FOAMED SHEET

Abstract

The foamed sheet includes a foamed body having an average cell diameter of 10 to 200 μm, and a compression set at 80° C. of not more than 80% and an impact absorption change rate of not more than ±20% defined by: Impact absorption change rate (%)={(an impact absorption rate b after high-temperature compression−an initial impact absorption rate a)/the initial impact absorption rate a}×100, where the impact absorption rate a is an impact absorption rate (%) of a test piece A and the impact absorption rate b (%) after high-temperature compression is an impact absorption rate (%) acquired by storing the test piece A at 80° C. for 72 hours in the state of being compressed by 60% with respect to the initial thickness of the test piece A, thereafter releasing the compression state, and thereafter conducting measurement after a lapse of 24 hours at 23° C.

Description

TECHNICAL FIELD

The present invention relates to a foamed sheet which, even if having a very small thickness, is excellent in impact absorption and excellent in heat resistance, and to an electric or electronic device using the foamed sheet.

BACKGROUND ART

There are conventionally used foamed materials when optical members such as image display members fixed on image display apparatuses such as liquid crystal displays, electroluminescence displays and plasma displays, display members installed on so-called “cellular phones,” “smartphones” and “personal digital assistants,” cameras and lenses are fixed on predetermined sites (for example, housings). As such foamed materials, there have been used compression-molded high-density fine-cell urethanic foamed bodies having a closed cell structure and low-density urethane and besides, polyethylenic foamed bodies having closed cells and an expansion ratio of about 30, and the like. Specifically, there are used, for example, a gasket (see Patent Literature 1) composed of a polyurethanic foamed body having an apparent density of 0.3 to 0.5 g/cm³, and a sealing material for electric or electronic devices (see Patent Literature 2) composed of a foamed structural body having an average cell diameter of 1 to 500 μm.

In recent years, along with the thickness reduction of electronic devices such as PCs (personal computers), tablet PCs, PDAs (personal digital assistants) and cellular phones, impact absorption sheets have been used on panel rear faces for the prevention of breakage of liquid crystal panels, organic EL panels and the like. Then, the thickness reduction is demanded also on the impact absorption sheets. In the case where conventional foamed materials are used as such impact absorption sheets, however, the sheets cannot exhibit sufficient impact absorption.

Further along with the function enhancement of electronic devices, in heat generating bodies such as electronic components, the amount of heat generation becomes large. When conventional foamed materials are used for electric or electronic devices having such heat generating bodies exhibiting a large amount of heat generation, a problem which arises is that the performance reduces due to heat accumulated in the interiors and the electric or electronic devices are broken due to impact when being dropped or otherwise.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2001-100216

Patent Literature 2: Japanese Patent Laid-Open No. 2002-309198

SUMMARY OF INVENTION

Technical Problem

Therefore, an object of the present invention is to provide a foamed sheet which, even if having a very small thickness, exhibits excellent impact absorption and is excellent in heat resistance in which the performance does not reduce even under high temperatures.

Another object of the present invention is to provide an electric or electronic device which, even if being down sized and reduced in thickness, and having a heat generating body exhibiting a large amount of heat generation, is hardly broken due to impact in dropping.

Solution to Problem

As a result of exhaustive studies in order to achieve the above objects, the present inventors have found that: a foamed sheet, comprising a foamed body having a specific average cell diameter and having a compression set at 80° C. of not more than 80% and a small difference between the impact absorption after the high-temperature (80° C.) compression and the initial impact absorption, even if the foamed sheet has a very small thickness, exhibits excellent impact absorption and is excellent in heat resistance as well; and an electric or electronic device installed with the foamed sheet, even if having a heat generating body exhibiting a large amount of heat generation, is not broken by impact in dropping. The present invention has been completed by carrying out further studies based on these findings.

That is, the present invention provides a foamed sheet comprising a foamed body having an average cell diameter of 10 to 200 μm, and having a compression set at 80° C. of not more than 80% and an impact absorption change rate of not more than ±20% as defined by the following.

Impact absorption change rate (%)={(an impact absorption rate b after high-temperature compression−an initial impact absorption rate a)/the initial impact absorption rate a}×100

The initial impact absorption rate a: an impact absorption rate (%) of a test piece A

The impact absorption rate b (%) after high-temperature compression: an impact absorption rate (%) acquired by storing the test piece A at 80° C. for 72 hours in the state of being compressed by 60% with respect to the initial thickness of the test piece A, thereafter releasing the compression state, and thereafter conducting measurement after a lapse of 24 hours at 23° C.

The impact absorption rate: a value defined by the following expression in an impact absorption test (the weight of an impactor: 28 g, the swing-up angle: 40°) (23° C.) using a pendulum impact tester.

Impact absorption rate (%)={(F₀−F₁)/F₀}×100 wherein F₀ is an impact force when the impactor is made to collide with a support plate alone; and F₁ is an impact force when the impactor is made to collide with the support plate of a structural body composed of the support plate and the test piece A.

In the foamed sheet, it is preferable that the thickness is 30 to 1,000 μm, and the apparent density of the foamed body is 0.2 to 0.7 g/cm³.

The foamed body preferably has a peak top of a loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C., the loss tangent (tan δ) being a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement.

The foamed body can be formed of at least one resin material selected from the group consisting of acrylic polymers, rubbers, urethanic polymers and ethylene-vinyl acetate copolymers.

The foamed body may be formed through step A of mechanically foaming an emulsion resin composition. Further the foamed body may be formed further through step B of coating a base material with the mechanically foamed emulsion resin composition followed by drying. Step B may comprise preliminary drying step B1 of drying the bubble-containing emulsion resin composition applied on the base material at not less than 50° C. and less than 125° C., and regular drying step B2 of thereafter further drying the resultant at not less than 125° C. and not more than 200° C.

In the foamed sheet, the compression set at 80° C. is preferably not more than 50%, and more preferably not more than 25%.

In the foamed sheet, the thickness is preferably 40 to 500 μm, and more preferably 50 to 300 μm.

The apparent density of the foamed sheet is preferably 0.21 to 0.6 g/cm³, and more preferably 0.22 to 0.5 g/cm³.

The foamed sheet may have a pressure-sensitive adhesive layer on one face or both faces of the foamed body.

The foamed sheet may be one to be used as an impact absorption sheet for electric or electronic devices.

The present invention also provides an electric or electronic device using the foamed sheet. The electric or electronic device includes one having a display member, and having a structure in which the foamed sheet is interposed between a housing of the electric or electronic device and the display member.

Advantageous Effects of Invention

The foamed sheet according to the present invention, since it comprises a foamed body having a specific average cell diameter and has a compression set at 80° C. of not more than 80% and an impact absorption change rate of as low as not more than ±20%, even if having a very small thickness, exhibits excellent impact absorption and is excellent in heat resistance as well. Hence, even in the case where the foamed sheet is used for electric or electronic devices having heat generating bodies exhibiting a large amount of heat generation, and the like, the performance as an impact absorption sheet does not reduce and a high reliability can be attained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic constitution view of a pendulum impact tester (impact testing apparatus).

FIG. 2 is a view illustrating a schematic constitution of a holding member of the pendulum impact tester (impact testing apparatus).

DESCRIPTION OF EMBODIMENTS

The foamed sheet according to the present invention comprises a foamed body having an average cell diameter of 10 to 200 μm. The lower limit of the average cell diameter of the foamed body is preferably 15 μm, and more preferably 20 μm; and the upper limit is preferably 150 μm, more preferably 130 μm, and still more preferably 100 μm. Since the average cell diameter is not less than 10 μm, the foamed sheet exhibits excellent impact absorption. Further since the average cell diameter is not more than 200 μm, the foamed sheet is excellent in compression recovery as well. Here, the maximum cell diameter of the foamed body is, for example, 40 to 800 μm, and the lower limit thereof is preferably 60 μm, and more preferably 80 μm; and the upper limit is preferably 400 μm, and more preferably 220 μm. Further the minimum cell diameter of the foamed body is, for example, 5 to 70 μm, and the lower limit thereof is preferably 8 μm, and more preferably 10 μm; and the upper limit is preferably 60 μm, and more preferably 50 μm.

In the foamed sheet according to the present invention, the compression set at 80° C. is not more than 80%, preferably not more than 50%, more preferably not more than 25%, and especially preferably not more than 10%.

A compression set test at 80° C. can be carried out according to the provision of JIS K6262. The compression set (%) is determined by the following expression.

