JP2015110781A - Foam sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a foam sheet capable of exerting excellent impact absorption properties even if the thickness is very small.SOLUTION: A foam sheet is configured with a foam body with thickness of 30-500 μm, density of 0.2-0.4 g/cmand an average cell diameter of 10-150 μm. In the foam body, a loss tangent (tanδ) being a ratio of storage modulus and loss modulus at angular frequency 1 rad/s by dynamic viscoelasticity measurement has a peak top within a range of -30°C or higher and 30°C or lower. The form body is obtained through a step A of mechanically foaming an emulsion resin composition. The maximum value of the loss tangent (tanδ) in the foam body in a range of -30°C or higher and 30°C or lower is preferably 0.2 or higher.

Description

  The present invention relates to a foam sheet having excellent shock absorption even when 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 a density of 0.3 to 0.5 g / cm ³ , and an electric / electronic product made of a foamed structure having an average cell diameter of 1 to 500 μm. Equipment sealing materials (see Patent Document 2) and the like are used.

JP 2001-100216 A JP 2002-309198 A

  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. Due to the further thinning of electronic devices in recent years, it has become desirable to further reduce the thickness of members such as the shock absorbing sheet used therein, and the adhesiveness at the time of lamination of the shock absorbing sheet and other members is increased. There is a demand for thinning of the adhesive layer or no adhesive layer.

  Furthermore, in recent years, with the spread of touch-panel-equipped devices such as smartphones and mobile terminals, there are increasing opportunities for users to touch and push the screen using a jig (stylus) such as a finger or a touch pen. These touch panel-equipped devices are becoming increasingly thinner, and members (for example, a display panel represented by an LCD panel and a touch panel for knowing a touched place) are also becoming thinner. . Therefore, when the user presses the screen, the display panel and the touch panel are very easy to bend (bend). When the bending occurs, the display panel may interfere with other members (for example, a circuit board, a battery, etc.) inside the bending and display unevenness (a ripple-like bleeding pattern) may occur on the display panel. As a cause of the occurrence of display unevenness, it is considered that the display panel is pressed against other members in the interior, stress is applied to the display panel, and the alignment of liquid crystal molecules is disturbed.

  In response to the problem of display unevenness (ripple blurring pattern) that may occur due to stress applied to the display panel, even if a gap is provided around the display panel and the display panel is deformed, the stress is applied to the display panel. It has been proposed to make it difficult to apply, to change the design of the display panel and touch panel to make it thicker and hard to bend, and to increase the rigidity of the housing of the device with touch panel to make it difficult to deform. . However, there is an adverse effect that the touch panel-equipped device becomes thicker and heavier. For this reason, touch-panel-equipped devices are required to solve the problem of display unevenness (ripple blurring pattern) that may occur due to stress applied to the display panel, while responding to the thinning that is progressing remarkably. ing.

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 foamed sheet that can prevent misalignment without laminating an adhesive layer when laminating other members 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.
Still another object of the present invention is to display excellent shock absorption even when the thickness is very small, and to display unevenness in the display unit due to a user's touch operation when used in a device equipped with a touch panel. It is in providing the foam sheet which can suppress generation | occurrence | production of highly.
Still another object of the present invention is to provide a touch panel-equipped device that is not easily damaged by an impact at the time of dropping and that highly suppresses occurrence of display unevenness in a display unit due to a user's touch operation.

  As a result of intensive studies to achieve the above object, the inventors of the present invention have a low loss and a loss tangent that 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. When the foam sheet is composed of a foam having a peak top in the range of (tan δ) of −30 ° C. or higher and 30 ° C. or lower, even if it is a thin thickness of 30 to 500 μm, the shock absorption is remarkably excellent. Electrical / electronic devices using foam sheets are less likely to cause damage to the display device due to impact, etc. even when dropped on the ground, and the impact absorption rate is high even for weak impacts. When used, it has been found that even when the touch-panel-equipped device is thin, it is possible to highly suppress the occurrence of display unevenness in the display unit accompanying the user's touch operation. Was completed.

