Foam sheets and electronic devices

Abstract

To provide a foam sheet having excellent impact absorption property in a wide temperature zone.SOLUTION: There is provided a foam sheet. The foam sheet includes a foam layer having an average flat degree of a cell calculated by the following (method for calculating average flat degree of cell) of more than 1. (Method for calculating average flat degree of cell) The method measures each of a length in a vertical direction and a length in a horizontal direction of each of the cells, and calculates a flat degree calculated by the following expression, in a cross section (within 2 mm in horizontal direction) perpendicular to a sheet surface. Flat degree=length in vertical direction / length in horizontal direction. The method averages the flat degrees obtained for the respective cells to determine the average flat degree.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to foamed sheets and electronic devices. [Background technology]

[0002] In recent years, thin foam materials have been used as shock-absorbing materials (back cushions) in displays used in mobile phones and smartphones. Furthermore, in recent years, displays have evolved to include shapes that can deform, and the shock-absorbing materials used in such flexible displays are required to be able to adapt to these changes in shape. Also, unlike conventional displays, freely deformable displays are not housed in rigid casings, so the shock-absorbing materials installed on their backs are subjected to more stress from the external environment than ever before. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2016 / 047611 [Patent Document 1] Japanese Patent Publication No. 2014-70174 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] As mentioned above, the deformation of the display makes it more susceptible to external stimuli than ever before, requiring the thin foam material to have shock absorption capabilities across a wide temperature range. However, conventional thin foams exhibit temperature-dependent shock absorption, becoming particularly hard and less shock-absorbing at low temperatures (-30°C). This is due to the foam's composition and structure, as it is designed to achieve higher shock absorption under typical operating conditions (room temperature). If the glass transition temperature (Tg) of the foam is lowered as a common method to maintain hardness at low temperatures, the hardness at room temperature decreases, making it impossible to obtain sufficient shock absorption.

[0005] This invention addresses the above-mentioned problems and aims to provide a foamed sheet that has excellent shock absorption properties over a wide temperature range. [Means for solving the problem]

[0006] One aspect of the present invention is a foamed sheet. The foamed sheet includes a foamed layer in which the average flatness of cells, calculated by the following method (method for calculating the average flatness of cells), is greater than 1. (Method for calculating the average flatness of cells) In a cross-section perpendicular to the sheet surface (within 2 mm horizontally), the vertical and horizontal lengths of each cell are measured, and the flatness is calculated using the following formula. Flatness = Vertical length / Horizontal length The average flatness is calculated by averaging the flatness values ​​obtained for each cell.

[0007] In the foamed sheet according to the above embodiment, the foamed layer may be formed of silicone foam. The foamed sheet according to the above embodiment may be used to protect electronic components. Another aspect of the present invention is an electronic device, which comprises a foamed sheet according to any of the above-described embodiments. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide technology relating to a foamed sheet that has excellent shock absorption properties over a wide temperature range. [Brief explanation of the drawing]

[0009] [Figure 1] FIG. 1(a)-(d) are cross-sectional SEM images of the foamed sheets of Examples 1-4, respectively. [Figure 2] FIG. 2(a)-(d) are cross-sectional SEM images of the foamed sheets of Examples 5-8, respectively. [Figure 3] FIG. 3(a)-(c) are cross-sectional SEM images of the foamed sheets of Comparative Examples 1-3, respectively.

MODE FOR CARRYING OUT THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail. In this specification, the notation "a to b" in the description of a numerical range represents a to b unless otherwise specified.

[0011] <Foamed sheet> The foamed sheet according to the embodiment will be described in detail below. The foamed sheet according to the embodiment includes a foamed layer having an average flatness of cells calculated by the following (method for calculating the average flatness of cells) greater than 1. (Method for calculating the average flatness of cells) In a cross-section (within 2 mm in the horizontal direction) orthogonal to the sheet surface, the vertical length and the horizontal length of each cell are measured respectively, and the flatness calculated by the following formula is calculated. Flatness = vertical length / horizontal length <000008�>The flatness obtained for each cell is averaged to obtain the average flatness. As the cross-section of the foamed sheet, a cross-section (image) imaged by SEM (scanning electron microscope) can be used.