CS={(t0−t1)/(t0−t2)}×100

CS: a compression set (%)

t0: an original thickness (mm) of a test piece

t1: a thickness (mm) of the test piece 30 min after the test piece is removed from a compression apparatus

t2: a thickness (mm) of the test piece in the state of being under a compressive strain

Here in the present invention, the compression set is a value when the test piece is compressed by 60%.

In the foamed sheet according to the present invention, the impact absorption change rate defined by the following is not more than ±20%, preferably not more than ±15%, and still more preferably not more than ±5%.

Impact absorption change rate (%)={(an impact absorption rate b after high-temperature compression−an initial impact absorption rate a)/the initial impact absorption rate a}×100

The initial impact absorption rate a: an impact absorption rate (%) of a test piece A

The impact absorption rate b (%) after high-temperature compression: an impact absorption rate (%) acquired by storing the test piece A at 80° C. for 72 hours in the state of being compressed by 60% with respect to the initial thickness of the test piece A, thereafter releasing the compression state, and thereafter conducting measurement after a lapse of 24 hours at 23° C.

The impact absorption rate: a value defined by the following expression in an impact absorption test (the weight of an impactor: 28 g, the swing-up angle: 40°) (23° C.) using a pendulum impact tester.

Impact absorption rate (%)={(F₀−F₁)/F₀}×100 wherein F₀ is an impact force when the impactor is made to collide with a support plate alone; and F₁ is an impact force when the impactor is made to collide with the support plate of a structural body composed of the support plate and the test piece A.

Since the foamed sheet according to the present invention has a low compression set at 80° C., and a low rate of the impact absorption change rate, even if the foamed sheet is compressed at a high temperature, cells hardly collapse and excellent thickness recovery is attained; and even in the case where the foamed sheet is subjected to an impact at a high temperature, high impact absorption is exhibited similarly as at normal temperature. Hence, also in the case where the foamed sheet is used for electric or electronic devices having heat generating bodies exhibiting a large amount of heat generation, and the like, even if an impact is applied on the electric or electronic devices in dropping or otherwise, breakage of the devices can be prevented.

The schematic constitution of a pendulum impact tester (impact testing apparatus) will be described by way of FIG. 1 and FIG. 2. As illustrated in FIG. 1 and FIG. 2, an impact testing apparatus 1 (pendulum tester 1) is constituted of a holding member 3 as a holding means to hold a test piece 2 (foamed sheet 2) by an arbitrary holding force, an impact applying member 4 to apply an impact stress on the test piece 2, a pressure sensor 5 as an impact force detecting means to detect an impact force on the test piece 2 by the impact applying member 4, and the like. Further the holding member 3 to hold the test piece 2 by an arbitrary holding force is constituted of a fixing jig 11, and a pressing jig 12 facing the fixing jig 11 and being slidable so that the test piece 2 is interposed and held between the fixing jig 11 and the pressing jig 12. Further the pressing jig 12 is provided with a pressure adjusting means 16. Further the impact applying member 4 to apply an impact force on the test piece 2 held by the holding member 3 is constituted of a support rod 23 (shaft 23) whose one end 22 is rotatably supported on a support column 20 and whose other end side has an impactor 24, and an arm 21 to lift and hold the impactor 24 at a predetermined angle. Here since a steel ball is used as the impactor 24, by providing an electromagnet 25 on one end of the arm, the impactor 24 is enabled to be unifiedly lifted at the predetermined angle. Further the pressure sensor 5 to detect an impact force acting on the test piece 2 by the impact applying member 4 is provided on the face side of the fixing jig 11 opposite to the face thereof contacting the test piece.

The impactor 24 is a steel ball (iron ball). Further the angle (swing-up angle a in FIG. 1) at which the impactor 24 is lifted by the arm 21 is 40°. The weight of the steel ball (iron ball) is 28 g.

As illustrated in FIG. 2, the test piece 2 (foamed sheet 2) is interposed between the fixing jig 11 and the pressing jig 12 through a support plate 28 constituted of a highly elastic plate material such as a resinous plate material (acryl plate, polycarbonate plate or the like) or a metal plate material.

The impact absorption is calculated by the expression described before by using the above impact testing apparatus, and determining an impact force F₀ measured by closely fixing the fixing jig 11 and the support plate 28 on each other and then making the impactor 24 collide with the support plate 28, and an impact force F₁ measured by inserting and closely fixing the test piece 2 between the fixing jig 11 and the support plate 28 and then making the impactor 24 collide with the support plate 28. Here, the impact testing apparatus is a similar apparatus as used in Example 1 of Japanese Patent Laid-Open No. 2006-47277.

The foamed sheet according to the present invention has excellent impact absorption while having a small thickness. The impact absorption rate (the weight of the impactor: 28 g, the swing-up angle: 40°) is usually 5 to 70%, and the lower limit is preferably 10%, more preferably 20%, and still more preferably 28%; and the upper limit is preferably 60%.

The thickness of the foamed sheet according to the present invention is not especially limited, but is, for example, 30 to 1,000 μm, and the lower limit thereof is more preferably 40 μm, and still more preferably 50 μm; and the upper limit is more preferably 500 μm, still more preferably 300 μm, and especially preferably 200 μm. When the thickness of the foamed sheet is not less than 30 μm, cells can be incorporated uniformly, and better impact absorption can be exhibited. Further by making the thickness of the foamed sheet to be not more than 1,000 μm, the foamed sheet can easily conform to fine clearances. The foamed sheet according to the present invention, even if having as small a thickness as 30 to 1,000 μm, is excellent in impact absorption.

In the present invention, from the viewpoint of the impact absorption, the ratio of the average cell diameter (μm) to the thickness (μm) of the foamed sheet (the former/the latter) is preferably in the range of 0.1 to 0.9. The lower limit of the ratio of the average cell diameter (μm) to the thickness (μm) of the foamed sheet is preferably 0.2, and more preferably 0.3; and the upper limit is preferably 0.85, and more preferably 0.8.

The apparent density of the foamed body constituting the foamed sheet according to the present invention is not especially limited, but is preferably 0.2 to 0.7 g/cm³. The lower limit thereof is preferably 0.21 g/cm³, and more preferably 0.22 g/cm³; and the upper limit is preferably 0.6 g/cm³, more preferably 0.5 g/cm³, and especially preferably 0.4 g/cm³. When the apparent density of the foamed body is not less than 0.2 g/cm³, a high strength can be maintained; and when it is not more than 0.7 g/cm³, higher impact absorption is exhibited. Further when the apparent density of the foamed body is in the range of 0.2 to 0.4 g/cm³, much higher impact absorption is exhibited.

In the case where the thickness of the foamed sheet is large in some degree, the impact absorption can be regulated by selection of the average cell diameter, the apparent density and the like; in the case where the thickness of the foamed sheet is very small (for example, the thickness is 30 to 500 μm), however, the impact cannot sufficiently be absorbed only by regulation of these properties, in some cases. This is because in the case where the thickness of the foamed sheet is very small, cells in the foamed body instantly collapse by an impact and the impact buffering function by cells disappears. From such a viewpoint, it is preferable that the peak top of the loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement of the foamed body, is present in the range of not less than −30° C. and not more than 30° C. In such a way, even after cells collapse, the constituting material of the foamed body exhibits more a function of buffering impacts.

The lower limit of the temperature range where the peak top of the loss tangent is present is more preferably −25° C., still more preferably −20° C., and especially preferably −10° C.; and the upper limit is more preferably 20° C., and still more preferably 10° C. In the case of materials having not less than two peak tops of the loss tangent, at least one of the peak tops is desirably in the above range. When the peak temperature is not less than −30° C., better compression recovery is exhibited. Further when the peak temperature is not more than 30° C., higher flexibility is exhibited and better impact absorption is exhibited.

It is preferable that the peak top intensity (maximum value) of the loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C. is higher from the viewpoint of the impact absorption, and the peak top intensity is, for example, not less than 0.2, and preferably not less than 0.3. The upper limit value of the peak top intensity (maximum value) is, for example, 2.0.

The peak temperature of the loss tangent (tan δ) contributes to the impact absorption of the foamed body in such a manner, in many cases. Although the reason is not completely clear why when the peak top of the loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement of the foamed body, is present in the range of not less than −30° C. and not more than 30° C., the impact absorption of the foamed sheet becomes high, it is presumably due to that the peak of the loss tangent (tan δ) is present in frequencies corresponding to those of impacts. That is, it is presumed that since the range of not less than −30° C. and not more than 30° C. of the loss tangent (tan δ) is reduced to a range of frequencies corresponding to dropping impacts of a structural material based on the temperature-time conversion rule in viscoelasticity measurements, foamed sheets having a peak temperature of the loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C. exhibit higher impact absorption. Further the storage elastic modulus is a resilient force to an impact energy applied on the foamed sheet; and when the storage elastic modulus is high, the impact is repulsed as it is. By contrast, the loss elastic modulus is a physical property which converts an impact energy applied on the foamed sheet to heat; since a higher loss elastic modulus causes an impact energy to be converted to more heat, the impact is absorbed and the deformation is reduced. It is presumed from this that foamed sheets which convert impacts to more heat and exhibit a lower resilient force, that is, exhibit a higher loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus, exhibit a higher impact absorption rate.

