WO2016121788A1 - Low-adhesiveness foam sheet - Google Patents

Abstract

A foam sheet is provided which is extremely thin and exhibits excellent shock absorption. This foam sheet is configured from a foam body which has a thickness of 30-500 µm, an apparent density of 0.2-0.7 g/cm³, and an average cell diameter of 10-150 µm. In shock absorption tests using a pendulum impact tester, the value R obtained by dividing the shock absorption rate (%) defined in the specification, by the thickness (μm) of the foam sheet is 0.20 or greater given an impactor weight of 28g and a swing-up angle of 40°, and it holds of at least one surface of the foam sheet that in a 23°C environment, when pressing a cylindrical probe having a diameter φ of 5 mm and made of SUS 304 against said at least one surface with the conditions of a pressing speed of 30 mm/min, a pressing load of 100 gf, and a pressing time of 1 second, and separating said probe therefrom at a separation speed of 30 mm/min, the maximum load value is less than or equal to 100 kN/m².

Description

Low adhesive foam sheet

The present invention relates to a foam sheet excellent in shock absorption even if the thickness is small, and an electric / electronic device using the foam sheet.

Conventionally, an image display member fixed to an image display device such as a liquid crystal display, an electroluminescence display, a plasma display, a display member attached to a so-called “mobile phone”, “smart phone”, “portable information terminal”, camera, A foam material is used when an optical member such as a lens is fixed to a predetermined part (for example, a housing). Examples of such a foam material include a low-foam, fine-cell urethane-based foam having a closed cell structure and a product obtained by compression molding a highly foamed urethane, and a polyethylene-based foam having a closed cell and an expansion ratio of about 30 times. It was used. Specifically, for example, a gasket (see Patent Document 1) made of a polyurethane foam having an apparent density of 0.3 to 0.5 g / cm ³ , or an electrical / electric structure made of a foam structure having an average cell diameter of 1 to 500 μm. A sealing material for electronic devices (see Patent Document 2) is used.

However, in recent years, as products to which optical members (image display devices, cameras, lenses, etc.) are mounted are becoming thinner and thinner, the clearance of the portion where the foam material is used tends to decrease remarkably. As the clearance decreases, it is necessary to reduce the thickness of the foamed member. However, when the thickness of the conventional foamed material is reduced, sufficient shock absorption is not exhibited. Therefore, for example, when an electric / electronic device with a display member such as “smartphone” is dropped on the ground or the like, there is a demand for a foam sheet that absorbs an impact at the time of collision and prevents the display member from being damaged.

In addition to shock absorbing sheets used to prevent damage to display members, etc., as electronic devices such as PCs (personal computers), tablet PCs, PDAs (personal personal digital assistants), and mobile phones become more sophisticated. These members (for example, a heat conductive layer) are stacked and incorporated. If the shock-absorbing sheet has adhesiveness, it will be difficult to peel off and correct it when assembled and the yield will be reduced. Is desired.

Japanese Patent Laid-Open No. 2001-100216 JP 2002-309198 A

Accordingly, an object of the present invention is to provide a foamed sheet that exhibits excellent shock absorption even when the thickness is very small.
Another object of the present invention is to provide a foam sheet that can be corrected in position when other members are laminated in addition to the above characteristics.
Another object of the present invention is to provide an electric / electronic device that is less likely to be damaged by an impact at the time of dropping even if it is reduced in size and thickness.

As a result of intensive investigations to achieve the above object, the present inventors have found that a foam sheet having a specific apparent density, a specific average cell diameter, a specific shock absorption characteristic, and a low surface tackiness is used again. In addition to excellent releasability, even a thin thickness of 30 to 500 μm is remarkably excellent in impact absorption. Therefore, when such a foam sheet is laminated with another member, position correction is easy. It has been found that the built-in electric / electronic devices are less likely to be damaged by a shock or the like even when dropped on the ground. The present invention has been completed based on the above findings and further studies.

That is, the present invention is a foam sheet composed of a foam having a thickness of 30 to 500 μm, an apparent density of 0.2 to 0.7 g / cm ³ , and an average cell diameter of 10 to 150 μm, In an impact absorption test using a pendulum type impact tester, a value R obtained by dividing the impact absorption rate (%) defined by the following formula by the thickness (μm) of the foamed sheet is 28 g in weight of the impactor. When the angle is 40 °, it is 0.20 or more, and at least one surface of the foam sheet is a 23 ° C. environment, a SUS304 cylindrical probe having a diameter φ of 5 mm, an indentation speed of 30 mm / min, an indentation load. Provided is a foamed sheet having a maximum load value of 100 kN / m ² or less when pressed at 100 gf and a pressing time of 1 second and peeled at a peeling speed of 30 mm / min.
Impact absorption rate (%) = {(F ₀ −F ₁ ) / F ₀ } × 100
(In the above formula, F ₀ is the impact force when the impactor collides only with the support plate, and F ₁ causes the impactor to collide with the support plate of the structure composed of the support plate and the foam sheet. Is the impact force when

The foam has a peak top in a range where a loss tangent (tan δ), which is a ratio of a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement, is −50 ° C. or higher and 50 ° C. or lower. It is preferable. The loss tangent (tan δ) preferably has a peak top in the range of −40 ° C. to 40 ° C., more preferably has a peak top in the range of −30 ° C. to 30 ° C., and −20 ° C. It is particularly preferable to have a peak top in the range of 20 ° C. or lower.

The maximum value of the loss tangent (tan δ) in the foam in the range of −50 ° C. to 50 ° C. is preferably 0.2 or more.

The initial elastic modulus of the foam in a tensile test at a tensile speed of 300 mm / min in an environment of 23 ° C. is preferably 10 N / mm ² or less. Also, the initial elastic modulus is more preferably 0.1 N / mm ² or more 10 N / mm ² or less, and more preferably 0.1 N / mm ² or more 5N / mm ² or less.

The foam can be formed of at least one resin material selected from the group consisting of acrylic polymer, rubber, urethane polymer, styrene polymer, and ethylene-vinyl acetate copolymer.

The foam may contain a crosslinking agent.

The foam may contain a filler.

The foam may be formed of an emulsion resin composition.

The foam may be formed through Step A of mechanically foaming the resin composition. In this case, the foam may be further formed through a step B in which a mechanically foamed emulsion resin composition is applied onto a substrate and dried.

The pressure-sensitive adhesive layer may be provided on one side or both sides of the foam.

The foamed sheet can be used as an impact absorbing sheet for electric / electronic devices.

The present invention also provides an electric / electronic device in which the foam sheet is used.

This electric / electronic device is an electric / electronic device provided with a display member, wherein the foam sheet is sandwiched between a housing of the electric or electronic device and the display member. Includes electronics.

Even if the foamed sheet of the present invention is thin, it has excellent shock absorption, and when other members (for example, a heat conductive layer) are laminated, they are peeled off when they are misaligned and assembled. The position can be corrected. Even if an electric / electronic device using the foamed sheet of the present invention falls on the ground or the like, it is possible to prevent damage to the display or the like due to impact.

It is a schematic block diagram of a pendulum type impact tester (impact test device). It is a figure which shows schematic structure of the holding member of a pendulum type impact tester (impact test apparatus).

