Foam sheets and electronic devices

Abstract

To provide a foam sheet which has excellent impact absorption property in a wide temperature zone, and is suitable for protecting a back face of a display device.SOLUTION: There is provided a foam sheet. The foam sheet includes a foam layer formed of a silicone foam, and is used for protecting a back face of a display device.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 2] 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. Furthermore, if the display itself deforms, it is not possible to protect the back of the display with a rigid, sturdy casing like conventional displays. Therefore, there is a need for new protective materials suitable for protecting the back of the display.

[0005] In view of the above-mentioned problems, the present invention aims to provide a foamed sheet that has excellent shock absorption over a wide temperature range and is suitable for protecting the back of a display device. [Means for solving the problem]

[0006] One aspect of the present invention is a foamed sheet, which comprises a foamed layer formed of silicone foam and is used to protect the back of a display device.

[0007] In the foamed sheet according to the above-described embodiment, the foamed layer may have an average flatness of 0.5 or more cells, calculated by the following (method for calculating the average flatness of cells). (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.

[0008] Furthermore, the density of the foamed layer is 200 kg / m³. 3 That's fine too.

[0009] Another aspect of the present invention is an electronic device, which has one of the above-described foam sheets on its back. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a technology relating to a foam sheet that has excellent shock absorption properties over a wide temperature range and is suitable for protecting the back of a display device. [Brief explanation of the drawing]

[0011] [Figure 1] Figures 1(a) to 1(d) are cross-sectional SEM images of the foam sheets of Examples 1 to 4, respectively. [Figure 2] Figures 2(a) to (e) are cross-sectional SEM images of the foamed sheets of Examples 5 to 9, respectively. [Figure 3] Figures 3(a) and 3(b) are cross-sectional SEM images of the foamed sheets of Comparative Examples 1 and 2, respectively. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described in detail below. In this specification, unless otherwise specified, the notation "a~b" in the description of numerical ranges means a or greater and b or less.

[0013] <Foam sheet> The foamed sheet according to the embodiment will be described in detail below. The foamed sheet according to this embodiment comprises a foamed layer formed from silicone foam. It is used to protect the back of a display device. The foamed layer formed from silicone foam has little temperature dependence of hardness, and in particular, the hardness increase at low temperatures is small. For this reason, it has excellent shock absorption over a wide temperature range from low temperatures (e.g., -30°C) to room temperature (e.g., 23°C), and is suitable for protecting the back of a display device. Furthermore, because the foamed sheet according to this embodiment has excellent curvature and flexibility, it is suitable for protecting the back of a deformable display device that is flexible or bendable and does not have a rigid housing. Examples of the display device include a rollable display device, a foldable display, and a flexible display. The foamed sheet described above is preferably used as a shock absorber (back cushion) on the back surface opposite to the display surface of the display device.

[0014] <<Composition of the foamed layer>> The foamed layer constituting the foamed sheet according to the embodiment is formed of a silicone foam. The form of the silicone foam is not particularly limited, and examples include a self-foaming reaction type silicone foam and a silicone foam using an emulsion. The self-foaming reaction type silicone foam is a foam in which two liquid silicone materials are mixed and stirred, and foamed by gas (hydrogen) generated during curing to form bubbles (cells). The silicone foam using an emulsion is a foam obtained by emulsifying silicone and water to prepare an emulsion composition, mechanically foaming the emulsion composition, and then undergoing a process of curing (drying).

[0015] <<Thickness of the foamed layer>> The thickness of the foamed 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.

[0016] <<Density of the foamed layer>> The lower limit of the density of the foamed layer is preferably 200 kg / m 3 or more, more preferably 三百 kg / m [[ID=二十三]] 3 or more, and even more preferably 四百 kg / m 3 or more. On the other hand, the upper limit of the density of the foamed 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 foamed layer is within the above range, the compression residual strain can be reduced.

[0017] <<Average diameter of cells>> It should be noted that there seem to be some incorrect or incomplete expressions in the original text such as "三百 kg / m" and "四百 kg / m" which should be corrected to the correct numerical values in the actual translation. Here, the translation is presented based on the existing text as accurately as possible.The lower limit of the average diameter of cells 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 diameter of cells 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 diameter of the cells is calculated according to the following calculation method. (Method for calculating the average diameter of cells) In a cross-section perpendicular to the sheet surface (within 2 mm horizontally), the vertical length of each cell is measured, and the obtained vertical lengths for each cell are averaged to obtain the average diameter. The sheet cross-section for calculating the average diameter of the cells can be imaged using a scanning electron microscope (SEM).

