JP7833282B2 - Foam sheets and electronic devices - Google Patents

Description

This invention relates to foamed sheets and electronic devices.

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.

International Publication No. 2016/047611 Japanese Patent Publication No. 2014-70174

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.
Silicone materials are cited as a solution to these challenges. In particular, silicone foam is suitable for achieving thinness and higher shock absorption. However, we have found that depending on the shape of the air bubbles (cells) and the dispersion (or distribution) of the air bubbles in the silicone foam, there is a problem in that wrinkles may form on the surface of the silicone foam when it is bent.

This invention addresses the aforementioned problems and aims to provide a foamed sheet that exhibits excellent shock absorption over a wide temperature range and suppresses wrinkle formation during bending.

One aspect of the present invention is a foamed sheet. The foamed sheet comprises a foamed layer formed of silicone foam, and when the foamed layer is divided equally 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), and S calculated by the formula S = t ₅ - (t ₁ + t ₁₀ ) / 2 is 10% or more.

In the foamed sheet according to the above embodiment, it is preferable that _t5 is 50% or more. The density of the foamed layer may be 200 kg/ ^m³ or more. It may also be used for protecting electronic components.

Another aspect of the present invention is an electronic device. This electronic device comprises a foamed sheet according to any of the above-described embodiments.

According to the present invention, it is possible to provide a technology for a foamed sheet that exhibits excellent shock absorption over a wide temperature range and suppresses the occurrence of wrinkles when bent.

Figures 1(a) to 1(f) are cross-sectional SEM images of the foamed sheets of Examples 1 to 6, respectively. Figures 2(a) to 2(d) are cross-sectional SEM images of the foamed sheets of Comparative Examples 1 to 4, respectively.

The embodiments of the present invention will be described in detail below. In this specification, unless otherwise specified, the notation "a to b" in the description of numerical ranges indicates a range of a or greater and b or less.

<Foam sheet>
The foamed sheet according to the embodiment will be described in detail below.
The foamed sheet according to the embodiment comprises a foamed layer formed of silicone foam. When the foamed layer is divided equally into 10 layers in the vertical direction, and the void ratio (%) of each layer is denoted as t _i (where i is the order of each layer counted from the surface layer), the value S calculated by the formula S = t ₅ - (t ₁ + t ₁₀ ) / 2 is 10% or more. It is more preferable that S be 20% or more, and even more preferable that it be 30% or more.

The foamed sheet according to this embodiment exhibits minimal temperature dependence of hardness, particularly a small increase in hardness at low temperatures. Therefore, it possesses excellent shock absorption over a wide temperature range, from low temperatures (e.g., -30°C) to room temperature (e.g., 23°C). Furthermore, the foamed sheet according to this embodiment has the excellent property of suppressing wrinkle formation on its surface when bent.

<<Composition of the foamed layer>>
The foamed layer constituting the foamed sheet according to this embodiment is formed of silicone foam.
It is preferable that the silicone foam is a self-foaming reaction type silicone foam. A self-foaming reaction type silicone foam is a foam in which bubbles (cells) are formed by foaming due to gas (hydrogen) generated during curing when two liquid silicone materials are mixed and stirred.

<<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.

<<Density of the foamed layer>>
The lower limit of the density of the foamed layer is preferably 200 kg/ ^m³ or more, more preferably 300 kg/ ^m³ or more, and even more preferably 400 kg/ ^m³ or more. On the other hand, the upper limit of the density of the foamed layer is preferably 900 kg/ ^m³ or less, more preferably 800 kg/ ^m³ or less, and even more preferably 700 kg/ ^m³ or less. By having the density of the foamed layer within the above range, the compression set can be reduced.

<<Average cell diameter>>
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).

<<Voidity of the foamed layer>>
When the foamed layer is divided into 10 uniform layers in the vertical direction, and the void ratio (%) of each layer is denoted as t _i (where i is the order of each layer counted from the surface), then t ₅ is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, and particularly preferably 80% or more.