The foam sheet of the present invention is composed of a foam having a thickness of 30 to 500 μm, an apparent density of 0.2 to 0.7 g / cm ³ , and an average cell diameter of 10 to 150 μm. In an impact absorption test using a testing machine, a value R obtained by dividing the impact absorption rate (%) defined by the formula described later by the thickness (μm) of the foamed sheet is a weight of impactor of 28 g and a swing-up angle of 40. In the case of °, it is 0.20 or more, and at least one surface of the foamed sheet is a 23 ° C. environment, a SUS304 cylindrical probe having a diameter φ of 5 mm, an indentation speed of 30 mm / min, an indentation load. The maximum load value is 100 kN / m ² or less when pressed under the condition of 100 gf and pressing time of 1 second and peeled off at a peeling speed of 30 mm / min. Therefore, it is excellent in shock absorption, and when the other members are laminated, the position can be easily corrected even if they are misaligned and assembled.

The thickness of the foamed sheet of the present invention is 30 to 500 μm. The lower limit is preferably 40 μm, more preferably 50 μm, and the upper limit is preferably 400 μm, more preferably 300 μm, and even more preferably 200 μm. In this invention, since the thickness of a foamed sheet is 30 micrometers or more, it can contain a bubble uniformly and can exhibit the outstanding impact absorption. Moreover, since the thickness of the foamed sheet is 500 μm or less, it can easily follow a minute clearance. The foamed sheet of the present invention is excellent in impact absorbency despite being as thin as 30 to 500 μm.

The apparent density of the foam constituting the foam sheet of the present invention is 0.2 to 0.7 g / cm ³ . The lower limit is preferably 0.21 g / cm ³ , more preferably 0.22 g / cm ³ , and the upper limit is preferably 0.6 g / cm ³ , more preferably 0.5 g / cm ³ , still more preferably 0.00. 4 g / cm ³ . When the apparent density of the foam is 0.2 g / cm ³ or more, the strength can be maintained, and when it is 0.7 g / cm ³ or less, high impact absorbability is exhibited. Further, when the apparent density of the foam is in the range of 0.2 to 0.4 g / cm ³ , even higher impact absorbability is exhibited.

The average cell diameter of the foam is 10 to 150 μm. The lower limit is preferably 15 μm, more preferably 20 μm, and the upper limit is preferably 140 μm, more preferably 130 μm, and even more preferably 100 μm. When the average cell diameter is 10 μm or more, excellent impact absorbability is exhibited. Moreover, since the average cell diameter is 150 μm or less, the compression recovery property is also excellent. The maximum cell diameter of the foam is, for example, 40 to 400 μm, and the lower limit is preferably 60 μm, more preferably 80 μm, and the upper limit is preferably 300 μm, more preferably 220 μm. The minimum cell diameter of the foam is, for example, 5 to 70 μm, and the lower limit is preferably 8 μm, more preferably 10 μm, and the upper limit is preferably 60 μm, more preferably 50 μm.

The average cell diameter is determined by observing an image containing about 20 to 40 cells in the cross section of the foam with a scanning electron microscope (SEM), a digital microscope, or the like. In order from the largest cell, the area of the cell is measured at 20 points or more, and an average value of the diameter of the circle is calculated from the area by image analysis.

In the present invention, the ratio of the average cell diameter (μm) to the thickness of the foamed sheet (μm) (the former / the latter) is preferably in the range of 0.2 to 0.9 from the viewpoint of shock absorption. The lower limit of the ratio of the average cell diameter (μm) to the thickness of the foamed sheet (μm) is preferably 0.25, more preferably 0.3, and the upper limit is preferably 0.85, more preferably 0. .8.

The foamed sheet of the present invention has excellent impact absorbability while being thin. That is, in an impact absorption test using a pendulum type impact tester (impactor weight 28 g, swing-up angle 40 °), the impact absorption rate (%) defined by the following formula is the thickness of the foam sheet (μm). When the impact absorption rate R per unit thickness is obtained by dividing by R, the R is 0.20 or more. R is preferably 0.25 or more, more preferably 0.28 or more. The upper limit value of R is, for example, about 0.5.
Impact absorption rate (%) = {(F ₀ −F ₁ ) / F ₀ } × 100
(In the above formula, F ₀ is the impact force when the impactor collides only with the support plate, and F ₁ causes the impactor to collide with the support plate of the structure composed of the support plate and the foam sheet. Is the impact force when

The impact absorption rate varies depending on the thickness of the foamed sheet, etc., but is usually 10 to 70%, and the lower limit is preferably 20%, more preferably 30%, still more preferably 35%. The upper limit is preferably 60%.

A schematic configuration of a pendulum type impact tester (impact test apparatus) will be described with reference to FIGS. As shown in FIGS. 1 and 2, the impact test apparatus 1 (pendulum tester 1) includes a holding member 3 as a holding means for holding the test piece 2 (foamed sheet 2) with an arbitrary holding force, and the test piece 2 An impact load member 4 for applying an impact stress to the test piece, a pressure sensor 5 as an impact force detection means for detecting an impact force of the impact load member 4 against the test piece 2 and the like. The holding member 3 that holds the test piece 2 with an arbitrary holding force includes a fixing jig 11 and a holding jig 12 that is slidable so as to sandwich and hold the test piece 2 facing the fixing jig 11. It is configured. Further, the pressing jig 12 is provided with a pressing pressure adjusting means 16. Further, the impact load member 4 for applying an impact force to the test piece 2 held by the holding member 3 is supported so that one end 22 is pivotally supported with respect to the column 20 and an impactor 24 is provided on the other end side. It is composed of a rod 23 (shaft 23) and an arm 21 that lifts and holds the impactor 24 at a predetermined angle. Here, since a steel ball is used as the impactor 24, it is possible to lift the impactor 24 integrally by a predetermined angle by providing an electromagnet 25 at one end of the arm. Furthermore, the pressure sensor 5 that detects the impact force acting on the test piece 2 by the impact load member 4 is provided on the opposite side of the surface of the fixing jig 11 that contacts the test piece.

The impactor 24 is a steel ball (iron ball). In the present invention, the angle at which the impactor 24 is lifted by the arm 21 (the swing angle a in FIG. 1) is 40 °, and the weight of the steel ball (iron ball) is 28 g.

As shown in FIG. 2, the test piece 2 (foamed sheet 2) is a highly elastic plate material such as a resin plate (acrylic plate, polycarbonate plate, etc.) or a metal plate between the fixing jig 11 and the holding jig 12. It is clamped via the support plate 28 configured.

The impact absorbability is an impact force F ₀ measured by causing the impactor 24 to collide with the support plate 28 after tightly fixing the fixing jig 11 and the support plate 28 using the impact test apparatus described above. The impact force F ₁ measured by causing the impactor 24 to collide with the support plate 28 after inserting the test piece 2 between the fixing jig 11 and the support plate 28 and fixing the test piece 2 tightly is calculated. The Note that the impact test apparatus is the same apparatus as that of Example 1 of JP-A-2006-47277.

Shock absorption can be adjusted by selecting the average cell diameter, apparent density, and the like. However, when the thickness of the foamed sheet is very small, it may not be possible to sufficiently absorb the impact only by adjusting these characteristics. This is because when the thickness of the foam sheet is very thin, the bubbles in the foam are immediately crushed by the impact and the shock buffering function by the bubbles is lost. From such a viewpoint, the peak top of the loss tangent (tan δ), which is the ratio of the storage elastic modulus and the loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the foam, is −50 ° C. to 50 ° C. The following range is preferable. By setting the peak top of the loss tangent (tan δ) in the above range, the constituent material of the foam exhibits a function of buffering the impact even after the bubbles are crushed.