With respect to the foamed body constituting the foamed sheet according to the present invention, the composition, the cell structure and the like thereof are not especially limited as long as the foamed body has the above properties. The cell structure may be any of an open cell structure, a closed cell structure and a semi-open semi-closed cell structure. From the viewpoint of the impact absorption, an open cell structure and a semi-open semi-closed cell structure are preferable.

The foamed body can be constituted of a resin composition containing a resin material (polymer). Here, it is preferable that the peak top of the loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement of the resin composition in an unfoamed state [the resin composition (solid material) in the case of not being foamed], is in the range of not less than −30° C. and not more than 30° C. The lower limit of the temperature range where the peak top of the loss tangent is present is more preferably −25° C., still more preferably −20° C., and especially preferably −10° C.; and the upper limit is more preferably 20° C., and still more preferably 10° C. In the case of materials having not less than two peak tops of the loss tangent, at least one of the peak tops is desirably in the above range. A higher peak top intensity of the loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C. of the resin composition (solid material) is preferable from the viewpoint of the impact absorption, wherein the value of the peak top intensity corresponds to a value obtained by dividing a peak top intensity of the loss tangent (tan δ) in the range of not less than −30° C. and less than 30° C. of the foamed body by an apparent density (g/cm³) of the foamed body. The peak top intensity of the loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C. of the resin composition (solid material) is preferably not less than 0.9 (g/cm³)⁻¹, and the upper limit is, for example, about 3 (g/cm³)⁻¹.

A resin material (polymer) constituting the foamed body is not especially limited, and publicly or commonly known resin materials constituting foamed bodies can be used. Examples of the resin material include acrylic polymers, rubbers, urethanic polymers and ethylene-vinyl acetate copolymers. Among these, from the viewpoint of the impact absorption, acrylic polymers, rubbers and urethanic polymers are preferable. The resin material (polymer) constituting the foamed body may be of a single kind or of not less than two kinds.

In order to make the peak top of the loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement of the foamed body, to be in the range of not less than −30° C. and not more than 30° C., Tg of the resin material (polymer) can be made to be an index or an indication. The resin material (polymer) can be selected, for example, from resin materials (polymers) having a Tg in the range of not less than −50° C. and less than 50° C. (the lower limit is preferably −40° C., and more preferably −30° C.; and the upper limit is preferably 40° C., and more preferably 30° C.)

The acrylic polymer is preferably one formed of, as essential monomer components, a monomer having a Tg of its homopolymer of not less than −10° C. and a monomer having a Tg of its homopolymer of less than −10° C. By using such an acrylic polymer and regulating the amount ratios of the former monomer and the latter monomer, there can comparatively easily be obtained a foamed body having a peak top of the loss tangent (tan δ), which is a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement, of not less than −30° C. and not more than 30° C.

Here, a “glass transition temperature (Tg) when a homopolymer is formed” (simply referred to as “Tg of a homopolymer” in some cases) in the present invention means a “glass transition temperature (Tg) of a homopolymer of the corresponding monomer”; and numerical values are specifically cited in “Polymer Handbook” (3rd edition, John Wiley & Sons, Inc., 1987). Here, Tgs of homopolymers of monomers which are not described in the above literature are values acquired, for example, by the following measurement method (see Japanese Patent Laid-Open No. 2007-51271). That is, 100 parts by weight of a monomer, 0.2 parts by weight of 2,2′-azobisisobutyronitrile, and 200 parts by weight of ethyl acetate as a polymerization solvent are charged in a reaction vessel equipped with a thermometer, a stirrer, a nitrogen introducing tube and a refluxing cooling tube, and stirred for 1 hour under the introduction of nitrogen gas. After oxygen in the polymerization system is removed in such a way, the system is heated up to 63° C. and allowed to react for 10 hours. Then, the system is cooled to room temperature to thereby obtain a homopolymer solution having a solid content concentration of 33% by weight. Then, the homopolymer solution is cast and applied on a separator, and dried to thereby fabricate a test sample (sheet-like homopolymer) having a thickness of about 2 mm. Then, the test sample is punched out into a disc of 7.9 mm in diameter, and interposed between parallel plates; and the viscoelasticity is measured by using a viscoelasticity tester (ARES, manufactured by Rheometric Scientific, Inc.) and in a temperature region of −70 to 150° C. at a temperature-rise rate of 5° C./min in a shearing mode under a shearing strain of 1 Hz in frequency, and the peak top temperature in tan δ is defined as Tg of the homopolymer. Here, also Tg of the resin material (polymer) can be measured by this method.

In a monomer having a Tg of its homopolymer of not less than −10° C., the Tg is, for example, −10° C. to 250° C., preferably 10 to 230° C., and more preferably 50 to 200° C.

Examples of the monomer having a Tg of its homopolymer of not less than −10° C. include (meth)acrylonitrile; amide group-containing monomers such as (meth) acrylamide and N-hydroxyethyl(meth)acrylamide; (meth)acrylic acid; alkyl (meth)acrylates having a Tg of their homopolymer of not less than −10° C., such as methyl methacrylate and ethyl methacrylate; isobornyl (meth)acrylate; heterocycle-containing vinyl monomers such as N-vinyl-2-pyrrolidone; and hydroxyl group-containing monomers such as 2-hydroxyethyl methacrylate. These can be used singly or in combinations of not less than two. Among these, (meth)acrylonitrile (particularly acrylonitrile) is especially preferable. When (meth)acrylonitrile (particularly acrylonitrile) is used as a monomer having a Tg of its homopolymer of not less than −10° C., probably because of the strong intermolecular interaction, the peak top intensity of the loss tangent (tan δ) of the foamed body can be made high.

In the monomer having a Tg of its homopolymer of less than −10° C., the Tg is, for example, not less than −70° C. and less than −10° C., preferably −70° C. to −12° C., and more preferably −65° C. to −15° C.

Examples of the monomer having a Tg of its homopolymer of less than −10° C. include alkyl (meth)acrylates having a Tg of their homopolymer of less than −10° C., such as ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. These can be used singly or in combinations of not less than two. Among these, C_2-8 alkyl acrylates are especially preferable.

The content of a monomer having a Tg of its homopolymer of not less than −10° C. with respect to the whole monomer components (total amount of monomer components) forming the acrylic polymer is, for example, 2 to 30% by weight, and the lower limit is preferably 3% by weight, and more preferably 4% by weight; and the upper limit is preferably 25% by weight, and more preferably 20% by weight. The content of a monomer having a Tg of its homopolymer of less than −10° C. with respect to the whole monomer components (total amount of monomer components) forming the acrylic polymer is, for example, 70 to 98% by weight, and the lower limit is preferably 75% by weight, and more preferably 80% by weight; and the upper limit is preferably 97% by weight, and more preferably 96% by weight.

Here, when the monomer forming the acrylic polymer contains a nitrogen atom-containing copolymerizable monomer, and when an emulsion resin composition is subjected to shearing by a mechanical stirring or the like to be thereby caused to foam, the viscosity of the composition decreases and it becomes easy for a large number of bubbles to be entrapped in the emulsion; and thereafter, when the emulsion resin composition containing bubbles is applied on a base material and dried in its standing-still state, since the composition becomes easily aggregated and the viscosity increases, and the bubbles are held in the composition and it becomes difficult for the bubbles to diffuse outside, a foamed body excellent in the foamed property can be obtained.

Examples of the nitrogen atom-containing copolymerizable monomer (nitrogen atom-containing monomer) include cyano group-containing monomers such as (meth)acrylonitrile; lactam ring-containing monomers such as N-vinyl-2-pyrrolidone; and amide group-containing monomers such as (meth)acrylamide, N-hydroxyethyl(meth)acrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide and diacetoneacrylamide. Among these, preferable are cyano group-containing monomers such as acrylonitrile, and lactam ring-containing monomers such as N-vinyl-2-pyrrolidone. The nitrogen atom-containing monomers can be used singly or in combinations of not less than two.

