CN115819827A

CN115819827A - Crosslinked resin foam sheet, method for producing same, and adhesive tape

Info

Publication number: CN115819827A
Application number: CN202211555302.6A
Authority: CN
Inventors: 永井麻美; 滨田哲史
Original assignee: Sekisui Chemical Co Ltd
Current assignee: Sekisui Chemical Co Ltd
Priority date: 2017-03-31
Filing date: 2018-03-30
Publication date: 2023-03-21
Also published as: KR20190129886A; CN110446747B; CN110446747A; JP6901476B2; JPWO2018181982A1; WO2018181982A1

Abstract

The crosslinked resin foam sheet according to the present invention is a crosslinked resin foam sheet having independent cells and having a compressive strength (C) measured with respect to a sample width of 20mm ₂₀ ) Compressive Strength (C) measured at a sample width of 1mm ₁ ) The reduction rate of (2) is 60% or less. According to the present invention, a resin foam sheet having excellent impact resistance can be provided which can prevent excessive softening even when the width is reduced.

Description

Crosslinked resin foam sheet, method for producing same, and adhesive tape

The invention of the application is a divisional application with application number 201880018702.9, the name of the invention is a crosslinked resin foamed sheet, a manufacturing method thereof, an adhesive tape and application date of 2018, 3 months and 30 days.

Technical Field

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

Background

Conventionally, a sealing material or an impact absorbing material formed of a foamed sheet has been used for electronic devices such as a cellular phone, a camera, a game machine, an electronic notebook, a tablet terminal, and a notebook personal computer. These sealing materials and impact absorbing materials are sometimes used as tapes or the like having a foamed sheet as a base material. For example, in general, the display device in the electronic apparatus has a structure in which a protective panel is provided on a display panel such as an LCD, and in order to attach the protective panel to a frame portion outside the display panel, an adhesive tape having a foam sheet as a base material is used.

As a foamed sheet used in an electronic device, a crosslinked polyolefin resin foamed sheet obtained by foaming and crosslinking a foamable polyolefin resin sheet containing a thermal decomposition type foaming agent is known (for example, see patent document 1).

Documents of the prior art

Patent literature

Patent document 1: international publication No. 2005/007731

Disclosure of Invention

Problems to be solved by the invention

However, recently, miniaturization of electronic devices has been advanced, and further, high functionality of various parts has been advanced, so that restriction of internal space of the electronic devices has been increased, and the width of the foam sheet used in the electronic devices tends to be narrowed. For example, the frame portion outside the display panel is narrowed in width due to the miniaturization of electronic equipment and the upsizing of display devices, and the tape attached to the frame portion is also narrowed in width.

However, if the width of the foamed sheet is narrowed, the force acting on the unit area becomes large and the material is likely to be broken, and therefore, the foamed sheet may be broken by an impact when the electric device is dropped or the like. Further, since the cells of the foamed sheet are open at the end face of the sheet and exhibit behavior of continuous cells, if the width of the sheet is narrow, the sheet may become too soft, causing adhesion failure or the like.

Therefore, there is a demand for a foamed sheet that can be prevented from becoming too soft even when the width is reduced, and that can improve the durability such as impact resistance.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a resin foamed sheet which is prevented from becoming too soft even when the width is narrowed, and has excellent impact resistance.

Means for solving the problems

The inventors have conducted intensive studies and found that the above problems can be solved by suppressing the rate of decrease in compressive strength when the sample width of the crosslinked resin foamed sheet is changed from thick to thin, and thus the following invention has been accomplished. Namely, the present invention provides the following [1] to [12].

[1]A crosslinked resin foamed sheet having independent cells and having a compressive strength (C) measured with respect to a sample width of 20mm ₂₀ ) Compressive Strength (C) measured at a sample width of 1mm ₁ ) The reduction rate of (D) is 60% or less.

[2] The crosslinked resin foamed sheet according to the above [1], wherein the average cell diameter in both the MD direction and the TD direction is 100 μm or less.

[3] The crosslinked resin foamed sheet according to the above item [1] or [2], having a thickness of 0.03 to 0.50mm.

[4] The crosslinked resin foamed sheet according to any one of the above [1] to [3], which comprises a polyolefin resin.

[5] The crosslinked resin foamed sheet according to the above [4], wherein the polyolefin resin is a polyethylene resin.

[6] The crosslinked resin foamed sheet according to item [4] above, wherein the polyolefin resin is a linear low-density polyethylene polymerized by using a metallocene compound as a polymerization catalyst.

[7] The crosslinked resin foamed sheet according to any one of the above [1] to [6], wherein the degree of crosslinking is 30% by mass or more.

[8]According to the above [1]～[7]The crosslinked resin foam sheet described in any one of the above items, having a foaming ratio of 1.2 to 4.0cm ³ /g。

[9] The crosslinked resin foamed sheet according to any one of the above [1] to [8], having a width of 5mm 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 thermal decomposition type foaming agent.