That is, the present invention has a thickness of 30 to 500 μm, a density of 0.2 to 0.4 g / cm ³ , an average cell diameter of 10 to 150 μm, and an angular frequency of 1 rad / s in dynamic viscoelasticity measurement. Loss tangent (tan δ), which is the ratio of storage elastic modulus and loss elastic modulus, has a peak top in the range of −30 ° C. or higher and 30 ° C. or lower, and is formed through Step A for mechanically foaming the emulsion resin composition. A foam sheet made of a foam is provided.

  In this foam sheet, it is preferable that the maximum value of the loss tangent (tan δ) in the foam in the range of −30 ° C. to 30 ° C. is 0.2 or more.

Moreover, it is preferable that 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 1 N / mm ² or less.

Furthermore, 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 of impactor weight. When the swing angle is 30 °, it is preferably 0.15 or more.
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 shear adhesive strength (23 ° C., tensile speed 50 mm / min) of at least one surface of the foam to the SUS304BA plate is preferably 0.5 N / 100 mm ² or more.

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

  The foam may be further formed through a step B in which a mechanically foamed emulsion resin composition is coated on a substrate and dried. The step B includes a preliminary drying step B1 for drying the bubble-containing emulsion resin composition coated on the substrate at 50 ° C. or higher and lower than 125 ° C., and then a main drying step B2 for further drying at 125 ° C. or higher and 200 ° C. or lower. You may go out.

  The foam sheet may have an adhesive layer on one or both sides of the foam.

  The foamed sheet can be used as an impact absorbing sheet for electric / electronic devices. Further, the foam sheet may be a foam sheet used in a touch panel mounted device.

  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.

  The present invention further provides a touch panel mounted device in which the foam sheet is used. In this touch panel mounting apparatus, it has the said foam sheet, a display panel, and a touch panel, and the said foam sheet may be arrange | positioned in the space of the back side of the said display panel.

The foamed sheet of the present invention has a low density and 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 more and 30 ° C. Since it is comprised with the foam which has a peak top in the following ranges, even if thickness is thin, it is excellent in shock absorption. In addition, those having a shear adhesive strength of at least 0.5 N / 100 mm ² on the SUS304BA plate on at least one surface of the foam have an adhesive layer when laminating other members (such as a thermoelectric layer). Misalignment can be prevented without doing so. Therefore, 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.
In addition, since the foam sheet of the present invention has a low compression load, even when the display panel or the touch panel is bent or deformed due to a user's touch operation when used in a touch panel mounted device, The generated force can be dispersed and absorbed effectively. Therefore, the foamed sheet of the present invention can highly suppress the occurrence of display unevenness (ripple-like blurring pattern) in the display unit that may be caused by stress applied to the display panel. For this reason, the foam sheet of this invention can be used suitably for a touchscreen mounting apparatus.

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). It is the upper surface schematic of a stress relaxation tester. It is a schematic sectional drawing of a stress relaxation tester. 3 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Example 1. 6 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Example 2. 6 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Example 3. 10 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Example 4. 10 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Example 5. 6 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Comparative Example 1. 10 is a pressure image of pressure acting on pressure-sensitive paper in a confirmation test for occurrence of display unevenness in Comparative Example 3.

The foamed sheet of the present invention has a thickness of 30 to 500 μm, a density of 0.2 to 0.4 g / cm ³ , and a storage elastic modulus and a loss elastic modulus at an angular frequency of 1 rad / s in dynamic viscoelasticity measurement. The loss tangent (tan δ), which is the ratio of the above, is made of a foam having a peak top in the range of −30 ° C. to 30 ° C. Therefore, it has a desired shock absorption. In the present specification, the density of the foam means “apparent density”.

  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 the thickness of 30 to 500 μm.