[0012] The foamed sheet according to the embodiment has little temperature dependence of hardness, and particularly little increase in hardness at low temperatures. Therefore, it has excellent shock absorption in a wide temperature range from low temperature (for example, -30°C) to normal temperature (for example, 23°C).

[0013] <<Composition of the foamed layer>> The foam layer constituting the foamed sheet according to the embodiment is preferably formed of a silicone foam. The silicone foam is preferably a self-foaming reaction type silicone foam. The self-foaming reaction type silicone foam is a foam in which two liquid silicone materials are mixed and stirred, and foamed by the gas (hydrogen) generated during curing to form voids (cells).

[0014] <<Thickness of the foam layer>> The thickness of the foam layer is not particularly limited, but is preferably 0.1 mm to 10 mm, more preferably 0.2 mm to 5 mm, and even more preferably 0.3 mm to 3 mm.

[0015] <<Density of the foam layer>> The lower limit of the density of the foam layer is preferably 200 kg / m 3 or more, more preferably 300 kg / m 3 or more, and even more preferably 400 kg / m 3 or more. On the other hand, the upper limit of the density of the foam layer is preferably 900 kg / m 3 or less, more preferably 800 kg / m 3 or less, and even more preferably 700 kg / m 3 or less. When the density of the foam layer is within the above range, the compression residual strain can be reduced.

[0016] <<Average cell diameter>> The lower limit of the average cell diameter in the foam layer is preferably 250 μm or more, more preferably 300 μm or more, and even more preferably 500 μm or more. On the other hand, the upper limit of the average cell diameter in the foam layer is preferably 1000 μm or less, more preferably 900 μm or less, and even more preferably 800 μm or less. The average cell diameter is calculated according to the following calculation method. (Calculation method of average cell diameter) In a cross section (within 2 mm in the horizontal direction) perpendicular to the sheet surface, the vertical length of each cell is measured, and the vertical lengths obtained for each cell are averaged to obtain the average diameter. The sheet cross section for calculating the average cell diameter can be imaged by SEM (scanning electron microscope).

[0017] <<Porosity of the foamed layer>> When the foam layer is divided vertically into 10 equal layers, the void ratio (%) of each layer is given by t i When i is the order of each layer counted from the surface, S = t5 - (t1 + t 10 The value of S calculated by the formula ) / 2 is preferably 10% or more, preferably 20% or more, and more preferably 30% or more. Also, t5 is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, and particularly preferably 80% or more. The method for calculating the porosity of each layer will be described later.

[0018] <<Compression residual strain of foamed sheet>> The compression set (%) of the foamed sheet is preferably 20% or less, more preferably 10% or less, even more preferably 6% or less, and particularly preferably 4% or less. The compression set (%) is calculated according to JIS K6401 by compressing a 50 x 50 mm foam sheet by 50% in the thickness direction, leaving it to stand at a specified temperature (70°C) for 22 hours, then releasing the compressive stress at room temperature, measuring the thickness of the foam sheet after 30 minutes (thickness after release), and using the following formula. Compression residual strain (%) = (thickness before compression - thickness after release) / thickness before compression × 100

[0019] <<Hardness of foam sheet (25% CLD)>> The hardness (25%CLD(kPa)) in environments of -30°C and 23°C is preferably 200kPa or less, more preferably 150kPa or less, and even more preferably 100kPa or less. The hardness (25%CLD(kPa)) is defined as the compressive stress obtained when a φ50mm sample is compressed by 25% at a speed of 1mm / min, based on JIS K6254. Furthermore, the rate of change in hardness (25%CLD) calculated by the following formula is preferably 30% or less, more preferably 20% or less, even more preferably 10% or less, and particularly preferably 6% or less. Rate of change (%) = {Hardness at <-30℃ (25%CLD)> - <Hardness at 23℃ (25%CLD)>} / <Hardness at 23℃ (25%CLD)> × 100

[0020] <<Adhesive strength with tape>> The adhesive strength (N / 24mm) between the foam sheet and the silicone adhesive tape is preferably 1N / 24mm or higher, more preferably 2N / 24mm or higher, even more preferably 3N / 24mm or higher, and particularly preferably 4N / 24mm or higher. The adhesive strength (N / 24mm) between the foam sheet and the silicone adhesive tape is measured in accordance with JIS Z0237. Details regarding the measurement conditions for adhesive strength will be described later.