[11] A method for producing a crosslinked resin foamed sheet according to any one of the above [1] to [10], wherein a foamable composition containing a resin and a thermal decomposition type foaming agent is crosslinked and heated to foam the thermal decomposition type foaming agent.

[12] An adhesive tape comprising the crosslinked resin foam sheet according to any one of [1] to [10] above and an adhesive layer provided on at least one surface of the crosslinked resin foam sheet.

Effects of the invention

According to the present invention, a resin foam sheet having excellent impact resistance can be provided, which is prevented from becoming too soft even when the width is reduced.

Drawings

FIG. 1 is a schematic view of an impact resistance testing apparatus.

Detailed Description

The present invention will be described in detail below with reference to embodiments.

[ crosslinked resin foam sheet ]

The crosslinked resin foamed sheet of the present invention (hereinafter also simply referred to as "foamed sheet") is a foamed sheet having independent cells, and is a foamed sheet having a compressive strength (C) measured with respect to a sample width of 20mm ₂₀ ) Compressive Strength (C) measured at a sample width of 1mm ₁ ) The reduction rate of (3) is 60% or less. The rate of decrease in compressive strength was determined by (C) ₂₀ -C ₁ )/C ₂₀ And (6) calculating.

The foamed sheet of the present invention can maintain the compressive strength at a high level even if the sample width is narrowed by reducing the rate of decrease in the compressive strength. Therefore, even when the sheet width is narrowed, the foam sheet can be prevented from becoming too soft, and a sticking failure or the like which occurs when a tape having the foam sheet as a base material is stuck can be prevented from easily occurring. Further, even when the foamed sheet is narrow, the mechanical strength such as impact resistance can be maintained at a high level, and the foamed sheet can be suitably used as an impact absorbing material or the like in a miniaturized electronic device.

In order to obtain a foamed sheet having suitable flexibility and good impact resistance, the reduction rate of the compressive strength is preferably 55% or less, more preferably 40% or less, and still more preferably 30% or less. The lower the reduction rate of the compressive strength, the better, but 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 preferably within the above range. If the reduction rate of both is within the above range, various performances such as adhesiveness and impact resistance are improved under various use conditions. The measurement methods of 10% and 25% compressive strength are shown in examples described later.

[ independent bubble Rate ]

The foamed sheet of the present invention has independent bubbles. The term "having independent bubbles" means that the ratio of independent bubbles to the total bubbles (referred to as "independent bubble ratio") is 70% or more. The isolated bubble rate is preferably 75% or more, more preferably 90% or more.

The independent bubble rate can be determined in accordance with ASTM D2856 (1998). Examples of commercially available measuring instruments include a dry automatic densitometer アキュピック 1330.

The independent bubble ratio is more specifically measured in the following manner. A test piece having a planar square shape with a side of 5cm and a certain thickness was cut out from the foamed sheet. The thickness of the test piece was measured, and the apparent volume V of the test piece was calculated ₁ And measuring the weight W of the test piece ₁ . Next, the apparent volume V occupied by the bubbles is calculated from the following formula ₂ . The density of the resin constituting the test piece was 1g/cm ³ 。

Apparent volume V occupied by bubbles ₂ ＝V ₁ -W ₁

Next, the test piece was allowed to sink in distilled water at 23 ℃ to a depth of 100mm from the water surface, and a pressure of 15kPa was applied to the test piece for 3 minutes. Then, the test piece was taken out of the water, and the water adhered to the surface of the test piece was removed to measure the weight W of the test piece ₂ The continuous bubble ratio F was calculated based on the following formula ₁ And the independent bubble rate F ₂ 。

Ratio of continuous bubble F ₁ (％)＝100×(W ₂ -W ₁ )/V ₂

Independent bubble rate F ₂ (％)＝100-F ₁

[ average bubble diameter ]

The average cell diameter in both MD and TD is preferably 100 μm or less, more preferably 80 μm or less, and still more preferably 70 μm or less. Such bubbles having an average bubble diameter are generally called fine bubbles. The foamed sheet has fine bubbles, so that even when the sheet width is narrowed, a large number of independent bubbles exist between the narrow widths.

The end face of the foamed sheet is cut off with bubbles and exhibits behavior like open cells, which causes a decrease in compressive strength, but since the cut-off cells are fine cells and a large number of independent cells exist between narrow widths, the decrease in compressive strength due to the bubbles at the end face of the sheet can be minimized. Therefore, the rate of decrease in compressive strength can be reduced by setting the average cell diameter of the cells within the above range.

From the viewpoint of ease of production, the average cell diameters in the MD and TD are preferably 10 μm or more, more preferably 20 μm or more, and still more preferably 30 μm or more, respectively.