<<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 set (%) = (Thickness before compression - Thickness after release) / Thickness before compression × 100

<<Hardness of the foam sheet (25% CLD)>>
The hardness (25% CLD (kPa)) in environments of -30°C and 23°C is preferably 200 kPa or less, more preferably 150 kPa or less, and even more preferably 100 kPa or less.
The hardness (25% CLD (kPa)) is defined as the compressive stress obtained when a φ50 mm sample is compressed by 25% at a speed of 1 mm/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°C (25% CLD)> - <Hardness at 23°C (25% CLD)>} / <Hardness at 23°C (25% CLD)> × 100

<<Adhesive strength with tape>>
The adhesive strength (N/24mm) between the foam sheet and the silicone adhesive tape is preferably 1N/24mm or more, more preferably 2N/24mm or more, even more preferably 3N/24mm or more, and particularly preferably 4N/24mm or more.
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.

<Method 1 for 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 are 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.

Specific examples of platinum catalysts include chloroplatinic acid, elemental platinum, chloroplatinic acid hexahydrate, chloroplatinic acid complexes 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, e.g., Karstedt catalysts, [Cp*Ru(MeCN) ₃ ] _PF6 , [ _PtCl2 (cyclooctadiene)], solid platinum supported on a carrier (e.g., alumina, silica, or carbon black), platinum-vinylsiloxane complexes (e.g., Ptn( _{ViMe2SiOSiMe2Vi} )c and Pt _{[(MeViSiO)4 ]} d)), platinum-phosphine complexes (e.g., Pt( _PPh3 ) Pt(PBU3) ₄ )), and platinum-phosphite 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.

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 described 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 shape of the foam to be obtained, but is typically 100:1 to 100:50.

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. Furthermore, liquid A may contain titanium dioxide, aluminum hydroxide, calcium carbonate, etc., as fillers for viscosity adjustment and for imparting functional properties 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.

The reaction time during 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 also adjusted as appropriate depending on the density of the foam to be obtained and the cell morphology, but is usually room temperature.

The foaming ratio, density, and porosity of the foam layer, as well as the average diameter of the cells within 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 within the foam layer can be adjusted by optimizing the temperature conditions during curing foaming (molding).

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

＜＜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 it with an emulsion containing resins other than silicone resins, such as acrylic resins, polyurethane resins, polyester resins, or polyepoxy resins.

<<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.

<<Foaming and curing 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.

<<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.

<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.

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. This mechanical flossing method allows for the production of foamed sheets with densities suitable for various applications by adjusting the mixing ratio of the emulsion composition and air.

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

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.

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

<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.

<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.

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

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

(Example 1)
The method for producing the foamed sheet in Example 1 will now be described.
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.

(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.

(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.

(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.

(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.

(Example 6)
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 6.

(Comparative Example 1)
Comparative Example 1 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 No. 400) manufactured by MIPOX Corporation.

(Comparative Example 2)
Comparative Example 2 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 No. 400) manufactured by MIPOX Corporation.

(Comparative Example 3)
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 3.

(Comparative Example 4)
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 4.

<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 a 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(f) show the cross-sectional SEM images of the foam sheets of Examples 1 to 6, respectively. Figures 2(a) to 2(d) show the cross-sectional SEM images of the foam sheets of Comparative Examples 1 to 4, respectively.

(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".

(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.

(Average diameter of cells)
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 vertical lengths obtained for each cell was used to determine the average diameter.

(Cell flatness)
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 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 obtained for each cell.

