ethylene propylene thermoplastic elastomer such as EPDM, and a rubber component.

(Pyrolytic foaming agent)
The foam sheet of the present invention is preferably formed by foaming a foamable composition containing the above resin and a pyrolytic foaming agent. Moreover, as a thermal decomposition type foaming agent, it is preferable to use a thing with a particle size of less than 15 micrometers. By using a particle having a particle size of less than 15 μm, it becomes easy to reduce the bubble diameter of the foamed sheet and the variation in the bubble diameter in combination with the relatively high degree of crosslinking as described above. The particle size of the pyrolytic foaming agent is preferably 2 to 14 μm, more preferably 5 to 13 μm. Further, as described above, the variation in the particle diameter of the foaming agent is preferably small in order to suppress the variation in the bubble diameter.
The particle size of the pyrolytic foaming agent is a value measured by a laser diffraction method and means a particle size (D50) corresponding to a cumulative frequency of 50%.

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 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 amount of the thermally decomposable foaming agent in the foamable composition is preferably 1 to 10 parts by weight, more preferably 1.5 to 5 parts by weight, and further preferably 2 to 4 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 foam sheet]
The method for producing the foamed sheet is not particularly limited. For example, the foamed sheet is produced by crosslinking a foamable composition containing a resin and a pyrolyzable foaming agent and heating the foamable composition to foam. 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 : A step of stretching the foam sheet in one or both of the MD direction and the TD direction

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.

The stretching of the foam sheet in the step (4) 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 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 1.5 to 4.0 times.
When the draw ratio is set to the above lower limit or more, the flexibility and tensile strength of the foam sheet are likely to be good. 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. Further, the step (4), that is, stretching of the foam sheet may be omitted.

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 and has high impact resistance and moderate flexibility even when the width is narrowed, it can be suitably used in 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 which has moderate softness | flexibility as a base material.

The pressure-sensitive adhesive tape includes, for example, a foam sheet and a pressure-sensitive adhesive layer provided on at least one surface of the foam sheet, but a double-sided pressure-sensitive adhesive tape provided with a pressure-sensitive adhesive layer on both sides is preferable.
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 for forming the pressure-sensitive adhesive layer on at least one surface of the foamed sheet is not particularly limited, but the following methods are exemplified. For example, a method of applying an adhesive on at least one surface of a foam sheet using a coater or the like, a method of spraying and applying an adhesive using a spray on at least one surface of a resin foam sheet, on at least one surface of a foam sheet Examples thereof include a method of applying a pressure-sensitive adhesive using a brush, a method of transferring a pressure-sensitive adhesive layer formed on a release sheet to at least one surface of a foamed sheet, and the like.

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)
<Closed cell ratio>
It was measured according to the method described in the specification.
<Average bubble diameter>
The average cell diameter was measured by the method described in the specification.

<Compressive strength>
The 10% and 25% compressive strengths (C ₂₀ ) are JIS except that a foam sheet is punched into 20 mm × 20 mm to obtain a sample, and the sample is used to obtain a thickness of one sheet. Measurement was performed according to K6767.
10% and 25% compressive strength (C ₁ ) was obtained by punching a foam sheet into 1.0 mm × 20 mm to obtain samples, and placing the 20 samples on a measuring machine so that the sheets do not overlap each other. % And 25% compression strength (C ₂₀ ) were measured in the same manner. In addition, the foam sheet of a present Example and the comparative example was sampled so that the longitudinal direction of a sample might correspond to MD direction.

<Impact resistance>
(Adjustment of impact resistance evaluation sample)
The adhesive layer obtained by the following method was laminated | stacked on both surfaces of the foam sheet obtained by the Example and the comparative example, and the double-sided adhesive tape which uses a foam sheet as a base material was produced in the following ways.
(Production method of double-sided adhesive tape)
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 surface of the release paper, and dried at 100 ° C. for 5 minutes to form an acrylic adhesive layer having a thickness of 30 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 impact resistance test equipment)
FIG. 1 shows a schematic diagram of an impact resistance test apparatus.
The impact resistance test apparatus was produced by the following procedure.
First, the double-sided adhesive tape obtained above was punched out so that the outer diameter was 15.0 mm in width, 15.0 mm in length, the inner diameter was 13.6 mm in width, and 13.6 mm in length. A test piece 1 having a square frame shape of 7 mm was prepared.
Next, as shown in FIG. 1 (a), a polycarbonate or SUS-made adherence plate 3 having a square hole 2 provided in the center is prepared, and the test piece 1 from which the release paper has been peeled off is used as the adherence plate 3 Affixed over the entire outer periphery of the hole 2 on the upper surface.
Next, a glass-made adherend plate 4 of a size covering the hole 2 was laminated and pasted on the test piece 1, and the hole 2 was covered to assemble an impact resistance test apparatus.
Then, with the impact resistance test apparatus turned upside down and with the adherend plate 3 on the upper surface, a pressure of 5 kgf is applied for 5 seconds from the adherend plate 3 side, and the adherend plate 3 and the test piece positioned vertically. And were allowed to stand at room temperature for 36 hours. In the impact resistance test, the case where the adherend plate 3 was made of polycarbonate (PC) and SUS was evaluated.

