JP7833281B2 - 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.

This invention addresses the aforementioned problems and aims to provide a foamed sheet with excellent shock absorption properties across a wide temperature range.

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 obtained for each cell.

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.

According to the present invention, it is possible to provide technology related to a foamed sheet that has excellent shock absorption properties over a wide temperature range.

Figures 1(a) to 1(d) are cross-sectional SEM images of the foamed sheets of Examples 1 to 4, respectively. Figures 2(a) to 2(d) are cross-sectional SEM images of the foamed sheets of Examples 5 to 8, respectively. Figures 3(a) to 3(c) are cross-sectional SEM images of the foamed sheets of Comparative Examples 1 to 3, 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 includes a foamed layer in which the average flatness of the cells, calculated by the following (method for calculating the average flatness of the 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 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.

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).

<<Composition of the foamed layer>>
The foamed layer constituting the foamed sheet according to the embodiment is preferably 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 two liquid silicone materials are mixed and stirred, causing foaming due to the gas (hydrogen) generated during curing, and voids (cells) are formed.

<<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 foam layer is divided vertically into 10 uniform layers, and the void ratio (%) of each layer is denoted as t _i (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 preferably 10% or more, preferably 20% or more, and more preferably 30% or more. Furthermore, t ₅ 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 void ratio of each layer will be described later.

<<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 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 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 foamed layer, as well as the average diameter of the cells within the foamed layer, can be adjusted by optimizing the temperature during curing foaming (molding), the amount of foaming aid, and the ratio of solution A to solution B (the amount of methylhydrogenpolysiloxane with SiH groups at both ends and side chains). Furthermore, the flatness of the cells within the foamed layer can be adjusted by optimizing the temperature conditions during curing foaming (molding).

<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.

Table 1 shows the raw materials and their proportions (parts by mass) used in the preparation of each foamed sheet 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 in the preparation of the foamed sheets. When preparing 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 predetermined 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.

(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)
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 No. 400) manufactured by MIPOX Corporation.

(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 No. 400) manufactured by MIPOX Corporation.

(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.

(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.

(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.

(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.

<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.

(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 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.

(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).

Claims (6)

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,
A foamed sheet having a foamed layer density of 400 kg/ ^m³ or more and 900 kg/ ^m³ or less.
(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. 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. 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 obtained for each cell. 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 obtained for each cell. A foamed sheet according to claim 1 or 2, used for protecting electronic components. An electronic device comprising a foamed sheet according to any one of claims 1, 2, or 5.