The average cell diameter is measured in the following manner.

A foam piece obtained by cutting a foam piece into 50mm square pieces was prepared as a foam sample for measurement. It was immersed in liquid nitrogen for 1 minute and then cut in the thickness direction along the MD direction and TD direction, respectively, using razor blades. The cross section was photographed at 200-fold magnification using a digital microscope ("VHX-900" manufactured by キーエンス), and the bubble diameters were measured for all bubbles present in a cut surface having a length of 2mm in each of the MD direction and TD direction, and this operation was repeated 5 times. Then, the average value of all the cells is set as the average cell diameter in the MD direction and the TD direction, respectively.

The MD direction is a Machine direction and is a direction corresponding to the extrusion direction or the like, and the TD direction is a Transverse direction and is a direction orthogonal to the MD direction and is a direction parallel to the sheet surface of the foamed sheet. 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, the fine bubbles preferably have small fluctuations in bubble diameter in the MD and TD directions. Therefore, the standard deviation of the cell diameter in both 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 0 μm as low as possible, but is preferably 1 μm or more from the viewpoint of practical use. The standard deviation of the cell diameters in the MD and TD is calculated based on the respective cell diameters measured to obtain the average cell diameters in the MD and TD.

If the fluctuation in the diameter of the bubbles is small, the size of the bubble walls between the bubbles tends to be uniform, and therefore the bubble walls having low mechanical strength become small. In addition, large bubbles are less likely to be present in the end faces of the foamed sheet. Therefore, even if the width of the foamed sheet is narrowed, the compressive strength is stabilized, and the rate of decrease in the compressive strength is easily decreased.

[ degree of crosslinking ]

The foam sheet is a crosslinked foam, and the degree of crosslinking is preferably 30% by mass or more. The crosslinking degree is more preferably 35 to 65% by mass, and still more preferably 40 to 49% by mass. By setting the degree of crosslinking to be equal to or higher than these lower limit values, the cells of the crosslinked foamed resin sheet can be easily made fine, and the fluctuation in the size of each cell can be easily made small. Further, when the upper limit value is not more than the above, the foam is easily appropriately foamed, and the expansion ratio is easily increased. In the foam sheet, by increasing the expansion ratio, the flexibility is easily improved and the compressive strength is easily adjusted to an appropriate value.

[ dimension of resin foam sheet ]

The thickness of the foamed sheet is preferably 0.03 to 0.5mm. When the thickness is 0.03mm or more, the impact resistance and flexibility of the foamed sheet can be easily secured. Further, if the thickness is 0.5mm or less, the thickness can be reduced, and the method can be applied to a miniaturized electronic device. From these viewpoints, the thickness of the resin foam sheet is more preferably 0.08 to 0.40mm, and still more preferably 0.10 to 0.25mm.

The width of the foamed sheet is preferably narrow, and specifically, the foamed sheet is preferably processed into a fine line shape. For example, the foam sheet may be used so that its width is 5mm or less, preferably 3mm or less, and more preferably 1mm or less. If the width of the resin foam sheet is narrowed, the resin foam sheet can be applied to the inside of a miniaturized electronic device. Further, the foamed sheet of the present invention can maintain impact resistance and flexibility well even if the width is narrowed.

The lower limit of the width of the foamed sheet is not particularly limited, and may be, for example, a foamed sheet of 0.1mm or more, or a foamed sheet of 0.2mm or more. The planar shape of the foamed sheet is not particularly limited, and may be a rectangular shape, a frame shape, an L shape, a コ -shaped shape, or the like. However, other than these shapes, the shape may be any other shape such as a normal quadrangle or a circle.

[ expansion ratio ]

The expansion ratio of the foam sheet is preferably 1.2 to 4.0cm ³ (iv) g. By setting the expansion ratio to 1.2cm ³ At least g, the foam sheet has good compressive strength and flexibility, and the foam sheet is easily improved in impact absorption and sealing properties. On the other hand, by making it 4.0cm ³ The mechanical strength is increased by a factor of/g or less, and the impact resistance is easily improved. Further, the average bubble diameter and the fluctuation of the bubble diameter are also easily made small.

From the above viewpoint, the expansion ratio is more preferably 1.3 to 3.5cm ³ Per g, more preferably 2.0 to 3.0cm ³ (ii) in terms of/g. In the present invention, the amount of the surfactant is determined in accordance with JIS K7222The density of the foamed sheet was determined, and the reciprocal of the density was defined as the expansion ratio.

[ compressive Strength ]

10% compressive Strength (C) at 1mm Width of sample of foamed sheet ₁ ) Preferably 30 to 600kPa, more preferably 40 to 350kPa, and still more preferably 100 to 200kPa.

Further, the sample width of the foamed sheet was 25% compressive strength (C) at 1mm ₁ ) Preferably 150 to 1500kPa, more preferably 250 to 1400kPa, and still more preferably 270 to 600kPa.