The density of the foam constituting the foam sheet of the present invention is 0.2 to 0.4 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 0.39 g / cm ³ , more preferably 0.38 g / cm ³ , still more preferably 0.00. 35 g / cm ³ . When the density of the foam is 0.2 g / cm ³ or more, the strength can be maintained, and when it is 0.4 g / cm ³ or less, high impact absorbability is exhibited. Further, when the 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, for example, 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, more excellent impact absorbability is exhibited. Moreover, when the average cell diameter is 100 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.

  In the present invention, from the viewpoint of impact absorption, 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. 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 peak top of loss tangent (tan δ), which is the ratio of storage elastic modulus and loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the foam, is in the range of −30 ° C. to 30 ° C. The lower limit of the temperature range where the peak tangent of the loss tangent exists is preferably −25 ° C., more preferably −20 ° C., further preferably −10 ° C., and the upper limit is preferably 20 ° C., more preferably 10 ° C. It is. 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 −30 ° C. or higher, excellent compression recoverability is exhibited. Moreover, when the peak temperature is 30 ° 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 −30 ° C. or higher and 30 ° 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 the peak top strength of the loss tangent (tan δ) greatly contribute to the impact absorption of the foam. When the peak top of loss tangent (tan δ), which is the ratio of storage elastic modulus and loss elastic modulus at an angular frequency of 1 rad / s in the dynamic viscoelasticity measurement of the foam, is in the range of −30 ° C. or higher and 30 ° C. or lower, 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 −30 ° C. or higher and 30 ° C. or lower 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 30 ° 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. .

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 5 N / mm ² or less, more preferably 3 N / mm ^2. Hereinafter, it is more preferably 1 N / mm ² or less. The lower limit value of the initial elastic modulus is, for example, 0.1 N / mm ² .

  As a foam which comprises the foam sheet of this invention, if it has the said characteristic, the composition, bubble structure, etc. will not be restrict | limited in particular. 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 foamed sheet of the present invention has excellent impact absorbability while being thin. In addition, it has high shock absorption even for very weak shocks, and exhibits excellent shock absorption regardless of the magnitude of the shock. For example, in an impact absorption test using a pendulum type impact tester, the impact absorption rate (%) defined by the following formula is divided by the thickness (μm) of the foamed sheet, and the impact absorption rate R per unit thickness is calculated. Ask.
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

  When the impactor weight is 28 g and the swing angle is 30 ° (hereinafter, sometimes referred to as “low impact condition (a)”), R is R1, when the impactor weight is 28 g and the swing angle is 40 ° (hereinafter referred to as “low impact condition (a)”) , R may be referred to as “low impact condition (b)”), and R when the weight of the impactor is 96 g and the swing angle is 47 ° (hereinafter sometimes referred to as “high impact condition”). When R3 is set, in the foamed sheet of the present invention, R1 is, for example, 0.15 or more, R2 is, for example, 0.15 or more, and R3 is, for example, 0.10 or more. The upper limit value of R (R1, R2, R3) is, for example, about 0.5.

  The impact absorption rate varies depending on the thickness of the foamed sheet and the like, but is usually 5 to 80%, the lower limit is preferably 7%, and the upper limit is preferably 60%. The impact absorption rate (%) in the case of the high impact condition is preferably 14% or more, more preferably 15% or more. The impact absorption rate (%) in the low impact condition (a) is preferably 18% or more, and more preferably 25% or more.

  Moreover, in this invention, as mentioned above, it has the outstanding shock absorption property irrespective of the magnitude | size of an impact. For example, in the foam sheet of the present invention, the sum of R1 and R3 (R1 + R3) is usually 0.25 or more (for example, 0.25 to 1.0), preferably 0.28 or more (for example, 0.28 to 1.0), and the ratio of R1 and R3 (R1 / R3) is usually in the range of 0.5 to 3.0, preferably 0.8 to 2.8. In the foam sheet of the present invention, the difference between R1 and R3 (R1-R3) is usually within ± 0.25, preferably within ± 0.21. Further, the sum of R1 and R2 (R1 + R2) is usually 0.40 or more (for example, 0.40 to 1.0), preferably 0.45 or more (for example, 0.45 to 1.0), and The ratio of R1 and R2 (R1 / R2) is usually in the range of 0.5 to 2.5, preferably 0.8 to 2.0.