(porosity)
When the foam layer of each foam sheet is divided vertically into 10 equal layers, the void ratio (%) of each layer is denoted as t _i (where i is the order of each layer counted from the surface). The void ratio (%) of each layer _t1 , _t5 , and _t10 was calculated using a scanning electron microscope (SEM, VHX-D510, manufactured by Keyence Corporation) to photograph the cross-section of the foam layer and the resulting cross-sectional image. The evaluation area is (total thickness of the foam layer) / 10 × 2 mm horizontally. The captured cross-sectional image was separated into the resin skeleton and void areas based on the brightness of the color using the attached image processing software, and the area of the void area was calculated. Areas where the resin skeleton and void areas could not be cleanly separated by the brightness of the color were manually repaired. The void ratio is calculated as follows using the above evaluation area ((total thickness of the foam layer) / 10 × 2 mm) and the area of the void area.
Void ratio (%) = (Area of voids) / Evaluated 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=t ₅ −AVE(t ₁ +t ₁₀ )

(Compression residual strain)
The compressive residual 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 set (%) = (Thickness before compression - Thickness after release) / Thickness before compression × 100

(Hardness (25% CLD))
The 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 φ50 mm sample is compressed by 25% at a rate of 1 mm/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°C (25% CLD)> - <Hardness at 23°C (25% CLD)>} / <Hardness at 23°C (25% CLD)> × 100
As shown in Table 1, the foamed sheets of Examples 1 to 5 showed a hardness change rate (%) of 10% or less, confirming low temperature dependence. In contrast, the foamed sheets of Comparative Examples 3 and 4 showed a hardness change rate (%) of 50% or more, indicating high temperature dependence.

(Adhesion strength with tape)
In accordance with JIS Z0237, a foam sheet was cut to a width of 24 mm x 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 together by two passes 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).

(Presence or absence of wrinkles)
Each foam sheet (40 mm wide, 100 mm long) was wrapped around a cylindrical polyethylene rod (Φ32.5 mm, Φ18.5 mm), and it was visually determined whether or not wrinkles formed on the surface of the foam sheet.
The criteria for determining whether or not wrinkles have formed are as follows:
A: No wrinkles appear on the entire surface of the foamed sheet exposed after wrapping. B: One wrinkle is visible on the entire surface of the foamed sheet exposed after wrapping. C: Two or more wrinkles are visible on the entire surface of the foamed sheet exposed after wrapping. As shown in Table 1, it was confirmed that the foamed sheets of Examples 1 to 6 suppressed the occurrence of wrinkles on the sheet surface when bent. On the other hand, the foamed sheet of Comparative Example 2 developed multiple wrinkles on the sheet surface when bent. Note that the foamed sheet of Comparative Example 1 could not be evaluated for the presence or absence of wrinkles because the surface layer had been removed.

Claims (7)

It has a foamed layer formed from silicone foam,
When the foamed layer is divided vertically into 10 equal 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 = t ₅ - (t ₁ + t ₁₀ )/2 is 46 % or more .
A foamed sheet having a foamed layer density of 400 kg/m³ ^or more and 900 kg/m³ ^or less. The foamed sheet according to claim 1, wherein _t5 is 50% or more. The foamed sheet according to claim 1 or 2 , wherein the foamed layer comprises at least one selected from silica, titanium oxide, aluminum hydroxide, and calcium carbonate . It has a foamed layer formed from silicone foam,
When the aforementioned foamed layer is divided into 10 equal layers in the vertical direction, the void ratio (%) of each layer is given by t _i When i is the order of each layer counted from the surface, S = t ₅ - (t ₁ +t ₁₀ A method for manufacturing a foamed sheet, wherein S, calculated by the formula )/2, is 10% or more,
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. It has a foamed layer formed from silicone foam,
When the aforementioned foamed layer is divided into 10 equal layers in the vertical direction, the void ratio (%) of each layer is given by t _i When i is the order of each layer counted from the surface, S = t ₅ - (t ₁ +t ₁₀ A method for manufacturing a foamed sheet, wherein S, calculated by the formula )/2, is 10% or more,
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. A foamed sheet according to any one of claims 1 to 3, used for protecting electronic components. An electronic device comprising a foamed sheet according to any one of claims 1 to 3 or 6 .