The lower limit of the temperature range where the peak tangent of the loss tangent exists is preferably −40 ° C., more preferably −30 ° C., further preferably −20 ° C., and the upper limit is preferably 40 ° C., more preferably 30 ° C. More preferably, it is 20 ° C. In the case of a material having two or more peak tops of loss tangents, it is desirable that at least one of them falls within the above range. When the peak temperature is −50 ° C. or higher, excellent compression recovery is exhibited. Moreover, when the peak temperature is 50 ° C. or lower, high flexibility is exhibited and excellent shock absorption is exhibited.

The peak top strength (maximum value) of loss tangent (tan δ) in the range of −50 ° C. or higher and 50 ° C. or lower is preferably higher from the viewpoint of shock absorption, for example 0.2 or higher, preferably 0.3 or higher. The upper limit value of the peak top intensity (maximum value) is, for example, 2.0.

Thus, the peak temperature and peak top strength of the loss tangent (tan δ) greatly contribute to the shock absorption of the foam. When the peak top of the loss tangent (tan δ), which is the ratio of the storage elastic modulus and the loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the foam, is in the range of −50 ° C. or more and 50 ° C. or less, The reason why the foam sheet has high impact absorbability is not necessarily clear, but it is presumed that the loss tangent (tan δ) peak exists at a location that matches the frequency of impact. That is, the range where the loss tangent (tan δ) is −50 ° C. or more and 50 ° C. or less is converted to the frequency range corresponding to the drop impact of the structure by the temperature-time conversion rule in the viscoelasticity measurement. It is presumed that the shock absorption becomes higher as the foamed sheet has a peak temperature of the loss tangent (tan δ) in the range of 50 ° C. or lower. The storage elastic modulus is a repulsive force with respect to the impact energy applied to the foam sheet. If the storage elastic modulus is high, the impact is repelled as it is. On the other hand, the loss elastic modulus is a physical property that changes impact energy applied to the foam sheet to heat, and the higher the loss elastic modulus is, the more the impact energy is changed to heat, so the impact is absorbed and the strain is reduced. From this, it is surmised that a foam sheet having a larger loss tangent (tan δ), which is a ratio of the storage elastic modulus and the loss elastic modulus, has a higher impact absorption rate by changing the impact to heat and reducing the repulsive force. .

In the foamed sheet of the present invention, it is desirable that at least one surface of the foamed sheet has low adhesiveness (tackiness) from the viewpoint of position correction. The adhesiveness can be measured by a known tacking tester using a probe tack method. At least one surface of the foamed sheet is a 23 ° C. environment, and a SUS304 cylindrical probe having a diameter φ of 5 mm is pressed at a speed of 30 mm / The maximum load value is 100 kN / m ² or less when pressed under the conditions of min, indentation load 100 gf, and press time 1 second, and peeled off at a peeling speed of 30 mm / min. It said maximum load value, preferably, 80 kN / m ² or less, more preferably 60 kN / m ^2, more preferably not more than 50 kN / m ^2. The lower limit value of the maximum load value is 0. That is, the foam sheet of the present invention has a very low surface tack. Therefore, when laminating other members on the foamed sheet, even if they are misaligned and assembled, they can be easily removed to correct the position.

As a means for reducing the tackiness of the surface of the foamed sheet, for example, the crosslinking density of the foam is increased, an appropriate amount of filler is added, or the resin material constituting the foam has a high Tg (for example, 50 to 200). ° C) polymer (for example, styrene-based polymer, etc.) may be blended in an appropriate amount.

The initial elastic modulus of the foam is preferably low from the viewpoint of impact absorption. The initial elastic modulus (a value calculated from a slope at the time of 10% strain in a tensile test under a 23 ° C. environment and a tensile speed of 300 mm / min) is preferably 10 N / mm ² or less, more preferably 5 N / mm ^2. It is as follows. The lower limit value of the initial elastic modulus is, for example, 0.1 N / mm ² .

The foam constituting the foam sheet of the present invention is not particularly limited in its composition and cell structure as long as it has the above characteristics. The cell structure may be any of an open cell structure, a closed cell structure, and a semi-continuous semi-closed cell structure. From the viewpoint of impact absorption, an open cell structure and a semi-open semi-closed cell structure are preferable.

The foam can be constituted by a resin composition containing a resin material (polymer). In addition, the loss which is a ratio of the storage elastic modulus and the loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the resin composition in an unfoamed state [resin composition when not foamed (solid matter)] The peak top of tangent (tan δ) is preferably in the range of −50 ° C. to 50 ° C. The lower limit of the temperature range where the peak tangent of the loss tangent exists is preferably −40 ° C., more preferably −30 ° C., further preferably −20 ° C., and the upper limit is preferably 40 ° C., more preferably 30 ° C. More preferably, it is 20 ° C. In the case of a material having two or more peak tops of loss tangents, it is desirable that at least one of them falls within the above range. Loss tangent (tan δ) peak top strength of the resin composition (solid) in the range of −50 ° C. to 50 ° C. (this value is the loss tangent of the foam in the range of −50 ° C. to 50 ° C.) The peak top strength of (tan δ) (corresponding to the value obtained by dividing the apparent density (g / cm ³ ) of the foam) is preferably higher from the viewpoint of impact absorption. For example, the peak top intensity of the loss tangent (tan δ) in the range of −50 ° C. to 50 ° C. of the resin composition (solid material) is preferably 0.9 (g / cm ³ ) ⁻¹ or more, The upper limit is, for example, about 3 (g / cm ³ ) ⁻¹ .

Further, the initial elastic modulus (23 ° C., tensile speed 300 mm / min) of the unfoamed resin composition (solid material) is desirably lower, preferably 50 N / mm ² or less, more preferably 30 N / mm ^2. It is as follows. The lower limit value of the initial elastic modulus is, for example, 0.3 N / mm ² .

The resin material (polymer) constituting the foam is not particularly limited, and a known or well-known resin material constituting the foam can be used. Examples of the resin material include acrylic polymer, rubber, urethane polymer, styrene polymer, and ethylene-vinyl acetate copolymer. Among these, acrylic polymer, rubber, urethane polymer, and styrene polymer are preferable from the viewpoint of impact absorption. One type of resin material (polymer) constituting the foam may be used alone, or two or more types may be used.

From the viewpoint of obtaining a foam sheet that is easy to correct the position by reducing the tackiness of the sheet surface while maintaining excellent shock absorption, from acrylic polymer, rubber, urethane polymer and ethylene-vinyl acetate copolymer It is preferable to use a combination of at least one resin material selected from the group and a styrenic polymer. In this case, the amount of the styrene polymer used is, for example, 100 parts by weight of at least one resin material selected from the group consisting of an acrylic polymer, rubber, urethane polymer, and ethylene-vinyl acetate copolymer. The lower limit thereof is preferably 1 part by weight, more preferably 5 parts by weight, and the upper limit thereof is preferably 80 parts by weight, more preferably 50 parts by weight.

The peak top of the loss tangent (tan δ), which is the ratio of the storage elastic modulus and the loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the foam, is in the range of −50 ° C. to 50 ° C. For this purpose, the Tg of the resin material (polymer) can be used as an index or a guide. For example, the resin material (polymer) constituting the foam has a Tg of −50 ° C. or more and less than 50 ° C. (lower limit is preferably −40 ° C., more preferably −30 ° C., upper limit is preferably 40 ° C., more preferably The resin material (polymer) can be selected so as to be in the range of 30 ° C.