  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). Further, the angle at which the impactor 24 is lifted by the arm 21 (the swing angle a in FIG. 1) is about 30 ° to 47 ° (in the embodiment, measured under three conditions of 30 °, 40 °, and 47 °). . The weight of the steel ball (iron ball) is about 20 to 100 g. In Examples described later, (1) steel ball (iron ball) weight 28 g, swing angle 30 °, (2) steel ball (iron ball) weight 28 g, swing angle 40 °, (3) steel ball ( The measurement is performed under three conditions: a weight of 96 g of iron balls and a swing angle of 47 °.

  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 The impact test apparatus is the same apparatus as that of Example 1 of JP-A-2006-47277.

  When the thickness of the foam sheet is large to some extent, the impact absorption can be adjusted by selecting the average cell diameter, density, etc., but when the thickness of the foam sheet is very small (for example, a thickness of 30 to 500 μm). ), It is not possible to absorb the shock sufficiently 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. In the present invention, as described above, 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 −30 ° C. or higher. Since it exists in the range of 30 degrees C or less, even after a bubble collapses, the constituent material of a foam exhibits the function which buffers an impact.

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 −30 ° C. to 30 ° C. The lower limit of the temperature range where the peak tangent of the loss tangent exists is preferably −25 ° C., more preferably −20 ° C., further preferably −10 ° C., and the upper limit is preferably 20 ° C., more preferably 10 ° C. It is. 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. Peak top strength of loss tangent (tan δ) in the range of −30 ° C. or higher and 30 ° C. or lower of the resin composition (solid) (this value is the loss tangent in the range of −30 ° C. or higher and lower than 30 ° C. of the foam) The peak top strength of (tan δ) is equivalent to a value obtained by dividing the peak top strength by the foam density (g / cm ³ ). For example, the peak top strength of the loss tangent (tan δ) in the range of −30 ° C. to 30 ° C. of the resin composition (solid material) is preferably 0.9 (g / cm ³ ) ⁻¹ or more, The upper limit is about 3, for example.

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 polymers, rubbers, urethane polymers, and ethylene-vinyl acetate copolymers. Among these, acrylic polymers, rubbers, and urethane polymers 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.

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

  As the acrylic polymer, an acrylic polymer formed by using 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 is preferable. 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 −30 ° C. or higher and 30 ° C. or lower 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). In addition, Tg of the homopolymer of the monomer which is not described in the said literature says the value (refer Unexamined-Japanese-Patent No. 2007-51271) obtained by the following measuring methods, for example. 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. Then, 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 of 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 the 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., and more preferably 50 to 200 ° C.

  Examples of monomers having a Tg of −10 ° C. or higher as homopolymers described above include (meth) acrylonitrile; amide group-containing monomers such as (meth) acrylamide and N-hydroxyethyl (meth) acrylamide; (meth) acrylic acid; methacrylic acid (Meth) acrylic acid alkyl ester having a homopolymer such as methyl or ethyl methacrylate having a Tg of −10 ° C. or higher; (meth) acrylic acid isobornyl; heterocyclic ring-containing vinyl monomer 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 Tg of −10 ° C. or higher for the homopolymer, 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 homopolymer 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 the homopolymer of −10 ° C. or higher with respect to all monomer components (total monomer component) forming the acrylic polymer is, for example, 2 to 30% by weight, 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.

  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.

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

  For example, an optional surfactant may be included for reducing the bubble diameter and stabilizing the foamed 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 intensity | strength of a foam, heat resistance, and moisture resistance, arbitrary crosslinking agents 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 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. 0.01 parts by weight and the upper limit is preferably 9 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 [solid content (nonvolatile content)] of the thickener 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.