[0018] <<Cell flatness>> The average flatness of the cells calculated according to the method described below (Method for calculating the average flatness of the cells) is preferably 0.5 or higher, more preferably greater than 1, and even more preferably 1.5 or higher. (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. A cross-sectional image of the foamed sheet, captured by a scanning electron microscope (SEM), can be used as the cross-section.

[0019] <<Porosity of the foamed layer>> When the foam layer is divided vertically into 10 uniform layers, and the void ratio (%) of each layer is denoted as ti (where i is the order of each layer counted from the surface), the value of S calculated by the formula S = t5 - (t1 + t10) / 2 is preferably 1% or more, more preferably 10% or more, even more preferably 20% or more, and particularly preferably 30% or more. Furthermore, t5 is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, and particularly preferably 80% or more.

[0020] <<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

[0021] <<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

[0022] <<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.

[0023] <Method for manufacturing foamed sheets 1> 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] The foaming ratio, density, and porosity of the foam layer, as well as the average diameter of the cells in the foam 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 (amount of Si-H added). Furthermore, the flatness of the cells in the foam layer can be adjusted by optimizing the temperature conditions during curing foaming (molding).

[0029] <Method for manufacturing foamed sheets 2> Foamed sheets can also be obtained by foaming / curing a silicone emulsion composition.

[0030] <<Raw materials>> The silicone resin used in the silicone emulsion composition is not particularly limited as long as it contains a silane compound as a raw material monomer, and dimethyl silicone, methylphenyl silicone, and various modified silicones (for example, amino-modified silicone, epoxy-modified silicone, polyether-modified silicone emulsion, alkyl-modified silicone emulsion, fluorine-modified silicone, etc.) can be used. A silicone emulsion composition can be produced, for example, by blending raw material monomers of a resin component in an aqueous medium and emulsion polymerization of the raw material monomers in the presence of various additives such as emulsifiers and polymerization initiators. Furthermore, a silicone emulsion composition may also be used by blending an emulsion containing resins other than silicone resins, such as acrylic resins, polyurethane resins, polyester resins, or polyepoxy resins.

[0031] <<Preparation process>> In the preparation process, the aforementioned raw materials are mixed to prepare a silicone emulsion composition, which is the raw material mixture for the foamed sheet. The mixing method is not particularly limited, but for example, the components can be mixed while stirring in a container such as a mixing tank.

[0032] <<Foaming and hardening process>> In the foaming and curing process, a predetermined foaming gas is added to the silicone emulsion composition obtained in the preparation process, and these are thoroughly mixed to create a state in which numerous bubbles exist in the silicone emulsion composition (foamed emulsion composition). This foaming and curing process is usually carried out by thoroughly mixing the liquid foam sheet raw material mixture obtained in the raw material preparation process with the foaming gas using a mixing device such as a mixing head.

[0033] <<Foaming Gas>> The foaming gas mixed into the emulsion composition during the stirring and foaming process forms bubbles (cells) in the foam, and the amount of foaming gas added determines the foaming ratio and density of the resulting foam. To adjust the density of the foamed sheet, the required weight of the foamed sheet raw material is calculated from the desired density of the foamed sheet and the volume of the foamed sheet raw material (for example, the internal volume of the mold into which the foamed sheet raw material is injected), and the amount of foaming gas is determined so that the desired volume is achieved with this weight. Air is mainly used as the foaming gas, but other inert gases such as nitrogen, carbon dioxide, helium, and argon can also be used.

[0034] <Foaming Method / Foaming Conditions> One foaming method used in the foam preparation method according to this embodiment is the mechanical flossing method. The mechanical flossing method is a method in which air from the atmosphere is mixed into the silicone emulsion composition by stirring it with a stirring blade or the like, thereby causing foaming.

[0035] As for the stirring device, any stirring device commonly used in the mechanical flossing method can be used without particular restriction, but for example, a homogenizer, dissolver, or mechanical flossing machine can be used. With this mechanical flossing method, by adjusting the mixing ratio of the emulsion composition and air, foamed sheets of a density suitable for various applications can be obtained.

[0036] The mixing time between the silicone emulsion composition and air is adjusted as appropriate depending on the density, porosity, and cell morphology of the desired foam, but is usually 1 to 10 minutes, preferably 2 to 6 minutes. The mixing temperature is adjusted as appropriate depending on the density and cell morphology of the desired foam, but is usually room temperature.