The acrylic polymer is preferably an acrylic polymer formed with a monomer having a homopolymer Tg of −10 ° C. or more and a monomer having a homopolymer Tg of less than −10 ° C. as essential monomer components. Using such an acrylic polymer, the ratio of the storage elastic modulus and loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement is obtained by adjusting the amount ratio of the former monomer and the latter monomer. A foam having a loss tangent (tan δ) peak top of −50 ° C. or more and 50 ° C. or less can be obtained relatively easily.

In the present invention, “glass transition temperature (Tg) when forming a homopolymer” (sometimes simply referred to as “Tg of homopolymer”) means “glass transition temperature (Tg of homopolymer of the monomer)”. ) ”, Specifically,“ Polymer Handbook ”(3rd edition, John Wiley & Sons, Inc., 1987). The Tg of a homopolymer of a monomer not described in the above document refers to, for example, a value obtained by the following measurement method (see JP 2007-51271 A). That is, in a reactor equipped with a thermometer, a stirrer, a nitrogen introduction tube and a reflux condenser, 100 parts by weight of monomer, 0.2 part by weight of 2,2′-azobisisobutyronitrile, and ethyl acetate 200 as a polymerization solvent. A part by weight is charged and stirred for 1 hour while introducing nitrogen gas. After removing oxygen in the polymerization system in this way, the temperature is raised to 63 ° C. and the reaction is carried out for 10 hours. Next, the mixture is cooled to room temperature to obtain a homopolymer solution having a solid concentration of 33% by weight. Next, this homopolymer solution is cast-coated on a separator and dried to prepare a test sample (sheet-like homopolymer) having a thickness of about 2 mm. This test sample was punched into a disk shape having a diameter of 7.9 mm, sandwiched between parallel plates, and subjected to a shear strain at a frequency of 1 Hz using a viscoelasticity tester (ARES, manufactured by Rheometrics). Viscoelasticity is measured in a shear mode at a heating rate of 150 ° C. and 5 ° C./min, and the peak top temperature of tan δ is defined as Tg of the homopolymer. The Tg of the resin material (polymer) can also be measured by this method.

In a monomer having a homopolymer Tg of −10 ° C. or higher, the Tg is, for example, −10 ° C. to 250 ° C., preferably 10 to 230 ° C., more preferably 50 to 200 ° C.

Examples of the homopolymer having a Tg of −10 ° C. or more include, for example, (meth) acrylonitrile; amide group-containing monomers such as (meth) acrylamide and N-hydroxyethyl (meth) acrylamide; (meth) acrylic acid; methacrylic acid (Meth) acrylic acid alkyl esters having homopolymers such as methyl and ethyl methacrylate having a Tg of −10 ° C. or higher; (meth) acrylic acid isobornyl; heterocycle-containing vinyl monomers such as N-vinyl-2-pyrrolidone; Examples thereof include hydroxyl group-containing monomers such as ethyl methacrylate. These can be used individually by 1 type or in combination of 2 or more types. Among these, (meth) acrylonitrile (especially acrylonitrile) is particularly preferable. When (meth) acrylonitrile (especially acrylonitrile) is used as a monomer having a homopolymer Tg of −10 ° C. or higher, the peak top strength of the loss tangent (tan δ) of the foam is increased because of the strong intermolecular interaction. be able to.

In a monomer having a Tg of less than −10 ° C., the Tg is, for example, −70 ° C. or more and less than −10 ° C., preferably −70 ° C. to −12 ° C., more preferably −65 ° C. to −15 ° C. .

Examples of the homopolymer having a Tg of less than −10 ° C. include, for example, (meth) acrylic acid alkyl esters having a homopolymer Tg of less than −10 ° C., such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc. Is mentioned. These can be used individually by 1 type or in combination of 2 or more types. Among these, acrylic acid C _2-8 alkyl ester is particularly preferable.

The content of the monomer having a Tg of -10 ° C. or more of the homopolymer is, for example, 2 to 30% by weight with respect to all the monomer components forming the acrylic polymer (total amount of monomer components), and the lower limit is preferably 3%. %, More preferably 4% by weight, and the upper limit is preferably 25% by weight, more preferably 20% by weight. In addition, the content of the monomer having a Tg of the homopolymer of less than −10 ° C. with respect to all the monomer components forming the acrylic polymer (total amount of monomer components) is, for example, 70 to 98% by weight, and the lower limit is preferably The upper limit is preferably 97% by weight, more preferably 96% by weight.

If the monomer that forms the acrylic polymer contains a nitrogen atom-containing copolymerizable monomer, the viscosity of the composition decreases when the emulsion resin composition is foamed by applying mechanical shearing or the like. Many bubbles are likely to be taken into the emulsion, and when the emulsion resin composition containing the bubbles is applied onto a substrate and dried in a stationary state, the composition tends to aggregate and the viscosity is increased. The foam rises and the bubbles are retained in the composition and hardly diffused to the outside, so that a foam having excellent foaming characteristics 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; (meth) acrylamide, N And amide group-containing monomers such as hydroxyethyl (meth) acrylamide, N-methylolacrylamide, N, N-dimethylacrylamide, N, N-diethylacrylamide, and diacetoneacrylamide. Among these, cyano group-containing monomers such as acrylonitrile and lactam ring-containing monomers such as N-vinyl-2-pyrrolidone are preferable. A nitrogen atom containing monomer can be used individually by 1 type or in combination of 2 or more types.

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

In addition, in the acrylic polymer having a structural unit derived from such a nitrogen atom-containing monomer, in addition to the structural unit derived from the nitrogen atom-containing monomer, an acrylic acid C _2-18 alkyl ester (particularly acrylic acid C _{2 -8} alkyl ester) is preferred. Acrylic acid C _2-18 alkyl ester can be used alone or in combination of two or more. In such an acrylic polymer, the content of structural units derived from acrylic acid C _2-18 alkyl esters (particularly acrylic acid C _2-8 alkyl esters) is based on the total structural units constituting the acrylic polymer. The lower limit is more preferably 75% by weight, still more preferably 80% by weight, and the upper limit is more preferably 97% by weight, still more preferably 96% by weight. .

The rubber may be natural rubber or 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, nitrile rubber (NBR), methyl methacrylate-butadiene rubber (MBR), and silicone rubber are preferable.

Examples of the urethane polymer include polycarbonate polyurethane, polyester polyurethane, and polyether polyurethane.

As the ethylene-vinyl acetate copolymer, a known or well-known ethylene-vinyl acetate copolymer can be used.

As the styrene polymer, a known or well-known styrene polymer can be used.

The foam constituting the foam sheet may contain a surfactant, a cross-linking agent, a thickener, a rust preventive agent, and other additives as required in addition to the resin material (polymer).

For example, an optional surfactant may be included for the purpose of reducing the bubble diameter and stabilizing the foam. The surfactant is not particularly limited, and any of an anionic surfactant, a cationic surfactant, a nonionic surfactant, an amphoteric surfactant, and the like may be used. From the viewpoint of the stability of the foam, an anionic surfactant is preferable, and a fatty acid ammonium surfactant such as ammonium stearate is more preferable. Surfactant may be used individually by 1 type and may be used in combination of 2 or more type. Different surfactants may be used in combination, for example, an anionic surfactant and a nonionic surfactant, or an anionic surfactant and an amphoteric surfactant may be used in combination.