(Judgment of impact resistance)
As shown in FIG. 1 (b), the produced impact resistance test apparatus is fixed to the support base 5, and an iron ball 6 weighing 50 g passing through the hole 2 formed in the adherend plate 3 is attached. , Dropped to pass through hole 2. Gradually increase the height at which the iron ball is dropped, and measure the height at which the iron ball was dropped when the test piece and the adherend peeled off due to the impact applied by dropping the iron ball to evaluate impact resistance did. When the adherend plate 3 was polycarbonate, it was 32 cm or more, and when it was SUS, it was evaluated as “A”. When the adherend plate 3 is made of polycarbonate, it is less than 32 cm, and when it is made of SUS, it is 38 cm or more, or when the adherend plate 3 is made of polycarbonate, it is more than 32 cm, and when made of SUS, it is less than 38 cm. This case was evaluated as “B”. The case where the adherend plate 3 was less than 32 cm when it was polycarbonate and less than 38 cm when it was made of SUS was evaluated as “C”.

[Example 1]
100 parts by mass of a linear low-density polyethylene resin (resin A) obtained by a polymerization catalyst of a metallocene compound, 3.4 parts by mass of azodicarbonamide having a particle size of 13 μm as a thermal decomposable foaming agent, and a cell nucleus regulator 1.0 part by mass of zinc oxide (trade name “OW-212F” manufactured by Sakai Chemical Industry Co., Ltd.) and 0.5 part by mass of an antioxidant were supplied to the extruder. Next, it was melt-kneaded at 130 ° C. and extruded into a long resin sheet having a thickness of 260 μm. As the resin A, a trade name “Affinity PL1850” (density 0.902 g / cm ³ ) manufactured by Dow Chemical Co., Ltd. was used.
Next, 7 Mrad of an electron beam with an acceleration voltage of 500 kV was irradiated on both surfaces of the long resin sheet to crosslink the resin sheet. Thereafter, the cross-linked resin sheet was continuously fed into a foaming furnace maintained at 250 ° C. with hot air and an infrared heater and heated to be foamed to obtain a foamed sheet having a thickness of 300 μm.
Subsequently, the obtained foam sheet was continuously sent out from the foaming furnace. The foam sheet is stretched at a stretch ratio of 2.0 times in the TD direction while maintaining the temperature of the both surfaces at 200 to 250 ° C., and the foam sheet is supplied to the foam furnace. The foamed sheet was stretched also in the MD direction by winding the foamed sheet at a winding speed higher than the feeding speed (supply speed) to obtain a foamed sheet. The winding speed of the foam sheet was adjusted in consideration of the expansion in the MD direction due to foaming of the resin sheet itself. The obtained foamed sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.

[Examples 2 to 7, Comparative Examples 1 to 3]
The composition of the polyolefin resin composition was changed as shown in Tables 1 and 2, and the dose at the time of crosslinking was adjusted to be the degree of crosslinking in Tables 1 and 2, and the TD draw ratio was 2.0 times to It implemented similarly to Example 1 except the point adjusted to 3.5 time.
In addition, the polyolefin resin used in Example 6 is as follows.
Resin B: Ethylene / vinyl acetate copolymer resin, manufactured by Mitsubishi Chemical Corporation, trade name “Novatec EVA”
Resin C: Linear low density polyethylene, manufactured by Prime Polymer Co., Ltd., trade name “Evaflex 460-H”

As described above, in Examples 1 to 7, the rate of decrease in compressive strength ((C ₂₀ -C ₁ ) / C ₂₀ ) is 60% or less, and it does not become too flexible even when the sheet width is narrowed. And the impact resistance was sufficiently high. On the other hand, in Comparative Examples 1 to 3, the rate of decrease in compressive strength was higher than 60%. Therefore, when the sheet width was narrowed, the sheet became too flexible and the impact resistance could not be sufficiently increased.

DESCRIPTION OF SYMBOLS 1 Test piece 2 Hole 3 Magnesium adherend board 4 Glass adherend board 5 Support stand 6 Iron ball

Claims

A crosslinked resin foamed sheet having closed cells, wherein the rate of decrease in compressive strength (C 1 ) measured at a sample width of 1 mm relative to the compressive strength (C 20 ) measured at a sample width of 20 mm is 60% or less. Resin foam sheet.
The cross-linked resin foamed sheet according to claim 1, wherein both the average cell diameter in the MD direction and the TD direction are 100 µm or less.
The cross-linked resin foam sheet according to claim 1 or 2, wherein the thickness is 0.03 to 0.50 mm.
The crosslinked resin foamed sheet according to any one of claims 1 to 3, comprising a polyolefin resin.
The cross-linked resin foam sheet according to claim 4, wherein the polyolefin resin is a polyethylene resin.
The cross-linked resin foamed sheet according to claim 4, wherein the polyolefin resin is a linear low density polyethylene polymerized with a polymerization catalyst of a metallocene compound.
The crosslinked resin foam sheet according to any one of claims 1 to 6, wherein the degree of crosslinking is 30% by mass or more.
The cross-linked resin foam sheet according to any one of claims 1 to 7, wherein an expansion ratio is 1.2 to 4.0 cm 3 / g.
The crosslinked resin foam sheet according to any one of claims 1 to 8, wherein the width is 5 mm or less.
The cross-linked resin foam sheet according to any one of claims 1 to 9, wherein a foamable composition containing a resin and a thermally decomposable foaming agent is foamed.
A method for producing a crosslinked resin foam sheet according to any one of claims 1 to 10,
A method for producing a crosslinked resin foamed sheet, wherein a foamable composition containing a resin and a thermally decomposable foaming agent is crosslinked and heated to foam the thermally decomposable foaming agent.
An adhesive tape comprising the crosslinked resin foam sheet according to any one of claims 1 to 10 and an adhesive layer provided on at least one surface of the crosslinked resin foam sheet.