By making the compressive strength (C) ₁ ) Within the above range, the foamed sheet has appropriate flexibility and is likely to have improved impact resistance. In addition, excessive softening and reduction in adhesion properties are prevented.

[ polyolefin resin ]

As the resin used for the resin foamed sheet, various resins can be used, but among them, a polyolefin resin is preferably used. By using the polyolefin resin, the average cell diameter and the fluctuation in cell diameter can be reduced while ensuring appropriate flexibility of the resin foamed sheet.

Examples of the polyolefin resin include a polyethylene resin, a polypropylene resin, an ethylene-vinyl acetate copolymer, and a mixture thereof, and among them, a polyethylene resin is preferable.

Examples of the polyethylene resin include those obtained by polymerization using a polymerization catalyst such as a ziegler-natta compound, a metallocene compound, or a chromium oxide compound, and those obtained by polymerization using a metallocene compound as a polymerization catalyst are preferably used.

The polyethylene resin is preferably linear low-density polyethylene. By using the 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 by using a polymerization catalyst such as a metallocene compound. Further, the linear low-density polyethylene is more preferably a linear low-density polyethylene obtained by copolymerizing ethylene (for example, 75 mass% or more, preferably 90 mass% or more based on the total monomer amount) and, if necessary, a small amount of an α -olefin.

Specific examples of the α -olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Among them, an α -olefin having 4 to 10 carbon atoms is preferable.

The density of the polyethylene resin, for example, the linear low-density polyethylene described above is preferably 0.870 to 0.910g/cm ³ More preferably 0.875 to 0.907g/cm ³ More preferably 0.880 to 0.905g/cm ³ . As the polyethylene resin, a plurality of polyethylene resins may be used, and further, a polyethylene resin outside the above-mentioned density range may be added.

(metallocene compound)

Examples of the metallocene compound include compounds such as bis (cyclopentadienyl) metal complexes having a structure in which a transition metal is sandwiched between pi-electron-based unsaturated compounds. More specifically, there may be mentioned compounds in which 1 or 2 or more cyclopentadiene rings or the like are present as ligands (ligands) on tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, platinum and the like.

Such metallocene compounds have uniform properties of active sites, and each active site has the same activity degree. Since the polymer synthesized using the metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition distribution, and the like, when a sheet containing the polymer synthesized using the metallocene compound is crosslinked, the crosslinking proceeds uniformly. Since the sheet obtained by uniform crosslinking is uniformly foamed, the fluctuation in the cell diameter is easily reduced as described above. Further, since the foam sheet can be uniformly stretched, the thickness of the foam sheet can be easily made uniform.

Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. These cyclic compounds may be substituted with hydrocarbyl, substituted hydrocarbyl or hydrocarbon-substituted metalloid radicals. Examples of the hydrocarbon group include methyl, ethyl, various propyl groups, various butyl groups, various pentyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various cetyl groups, and phenyl groups. The term "various" means that various normal, secondary, tertiary and iso isomers are included.

In addition, a substance obtained by polymerizing a cyclic compound into an oligomer may be used as a ligand.

Further, in addition to the pi-electron-based unsaturated compound, monovalent anion ligands such as chlorine and bromine, divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, phenoxides (aryloxides), amides, arylamides, phosphides, arylphosphides, and the like can be used.

Examples of the metallocene compound containing a tetravalent transition metal and a ligand include cyclopentadienyl tris (dimethylamide) titanium, methylcyclopentadienyl tris (dimethylamide) titanium, bis (cyclopentadienyl) titanium dichloride, and dimethylsilyl tetramethylcyclopentadienyl-tert-butylamidozirconium dichloride.

The metallocene compound functions as a catalyst in the polymerization of various olefins by being combined with a specific cocatalyst (co-catalyst). Specific examples of the cocatalyst include Methylaluminoxane (MAO) and boron compounds. The proportion of the cocatalyst to the metallocene compound is preferably 10 to 100 ten thousand mol times, and more preferably 50 to 5,000 mol times.

When the above-mentioned linear low-density polyethylene is used as the polyolefin resin contained in the foamed sheet, the above-mentioned linear low-density polyethylene may be used alone, or may be used in combination with another polyolefin resin, for example, in combination with another polyolefin resin described below. When the other polyolefin resin is contained, the proportion of the other polyolefin resin to the linear low-density polyethylene (100 mass%) is preferably 40 mass% or less, more preferably 30 mass% or less, and still more preferably 20 mass% or less.

Examples of the ethylene-vinyl acetate copolymer (EVA) used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50 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), and the mass ratio (EVA/PE) thereof is preferably 10/90 to 90/10, more preferably 20/80 to 80/20, and still more preferably 50/50 to 80/20.