[0021] <Method of manufacturing foamed sheets> One method for forming silicone foam involves mixing and stirring two-part liquid silicone components to initiate a reaction and obtain the silicone foam. Specifically, a self-foaming silicone foam is obtained by foaming (due to the generated hydrogen gas) and curing through the following reaction carried out in the presence of a catalyst such as a platinum catalyst. "Reactions between silanol group-containing organopolysiloxanes such as hydroxyl-terminated polydimethylsiloxane, or hydroxyl-containing compounds (foaming agents), and organohydrogenpolysiloxanes such as methylhydrogenpolysiloxane having SiH groups at both ends and in the side chains," and "Reactions between vinyl group-containing organopolysiloxanes such as dimethylpolysiloxane, which is sealed at both ends with dimethylvinylsiloxy groups, and organohydrogenpolysiloxanes such as methylhydrogenpolysiloxane having SiH groups at both ends and in the side chains." Furthermore, when mixing and stirring the two-part liquid silicone raw materials, an inert gas such as air or nitrogen may be added. This allows for the formation of more uniform cells, as the inert gas acts as a foaming nucleus.

[0022] Specific examples of platinum catalysts include chloroplatinic acid, elemental platinum, chloroplatinic acid hexahydrate, complexes of chloroplatinic acid with sym-divinyltetramethyldisiloxane, dichloro-bis(triphenylphosphine)platinum(II), cis-dichloro-bis(acetonitrile)platinum(II), dicarbonyldichloroplatinum(II), platinum chloride, and platinum oxide, zero-valent platinum metal complexes, such as Karrstedt catalysts, [Cp*Ru(MeCN)3]PF6, [PtCl2(cyclooctadiene)], and solid platinum supported on a support (e.g., aluminum Platinum-siloxane complexes (e.g., Ptn(ViMe2SiOSiMe2Vi)c and Pt[(MeViSiO)4]d)), platinum-phosphine complexes (e.g., Pt(PPh3)4 and Pt(PBU3)4)), and platinum-phosphine complexes (e.g., Pt[P(Oph)3]4 and Pt[P(Obu)3]4)), where "Me" represents methyl, "Bu" represents butyl, "Vi" represents vinyl, and "Ph" represents phenyl, and c and d represent integers.

[0023] As hydroxyl group-containing compounds (foaming aids), alcohols such as benzyl alcohol and ethanol, and water can be used. In this case, a stock solution containing a vinyl group-containing organopolysiloxane (main polymer), a hydroxyl group-containing compound (foaming aid), and a catalyst may be prepared as solution A, and a stock solution containing a vinyl group-containing organopolysiloxane (main polymer) and an organohydrogenpolysiloxane (crosslinking agent) may be prepared as solution B. The foaming reaction and curing reaction may then proceed by mixing and stirring solutions A and B. The number-average molecular weight of the main polymer described above is preferably 500 to 100,000, more preferably 1,000 to 70,000, and even more preferably 1,500 to 50,000. The number-average molecular weight refers to the value measured by gel permeation chromatography (GPC) using standard polystyrene. As mentioned above, when using two liquids consisting of liquid A and liquid B, the mixing ratio (mass ratio) of liquid A to liquid B depends on the density and cell morphology of the foam to be obtained, but is typically 100:1 to 100:50.

[0024] The above-mentioned liquid A may contain silica as a reinforcing material. The amount of silica added is not particularly limited, but is between 0% and 40% by mass based on the total mass of liquid A. Liquid A may also contain titanium dioxide, aluminum hydroxide, calcium carbonate, etc., as fillers for viscosity adjustment and for imparting functionalities such as strength and flame retardancy. The total content of these fillers is not particularly limited, but is between 0% and 50% by mass based on the total mass of liquid A.

[0025] The reaction time for hydrogen generation is adjusted as appropriate depending on the density of the foam to be obtained and the cell morphology, but is usually 1 to 10 minutes, preferably 2 to 6 minutes. The mixing temperature is adjusted as appropriate depending on the density of the foam to be obtained and the cell morphology, but is usually room temperature.

[0026] The foaming ratio, density, and porosity of the foamed layer, as well as the average diameter of the cells in the foamed layer, can be adjusted by optimizing the temperature during curing foaming (molding), the amount of foaming aid, and the ratio of liquid A to liquid B (the amount of methylhydrogenpolysiloxane with SiH groups at both ends and side chains). Furthermore, the flatness of the cells in the foamed layer can be adjusted by optimizing the temperature conditions during curing foaming (molding).