  Moreover, arbitrary rust preventives may be included for the corrosion prevention of the metal member adjacent to a 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 an alkyl group having 1 to 6 carbon atoms (preferably 1 to 3 carbon atoms) such as a methyl group, an ethyl group, a propyl group, an isopropyl group, and a butyl group; a methoxy group, an ethoxy group, and 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— Mono or di (C _1-10 alkyl) amino-C _1-6 alkyl group such as bis (2-ethylhexyl) aminomethyl group; mercapto group; carbon number 1 such as methoxycarbonyl group, ethoxycarbonyl group Carboxyl group; carboxy-C _1-6 alkyl group such as carboxymethyl group; carboxy-C _1-6 alkylthio group such as 2-carboxyethylthio 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, a compound in which an azole ring forms a condensed ring with an aromatic ring such as a benzene ring is preferable. Among them, a benzotriazole compound (a compound having a benzotriazole ring), a benzothiazole compound ( A compound having a benzothiaazole 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.

  An azole ring containing compound may be used individually by 1 type, and may be used in combination of 2 or more type.

  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.

  In addition, any appropriate other component may be included within a range not impairing the impact absorbability. 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, flame retardants, and light stability. Agents, ultraviolet absorbers, colorants (such as pigments and dyes), pH adjusters, solvents (organic solvents), thermal polymerization initiators, photopolymerization initiators, and the like.

  Examples of the filler include silica, clay (mica, talc, smectite, etc.), alumina, titania, zinc oxide, tin oxide, zeolite, graphite, carbon nanotube, inorganic fiber (carbon fiber, glass fiber, etc.), and organic fiber. And metal powder (silver, copper, etc.). In addition, piezoelectric particles (such as titanium oxide), conductive particles, thermally conductive particles (such as boron nitride), and organic fillers (such as silicone powder) can be added as fillers.

  The foam sheet of the present invention can be produced by subjecting a resin composition containing a resin material (polymer) constituting a 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 to disperse a gas component such as air or nitrogen in a polymer solution and form bubbles by mechanical mixing. 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 the resin composition containing the resin material (polymer) to be subjected to foam molding, a resin solution in which the resin material is dissolved in a solvent may be used. From the viewpoint of cellularity, it is preferable to use an emulsion containing the resin material. . As an emulsion, you may blend and use 2 or more types of emulsion.

  A higher solid content concentration of the emulsion is preferable 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 this invention, the method of producing a foam through the process (process A) of foaming mechanically by foaming an emulsion resin composition is preferable. 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 the 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 includes a preliminary drying step B1 for drying the bubble-containing emulsion resin composition applied on the substrate at 50 ° C. or higher and lower than 125 ° C., and then a main drying step B2 for further drying at 125 ° C. or higher and 200 ° C. or lower. Preferably it is.

  By providing the preliminary drying step B1 and the main drying step B2, it is possible to prevent coalescence of bubbles and bursting of bubbles 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 preliminary drying process B1 is 0.5 minute-30 minutes, for example, Preferably it is 1 minute-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 this drying process B2 is 0.5 minute-30 minutes, for example, Preferably it is 1 minute-15 minutes.

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

The density of the foam can be adjusted by adjusting the amount of gas (gas) component incorporated into the emulsion resin composition during mechanical stirring to obtain a foam sheet having a density of 0.2 to 0.4 g / cm ^3. .

  The foamed sheet of the present invention may have an adhesive layer (adhesive layer) on one or both sides 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. In addition, when the foam which comprises the foamed sheet of this invention has a fine tack property, a member etc. can be fixed without providing an adhesive layer.