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

Moreover, in order to improve the strength, heat resistance, and moisture resistance of the foam, an arbitrary cross-linking agent may be included. The crosslinking agent is not particularly limited, and any of oil-soluble and water-soluble may be used. Examples of the crosslinking agent include epoxy, oxazoline, isocyanate, carbodiimide, melamine, and metal oxide. Among these, an oxazoline-based crosslinking agent is preferable.

The addition amount [solid content (nonvolatile content)] of the crosslinking agent is, for example, 0 to 30 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)], and the lower limit is preferably 0. 0.01 parts by weight, and the upper limit is preferably 20 parts by weight. From the viewpoint of reducing the tackiness of the surface of the foamed sheet, the addition amount of the crosslinking agent [solid content (nonvolatile content)] has, for example, a resin material [polymer, particularly a crosslinking point (hydroxyl group, carboxyl group, etc.). Polymer] [Solid content (nonvolatile content)] is preferably 0.5 to 30 parts by weight with respect to 100 parts by weight, and the lower limit thereof is more preferably 1 part by weight, still more preferably 2 parts by weight. More preferably, it is 25 parts by weight, still more preferably 20 parts by weight.

Furthermore, in order to improve the stability of the foamed foam and the film forming property, an optional thickener may be included. The thickener is not particularly limited, and examples thereof include acrylic acid type, urethane type, and polyvinyl alcohol type. Of these, polyacrylic acid thickeners and urethane thickeners are preferred.

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

Also, an arbitrary rust inhibitor may be included to prevent corrosion of the metal member adjacent to the foam sheet. As the rust inhibitor, an azole ring-containing compound is preferable. When an azole ring-containing compound is used, it is possible to achieve both high levels of corrosion prevention for metals and adhesion to adherends.

The azole ring-containing compound may be a compound having a 5-membered ring containing one or more nitrogen atoms in the ring. For example, a diazole (imidazole, pyrazole) ring, triazole ring, tetrazole ring, oxazole ring, isoxazole And compounds having a ring, a thiazole ring, or an isothiazole ring. These rings may be condensed with an aromatic ring such as a benzene ring to form a condensed ring. Examples of the compound having such a condensed ring include a compound having a benzimidazole ring, a benzopyrazole ring, a benzotriazole ring, a benzoxazole ring, a benzoisoxazole ring, a benzothiazole ring, or a benzoisothiazole ring.

The azole ring and the condensed ring (benzotriazole ring, benzothiazole ring, etc.) each may have a substituent. Examples of the substituent include alkyl groups having 1 to 6 carbon atoms (preferably 1 to 3 carbon atoms) such as methyl group, ethyl group, propyl group, isopropyl group and butyl group; methoxy group, ethoxy group, isopropyloxy An alkoxy group having 1 to 12 carbon atoms (preferably 1 to 3 carbon atoms) such as a butoxy group; an aryl group having 6 to 10 carbon atoms such as a phenyl group, a tolyl group or a naphthyl group; an amino group; a methylamino group; (Mono or di) C _1-10 alkylamino group such as dimethylamino group; amino-C _1-6 alkyl group such as aminomethyl group, 2-aminoethyl group; N, N-diethylaminomethyl group, N, N— bis (2-ethylhexyl) mono or di (C _1-10 alkyl) such as aminomethyl group amino -C _1-6 alkyl group; a mercapto group; a methoxycarbonyl group, the number of carbon atoms such as an ethoxycarbonyl group An alkoxycarbonyl group having 1-6; carboxyl group; carboxyl -C such as 2-carboxyethyl thio group _1-6 alkylthio group; carboxy carboxy -C _1-6 alkyl group such as a methyl group N, N-bis (hydroxymethyl) N, N-bis (hydroxy-C _1-4 alkyl) amino-C _1-4 alkyl group such as aminomethyl group; sulfo group and the like. The azole ring-containing compound may form a salt such as a sodium salt or a potassium salt.

From the viewpoint of rust prevention action on metals, compounds in which the azole ring forms a condensed ring with an aromatic ring such as a benzene ring are preferred. A compound having a benzothiazole ring is particularly preferred.

Examples of the benzotriazole compounds include 1,2,3-benzotriazole, methylbenzotriazole, carboxybenzotriazole, carboxymethylbenzotriazole, and 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, or these A sodium salt etc. are mentioned.

Examples of the benzothiazole compound include 2-mercaptobenzothiazole, 3- (2- (benzothiazolyl) thio) propionic acid, or a sodium salt thereof.

The azole ring-containing compound may be used alone or in combination of two or more.

The addition amount [solid content (nonvolatile content)] of the rust inhibitor (for example, the azole ring-containing compound) [solid content (nonvolatile content)] is within a range that does not impair the adhesion to the adherend and the original properties of the foam. For example, 0.2 to 5 parts by weight is preferable with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. The lower limit is more preferably 0.3 parts by weight, still more preferably 0.4 parts by weight, and the upper limit is more preferably 3 parts by weight, still more preferably 2 parts by weight.

Further, any appropriate other component may be included within the range not impairing the shock absorption. Such other components may contain only 1 type and may contain 2 or more types. Examples of the other components include polymer components other than those described above, softeners, antioxidants, anti-aging agents, gelling agents, curing agents, plasticizers, fillers, reinforcing agents, foaming agents (such as baking soda), micro Capsules (thermally expandable microspheres, etc.), flame retardants, light stabilizers, UV absorbers, colorants (pigments, dyes, etc.), pH adjusters, solvents (organic solvents), thermal polymerization initiators, photopolymerization initiators, etc. Is mentioned. The addition amount [solid content (nonvolatile content)] of these components may be in a range that does not impair the adhesion to the adherend and the original properties of the foam. For example, resin material (polymer) [solid content (nonvolatile content) )] For example, a range of 0.2 to 100 parts by weight is preferable with respect to 100 parts by weight.

The addition amount [solid content (nonvolatile content)] of the foaming agent (e.g. baking soda) is preferably 0.5 to 20 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. is there. The addition amount [solid content (nonvolatile content)] of the microcapsule (thermally expandable microspheres) is more preferably 0.2 to 100 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. 10 parts by weight.

Examples of the filler include silica, clay (mica, talc, smectite, etc.), alumina, aluminum hydroxide, alkaline earth metal hydroxide (magnesium hydroxide, etc.), and alkaline earth metal carbide (calcium carbide). Etc.), titania, zinc oxide, tin oxide, zeolite, graphite, carbon nanotube, inorganic fiber (carbon fiber, glass fiber, potassium titanate fiber, etc.), organic fiber, metal powder (silver, copper, etc.), wax (polyethylene wax) , Polypropylene wax, etc.). As fillers, piezoelectric particles (titanium oxide, barium titanate, etc.), conductive particles (conductive titanium oxide, tin oxide, etc.), thermally conductive particles (boron nitride, etc.), organic fillers (silicone powder, polyethylene powder) , Polypropylene powder, etc.) can also be added.