[0027] <Uses of foamed sheets> The foamed sheet described above is suitable for use as a shock absorber for electronic components because it has excellent shock absorption (cushioning) properties over a wide temperature range. Examples of electronic components include display devices and electronic equipment components. Examples of display devices include flexible or bendable display devices, such as rollable displays, foldable displays, and flexible displays, and the foamed sheet described above is suitable for use as a shock absorber (back cushion) on the back side of the display device opposite to the display surface.

[0028] <Electronic equipment> The electronic device according to this embodiment includes the foam sheet described above. Specifically, examples include mobile phones, laptop computers, televisions, and computer monitors in which the foam sheet described above is used as a back cushion for a display device.

[0029] The embodiments of the present invention have been described above, but these are merely examples, and various other configurations can also be adopted. [Examples]

[0030] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0031] Table 1 shows the raw materials and their proportions (parts by mass) used to produce each of the foamed sheets in Examples 1-8 and Comparative Examples 1-3. Table 2 shows the composition and viscosity of the two starting materials (liquid A and liquid B) used to produce the foamed sheets. When producing foamed sheets using liquid silicone, the content of each component was adjusted so that the viscosity of both liquid A and liquid B matched the values ​​shown in Table 2. In Table 2, "wt%" indicates the content of a given component relative to the total mass of liquid A. The raw material viscosity (type B viscosity / Pa·s) listed in Table 2 was measured using a Brookfield rotational viscometer at a temperature of 25°C.

[0032] (Example 1) The method for producing the foamed sheet in Example 1 will be described below. Under room temperature conditions (25°C), the two liquids (liquid A and liquid B) were mixed in the proportions shown in Table 1 and stirred using a stirring device at a rotation speed of 100 rpm for a stirring time of 30 seconds. Subsequently, the stirred mixture was poured into a mold for foamed sheets (thickness 0.3 mm), and PET substrates were placed on both sides of the mold, sandwiching the mold between the pair of PET substrates. In this state, the mixture was reacted at a heating temperature of 60°C for a heating time of 3 minutes, and then, with one side of the PET substrate removed, the mixture was reacted further at a heating temperature of 120°C for a heating time of 3 minutes to obtain the foamed sheet of Example 1.

[0033] (Example 2) The foamed sheet of Example 2 was manufactured using the same method as in Example 1, except that a mold with a thickness of 0.4 mm was used.

[0034] (Example 3) The two liquids (liquid A and liquid B) were mixed and stirred in the proportions shown in Table 1, and the foamed sheet of Example 3 was produced using the same method as in Example 1, except that a mold with a thickness of 1.5 mm was used.

[0035] (Example 4) The two liquids (liquid A and liquid B) were mixed and stirred in the proportions shown in Table 1, and the foamed sheet of Example 4 was produced using the same method as in Example 1, except that a mold with a thickness of 0.4 mm was used.

[0036] (Example 5) The two liquids (liquid A and liquid B) were mixed and stirred in the proportions shown in Table 1, and the foamed sheet of Example 5 was produced using the same method as in Example 1, except that a mold with a thickness of 0.3 mm was used.

[0037] (Example 6) The foam sheet of Example 6 was obtained by removing the surface layer (skin layer) on one side of the foam sheet of Example 3 by polishing it to a thickness of 50 μm or more using abrasive paper (WTCC-S 400 grit) manufactured by MIPOX Corporation.

[0038] (Example 7) The foam sheet of Example 7 was obtained by removing the surface layer (skin layer) on one side of the foam sheet of Example 5 by polishing it to a thickness of 50 μm or more using abrasive paper (WTCC-S 400 grit) manufactured by MIPOX Corporation.

[0039] (Example 8) The two liquids (liquid A and liquid B) were mixed and stirred in the proportions shown in Table 1, and the foamed sheet of Example 8 was produced using the same method as in Example 1, except that a mold with a thickness of 0.5 mm was used.

[0040] (Comparative Example 1) Acrylic foam (manufactured by Iwatani Corporation, ISR-ACF SLN, 0.1 mm thick) was obtained and used as the foamed sheet for Comparative Example 1.