Examples of the polypropylene resin include polypropylene and a propylene- α -olefin copolymer containing 50 mass% or more of propylene. These may be used alone in 1 kind, or may be used in combination of 2 or more kinds.

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, and 1-octene, and among them, α -olefins having 6 to 12 carbon atoms are preferred.

In the case of using a polyolefin resin as the resin in the foamed sheet, the polyolefin resin may be used alone or a resin other than the polyolefin resin may be contained in the resin contained in the foamed sheet. In the foamed sheet, the proportion of the polyolefin resin relative to the total amount of the resin is preferably 60% by mass or more, more preferably 70% by mass or more, and further preferably 80% by mass or more.

Examples of the resin other than the polyolefin resin used in the foamed sheet include various elastomers such as styrene-based thermoplastic elastomers and ethylene propylene-based thermoplastic elastomers such as EPDM, and rubber components.

(thermal decomposition type foaming agent)

The foamed sheet of the present invention is preferably one obtained by foaming a foamable composition containing the resin and a thermal decomposition type foaming agent. Further, as the thermal decomposition type foaming agent, a foaming agent having a particle diameter of less than 15 μm is preferably used. By using a blowing agent having a particle diameter of less than 15 μm and by making the degree of crosslinking relatively high as described above, the fluctuation in the cell diameter and the cell diameter of the foamed sheet can be easily made small. The particle diameter of the thermal decomposition type foaming agent is preferably 2 to 14 μm, and more preferably 5 to 13 μm. In addition, as described above, in order to suppress fluctuation in the cell diameter, it is preferable that fluctuation in the particle diameter of the blowing agent is small.

The particle diameter of the thermal decomposition type foaming agent is a value measured by a laser diffraction method, and represents a particle diameter (D50) corresponding to a cumulative frequency of 50%.

As the thermal decomposition type foaming agent, an organic foaming agent or an inorganic foaming agent can be used. Examples of the organic blowing agent include azodicarbonamide, metal salts of azodicarboxylic acid (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N '-dinitrosopentamethylenetetramine, hydrazonodicarbonamide, hydrazine derivatives such as 4,4' -oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonylsemicarbazide.

Examples of the inorganic foaming agent include ammonium sulfate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate.

Among them, from the viewpoint of obtaining fine bubbles and from the viewpoint of economy and safety, an azo compound is preferable, and azodicarbonamide is particularly preferable. These thermal decomposition type foaming agent can be used alone or in combination of 2 or more.

The amount of the thermal decomposition type foaming agent to be incorporated in the foamable composition is preferably 1 to 10 parts by mass, more preferably 1.5 to 5 parts by mass, and still more preferably 2 to 4 parts by mass, per 100 parts by mass of the resin.

Further, the foamable composition preferably contains a cell nucleus-regulating agent in addition to the above resin and the thermal decomposition type foaming agent. Examples of the cell nucleus regulator include zinc compounds such as zinc oxide and zinc stearate, and organic compounds of citric acid and urea, and among them, zinc oxide is more preferable. In addition to the above-mentioned small-particle-diameter blowing agent, the fluctuation of the average cell diameter and the cell diameter can be easily made small by using the cell nucleus modifier. The amount of the cell nucleus-regulating agent is preferably 0.4 to 8 parts by mass, more preferably 0.5 to 5 parts by mass, and still more preferably 0.8 to 2.5 parts by mass, per 100 parts by mass of the resin.

The foamable composition may contain, if necessary, additives generally used in foams such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, and fillers.

[ Process for producing foam sheet ]

The method for producing the foamed sheet is not particularly limited, and for example, the foamed sheet is produced by crosslinking a foamable composition containing a resin and a thermal decomposition type foaming agent and heating the crosslinked composition to foam the thermal decomposition type foaming agent. The production method more specifically includes the following steps (1) to (4).

Step (1): a step of mixing a resin with an additive containing a thermal decomposition type foaming agent to form a sheet-like foamable composition (resin sheet)

Step (2): a step of irradiating the sheet-like foamable composition with ionizing radiation to crosslink the foamable composition

Step (3): a step of heating the crosslinked foamable composition to foam the thermal decomposition type foaming agent to obtain a foamed sheet

Step (4): stretching the foamed sheet in either or both of the MD and TD directions

In the step (1), the method for forming the resin sheet is not particularly limited, and for example, the resin and the additive are supplied to an extruder, and melt-kneaded, and the foamable composition is extruded from the extruder into a sheet shape to form the resin sheet.

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, or γ -ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of crosslinking of the resulting foam sheet is within the desired range, but is preferably 5 to 15Mrad, and more preferably 6 to 13Mrad.

In the step (3), the heating temperature at which the foamable composition is heated to foam the thermal decomposition type foaming agent may be equal to or higher than the foaming temperature of the thermal decomposition type foaming agent, but is preferably 200 to 300 ℃, and more preferably 220 to 280 ℃.