In the foamed sheet of the present invention, the shear adhesive force (measurement conditions: 23 ° C., tensile speed 50 mm / min) to the SUS304BA plate of at least one surface of the foam constituting the foamed sheet is 0.5 N / 100 mm ² or more. When the foamed sheet and other members (for example, a heat conductive layer) are used by being laminated, the other members are not peeled off even if the adhesive sheet is not provided on the foamed sheet, and misalignment can be prevented. The effect is obtained. Moreover, since it is not necessary to provide an adhesive layer, the thickness of the laminate of the foamed sheet and the other member can be reduced, which can contribute to further thinning of electrical / electronic devices and the like to be attached. Furthermore, the manufacturing efficiency of the laminate can be improved and the cost can be reduced. In addition, as an example of the layer structure of the laminated body of the foam sheet of this invention and other members (for example, heat conductive layer etc.), for example, other member / foam, other member / adhesive layer / foam, foam / Other members / foams, foams / adhesive layers / other members / adhesive layers / foams and the like can be mentioned.

The shear adhesive force of the foam to the SUS304BA plate can be adjusted, for example, by selecting the type of monomer constituting the resin material (polymer) constituting the foam and the composition ratio thereof. For example, as a monomer constituting the resin material (polymer such as acrylic polymer) constituting the foam, the homopolymer Tg is less than −10 ° C. (for example, −70 ° C. or more and less than −10 ° C., preferably −70 ° C. to −12 ° C., more preferably −65 ° C. to −15 ° C.) with respect to all monomer components (total amount of monomer components) constituting the resin material (polymer such as acrylic polymer) constituting the foam, For example, 70 to 98% by weight (the lower limit is preferably 75% by weight, and the upper limit is preferably 97% by weight) is used, and by appropriately selecting the type and amount of other monomers, the foam is sheared with respect to the SUS304BA plate. The adhesive force can be 0.5 N / 100 mm ² or more.

The lower limit of the shear adhesive strength of the foam to the SUS304BA plate is preferably 0.5 N / 100 mm ² , more preferably 0.7 N / 100 mm ² . Moreover, the upper limit of the shear adhesive force of the foam to the SUS304BA plate is not particularly limited, but is, for example, 100 N / 100 mm ² .

  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 foam sheet of the present invention is used for the electrical / electronic device 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”.

Moreover, since the foam sheet of the present invention has a very low density of 0.2 to 0.4 g / cm ³ and a low compressive load, when used in a touch panel mounted device, with the touch operation of the user, Even if the display panel or touch panel bends and deforms, the force generated by the deformation can be effectively dispersed and absorbed. Therefore, the foamed sheet of the present invention can highly suppress the occurrence of display unevenness (ripple-like blurring pattern) in the display unit that may be caused by stress applied to the display panel. For this reason, the foam sheet of this invention can be used suitably for a touchscreen mounting apparatus.

  The touch panel mounted device means a device having a display panel and mounted with a touch panel. The touch panel-equipped device is not particularly limited. For example, a mobile phone; a smartphone; a personal digital assistant (PDA); a tablet computer; various personal computers (computers) such as a desktop type, notebook type, and tablet type; plasma display, liquid crystal Various displays (monitors) such as displays and electroluminescence displays (organic EL displays); portable game machines; digital audio players; electronic book readers (devices for viewing electronic books, terminals dedicated to electronic books); wearable computers (wearable devices) Digital signage (electronic signage); Automatic teller machines (ATMs); Automatic ticket machines used to sell tickets, various cash vouchers, beverages, food, tobacco, magazines, newspapers, etc. Vending machines; TV receiver (TV); and the like electronic blackboard (interactive whiteboard).

  The foam mounting sheet of the present invention is used for the touch panel mounted device of the present invention. Examples of such a touch panel-mounted device include a device having the foam sheet, a display panel, and a touch panel, in which the foam sheet is disposed in a space on the back side of the display panel.

  EXAMPLES 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 (nonvolatile content).