[0041] (Comparative Example 2) Olefin foam (manufactured by Nitto Denko Corporation, SCF T-100, 0.8 mm thick) was obtained and used as the foamed sheet for Comparative Example 2.

[0042] (Comparative Example 3) As shown in Table 1, 70 parts by mass of silicone emulsion (Shin-Etsu Silicone KM-2002-T, manufactured by Shin-Etsu Chemical Co., Ltd., viscosity: 6.5 Pa·s), 30 parts by mass of acrylic emulsion (Boncoat AC-501, manufactured by DIC Corporation, viscosity: 6.0 Pa·s), and 3 parts by mass of TDI trimmer (Duranate TLA-100, manufactured by Asahi Kasei Corporation, viscosity: 0.5 Pa·s) were mixed and stirred to obtain a mixture. This mixture was poured into a mold (a mold for foamed sheets with a PET substrate placed on only one side), and a foam was obtained by mechanical foaming (mechanical flossing) (foaming conditions: screw rotation speed 500 rpm, foaming time 1 minute 30 seconds). The obtained foam was cured at a heating temperature of 120°C for 3 minutes to obtain the foamed sheet of Comparative Example 3.

[0043] <Evaluation Criteria> The following evaluations were performed on each obtained foam sheet. The results are shown in Table 1. In the following evaluation items, "cross-section" refers to the cross-sectional image (image) obtained by cutting the foam sheet with a plane perpendicular to its surface and imaging the resulting cross-section using a SEM. Figures 1(a) to 1(d) show the cross-sectional SEM images of the foam sheets of Examples 1 to 4, respectively. Figures 2(a) to 2(d) show the cross-sectional SEM images of the foam sheets of Examples 5 to 8, respectively. Figures 3(a) to 3(c) show the cross-sectional SEM images of the foam sheets of Comparative Examples 1 to 3, respectively.

[0044] (Density of the foam layer) The density of the foam layer of each foam sheet was measured in accordance with JIS K7222:2005 "Foamed plastics and rubber - Method for determining apparent density".

[0045] (Thickness of the foam layer) The thickness of the cross-section perpendicular to the surface of each foam sheet was measured using a dial thickness gauge.

[0046] (Average cell diameter) In the cross-section perpendicular to the surface of the foam sheet (within 2 mm in the horizontal direction), the vertical length of each cell was measured, and the average of the vertical lengths obtained for each cell was taken as the average diameter.

[0047] (Cell flatness) The flatness of the cells in the foam layer of each foam sheet was calculated according to the following calculation method. (Calculation method of average cell flatness) In the cross-section perpendicular to the sheet surface (within 2 mm in the horizontal direction), the vertical length and the horizontal length of each cell were measured respectively, and the flatness calculated by the following formula was calculated. Flatness = vertical length / horizontal length The flatness obtained for each cell was averaged to obtain the average flatness.

[0048] (Void fraction) When the foam layer of each foam sheet was evenly divided into 10 layers in the vertical direction, the void fraction (%) of each layer was t i (where i is the order of each layer counted from the surface layer), for the void fractions (%) of t1, t5, t 10 of each layer, a cross-section of the foam layer was photographed using a scanning electron microscope (SEM, manufactured by KEYENCE CORPORATION, VHX-D510), and calculated using the cross-sectional image. The evaluation area is (total thickness of the foam layer) / 10 × 2 mm in the horizontal direction. The photographed cross-sectional image was separated into the resin skeleton part and the void part by color brightness and darkness using the attached image processing software, and the area of the void part was calculated. For the parts where the resin skeleton part and the void part could not be clearly separated by color brightness and darkness, manual repair was performed. The void fraction was calculated as follows using the above evaluation area ((total thickness of the foam layer) / 10 × 2 mm) and the area of the void part. Void fraction (%) = (area of the void part) / evaluation area × 100 Furthermore, the difference in void ratio S (the difference between the void ratio of the central part of the foamed layer and the void ratio of the surface part of the foamed layer) was calculated using the following formula. S = t5 - AVE(t1 + t) 10 )

[0049] (Compression residual strain) The compression set (%) is calculated according to JIS K6401 by compressing a 50 x 50 mm foam sheet by 50% in the thickness direction, leaving it to stand at a specified temperature (70°C) for 22 hours, then releasing the compressive stress at room temperature, measuring the thickness of the foam sheet after 30 minutes (thickness after release), and using the following formula. Compression residual strain (%) = (thickness before compression - thickness after release) / thickness before compression × 100