The amount of the filler added is, for example, 0.3 to 100 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (non-volatile content)]. From the viewpoint of reducing the tackiness of the surface of the foam sheet, the amount of filler added is preferably 1 to 100 parts by weight, and the lower limit thereof is preferably 5 parts by weight, more preferably 10 parts by weight. The upper limit is preferably 80 parts by weight, more preferably 50 parts by weight. When clay such as mica is used as the filler, the addition amount is particularly in the range of 0.3 to 10 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. preferable. In addition, when piezoelectric particles are used as the filler, the addition amount thereof is particularly preferably in the range of 5 to 40 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. Further, when conductive particles are used as the filler, the addition amount thereof is particularly preferably in the range of 5 to 40 parts by weight with respect to 100 parts by weight of the resin material (polymer) [solid content (nonvolatile content)]. In addition, when piezoelectric particles and conductive particles are used in combination as fillers, the amount of generated charges can be adjusted by pressure. In this case, the ratio of the piezoelectric particles to the conductive particles is, for example, the former / the latter (weight ratio) = 10/90 to 90/10, preferably the former / the latter (weight ratio) = 20/80 to 80/20, More preferably, the former / the latter (weight ratio) = 30/70 to 70/30.

The foam sheet of the present invention can be produced by subjecting a resin composition containing a resin material (polymer) constituting the foam to foam molding. As the foaming method (bubble forming method), methods usually used for foam molding, such as physical methods and chemical methods, can be employed. In general, the physical method is a method in which gas components such as air and nitrogen are dispersed in a polymer solution and bubbles are formed by mechanical mixing (mechanical foam). The chemical method is a method of obtaining a foam by forming cells with a gas generated by thermal decomposition of a foaming agent added to a polymer base. From the viewpoint of environmental problems, a physical method is preferable. Bubbles formed by physical methods are often open cells.

As a resin composition containing a resin material (polymer) to be subjected to foam molding, a resin solution in which a resin material is dissolved in a solvent may be used. From the viewpoint of cellularity, an emulsion containing a resin material (emulsion resin composition) ) Is preferably used. That is, it is preferable that the foam which comprises the foam sheet of this invention is a foam of an emulsion resin composition. As an emulsion, you may blend and use 2 or more types of emulsion.

The solid content concentration of the emulsion is preferably higher from the viewpoint of film formability. The solid content concentration of the emulsion is preferably 30% by weight or more, more preferably 40% by weight or more, and further preferably 50% by weight or more.

In the present invention, a method of producing a foam through a step (Step A) in which a resin composition (for example, an emulsion resin composition) is foamed mechanically is preferable. That is, it is preferable that the foam which comprises the foam sheet of this invention is a mechanical foam of a resin composition (for example, emulsion resin composition). The foaming device is not particularly limited, and examples thereof include a high-speed shearing method, a vibration method, and a pressurized gas discharge method. Among these, the high-speed shearing method is preferable from the viewpoint of finer bubble diameter and production of a large capacity.

Bubbles when foamed by mechanical stirring are gas (gas) taken into the emulsion. The gas is not particularly limited as long as it is inert to the emulsion, and examples thereof include air, nitrogen, carbon dioxide and the like. Among these, air is preferable from the viewpoint of economy.

The foamed sheet of the present invention can be obtained through a step (Step B) in which a resin composition (for example, an emulsion resin composition) foamed by the above method is applied onto a substrate and dried. The substrate is not particularly limited, and examples thereof include a peeled plastic film (such as a peeled polyethylene terephthalate film), a plastic film (such as a polyethylene terephthalate film), and a heat conductive layer (a heat conductive layer described later). It is done. When coating is performed using the heat conductive layer as a base material, the adhesion between the foam layer and the heat conductive layer can be improved, and the efficiency of the drying process when the foam layer is produced can also be improved.

In the step B, a general method can be adopted as a coating method and a drying method. Step B is a pre-drying step B1 in which the bubble-containing resin composition (eg, emulsion resin composition) applied on the substrate is dried at 50 ° C. or higher and lower than 125 ° C., and then further dried at 125 ° C. or higher and 200 ° C. or lower. It is preferable that this drying process B2 is included.

By providing the preliminary drying step B1 and the main drying step B2, it is possible to prevent the bubbles from being coalesced and the bubbles to burst due to a rapid temperature rise. In particular, in the foam sheet having a small thickness, bubbles are united or ruptured due to a rapid rise in temperature, and therefore, the provision of the preliminary drying step B1 is significant. The temperature in the preliminary drying step B1 is preferably 50 ° C. or higher and 100 ° C. or lower. The time of the preliminary drying step B1 is, for example, 0.5 minutes to 30 minutes, preferably 1 minute to 15 minutes. Moreover, the temperature in this drying process B2 becomes like this. Preferably they are 130 degreeC or more and 180 degrees C or less, More preferably, they are 130 degreeC or more and 160 degrees C or less. The time of the main drying step B2 is, for example, 0.5 minutes to 30 minutes, preferably 1 minute to 15 minutes.

The average cell diameter, maximum cell diameter, and minimum cell diameter of the foam can be adjusted by adjusting the type and amount of the surfactant, and by adjusting the stirring speed and stirring time during mechanical stirring. A foam sheet having a range of 10 to 150 μm can be obtained.

The apparent density of the foam is an apparent density of 0.2 to 0.7 g / cm ³ by adjusting the amount of gas (gas) component taken into the resin composition (for example, emulsion resin composition) during mechanical stirring. A foam sheet having a density can be obtained.

The foamed sheet of the present invention may have a pressure-sensitive adhesive layer (adhesive layer) on one or both sides (particularly, a surface other than the low tack surface) of the foam. It does not specifically limit as an adhesive which comprises an adhesive layer, For example, any of an acrylic adhesive, a rubber adhesive, a silicone adhesive, etc. may be sufficient. Moreover, when providing an adhesive layer, you may laminate | stack the release liner which protects an adhesive layer until the time of use on the surface.

The foamed sheet of the present invention may be distributed on the market as a wound body (rolled material) wound in a roll shape.

The foamed sheet of the present invention is excellent in impact absorption even if the thickness is small. Therefore, for example, in an electrical / electronic device, various members or parts (for example, optical members) are used for attaching (attaching) a predetermined part (for example, a housing) to a member for electrical / electronic devices, In particular, it is useful as an impact absorbing sheet.

As an optical member that can be attached (attached) using the foam sheet of the present invention, for example, an image display member attached to an image display device such as a liquid crystal display, an electroluminescence display, a plasma display (particularly, a small image display). Members), display members such as touch panels attached to mobile communication devices such as so-called “mobile phones”, “smartphones” and “portable information terminals”, cameras and lenses (particularly small cameras and lenses), etc. Can be mentioned.

The electrical / electronic device of the present invention uses the foam sheet of the present invention. Such an electric / electronic device is, for example, an electric / electronic device provided with a display member, in which the foamed sheet is sandwiched between a housing of the electric or electronic device and the display member. Electric and electronic equipment having Examples of the electric / electronic devices include mobile communication devices such as so-called “mobile phones”, “smartphones”, and “portable information terminals”.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, “%” representing the content means% by weight. In addition, all the compounding parts (parts by weight) are values in terms of solid content (non-volatile content).

Example 1
100 parts by weight of acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)), fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, (Solid content 33%) (surfactant A) 2 parts by weight, carboxybetaine type amphoteric surfactant (“Amogen CB-H”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (surfactant B) 2 parts by weight, oxazoline-based crosslinking 10 parts by weight of agent (Epocross WS-500, manufactured by Nippon Shokubai Co., Ltd., solid content 39%), 1 part by weight of pigment (carbon black) (NAF-5091, manufactured by Daiichi Seika Kogyo Co., Ltd.), polyacrylic acid thickening agent 0.6 parts by weight of an agent (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight), solid content 28.7%) disperse ("Robomix It was caused blowing and stir-mixed in a PRIMIX Co., Ltd.). This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. It was dried for 4.5 minutes to obtain an open cell foam (foamed sheet) having a thickness of 130 μm, an apparent density of 0.28 g / cm ³ , and an initial elastic modulus of 1.15 N / mm ² .