The stretching of the foamed sheet in the step (4) may be performed after the foamed sheet is obtained by foaming the resin sheet, or may be performed while foaming the resin sheet. In the case where the foamed sheet is stretched after the foamed sheet is obtained by foaming the resin sheet, the foamed sheet may be continuously stretched while maintaining the molten state at the time of foaming without cooling the foamed sheet, or the foamed sheet may be stretched after cooling the foamed sheet and heating the foamed sheet again to bring the foamed sheet into a molten or softened state. The foamed sheet is easily thinned by stretching.

In step (4), the stretch ratio of stretching in one or both of the MD direction and the TD direction of the foamed sheet is preferably 1.1 to 5.0 times, and more preferably 1.5 to 4.0 times.

When the stretch ratio is not less than the lower limit, the flexibility and tensile strength of the foamed sheet tend to be improved. On the other hand, if the upper limit value is less than the upper limit value, the foamed sheet can be prevented from being broken during stretching or from being broken due to escape of foaming gas from the foamed sheet during foaming, and the foaming ratio can be prevented from being significantly reduced, whereby the foamed sheet can be improved in flexibility and tensile strength, and the quality thereof can be easily made uniform.

In addition, the foamed sheet may be heated to, for example, 100 to 280 ℃, preferably 150 to 260 ℃ at the time of stretching.

The foamed sheet obtained as described above may be cut by a known method such as press working and processed into a desired shape.

However, the present production method is not limited to the above, and a foamed sheet can be obtained by a method other than the above. For example, crosslinking may be performed by a method of mixing an organic peroxide in advance with the foamable composition and heating the foamable composition to decompose the organic peroxide, instead of irradiating ionizing radiation. Further, the step (4), i.e., stretching of the foamed sheet, may be omitted.

The use of the foam sheet is not particularly limited, but the foam sheet is preferably used in, for example, an electronic device. The foam sheet of the present invention has high impact resistance and appropriate flexibility even when the width is reduced, and therefore, the foam sheet can be applied to the interior of various portable electronic devices in which the space for disposing the foam sheet is small. Examples of the portable electronic device include a mobile phone, a camera, a game machine, an electronic notebook, a tablet terminal, a notebook personal computer, and the like. The foamed sheet can be used as an impact absorbing material or a sealing material in an electronic device.

Further, the adhesive tape can be used for an adhesive tape having a foamed sheet as a base material. Since the tape uses the foamed sheet of the present invention having appropriate flexibility as a base material, defective adhesion and the like are less likely to occur.

The adhesive tape includes, for example, a foam sheet and an adhesive layer provided on at least one surface of the foam sheet, and is preferably a double-sided adhesive tape having adhesive layers provided on both surfaces.

The thickness of the adhesive layer constituting the adhesive tape is preferably 5 to 200 μm. The thickness of the adhesive layer is more preferably 7 to 150. Mu.m, and still more preferably 10 to 100. Mu.m. If the thickness of the adhesive layer is in the range of 5 to 200 μm, the thickness of the construct fixed with the tape can be reduced.

The adhesive used in the adhesive layer is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, or the like can be used.

Further, a release sheet such as release paper may be further bonded to the adhesive layer.

The method of forming the adhesive layer on at least one surface of the foam sheet is not particularly limited, and the following methods can be exemplified. Examples thereof include a method of applying an adhesive to at least one surface of a foamed sheet using a coater such as a coater (coater), a method of spraying and applying an adhesive to at least one surface of a resin foamed sheet using a sprayer, a method of applying an adhesive to at least one surface of a foamed sheet using a brush, and a method of transferring an adhesive layer formed on a release sheet to at least one surface of a foamed sheet.

Examples

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

[ measurement method ]

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

< apparent Density and expansion ratio >

The resin foam sheet was measured for apparent density in accordance with JIS K7222, and the reciprocal thereof was defined as the expansion ratio.

< degree of crosslinking >

A test piece of about 100mg was taken out of the resin foam, and the weight A (mg) of the test piece was precisely weighed. Next, the test piece was subjected to 30cm xylene at 120 ℃ C ³ Immersed in the solvent, left to stand for 24 hours, and then filtered through a 200-mesh wire gauze to obtain insoluble matter on the wire gauze, followed by vacuum drying, and precisely weighing the weight B (mg) of the insoluble matter. From the obtained value, the degree of crosslinking (% by mass) was calculated by the following formula.

Degree of crosslinking (% by mass) =100 × (B/a)

< independent bubble Rate >

The measurement was carried out 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 >

About 10%, 25% compressive strength (C) ₂₀ ) The foamed sheets were each die-cut into 20mm × 20mm to obtain samples, and the samples were used to measure in accordance with JIS K6767 except that the thickness was 1 sheet.