[0050] (Hardness (25% CLD)) Hardness (25%CLD(kPa)) was measured under conditions of -30°C and 23°C. The hardness (25%CLD(kPa)) is defined as the compressive stress obtained when a φ50mm sample is compressed by 25% at a speed of 1mm / min, according to JIS K6254. Furthermore, the rate of change in hardness (25%CLD) was calculated based on the following formula. Rate of change (%) = {Hardness at <-30℃ (25%CLD)> - <Hardness at 23℃ (25%CLD)>} / <Hardness at 23℃ (25%CLD)> × 100 As shown in Table 1, the foamed sheets of Examples 1 to 8 showed a hardness change rate (%) of 10% or less, confirming low temperature dependence. In contrast, the foamed sheets of Comparative Examples 1 to 3 showed a hardness change rate (%) of 30% or more, indicating high temperature dependence.

[0051] (Adhesion strength with tape) In accordance with JIS Z0237, a foam sheet was cut to a width of 24 mm and a length of 130 mm, attached to a silicone adhesive tape (Ebisu Kasei Co., Ltd., TAPE #880WP, width 24 mm x length 130 mm x thickness 0.11 mm), pressed down twice with a 2 kg roller, and left at room temperature (23 ± 5 °C, 60 ± 20% RH) for 24 hours. Subsequently, an autograph was used to perform a test of pulling up at a speed of 300 mm / min (90° peel) under room temperature conditions (23 ± 5 °C, 60 ± 20% RH) to determine the adhesive strength (90° peel strength) (N / 24 mm).

[0052] [Table 1]

[0053] [Table 2]

[0054] [Table 3]

Claims

1. The foamed layer includes a cell with an average flatness of 1.42 or more and 1.9 or less, calculated according to the method described below (Method for calculating the average flatness of cells). The aforementioned foam layer is formed of silicone foam, The density of the aforementioned foamed layer is 400 kg / m³ ３ More than 900kg / m ３ The following is a foam sheet. (Method for calculating the average flatness of cells) In a cross-section perpendicular to the sheet surface (within 2 mm horizontally), the vertical and horizontal lengths of each cell are measured, and the flatness is calculated using the following formula. Flatness = Vertical length / Horizontal length The average flatness is calculated by averaging the flatness values ​​obtained for each cell.

2. The foamed sheet according to claim 1, wherein the foamed layer comprises at least one selected from silica, titanium oxide, aluminum hydroxide, and calcium carbonate.

3. The foamed layer includes a cell average flatness greater than 1, calculated according to the method below (Method for calculating the average flatness of cells), A method for manufacturing a foamed sheet in which the foamed layer is formed of silicone foam, It includes a process for mixing silicone raw materials, A method for producing a foamed sheet, using benzyl alcohol or ethanol, which are hydroxyl group-containing compounds, as one of the raw materials. (Method for calculating the average flatness of cells) In a cross-section perpendicular to the sheet surface (within 2 mm horizontally), the vertical and horizontal lengths of each cell are measured, and the flatness is calculated using the following formula. Flatness = Vertical length / Horizontal length The average flatness is calculated by averaging the flatness values ​​obtained for each cell.

4. The foamed layer includes a cell average flatness greater than 1, calculated according to the method below (Method for calculating the average flatness of cells), A method for manufacturing a foamed sheet in which the foamed layer is formed of silicone foam, A mixing process in which silicone raw materials are mixed, The system includes a heating step in which the mixture after the mixing step is heated in two stages, A method for manufacturing a foamed sheet, wherein in the second stage of heating in the aforementioned two-stage heating process, the pressure in the thickness direction is relieved during heating. (Method for calculating the average flatness of cells) In a cross-section perpendicular to the sheet surface (within 2 mm horizontally), the vertical and horizontal lengths of each cell are measured, and the flatness is calculated using the following formula. Flatness = Vertical length / Horizontal length The average flatness is calculated by averaging the flatness values ​​obtained for each cell.

5. A foam sheet according to claim 1 or 2, used for protecting electronic components.

6. An electronic device comprising a foamed sheet according to any one of claims 1, 2, or 5.

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