In an acrylic polymer having a structural unit originated from such a nitrogen atom-containing monomer, the content of the structural unit originated from the nitrogen atom-containing monomer is, with respect to the whole structural units constituting the acrylic polymer, preferably 2 to 30% by weight, and the lower limit thereof is more preferably 3% by weight, and still more preferably 4% by weight; and the upper limit thereof is more preferably 25% by weight, and still more preferably 20% by weight.

Further in an acrylic polymer having a structural unit originated from such a nitrogen atom-containing monomer, in addition to the structural unit originated from the nitrogen atom-containing monomer, a structural unit originated from a C_2-18 alkyl acrylate (particularly a C_2-8 alkyl acrylate) is preferably contained. The C_2-18 alkyl acrylates can be used singly or in combinations of not less than two. In such an acrylic polymer, the content of the structural unit originated from a C_2-18 alkyl acrylate (particularly a C_2-8 alkyl acrylate) is, with respect to the whole structural units constituting the acrylic polymer, preferably 70 to 98% by weight, and the lower limit thereof is more preferably 75% by weight, and still more preferably 80% by weight; and the upper limit thereof is more preferably 97% by weight, and still more preferably 96% by weight.

The rubber may be either of natural rubber and synthetic rubber. Examples of the rubber include nitrile rubber (NBR), methyl methacrylate-butadiene rubber (MBR), styrene-butadiene rubber (SBR), acrylic rubber (ACM, ANM), urethane rubber (AU) and silicone rubber. Among these, preferable are nitrile rubber (NBR), methyl methacrylate-butadiene rubber (MBR) and silicone rubber.

Examples of the urethanic polymer include polycarbonate-based polyurethane, polyester-based polyurethane and polyether-based polyurethane.

As the ethylene-vinyl acetate copolymer, publicly or commonly known ethylene-vinyl acetate copolymers can be used.

The foamed body constituting the foamed sheet, in addition to the resin material (polymer), may contain, as required, a surfactant, a crosslinking agent, a thickener, a rust preventive, a silicone-based compound and other additives.

For example, for the micronization of the cell diameter and the stabilization of cells foamed, an optional surfactant may be contained. The surfactant is not especially limited, and there may be used any of an anionic surfactant, a cationic surfactant, a nonionic surfactant, an amphoteric surfactant and the like; but from the viewpoint of the micronization of the cell diameter and the stabilization of cells foamed, an anionic surfactant is preferable, and a fatty acid ammonium-based surfactant, particularly ammonium stearate or the like, is more preferable. The surfactants may be used singly or in combinations of not less than two. Further dissimilar surfactants may be used concurrently, and for example, an anionic surfactant and a nonionic surfactant, or an anionic surfactant and an amphoteric surfactant may be used concurrently.

The amount [solid content (nonvolatile content)] of the surfactant to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], for example, 0 to 10 parts by weight, and the lower limit is preferably 0.5 parts by weight; and the upper limit is preferably 8 parts by weight.

Further in order to improve the strength, heat resistance and moisture resistance of the foamed body, an optional crosslinking agent may be contained. The crosslinking agent is not especially limited, and either of an oil-soluble one and a water-soluble one may be used. Examples of the crosslinking agent include epoxy-based, oxazoline-based, isocyanate-based, carbodiimide-based, melamine-based and metal oxide-based ones. Among these, oxazoline-based crosslinking agents are preferable.

The amount [solid content (nonvolatile content)] of the crosslinking agent to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], for example, 0 to 10 parts by weight, and the lower limit is preferably 0.01 parts by weight, and more preferably 0.1 parts by weight; and the upper limit is preferably 9 parts by weight, and more preferably 8 parts by weight.

Further for the stabilization of cells foamed and the improvement of the film formability, an optional thickener may be contained. The thickener is not especially limited, and includes acrylic acid-based, urethanic and polyvinyl alcoholic ones. Among these, polyacrylic acid-based thickeners and urethanic thickeners are preferable.

The amount [solid content (nonvolatile content)] of the thickener to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], for example, 0 to 10 parts by weight, and the lower limit is preferably 0.1 parts by weight; and the upper limit is preferably 5 parts by weight.

Further in order to prevent the corrosion of metal members adjacent to the foamed sheet, an optional rust preventive may be contained. The rust preventive is preferably an azole ring-containing compound. When an azole ring-containing compound is used, both the corrosion prevention of metals and the close adhesion with objects can be met simultaneously in high levels.

The azole ring-containing compound suffices as long as being a compound having 5-membered ring containing not less than one nitrogen atom in the ring, and examples include compounds having a diazole (imidazole, pyrazole) ring, a triazole ring, a tetrazole ring, an oxazole ring, an isoxazole ring, a thiazole ring or an isothiazole ring. These rings may be condensed with an aromatic ring such as a benzene ring to thereby form condensed rings. Examples of compounds having such a condensed ring include compounds having a benzimidazole ring, a benzopyrazole ring, a benzotriazol ring, a benzoxazole ring, a benzisoxazole ring, a benzothiazole ring or a benzisothiazole ring.

The azole ring and the condensed rings (benzotriazole ring, benzothiazole ring and the like) may each have a substituent. Examples of the substituent include alkyl groups having 1 to 6 carbon atoms (preferably having 1 to 3 carbon atoms) such as a methyl group, an ethyl group, a propyl group, an isopropyl group and a butyl group; alkoxy groups having 1 to 12 carbon atoms (preferably having 1 to 3 carbon atoms) such as a methoxy group, an ethoxy group, an isopropyloxy group and a butoxy group; aryl groups having 6 to 10 carbon atoms such as a phenyl group, a tolyl group and a naphthyl group; an amino group; (mono- or di-) C_1-10 alkylamino groups such as a methylamino group and a dimethylamino group; amino-C_1-6 alkyl groups such as an aminomethyl group and 2-aminoethyl group; mono- or di-(C_1-10 alkyl)amino-C_1-6 alkyl groups such as an N,N-diethylaminomethyl group and an N,N-bis(2-ethylhexyl)aminomethyl group; a mercapto group; alkoxycarbonyl groups having 1 to 6 carbon atoms such as a methoxycarbonyl group and an ethoxycarbonyl group; a carboxyl group; carboxy-C_1-6 alkyl groups such as a carboxymethyl group; carboxy-C_1-6 alkylthio groups such as 2-carboxyethylthio group; N,N-bis(hydroxy-C_1-4 alkyl)amino-C_1-4 alkyl groups such as an N,N-bis(hydroxymethyl)aminomethyl group; and a sulfo group. Further the azole ring-containing compound may form a salt such as a sodium salt or a potassium salt.

From the viewpoint of the rust preventive action on metals, preferable are compounds in which an azole ring forms a condensed ring with an aromatic ring such as a benzene ring; and among these, especially preferable are benzotriazole-based compounds (compounds having a benzotriazole ring) and benzothiazole-based compounds (compounds having a benzothiazole ring).

Examples of the benzotriazole-based compounds include 1,2,3-benzotriazole, methylbenzotriazole, carboxybenzotriazole, carboxymethylbenzotriazole, 1-[N,N-bis(2-ethylhexyl)aminomethyl]benzotriazole, 1-[N,N-bis(2-ethylhexyl)aminomethyl]methylbenzotriazole, 2,2′-[[(methyl-1H-benzotriazol-1-yl)methyl]imino]bisethanol, and sodium salts thereof.

Examples of the benzothiazole-based compounds include 2-mercaptobenzothiazole, 3-(2-(benzothiazolyl)thio)propionic acid, and sodium salts thereof.

The azole ring-containing compounds may be used singly or in combinations of not less than two.

The amount [solid content (nonvolatile content)] of the rust preventive (for example, the azole ring-containing compound) [solid content (nonvolatile content)] to be added suffices as long as being in the range of not impairing the close adhesion with objects and the intrinsic property of the foamed body, and is, for example, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], for example, preferably 0.2 to 5 parts by weight. The lower limit thereof is more preferably 0.3 parts by weight, and still more preferably 0.4 parts by weight; and the upper limit thereof is more preferably 3 parts by weight, and still more preferably 2 parts by weight.

Further in order to improve the recovery and the recovery speed of the thickness of the foamed sheet after being compressed, a silicone-based compound may be added. Further for the same purpose, a silicone-modified polymer (for example, a silicone-modified acrylic polymer, a silicone-modified urethanic polymer) may be used as at least a part of the resin material (polymer). These can be used singly or in combinations of not less than two.

As the silicone-based compound, preferable are silicone-based compounds having not more than 2,000 siloxane bonds. Examples of the silicone-based compounds include silicone oils, modified silicone oils and silicone resins.