About 10%, 25% compressive strength (C) ₁ ) Samples obtained by punching the foamed sheets into 1.0 mm. Times.20 mm were individually prepared, and 20 of the samples were placed on a measuring machine in such a manner that the sheets were not overlapped with each other, and the compression strength (C) was adjusted to 10% and 25% ₂₀ ) The measurement was performed in the same manner. The foam sheets of the present examples and comparative examples were sampled so that the longitudinal direction of the sample coincides with the MD direction.

< impact resistance >

(preparation of impact resistance evaluation sample)

Double-sided tapes each having a foamed sheet as a base material were produced in the following manner by laminating adhesive layers obtained by the following method on both sides of the foamed sheets obtained in examples and comparative examples.

(method of producing double-sided adhesive tape)

After 75 parts by mass of butyl acrylate, 22 parts by mass of 2-ethylhexyl acrylate, 3 parts by mass of acrylic acid, 0.2 part by mass of 2-hydroxyethyl acrylate and 80 parts by mass of ethyl acetate were charged into a reactor equipped with a thermometer, a stirrer and a cooling tube and replaced with nitrogen, the reactor was heated to start reflux. Next, 0.1 part by mass of azobisisobutyronitrile as a polymerization initiator was added to the reactor. This was refluxed for 5 hours to obtain a solution of the acrylic copolymer (z). The weight average molecular weight of the acrylic copolymer (z) thus obtained was measured by GPC using a "2690 separators Model" manufactured by Water as a column, and was 60 ten thousand.

To 100 parts by mass of the solid content of the acrylic copolymer (Z) contained in the obtained acrylic copolymer (Z) solution, 15 parts by mass of polymerized rosin ester having a softening point of 135 ℃, 125 parts by mass of ethyl acetate (manufactured by shin-ko chemical industries), and 2 parts by mass of an isocyanate-based crosslinking agent (コロネート L45, manufactured by imperial ソー) were added and stirred to obtain an adhesive (Z). The acrylic pressure-sensitive adhesive had a crosslinking degree of 33% by mass.

A release paper having a thickness of 150 μm was prepared, and the release-treated surface of the release paper was coated with an adhesive (Z) and dried at 100 ℃ for 5 minutes to form an acrylic adhesive layer having a thickness of 30 μm. The acrylic adhesive layer is bonded to the surface of a base material formed of a foam sheet. Next, in the same manner, the same acrylic adhesive layer as described above was also bonded to the surface on the opposite side of the substrate. Thus, a double-sided tape was obtained in which both sides were covered with a release paper having a thickness of 150 μm.

(preparation of impact resistance test apparatus)

FIG. 1 is a schematic view of an impact resistance testing apparatus.

The impact resistance test apparatus was manufactured in the following manner.

First, the double-sided tape obtained as described above was punched out so that the outer diameter was 15.0mm and the length was 15.0mm, and the inner diameter was 13.6mm and the length was 13.6mm, thereby producing a test piece 1 having a square frame shape with each frame side having a width of 0.7 mm.

Next, as shown in fig. 1 (a), a polycarbonate or SUS plate 3 having a rectangular hole 2 at the center is prepared, and the test piece 1 from which the release paper has been peeled is attached to the upper surface of the plate 3 over the entire circumference of the hole 2.

Next, a glass plate 4 to be bonded, which covers the hole 2 in size, is superimposed on the test piece 1, and an impact resistance test apparatus is attached so as to cover the hole 2.

Then, the impact resistance test apparatus was turned upside down, and a pressure of 5kgf was applied for 5 seconds from the side of the bonded plate 3 with the bonded plate 3 being on the upper surface, so that the bonded plate 3 and the test piece in the upper and lower positions were pressure-bonded, and left to stand at room temperature for 36 hours. In the impact resistance test, the cases where the adherend 3 was made of Polycarbonate (PC) and SUS were evaluated.

(determination of impact resistance)

As shown in fig. 1 (b), the impact resistance test apparatus thus produced was fixed to a support base 5, and iron balls 6 having a size of 50g passed through the holes 2 formed in the bonded plate 3 were dropped through the holes 2. The height of the falling iron ball was gradually increased, and the height of the falling iron ball when the test piece and the bonded plate were peeled off by the impact of the falling iron ball was measured to evaluate the impact resistance. The case where the height of the falling iron ball was 32cm or more when the bonded plate 3 was made of polycarbonate and the height of the falling iron ball was 38cm or more when the bonded plate 3 was made of SUS was evaluated as "a". The case where the height of the dropped iron ball was less than 32cm when the bonded plate 3 was made of polycarbonate and the height of the dropped iron ball was 38cm or more when the bonded plate 3 was made of SUS, or the case where the height of the dropped iron ball was 32cm or more when the bonded plate 3 was made of polycarbonate and the height of the dropped iron ball was less than 38cm when the bonded plate 3 was made of SUS was evaluated as "B". The case where the height of the falling iron balls was less than 32cm when the bonded panel 3 was made of polycarbonate and the height of the falling iron balls was less than 38cm when the bonded panel 3 was made of SUS was evaluated as "C".