[0037] The stirring speed during mixing is preferably 200 rpm or higher (more preferably 500 rpm or higher) to create fine bubbles, and preferably 2000 rpm or lower (more preferably 800 rpm or lower) to ensure smooth discharge of the foam from the foaming machine.

[0038] As described above, a foamed silicone emulsion composition (foamed silicone emulsion composition), in other words, a foamed sheet can be obtained.

[0039] <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.

[0040] The embodiments of the present invention have been described above, but these are merely examples, and various other configurations can also be adopted. [Examples]

[0041] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0042] (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.

[0043] (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.

[0044] (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.

[0045] (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.

[0046] (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.

[0047] (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.

[0048] (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.

[0049] (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.

[0050] (Example 9) 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 Example 9.

[0051] (Comparative Example 1) Acrylic foam (manufactured by Iwatani Corporation, ISR-ACF SLN) was obtained and prepared to a thickness of 0.1 mm to be used as the foamed sheet for Comparative Example 1.

[0052] (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.

[0053] <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) and 3(b) show the cross-sectional SEM images of the foam sheets of Comparative Examples 1 and 2, respectively.

[0054] (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".

[0055] (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.

[0056] (Average cell diameter) In a cross-section perpendicular to the surface of the foam sheet (within 2 mm horizontally), the vertical length of each cell was measured, and the average of the obtained vertical lengths for each cell was used to determine the average diameter.

[0057] (Cell flattening) The flatness of the cells in the foam layer of each foam sheet was calculated according to the following calculation method. (Method for calculating the average flatness of cells) In a cross-section perpendicular to the sheet surface (within 2 mm in the horizontal direction), measure the vertical length and horizontal length of each cell, and calculate the flatness calculated by the following formula. Flatness = Vertical length / Horizontal length Average the flatness obtained for each cell to obtain the average flatness.

[0058] (Void ratio) When the foam layer of each foam sheet is evenly divided into 10 layers in the vertical direction, the void ratio (%) of each layer is t i (where i is the order of each layer counted from the surface layer), for the void ratios (%) of t1, t5, and t 10 of each layer, use a scanning electron microscope (SEM, manufactured by Keyence Corporation, VHX-D510) to photograph the cross-section of the foam layer, and calculate using the cross-sectional image. The evaluation area is (the 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 the brightness and darkness of the color using the attached image processing software, and the area of the void part was calculated. For parts where the resin skeleton part and the void part could not be clearly separated by the brightness and darkness of the color, manual repair was performed. The void ratio is calculated as follows using the above evaluation area ((the total thickness of the foam layer) / 10 × 2 mm) and the area of the void part. Void ratio (%) = (Area of the void part) / Evaluation area × 100 Also, calculate the difference S in void ratio (the difference between the void ratio of the central part of the foam layer and the void ratio of the surface layer part) calculated by the following formula. S = t5 - AVE(t1 + t 10 )

[0059] (Compression residual strain) The compression residual strain (%) is based on JIS K6401. Compress a 50 × 50 mm foam sheet by 50% in the thickness direction, leave it standing at a predetermined temperature (70 °C) for 22 hours, then release the compression stress at room temperature and measure the thickness of the foam sheet (thickness after release) after 30 minutes. It is the value calculated by the following formula. Compression residual strain (%) = (Thickness before compression - Thickness after release) / Thickness before compression × 100

[0060] (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 extremely low temperature dependence. Furthermore, the foamed sheet of Example 9 showed a hardness change rate (%) of less than 50%, confirming sufficiently low temperature dependence. In contrast, the foamed sheets of Comparative Examples 1 and 2 showed a hardness change rate (%) of 50% or more, indicating high temperature dependence.

[0061] (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).

[0062] [Table 1]

[0063] [Table 2]

[0064] [Table 3]

Claims

1. It has a foamed layer formed from silicone foam, The foamed layer has an average cell flatness of 0.5 or more and 1.9 or less, calculated according to the following (method for calculating the average cell flatness). The density of the aforementioned foamed layer is 400 kg / m³ ３ More than 900kg / m ３ The following is a foam sheet used to protect the back of a deformable display device. (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. It has a foamed layer formed from silicone foam, A method for manufacturing a foam sheet used to protect the back of a deformable display device, It includes a process for mixing silicone raw materials, A method for manufacturing foamed sheets, using a hydroxyl group-containing compound as one of the raw materials.

4. It has a foamed layer formed from silicone foam, A method for manufacturing a foam sheet used to protect the back of a deformable display device, 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.

5. An electronic device having the foam sheet described in claim 1 or 2 provided on its back surface.

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