Examples of the silicone oils (straight silicone oils) include dimethyl silicone oils and methyl phenyl silicone oils.

Examples of the modified silicone oils include polyether-modified silicone oils (polyether-modified dimethyl silicone oils and the like), alkyl-modified silicone oils (alkyl-modified dimethyl silicone oils and the like), aralkyl-modified silicone oils (aralkyl-modified dimethyl silicone oils and the like), higher fatty acid ester-modified silicone oils (higher fatty acid ester-modified dimethyl silicone oils and the like) and fluoroalkyl-modified silicone oils (fluoroalkyl-modified dimethyl silicone oils and the like).

Among these, polyether-modified silicones are preferable. Examples of commercially available products of the polyether-modified silicone oils include straight chain-type ones such as “PEG 11 Methyl Ether Dimethicone,” “PEG/PPG-20/22 Butyl Ether Dimethicone,” “PEG-9 Methyl Ether Dimethicone,” “PEG-32 Methyl Ether Dimethicone,” “PEG-9 Dimethicone,” “PEG-3 Dimethicone” and “PEG-10 Dimethicone”; and branched chain-type ones such as “PEG-9 Polydimethylsiloxyethyl Dimethicone” and “Lauryl PEG-9 Polydimethylsiloxyethyl Dimethicone” (which are all manufactured by Shin-Etsu Silicone).

The silicone resins include straight silicone resins and modified silicone resins. Examples of the straight silicone resins include methyl silicone resins and methyl phenyl silicone resins. Examples of the modified silicone resins include alkyd-modified silicone resins, epoxy-modified silicone resins, acryl-modified silicone resins and polyester-modified silicone resins.

The total content of the silicone-based compound and the silicone chain moiety present in the silicone-modified polymer in the foamed body is, with respect to 100 parts by weight of the resin material (polymer) in the foamed body, for example, 0.01 to 5 parts by weight in terms of nonvolatile content (in terms of solid content). The lower limit of the total content is preferably 0.05 parts by weight, and more preferably 0.1 parts by weight; and the upper limit is preferably 4 parts by weight, and more preferably 3 parts by weight. In the case where the total content of the silicone component and the silicone chain moiety in the foamed body is in the above range, the recovery and the recovery speed after compression can be improved without impairing the properties as the foamed body.

Further the total content of the silicone-based compound and the silicone chain moiety present in the silicone-modified polymer in the foamed body is, for example, 0.01 to 5% by weight in terms of nonvolatile content (in terms of solid content). The lower limit of the total content is preferably 0.05% by weight, and more preferably 0.1% by weight; and the upper limit is preferably 4% by weight, and more preferably 3% by weight. In the case where the total content of the silicone component and the silicone chain moiety in the foamed body is in the above range, the recovery and the recovery speed after compression can be improved without impairing the properties as the foamed body.

The foamed body constituting the foamed sheet may contain optional other suitable components in the range of not impairing the impact absorption. Such other components may be contained in one kind thereof alone, or may be contained in not less than two kinds thereof. Examples of the other components include polymer components other than the above, softening agents, antioxidants, antiaging agents, gelling agents, curing agents, plasticizers, filling agents, reinforcing agents, foaming agents (sodium bicarbonate and the like), microcapsules (thermally expandable microballs and the like), flame retardants, light stabilizers, ultraviolet absorbents, coloring agents (pigments, dyes and the like), pH regulators, solvents (organic solvents), thermopolymerization initiators and photopolymerization initiators. The amounts [solid contents (nonvolatile contents)] of these components to be added suffices as long as being in the range of not impairing the close adhesion with objects and the intrinsic property of the foamed body, and are each, for example, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], preferably in the range of, for example, 0.2 to 60 parts by weight. The amount [solid content (nonvolatile content)] of the foaming agent (sodium bicarbonate or the like) to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], more preferably 0.5 to 20 parts by weight. The amount [solid content (nonvolatile content)] of the microcapsule (thermally expandable microball or the like) to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], more preferably 0.2 to 10 parts by weight. The amount [solid content (nonvolatile content)] of the filling agent to be added is, with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], more preferably 0.3 to 50 parts by weight.

Examples of the filling agents include silica, clay (mica, talc, smectite and the like), alumina, aluminum hydroxide, hydroxides of alkaline earth metals (magnesium hydroxide and the like), carbonate salts of alkaline earth metals (calcium carbonate and the like), titania, zinc oxide, tin oxide, zeolite, graphite, carbon black, carbon nanotubes, inorganic fibers (carbon fibers, glass fibers, potassium titanate fibers and the like), organic fibers, metal powders (silver, copper, and the like) and waxes (polyethylene wax, polypropylene wax, and the like). Further as the filling agent, there can also be added piezoelectric particles (titanium oxide, barium titanate and the like), electroconductive particles (electroconductive carbon black, electroconductive titanium oxide, tin oxide and the like), thermoconductive particles (boron nitride and the like), organic fillers (silicone powder, polyethylene powder, polypropylene powder and the like) and the like. In the case of using silica as the filling agent, the amount thereof to be added is, with respect to 100 parts by weight of the thermoplastic resin [solid content (nonvolatile content)], especially preferably in the range of 0.5 to 40 parts by weight. Further in the case of using clay such as mica as the filling agent, the amount thereof to be added is, with respect to 100 parts by weight of the thermoplastic resin [solid content (nonvolatile content)], especially preferably in the range of 0.3 to 10 parts by weight. Further in the case of using a piezoelectric particle as the filling agent, the amount thereof to be added is, with respect to 100 parts by weight of the thermoplastic resin [solid content (nonvolatile content)], especially preferably in the range of 5 to 40 parts by weight. Further in the case of using an electroconductive particle as the filling agent, the amount thereof to be added is, with respect to 100 parts by weight of the thermoplastic resin [solid content (nonvolatile content)], especially preferably in the range of 5 to 40 parts by weight. Further when a piezoelectric particle and an electroconductive particle are used in combination as the filling agent, the amount of charge to be generated can be regulated by the pressure. In this case, the ratio of the piezoelectric particle to the electroconductive particle, for example the former/the latter (weight ratio) is preferably 10/90 to 90/10, preferably 20/80 to 80/20, and still more preferably 30/70 to 70/30.

The foamed sheet according to the present invention can be produced by subjecting a resin composition containing the resin material (polymer) constituting the foamed body to expansion molding. As a foaming method (method of forming cells), there can be employed methods usually used for expansion molding, including physical methods and chemical methods. The physical methods generally involve dispersing a gas component such as air or nitrogen in a polymer solution, and forming bubbles by mechanical mixing. The chemical methods are ones in which cells are formed by a gas generated by thermal decomposition of a foaming agent added to a polymer base, to thereby obtain foamed bodies. From the viewpoint of environmental problems and the like, the physical methods are preferable. Cells to be formed by the physical methods are open cells in many cases.

As the resin composition containing the resin material (polymer) to be subjected to expansion molding, there may be used a resin solution in which the resin material is dissolved in a solvent, but from the viewpoint of the foamability, an emulsion containing the resin material is preferably used. Not less than two emulsions may be blended and used as the emulsion.

It is preferable from the viewpoint of the film formability that the solid content concentration of the emulsion is higher. The solid content concentration of the emulsion is preferably not less than 30% by weight, more preferably not less than 40% by weight, and still more preferably not less than 50% by weight.

In the present invention, a method is preferable in which a foamed body is fabricated through a step (step A) of mechanically foaming an emulsion resin composition to thereby foam the emulsion resin composition. A foaming apparatus is not especially limited, and examples thereof include apparatuses such as a high-speed shearing system, a vibration system, and a discharge system of a pressurized gas. Among these, from the viewpoint of the micronization of the cell diameter and the fabrication of a large volume, a high-speed shearing system is preferable.

Bubbles foamed by mechanical stirring are ones of gas entrapped in the emulsion. The gas is not especially limited as long as being inactive to the emulsion, and includes air, nitrogen and carbon dioxide. Among these, from the viewpoint of the economical efficiency, air is preferable.

By subjecting the emulsion resin composition foamed by the above method to a step (step B) of coating a base material with the emulsion resin composition followed by drying, the foamed sheet according to the present invention can be obtained. The base material is not especially limited, but examples thereof include release-treated plastic films (release-treated polyethylene terephthalate films and the like), plastic films (polyethylene terephthalate films and the like) and thermoconductive layers. In the case of coating by using a thermoconductive layer as the base material, the close adhesion of a foamed body layer with the thermoconductive layer can be improved and the efficiency of a drying step in the fabrication of the foamed body layer can also be improved.