[ example 1]

100 parts by mass of a linear low-density polyethylene resin (resin A) obtained using a metallocene compound as a polymerization catalyst, 3.4 parts by mass of azodicarbonamide having a particle size of 13 μm as a thermal decomposition type foaming agent, 1.0 part by mass of zinc oxide (made by Sakai chemical industry Co., ltd., trade name "OW-212F") as a cell nucleus regulator, and 0.5 part by mass of an antioxidant were supplied to an extruder. Subsequently, the resulting mixture was melt-kneaded at 130 ℃ to form a long resin sheet having a thickness of 260 μm. In addition, ダウ was used as resin a3238 Zxft 3238 trade name "アフィニティー PL1850" (density 0.902 g/cm) ³ )。

Subsequently, both sides of the elongated resin sheet were irradiated with electron beams 7Mrad having an acceleration voltage of 500kV to crosslink the resin sheet. Then, the crosslinked resin sheet was continuously fed into a foaming furnace maintained at 250 ℃ by a hot air and infrared heater and heated to foam, thereby obtaining a foamed sheet having a thickness of 300 μm.

Subsequently, the obtained foamed sheet is continuously fed out from the foaming furnace. Then, while maintaining the temperature of the foam sheet at both surfaces thereof at 200 to 250 ℃, the foam sheet is stretched at a stretch ratio of 2.0 times in the TD direction, and the foam sheet is wound at a winding speed higher than the feeding speed (feeding speed) of the foam sheet into the foaming furnace, whereby the foam sheet is also stretched in the MD direction, and a foam sheet is obtained. The winding speed of the foamed sheet is adjusted while taking into account the expansion in the MD direction caused by the 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 and comparative examples 1 to 3

The same procedure as in example 1 was repeated, except that the compounding of the polyolefin resin composition was changed as shown in tables 1 and 2, and the dose at the time of crosslinking was adjusted so as to obtain the crosslinking degrees in tables 1 and 2, and the TD stretch ratio was adjusted to 2.0 to 3.5 times.

The polyolefin resin used in example 6 is as follows.

Resin B: ethylene-vinyl acetate copolymer resin, product of Mitsubishi chemical corporation, trade name "ノバテック EVA"

Resin C: linear low-density polyethylene, available from Kyowa K.K. プライムポリマー, under the trade name "エバフレックス -H" [ Table 1]

As described above, in examples 1 to 7, the reduction ratio of compressive strength ((C) ₂₀ -C ₁ )/C ₂₀ ) 60% or less, the sheet does not become too soft even when the sheet width is narrowed, and the impact resistance is sufficiently high. On the other hand, in comparative examples 1 to 3, since the reduction rate of the compressive strength was higher than 60%, if the sheet width was narrowed, the sheet became too soft, and the impact resistance could not be sufficiently high.

Description of the symbols

1. Test piece

2. Hole(s)

3. Magnesium quilt sticking board

4. Glass quilt sticking board

5. Support table

6. An iron ball.

Claims

1. A crosslinked resin foamed sheet having independent cells and having a compressive strength C measured with respect to a sample width of 20mm ₂₀ Compressive strength C measured with a sample width of 1mm ₁ The reduction rate of (D) is 60% or less.

2. The crosslinked resin foamed sheet according to claim 1, wherein the average cell diameter in both the MD direction and the TD direction is 100 μm or less.

3. The crosslinked resin foamed sheet according to claim 1 or 2, having a thickness of 0.03 to 0.50mm.

4. The crosslinked resin foamed sheet according to any one of claims 1 to 3, comprising a polyolefin resin.

5. The crosslinked resin foamed sheet according to claim 4, the polyolefin resin being a polyethylene resin.

6. The crosslinked resin foamed sheet according to claim 4, wherein the polyolefin resin is a linear low-density polyethylene obtained by polymerization using a metallocene compound as a polymerization catalyst.

7. The crosslinked resin foamed sheet according to any one of claims 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 claims 1 to 7, having a foaming ratio of 1.2 to 4.0cm ³ /g。

9. The crosslinked resin foamed sheet according to any one of claims 1 to 8, having a width of 5mm or less.

10. The crosslinked resin foamed sheet according to any one of claims 1 to 9, which is obtained by foaming a foamable composition containing a resin and a thermal decomposition type foaming agent.

11. A method for producing a crosslinked resin foamed sheet according to any one of claims 1 to 10, wherein a foamable composition containing a resin and a thermal decomposition type foaming agent is crosslinked and heated to foam the thermal decomposition type foaming agent.

12. 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.