Example 2
100 parts by weight of acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)), fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, (Solid content 33%) (surfactant A) 2 parts by weight, carboxybetaine type amphoteric surfactant (“Amogen CB-H”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (surfactant B) 2 parts by weight, oxazoline-based crosslinking 4 parts by weight of agent (Epocross WS-500, manufactured by Nippon Shokubai Co., Ltd., solid content 39%), 1 part by weight of pigment (carbon black) (NAF-5091, manufactured by Daiichi Seika Kogyo Co., Ltd.), polyacrylic acid thickening agent Agent (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight), solid content 28.7%) 0.6 parts by weight, aluminum hydroxide ("Heidi" Ito H42 "was caused blowing and stirring and mixing in the Showa Denko Co., Ltd.) 40 parts by weight of the disperser (" Made in ROBOMIX "PRIMIX Co., Ltd.). This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. It was dried for 4.5 minutes to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, an apparent density of 0.30 g / cm ³ , and an initial elastic modulus of 0.82 N / mm ² .

Example 3
100 parts by weight of acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)), fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, (Solid content 33%) (surfactant A) 2 parts by weight, carboxybetaine type amphoteric surfactant (“Amogen CB-H”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (surfactant B) 2 parts by weight, oxazoline-based crosslinking 4 parts by weight of agent (Epocross WS-500, manufactured by Nippon Shokubai Co., Ltd., solid content 39%), 1 part by weight of pigment (carbon black) (NAF-5091, manufactured by Daiichi Seika Kogyo Co., Ltd.), polyacrylic acid thickening agent Agent (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight), solid content 28.7%) 0.6 parts by weight, potassium titanate fiber ("Te Mo D "manufactured by Otsuka Chemical Co., Ltd.) was caused foaming by stirring and mixing at 30 parts by weight disper (" manufactured ROBOMIX "PRIMIX Corporation). This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. It was dried for 4.5 minutes to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, an apparent density of 0.34 g / cm ³ , and an initial elastic modulus of 2.37 N / mm ² .

Example 4
Acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)) 100 parts by weight, styrene emulsion solution (solid content 53%, Boncoat SK-105 -E ", manufactured by DIC) 20 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid content 33%) (surfactant A) 2 parts by weight, carboxybetaine type amphoteric surfactant ( "Amorgen CB-H" (Daiichi Kogyo Seiyaku Co., Ltd.) (Surfactant B) 2 parts by weight, Oxazoline-based crosslinking agent ("Epocross WS-500" Nippon Shokubai Co., Ltd., solid content 39%) 4 parts by weight, pigment (Carbon black) ("NAF-5091" manufactured by Dainichi Seika Kogyo Co., Ltd.) 1 part by weight, polyacrylic acid thickener (ethyl acrylate-acrylic 0.6 parts by weight of a phosphoric acid copolymer (acrylic acid 20% by weight, solid content 28.7%) was stirred and mixed with a disper (“Robomix” Primix) to make foam. This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. After drying for 4.5 minutes, a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, an apparent density of 0.29 g / cm ³ , and an initial elastic modulus of 3.64 N / mm ² was obtained.

Example 5
100 parts by weight of acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)), fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, (Solid content 33%) (surfactant A) 2 parts by weight, carboxybetaine type amphoteric surfactant (“Amogen CB-H”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (surfactant B) 2 parts by weight, oxazoline-based crosslinking 4 parts by weight (“Epocross WS-500” manufactured by Nippon Shokubai Co., Ltd., solid content 39%), 1 part by weight pigment (carbon black) (“NAF-5091” manufactured by Dainichi Seikagaku), silica (average particle size 5 μm) , BET specific surface area 80 ~ 130m ² / g, an apparent specific gravity of 0.26 ~ 0.32g / cm ³⁾ 25 parts by weight of a disper ( "ROBOMIX" Priming It was caused blowing and stirring and mixing vinegar Co., Ltd.). This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. It was dried for 4.5 minutes to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, an apparent density of 0.30 g / cm ³ and an initial elastic modulus of 0.53 N / mm ² .

Comparative Example 1
100 parts by weight of acrylic emulsion solution (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)), fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, (Solid content 33%) (surfactant A) 1.5 parts by weight, carboxybetaine type amphoteric surfactant (“Amogen CB-H”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) (surfactant B) 1 part by weight, oxazoline -Based crosslinking agent (Epocross WS-500, manufactured by Nippon Shokubai Co., Ltd., solid content 39%) 0.35 parts by weight, polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight) , 0.78 parts by weight of solid content 28.7%) and 0.5 parts by weight of benzotriazole rust inhibitor (“SEETEC BT-NA”, manufactured by Cypro Kasei Co., Ltd.) The mixture was stirred and mixed with a spar (“Robomix” manufactured by Primics) to generate foam. This foamed composition was applied onto a release-treated PET (polyethylene terephthalate) film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 70 ° C. for 4.5 minutes, and 140 ° C. Dried for 4.5 minutes, foamed with an open cell structure having a thickness of 130 μm, an apparent density of 0.33 g / cm ³ , an initial elastic modulus of 0.32 N / mm ² , a maximum cell diameter of 110 μm, a minimum cell diameter of 20 μm, and an average cell diameter of 45 μm. A body (foamed sheet) was obtained.

Comparative Example 2
A commercially available polyethylene-based foam (“Bora La XLIM WF01”, manufactured by Sekisui Chemical Co., Ltd.) was used as the foam (foamed sheet). The foam had a thickness of 100 μm, an apparent density of 0.31 g / cm ³ , and an initial elastic modulus of 13.13 N / mm ² .

<Evaluation>
The following evaluation was performed about the foam (foamed sheet) obtained by the Example and the comparative example. The results are shown in Table 1. In Table 1, the part (part by weight) of each component in each example and comparative example is [in terms of solid content (nonvolatile content)]. “Em” indicates an emulsion.

(Average cell diameter)
An average cell diameter (μm) was obtained by capturing an enlarged image of the foam cross section with a low vacuum scanning electron microscope (“S-3400N scanning electron microscope” manufactured by Hitachi High-Tech Science Systems) and analyzing the image. The number of analyzed cells is 20. In the same manner, the minimum cell diameter (μm) and the maximum cell diameter (μm) of the foam were determined.

(Apparent density)
A foam (foamed sheet) is punched with a 100 mm × 100 mm punching blade mold, and the dimensions of the punched sample are measured. Further, the thickness is measured with a 1/100 dial gauge having a measurement terminal diameter (φ) of 20 mm. The volume of the foam was calculated from these values.
Next, the weight of the foam is measured with an upper pan balance having a minimum scale of 0.01 g or more. From these values, the apparent density (g / cm ³ ) of the foam was calculated.