As a coating method and a drying method in step B, usual methods can be employed. It is preferable that step B comprises preliminary drying step B1 of drying the bubble-containing emulsion resin composition applied on the base material at not less than 50° C. and less than 125° C., and regular drying step B2 of thereafter further drying the resultant at not less than 125° C. and not more than 200° C.

By providing preliminary drying step B1 and regular drying step B2, the coalescence of bubbles and the burst of bubbles due to a rapid temperature rise can be prevented. Particularly in the foamed sheet having a small thickness, since bubbles coalesce or burst in a rapid rise of the temperature, the significance of the provision of preliminary drying step B1 is large. The temperature in preliminary drying step B1 is preferably not less than 50° C. and not more than 100° C. The time of preliminary drying step B1 is, for example, 0.5 min to 30 min, and preferably 1 min to 15 min. Further the temperature in regular drying step B2 is preferably not less than 130° C. and not more than 180° C., and more preferably not less than 130° C. and not more than 160° C. The time of regular drying step B2 is, for example, 0.5 min to 30 min, and preferably 1 min to 15 min.

In the present invention, the average cell diameter, the maximum cell diameter and the minimum cell diameter of the foamed body can be controlled by regulation of the kind and the amount of the surfactant, and regulation of the stirring rate and the stirring time in the mechanical stirring.

Further the apparent density of the foamed body can be controlled by regulation of the amount of the gas component entrapped in the emulsion resin composition in the mechanical stirring.

Further the value of the compression set at 80° C. and the value of the impact absorption change rate can be controlled, for example, by regulation of the degree of crosslinking and Tg of the resin material (polymer) constituting the foamed body. More specifically, for example, by regulation of the amount of the crosslinking agent to be added, and regulation of the proportion accounted for by the monomer having a Tg of its homopolymer of not less than −10° C. in the whole monomer components forming the resin material (polymer), the value of the compression set at 80° C. and the value of the impact absorption change rate can be controlled in predetermined ranges. By increasing the amount of the crosslinking agent to be added, and increasing the proportion accounted for by the monomer having a Tg of its homopolymer of not less than −10° C. in the whole monomer components forming the resin material (polymer), the value of the compression set at 80° C. and the value of the impact absorption change rate can be made low.

The foamed sheet according to the present invention may have a pressure-sensitive adhesive layer on one face or on both faces of the foamed body. A pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is not especially limited, and may be any of an acrylic pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, a silicone-based pressure-sensitive adhesive and the like. Further in the case of providing the pressure-sensitive adhesive layer, a release liner to protect the pressure-sensitive adhesive layer until its usage may be laminated on its face. Here, in the case where the foamed body constituting the foamed sheet according to the present invention exhibits slight tackiness, members and the like can be fixed even without providing the pressure-sensitive adhesive layer.

The foamed sheet according to the present invention may be distributed to markets as a wound body (roll-like material) wound in a rolled form.

As described above, the foamed sheet according to the present invention, even if having a small thickness, is excellent in impact absorption. Further the foamed sheet is excellent in heat resistance, and even if being subjected to compressions or impacts under high temperatures (for example, about 80° C.), holds a force to recover its original shape (thickness). The foamed sheet according to the present invention has, for example, an 80° C. stress retention rate of not less than 68% as defined by the following. Here, in conventional foamed sheets, the 80° C. stress retention rate is usually low, and the stress relaxes at 80° C. and the recovering force decays.

<The 80° C. Stress Retention Rate>

A test piece (foamed sheet) is held in an atmosphere of 80° C. for 30 min; thereafter, by using a tensile tester, the test piece is set at 80° C. at an interchuck distance of 40 mm, stretched by 50% at a tensile rate of 500 mm/min, and thereafter held for 120 sec; a maximum load and a load after the 120 sec are measured, and the 80° C. stress retention rate is determined by the following expression.

80° C. stress retention rate (%)=[a load(N) after the 120 sec/a maximum load(N)]×100

Thus, the foamed sheet according to the present invention, since even if having a small thickness, being excellent in impact absorption and moreover excellent in heat resistance, is high in installation (adhesion) reliability even at high temperatures; and for example, in electric or electronic devices, the foamed sheet is useful as a member, particularly an impact absorption sheet, for electric or electronic devices to be used when various types of members or components (for example, optical members) are attached (installed) on predetermined sites (for example, housings).

Examples of optical members attachable (installable) by utilizing the foamed sheet according to the present invention include image display members (particularly small-size image display members) installed on image display apparatuses such as liquid crystal displays, electroluminescence displays and plasma displays, display members, such as touch panels, installed on mobile communication apparatuses such as so-called “cellular phones,” “smartphones” and “personal digital assistants,” cameras and lenses (particularly small-size cameras and lenses).

The electric or electronic device according to the present invention uses the foamed sheet according to the present invention. Such an electric or electronic device includes, for example, an electric or electronic device having a display member, and having a structure in which the foamed sheet is interposed between a housing of the electric or electronic device and the display member. Examples of the electric or electronic device include mobile communication apparatuses such as so-called “cellular phones,” “smartphones” and “personal digital assistants.”

EXAMPLES

Hereinafter, the present invention will be described in more detail by way of Examples, but the present invention is not limited to these Examples. Here, unless otherwise mentioned, “%” representing a content means % by weight. Here, the numbers of parts (parts by weight) blended are all values in terms of solid content (nonvolatile content).

Example 1

100 parts by weight of an acryl emulsion solution (the amount of the solid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part by weight of a silicone-based compound A (a dimethyl silicone oil, the number-average molecular weight Mn: 7.16×10³, the weight-average molecular weight Mw: 1.71×10⁴, the amount of the solid content (nonvolatile content): 100%), 3 parts by weight of a fatty acid ammonium-based surfactant (a water dispersion of ammonium stearate, the amount of the solid content: 33%), 2.0 parts by weight of an oxazoline-based crosslinking agent (“Epocros WS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of the solid content: 39%), 1 part by weight of a benzotriazole sodium salt (the solid content: 40%) (a rust preventive), and 0.8 parts by weight of a polyacrylic acid-based thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid: 20% by weight), the amount of the solid content: 28.7%) were stirred and mixed by a Disper (“Robomix,” manufactured by Primix Corp.) and thereby foamed. The foamed composition was applied on a release-treated PET (polyethylene terephthalate) film (the thickness: 38 μm, the trade name: “MRF#38,” manufactured by Mitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C. for 4.5 min to thereby obtain a foamed body (foamed sheet) of an open cell structure having a thickness of 100 μm, an apparent density of 0.34 g/cm³, a cell porosity of 65.7%, a maximum cell diameter of 72.5 μm, a minimum cell diameter of 28.5 μm and an average cell diameter of 45 μm.

Example 2

100 parts by weight of an acryl emulsion solution (the amount of the solid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part by weight of a silicone-based compound A (a dimethyl silicone oil, the number-average molecular weight Mn: 7.16×10³, the weight-average molecular weight Mw: 1.71×10⁴, the amount of the solid content (nonvolatile content): 100%), 3 parts by weight of a fatty acid ammonium-based surfactant (a water dispersion of ammonium stearate, the amount of the solid content: 33%), 0.35 parts by weight of an oxazoline-based crosslinking agent (“Epocros WS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of the solid content: 39%), 1 part by weight of a benzotriazole sodium salt (the solid content: 40%) (a rust preventive), and 0.8 parts by weight of a polyacrylic acid-based thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid: 20% by weight), the amount of the solid content: 28.7%) were stirred and mixed by a Disper (“Robomix,” manufactured by Primix Corp.) and thereby foamed. The foamed composition was applied on a release-treated PET (polyethylene terephthalate) film (the thickness: 38 μm, the trade name: “MRF#38,” manufactured by Mitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C. for 4.5 min to thereby obtain a foamed body (foamed sheet) of an open cell structure having a thickness of 100 μm, an apparent density of 0.45 g/cm³, a cell porosity of 54.5%, a maximum cell diameter of 87.5 μm, a minimum cell diameter of 48.5 μm and an average cell diameter of 65 μm.