Example 1
Synthetic rubber emulsion ("Luckstar 1570" manufactured by DIC, solid content 42.4%, NBR) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid content 33%) (surfactant A) 3.9 parts by weight, 0.74 parts by weight of urethane thickener (“Hydran Assister T10” manufactured by DIC, solid content 25.1%) is stirred and mixed with a disper (“Robomix” Primix) And foamed. This foamed composition was applied onto a release-treated PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 4.5 minutes at 70 ° C., and 4.5 minutes at 140 ° C. By drying, a foam (foamed sheet) having an open cell structure with a thickness of 150 μm, a density of 0.26 g / cm ³ , and an average cell diameter of 55 μm was obtained.

Example 2
Synthetic rubber emulsion (“Luckstar DM886” manufactured by DIC, solid content 48.6%, MBR) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid content 33%) 3.4 weight Part, 0.65 parts by weight of a urethane-based thickener (“Hydran Assister T10” manufactured by DIC, solid content 25.1%) was stirred and mixed with a disper (“Robomix” Primix) to make foam. . This foamed composition was applied onto a release-treated PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 4.5 minutes at 70 ° C., and 4.5 minutes at 140 ° C. It was dried to obtain a foam (foamed sheet) having an open cell structure having a thickness of 150 μm, a density of 0.28 g / cm ³ , and an average cell diameter of 88 μm.

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, 3 parts by weight of solid content (33%), polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight), solid content 28.7%) 1.3 parts by weight of disperse (" The mixture was stirred and mixed to produce foam. This foamed composition was applied onto a release-treated PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 4.5 minutes at 70 ° C., and 4.5 minutes at 140 ° C. It was dried to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, a density of 0.38 g / cm ³ , and an average cell diameter of 68 μm.

Example 4
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 cross-linking 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 Sipro 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. It was dried for 4.5 minutes to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, a density of 0.33 g / cm ³ , a maximum cell diameter of 110 μm, a minimum cell diameter of 20 μm, and an average cell diameter of 45 μm.

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, 3 parts by weight of solid content (33%), 2 parts by weight of oxazoline-based crosslinking agent (Epocross WS-500, manufactured by Nippon Shokubai Co., Ltd., 39% solid content), polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer) 0.78 parts by weight (20% by weight of acrylic acid), solid content 28.7%), 0.5 parts by weight of benzotriazole rust inhibitor (“SEETEC BT-NA”, manufactured by Sipro Kasei Co., Ltd.) The mixture was stirred and mixed to produce 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. It was dried for 4.5 minutes to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, a density of 0.24 g / cm ³ , a maximum cell diameter of 69.5 μm, a minimum cell diameter of 15 μm, and an average cell diameter of 34 μm.

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, 3 parts by weight of a solid content 33%), 0.78 parts by weight of a polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20% by weight), solid content 28.7%) The mixture was stirred and mixed to produce foam. This foamed composition was applied onto a release-treated PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics), 4.5 minutes at 70 ° C., and 4.5 minutes at 140 ° C. It was dried to obtain a foam (foamed sheet) having an open cell structure having a thickness of 130 μm, a density of 0.62 g / cm ³ , and an average cell diameter of 58 μm.

<Evaluation>
The following evaluation was performed about the foam (foamed sheet) obtained by the Example and the comparative example. The results are shown in Tables 1 and 2. Table 1 shows the number of parts (parts by weight) [converted to solid content (nonvolatile content)] of each component in each example and comparative example. “Em” indicates an acrylic emulsion solution.

(Average cell diameter)
An average cell diameter (μm) was determined by capturing an enlarged image of the foam cross-section with a low vacuum scanning electron microscope (“S-3400N type scanning electron microscope” manufactured by Hitachi High-Tech Science Systems) and analyzing the image. The analyzed number of bubbles is about 10-20. Moreover, the minimum cell diameter (micrometer) and the maximum cell diameter (micrometer) of the foam sheet were calculated | required by the method similar to an average cell diameter.