(Dynamic viscoelasticity)
A temperature dispersibility test was performed at an angular frequency of 1 rad / s in a film tension measurement mode of a viscoelasticity measurement apparatus (“ARES2KFRTN1-FCO” manufactured by TA Instruments Japan). The peak top temperature (° C.) and strength (maximum value) of loss tangent (tan δ), which is the ratio of storage elastic modulus E ′ and loss elastic modulus E ″ at that time, were measured.
The peak top temperature (° C.) of the loss tangent (tan δ) of the foam is described in the “tan δ temperature” column of Table 1, and the strength (maximum value) of the peak top is described in the “tan δ” column. In the “tan δ / apparent density” column, the value obtained by dividing the peak top strength (maximum value) by the apparent density of the foam [the peak top strength of tan δ of the material constituting the foam itself (excluding bubbles) Corresponds to (maximum value)].

(Initial elastic modulus)
The initial elastic modulus (N / mm ² ) calculated from the slope at 10% strain in a tensile test at a tensile speed of 300 mm / min in a 23 ° C. environment was evaluated.

(Shock absorption test)
An impact absorption test was performed using the above-described pendulum impact tester (impact test apparatus) (see FIGS. 1 and 2). The foamed sheets (sample size: 20 mm × 20 mm) obtained in the examples and comparative examples were subjected to an impact test under low impact conditions in which a 28 g iron ball was tilted by 40 °, and the impact absorption rate (%) was obtained.
Next, the impact absorption rate (%) was divided by the thickness (μm) of the foamed sheet to determine the impact absorption rate R per unit thickness.

As shown in Table 1, in the foamed sheets of the examples, the R was 0.20 or more, and excellent shock absorption was exhibited. On the other hand, in the foamed sheet of Comparative Example 2, R was less than 0.20.

(Probe tack test)
A tacking tester (“TAC-2”, manufactured by Resca) was used. In a 23 ° C environment, when a SUS304 cylindrical probe having a diameter of 5 mm is pressed from the top of the foam sheet under the conditions of an indentation speed of 30 mm / min, an indentation load of 100 gf, and a press time of 1 second, and peeled off at a delamination speed of 30 mm / min. The maximum load value (kN / m ² ) was measured.

As shown in Table 1, in the foamed sheet of the example, the maximum load value was 100 kN / m ² or less, and the tackiness was significantly lowered as compared with Comparative Example 1.

(Removability test)
The foamed sheet was cut into 100 mm × 100 mm and attached on a SUS304BA plate to obtain a measurement sample. A 1 kg roller was reciprocated once on a measurement sample placed horizontally and pressed. After crimping, the state of the foamed sheet when peeled again in the 180 ° C. direction at room temperature (23 ° C.) was measured.
(Double-circle): It does not stick to a SUS304BA board.
○: Although it sticks to the SUS304BA plate, it can be peeled off without changing the shape of the foam sheet.
X: The foamed sheet shape changes when it sticks to the SUS304BA plate and peels off.

As shown in Table 1, in the foam sheet of the example, even when peeled from the SUS304BA plate, the foam sheet returned to its original shape and could be re-applied, but Comparative Example 1 It could not be stretched or shredded to return to its original shape and re-applied.

The foamed sheet of the present invention is used, for example, in electrical / electronic devices when various members or parts (for example, optical members) are attached (attached) to a predetermined part (for example, a housing). It is useful as a member for electronic equipment, particularly as an impact absorbing sheet.

1 Pendulum type impact tester (impact tester)
2 Test piece (foamed sheet)
DESCRIPTION OF SYMBOLS 3 Holding member 4 Impact load member 5 Pressure sensor 11 Fixing jig 12 Holding jig 16 Pressure adjusting means 20 Support | pillar 21 Arm 22 End of support rod (shaft) 23 Support rod (shaft)
24 Impactor 25 Electromagnet 28 Support plate a Swing angle

Claims (19)

1. A foam sheet made of a foam having a thickness of 30 to 500 μm, an apparent density of 0.2 to 0.7 g / cm ³ and an average cell diameter of 10 to 150 μm. In the shock absorption test used, when the value R obtained by dividing the shock absorption rate (%) defined by the following formula by the thickness (μm) of the foam sheet is a weight of impactor of 28 g and a swing angle of 40 °. It is 0.20 or more, and at least one surface of the foamed sheet is a 23 ° C. environment, a SUS304 cylindrical probe having a diameter φ of 5 mm, a pressing speed of 30 mm / min, a pressing load of 100 gf, and a pressing time of 1 second. A foamed sheet having a maximum load value of 100 kN / m ² or less when pressed under the above conditions and peeled at a peeling speed of 30 mm / min.
   Impact absorption rate (%) = {(F ₀ −F ₁ ) / F ₀ } × 100
   (In the above formula, F ₀ is the impact force when the impactor collides only with the support plate, and F ₁ causes the impactor to collide with the support plate of the structure composed of the support plate and the foam sheet. Is the impact force when
2. The foam has a peak top in a range where a loss tangent (tan δ), which is a ratio of a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement, is −50 ° C. or higher and 50 ° C. or lower. The foam sheet according to claim 1.
3. The foam has a peak top in a range where a loss tangent (tan δ), which is a ratio of a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement, is −40 ° C. or higher and 40 ° C. or lower. The foam sheet according to claim 2.
4. The foam has a peak top in a range where a loss tangent (tan δ), which is a ratio of a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement, is −30 ° C. or higher and 30 ° C. or lower. The foam sheet according to claim 3.
5. The foam has a peak top in a range where a loss tangent (tan δ), which is a ratio of a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement, is −20 ° C. or higher and 20 ° C. or lower. The foam sheet according to claim 4.
6. The foamed sheet according to any one of claims 1 to 5, wherein the maximum loss loss tangent (tan δ) in the range of -50 ° C to 50 ° C is 0.2 or more.
7. The foam sheet according to any one of claims 1 to 6, wherein the foam has an initial elastic modulus of 10 N / mm ² or less in a tensile test in a 23 ° C environment at a tensile speed of 300 mm / min.
8. The foamed sheet according to claim 7, wherein the initial elastic modulus of the foam in a tensile test at a tensile speed of 300 mm / min in a 23 ° C environment is 0.1 N / mm ² or more and 10 N / mm ² or less.
9. The foamed sheet according to claim 8, wherein an initial elastic modulus of the foam in a tensile test at a tensile speed of 300 mm / min in a 23 ° C environment is 0.1 N / mm ² or more and 5 N / mm ² or less.
10. 10. The foam according to claim 1, wherein the foam is formed of at least one resin material selected from the group consisting of an acrylic polymer, rubber, urethane polymer, styrene polymer, and ethylene-vinyl acetate copolymer. The foam sheet of any one of Claims 1.
11. The foam sheet according to any one of claims 1 to 10, wherein the foam contains a crosslinking agent.
12. The foam sheet according to any one of claims 1 to 11, wherein the foam contains a filler.
13. The foam sheet according to any one of claims 1 to 12, wherein the foam is formed of an emulsion resin composition.
14. The foam sheet according to any one of claims 1 to 13, wherein the foam is formed through a step A of mechanically foaming the resin composition.
15. The foamed sheet according to claim 14, wherein the foam is further formed through a step B in which a mechanically foamed emulsion resin composition is coated on a substrate and dried.
16. The foam sheet according to any one of claims 1 to 15, which has an adhesive layer on one side or both sides of the foam.
17. The foam sheet according to any one of claims 1 to 16, which is used as an impact absorbing sheet for electrical and electronic equipment.
18. An electric / electronic device in which the foam sheet according to any one of claims 1 to 17 is used.
19. An electric / electronic device comprising a display member, wherein the foamed sheet according to any one of claims 1 to 17 is sandwiched between a housing of the electric or electronic device and the display member. The electric / electronic device according to claim 18.

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