Example 3

100 parts by weight of an acryl emulsion solution (the amount of the solid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part by weight of a silicone-based compound A (a dimethyl silicone oil, the number-average molecular weight Mn: 7.16×10³, the weight-average molecular weight Mw: 1.71×10⁴, the amount of the solid content (nonvolatile content): 100%), 3 parts by weight of a fatty acid ammonium-based surfactant (a water dispersion of ammonium stearate, the amount of the solid content: 33%), 0.35 parts by weight of an oxazoline-based crosslinking agent (“Epocros WS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of the solid content: 39%), 1 part by weight of a benzotriazole sodium salt (the solid content: 40%) (a rust preventive), and 0.8 parts by weight of a polyacrylic acid-based thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid: 20% by weight), the amount of the solid content: 28.7%) were stirred and mixed by a Disper (“Robomix,” manufactured by Primix Corp.) and thereby foamed. The foamed composition was applied on a release-treated PET (polyethylene terephthalate) film (the thickness: 38 μm, the trade name: “MRF#38,” manufactured by Mitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C. for 4.5 min to thereby obtain a foamed body (foamed sheet) of an open cell structure having a thickness of 120 μm, an apparent density of 0.26 g/cm³, a cell porosity of 73.7%, a maximum cell diameter of 57.5 μm, a minimum cell diameter of 15.3 μm and an average cell diameter of 30 μm.

Example 4

100 parts by weight of an acryl emulsion solution (the amount of the solid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part by weight of a silicone-based compound A (a dimethyl silicone oil, the number-average molecular weight Mn: 7.16×10³, the weight-average molecular weight Mw: 1.71×10⁴, the amount of the solid content (nonvolatile content): 100%), 3 parts by weight of a fatty acid ammonium-based surfactant (a water dispersion of ammonium stearate, the amount of the solid content: 33%), 1 part by weight of a benzotriazole sodium salt (the solid content: 40%) (a rust preventive), and 0.8 parts by weight of a polyacrylic acid-based thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid: 20% by weight), the amount of the solid content: 28.7%) were stirred and mixed by a Disper (“Robomix,” manufactured by Primix Corp.) and thereby foamed. The foamed composition was applied on a release-treated PET (polyethylene terephthalate) film (the thickness: 38 μm, the trade name: “MRF#38,” manufactured by Mitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C. for 4.5 min to thereby obtain a foamed body (foamed sheet) of an open cell structure having a thickness of 130 μm, an apparent density of 0.37 g/cm³, a cell porosity of 62.6%, a maximum cell diameter of 82.5 μm, a minimum cell diameter of 43.5 μm and an average cell diameter of 60 μm.

Comparative Example 1

45 parts by weight of a polypropylene [the melt flow rate (MFR): 0.35 g/10 min], 55 parts by weight of a mixture (MFR (230° C.): 6 g/10 min, JIS A-hardness: 79°, 30 parts by mass of a softening agent was blended in 100 parts by mass of a polyolefinic elastomer) of the polyolefinic elastomer and the softening agent (paraffinic extender oil), 10 parts by weight of magnesium hydroxide, 10 parts by weight of a carbon (the trade name: “Asahi #35,” manufactured by Asahi Carbon Co., Ltd.), 1 part by weight of stearic monoglyceride, and 1.5 parts by weight of a fatty acid amide (lauric acid bisamide) were kneaded in a twin-screw kneader, manufactured by The Japan Steel Works, Ltd. (JSW), at a temperature of 200° C., then extruded in a strand form, and water cooled and then formed into a pellet form. The pellet was charged in a single-screw extruder, manufactured by The Japan Steel Works, Ltd.; and a carbon dioxide gas was injected in the atmosphere of 220° C. at a pressure of 13 (after the injection, 12) MPa. The carbon dioxide gas was injected in a proportion of 5.6% by weight with respect to the total amount of the pellet. After the carbon dioxide gas was fully saturated, the resultant was cooled to a temperature suitable for being foamed, and thereafter extruded in a cylindrical form through a die; the resultant was passed through between a mandrel to cool the inner side face of the foamed body and an air ring for cooling the foamed body to cool the outside face of the cylindrical foamed body extruded from the ring die of the extruder; and a part of the diameter was cut and the cylindrical foamed body was unfolded into a sheet form to thereby obtain a long-size foamed body original sheet. In the long-size foamed body original sheet, the average cell diameter was 55 μm, and the apparent density was 0.041 g/cm³.

The long-size foamed body original sheet was cut (subjected to a slitting work) into a predetermined width; and by using a continuous slicing apparatus (slicing line), high-density layers on faces were separated away one by one to thereby obtain a resin foamed body.

By passing the resin foamed body through in the above continuous treatment apparatus in which the temperature of the induction heating roll was set at 160° C. and the gap was set at 0.20 mm, one face thereof was subjected to a melting treatment by the heat; and the resultant was subjected to a slitting work, and thereafter taken up to thereby obtain a wound body. Here, the taking-up speed was made to be 20 m/min.

Then, by rewinding the wound body, and passing the rewound body through in the above continuous treatment apparatus in which the temperature of the induction heating roll was set at 160° C. and the gap was set at 0.10 mm, a face thereof (untreated face) not having been subjected to a melting treatment was subjected to a melting treatment by the heat; and the resultant was subjected to a slitting work, and thereafter taken up to thereby obtain a foamed body (foamed sheet) whose both faces had been subjected to the heat melting treatment and which had an open cell structure having a thickness of 100 μm, an apparent density of 0.12 g/cm³, a cell porosity of 88%, a maximum cell diameter of 90 μm, a minimum cell diameter of 30 μm and an average cell diameter of 60 μm.

<Evaluations>

The foamed bodies (foamed sheets) obtained in the Examples and the Comparative Example were evaluated for the following. The results are shown in Table 1 and Table 2. In Table 1, there is shown the number of parts (parts by weight) [in terms of solid content (nonvolatile content)] of each component blended in each Example and Comparative Example. “Em” indicates an emulsion.

(The Average Cell Diameter)

The average cell diameter (μm) was determined by taking and image analyzing an enlarged image of a foamed body cross-section by a low-vacuum scanning electron microscope (“S-3400N type scanning electron microscope,” manufactured by Hitachi High-Tech Science Systems Corp.). Here, the number of cells analyzed was about 10 to 20. Similarly, the minimum cell diameter (μm) and the maximum cell diameter (μm) of the foamed sheet were determined.

(The Apparent Density)

A foamed body (foamed sheet) is punched out with a punching knife of 100 mm×100 mm, and the size of the punched-out sample is measured. Further the thickness is measured by a 1/100 dial gage having a diameter (φ) of its measuring terminal of 20 mm. The volume of the foamed body was calculated from these values.

Then, the weight of the foamed body is measured by an even balance whose minimum division is not less than 0.01 g. The apparent density (g/cm³) of the foamed body was calculated from these values.

(The Dynamic Viscoelasticity)

A temperature-dispersion test was carried out at an angular frequency of 1 rad/sec in a film tensile measurement mode of a viscoelasticity measuring apparatus (“ARES2KFRTN1-FCO,” manufactured by TA Instruments Japan Inc.). There was measured the temperature (° C.) and intensity (maximum value) of the peak top of the loss tangent (tan δ), which was a ratio of the loss elastic modulus E″ to the storage elastic modulus E′ at this time.

In the column of “tan δ Temperature” of Table 2, the temperatures (° C.) of peak tops of loss tangents (tan δ) of foamed bodies are indicated; and in the column of “tan δ Maximum Value,” intensities (maximum values) of the peak tops are indicated.

(The Compression Set Test)

The foamed sheets (sample size: 30 mm×30 mm) obtained in the Examples and the Comparative Example were used as test pieces. By using the test piece, the compression set test was carried out at 80° C. (according to the provision of JIS K6262). More specifically, the test piece was compressed (until the thickness of the compressed test piece became a thickness of 40% of its original thickness) in an atmosphere of 80° C., held in this state for 24 hours, thereafter released from the compressed state, and left as it was at 23° C. for 30 min; and the thickness of the test piece was measured at 23° C. Then, the compression set (%) at 80° C. was determined by the following expression.

CS={(t0−t1)/(t0−t2)}×100

CS: a compression set (%)

t0: an original thickness (mm) of a test piece

t1: a thickness (mm) of the test piece at 30 min after the test piece is removed from a compression apparatus

t2: a thickness (mm) of the test piece in the state of being under a compressive strain

(The Impact Absorption Change Rate)

For the foamed sheets (sample size: 20 mm×20 mm) (test pieces A) obtained in the Examples and the Comparative Example, by using the above-mentioned pendulum impact tester (impact testing apparatus) (see FIG. 1 and FIG. 2), an impact absorption test was carried out under the conditions of 23° C., a weight of an impactor of 28 g, and a swing-up angle of 40°. The impact absorption rate acquired at this time is defined as an initial impact absorption rate a.

Then, the test piece A was stored at 80° C. for 72 hours in the state of being compressed by 60% with respect to the initial thickness of the test piece A, and thereafter, the compression state was released; and thereafter, as in the above, an impact absorption test was carried out under the conditions of 23° C., a weight of an impactor of 28 g, and a swing-up angle of 40° after a lapse of 24 hours at 23° C. The impact absorption rate acquired at this time is defined as an impact absorption rate b after the high-temperature compression.

Then, the impact absorption change rate (%) was determined by the following expression.

Impact absorption change rate (%)={(the impact absorption rate b after high-temperature compression−the initial impact absorption rate a)/the initial impact absorption rate a}×100

Here, the impact absorption rate is a value defined by the following expression.

Impact absorption rate (%)={(F₀−F₁)/F₀}×100

wherein F₀ is an impact force when the impactor is made to collide with a support plate alone; and F₁ is an impact force when the impactor is made to collide with the support plate of a structural body composed of the support plate and the test piece A.

(The 80° C. Stress Retention Rate)

The foamed sheets [the shape and size of the samples: dumbbell No. 1 (see JIS K6251)] obtained in the Examples and the Comparative Example were each held in an atmosphere of 80° C. for 30 min, thereafter, by using a tensile tester, set at 80° C. at an interchuck distance of 40 mm, stretched by 50% at a tensile speed of 500 mm/min, and thereafter held for 120 sec; and the maximum load and the load after the 120 sec were measured and the 80° C. stress retention rate was determined by the following expression.

80° C. stress retention rate (%)=[a load (N) after 120 sec/a maximum load (N)]×100

	TABLE 1
		Silicone-	Surfactant
	Em	based	(Foaming		Thickener	Rust
	the	Compound	Agent)	Crosslinking	the	Preventive
	number	the number	the number	Agent	number of	the number
	of parts	of parts	of parts	the number of	parts	of parts
	blended	blended	blended	parts blended	blended	blended
Example	1	100	1	3	2	0.8	1
	2	100	1	3	0.35	0.8	1
	3	100	1	3	0.35	0.8	1
	4	100	1	3	0.35	0.8	1
Comparative	1	—	—	—	—	—	—
Example

	TABLE 2
							Impact
							Absorption
						Initial	Rate b after	Impact		80° C.
		Average			tanδ	Impact	High-	Absorption		Stress
		Cell	Apparent	tanδ	Maximum	Absorption	Temperature	Change	Compression	Retention
	Thickness	Diameter	Density	Temperature	Value	Rate a	Compression	Rate	Set	Rate
	(μm)	(μm)	(g/cm3)	(° C.)	(—)	(%)	(%)	(%)	(%)	(%)
Example	1	100	45	0.34	−3	0.37	33	32	−3	0	74
	2	100	65	0.45	−3	0.38	30	30	0	0	76
	3	120	30	0.26	−2	0.42	35.4	31.2	−12	14	73
	4	130	60	0.37	−3	0.37	34.8	31.3	−10	13	73
Comparative	1	100	60	0.12	0	0.21	26	13	−50	92	64
Example

INDUSTRIAL APPLICABILITY

The foamed sheet according to the present invention, since even if having a small thickness, being excellent in impact absorption and moreover excellent in heat resistance, is high in installation (adhesion) reliability even at high temperatures; and for example, in electric or electronic devices, the foamed sheet is useful as a member, particularly an impact absorption sheet, for electric or electronic devices to be used when various types of members or components (for example, optical members) are attached (installed) on predetermined sites (for example, housings). Examples of optical members attachable (installable) by utilizing the foamed sheet according to the present invention include image display members (particularly small-size image display members) installed on image display apparatuses such as liquid crystal displays, electroluminescence displays and plasma displays, display members, such as touch panels, installed on mobile communication apparatuses such as so-called “cellular phones,” “smartphones” and “personal digital assistants,” cameras and lenses (particularly small-size cameras and lenses). The electric or electronic device according to the present invention uses the foamed sheet according to the present invention. Such an electric or electronic device includes, for example, an electric or electronic device having a display member, and having a structure in which the foamed sheet is interposed between a housing of the electric or electronic device and the display member. Examples of the electric or electronic device include mobile communication apparatuses such as so-called “cellular phones,” “smartphones” and “personal digital assistants.”

REFERENCE SIGNS LIST

• • 1 PENDULUM IMPACT TESTER (IMPACT TESTING APPARATUS)
  • 2 TEST PIECE (FOAMED SHEET)
  • 3 HOLDING MEMBER
  • 4 IMPACT APPLYING MEMBER
  • 5 PRESSURE SENSOR
  • 11 FIXING JIG
  • 12 PRESSING JIG
  • 16 PRESSURE ADJUSTING MEANS
  • 20 SUPPORT COLUMN
  • 21 ARM
  • 22 ONE END OF SUPPORT ROD (SHAFT)
  • 23 SUPPORT ROD (SHAFT)
  • 24 IMPACTOR
  • 25 ELECTROMAGNET
  • 28 SUPPORT PLATE
  • a SWING-UP ANGLE

Claims

1. A foamed sheet, comprising a foamed body having an average cell diameter of 10 to 200 μm, and having a compression set at 80° C. of not more than 80% and an impact absorption change rate of not more than ±20% as defined by the following:

an impact absorption change rate (%)={(an impact absorption rate b after high-temperature compression−an initial impact absorption rate a)/the initial impact absorption rate a}×100,

wherein the initial impact absorption rate a: an impact absorption rate (%) of a test piece A;

the impact absorption rate b (%) after high-temperature compression: an impact absorption rate (%) acquired by storing the test piece A at 80° C. for 72 hours in the state of being compressed by 60% with respect to an initial thickness of the test piece A, thereafter releasing the compression state, and thereafter conducting measurement after a lapse of 24 hours at 23° C.; and

the impact absorption rate: a value defined by the following expression in an impact absorption test (a weight of an impactor: 28 g, a swing-up angle: 40°) (23° C.) using a pendulum impact tester: an impact absorption rate (%)={(F0−F1)/F0}×100

wherein F0 is an impact force when the impactor is made to collide with a support plate alone; and F1 is an impact force when the impactor is made to collide with the support plate of a structural body composed of the support plate and the test piece A.

2. The foamed sheet according to claim 1, wherein the foamed sheet has a thickness of 30 to 1,000 μm; and the foamed body has an apparent density of 0.2 to 0.7 g/cm3.

3. The foamed sheet according to claim 1, wherein the foamed body has a peak top of a loss tangent (tan δ) in the range of not less than −30° C. and not more than 30° C., the loss tangent (tan δ) being a ratio of a loss elastic modulus to a storage elastic modulus at an angular frequency of 1 rad/sec in a dynamic viscoelasticity measurement.

4. The foamed sheet according to claim 1, wherein the foamed body is formed of at least one resin material selected from the group consisting of acrylic polymers, rubbers, urethanic polymers and ethylene-vinyl acetate copolymers.

5. The foamed sheet according to claim 1, wherein the foamed body is formed through step A of mechanically foaming an emulsion resin composition.

6. The foamed sheet according to claim 5, wherein the foamed body is formed further through step B of coating a base material with the mechanically foamed emulsion resin composition followed by drying.

7. The foamed sheet according to claim 6, wherein step B comprises preliminary drying step B1 of drying the bubble-containing emulsion resin composition applied on the base material at not less than 50° C. and less than 125° C., and regular drying step B2 of thereafter further drying the resultant at not less than 125° C. and not more than 200° C.

8. The foamed sheet according to claim 1, wherein the foamed sheet has a compression set at 80° C. of not more than 50%.

9. The foamed sheet according to claim 8, wherein the foamed sheet has the compression set at 80° C. of not more than 25%.

10. The foamed sheet according to claim 2, wherein the foamed sheet has a thickness of 40 to 500 μm.

11. The foamed sheet according to claim 10, wherein the foamed sheet has a thickness of 50 to 300 μm.

12. The foamed sheet according to claim 2, wherein the foamed body has an apparent density of 0.21 to 0.6 g/cm3.

13. The foamed sheet according to claim 12, wherein the foamed body has an apparent density of 0.22 to 0.5 g/cm3.

14. The foamed sheet according to claim 1, wherein the foamed sheet has a pressure-sensitive adhesive layer on one face or both faces of the foamed body.

15. The foamed sheet according to claim 1, being used as an impact absorption sheet for an electric or electronic device.

16. An electric or electronic device, using a foamed sheet according to claim 1.

17. An electric or electronic device comprising a display member, a housing, and the foamed sheet according to claim 1 between the housing of the electric or electronic device and the display member.