(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 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.
In the column of “tan δ temperature” in Table 2, the peak top temperature (° C.) of the loss tangent (tan δ) of the foam is described, and in the “tan δ maximum value” column, the strength (maximum value) of the peak top is indicated. In the column of “tan δ maximum value / density”, the value obtained by dividing the intensity (maximum value) of the peak top by the density of the foam [peak of tan δ of the material constituting the foam itself (excluding bubbles) It corresponds to the top strength (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). For the foamed sheets (sample size: 20 mm × 20 mm) obtained in the examples and comparative examples, a low impact condition (a) in which a 28 g iron ball is tilted by 30 ° and a low impact condition in which a 28 g iron ball is tilted by 40 ° An impact test was performed under three conditions of (b) and a high impact condition in which a 96 g iron ball was tilted by 47 °, and the impact absorption rate (%) under each condition was determined.
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.

(Shearing adhesive strength)
The foamed sheet was cut into 25 mm × 25 mm, and SUS304BA plates were attached to both surfaces of the foamed material to obtain measurement samples. A 5 kg roller was reciprocated once and pressed against a measurement sample placed horizontally. After crimping, the sample was allowed to stand at room temperature (23 ° C.) for 30 minutes, then fixed to Tensilon so that the measurement sample was vertical, pulled at a pulling speed of 50 mm / min, and the shear adhesive force was measured. The measurement sample was measured by n = 2, and the average value was defined as shear adhesive strength (N / 100 mm ² ).

(Confirmation test for display unevenness)
A commercially available smartphone was disassembled. In this smartphone, there was a space between the lower part of the display panel and the housing, and the height of this space was 0.2 mm. This space was used as a gap for foam insertion.
Next, a sample to be evaluated was inserted into the gap for inserting the foam, and the disassembled smartphone was assembled again. In Comparative Example 2, nothing was inserted into the gap for foam insertion.
I started the smartphone and pressed the center of the screen with my thumb. The force at this time was approximately 12N. Then, whether or not display unevenness occurs was confirmed and evaluated according to the following criteria.
A: Display unevenness does not occur.
B: Although display unevenness occurs, the user can view without any problem and can operate the smartphone without any problem.
C: Due to display unevenness, the visibility is adversely affected, and the user feels uncomfortable with the operation of the smartphone.

(Stress relaxation test)
Using the stress relaxation tester shown below, a sample to be evaluated was inserted into the gap for foam insertion, and the stress relaxation property was evaluated.

The stress relaxation tester is shown in FIG. 3 and FIG. FIG. 3 is a schematic top view of the stress relaxation tester, and FIG. 4 is a schematic cross-sectional view of the stress relaxation tester (cross-sectional view taken along line AA ′ in FIG. 3). 3 and 4, 6 is a stress relaxation tester, 61 is a presser (indenter), 62 is a window lens, 63 is a liquid crystal display (LCD), 64 is a pressure sensitive paper, 65 Is a substrate (base plate), 661 and 662 are adhesive tapes, and 67 is a housing. In addition, a region 68 surrounded by a dotted line indicates a void for inserting a form. The stress relaxation testing machine 6 has a pressure sensitive paper 64 on the back side of the liquid crystal display 63 and a window lens 62 on the front side of the liquid crystal display 63. In the stress relaxation testing machine 6, a casing 67 is disposed on the substrate 65 via an adhesive tape 662, and the window lens 62 is fixed on the casing via the adhesive tape 661. There is an internal space partitioned by 661, an adhesive tape 662, a housing 67 and a substrate 68, and a laminated body of a liquid crystal display 63 and a pressure sensitive paper 64 is disposed in this internal space. Further, a foam insertion gap 68 is provided in the internal space below the pressure sensitive paper 64.
The window lens in this stress relaxation tester can be regarded as a touch panel.

The pusher 61 has a spherical tip shape with a diameter of 13 mm. As the adhesive tape 661, a double-sided adhesive tape (thickness 300 μm, trade name “double-sided adhesive tape HJ-90130B”, manufactured by Nitto Denko Corporation) was used. As the adhesive tape 662, a double-sided adhesive tape (thickness 30 μm, trade name “Double-sided adhesive tape No. 5603”, manufactured by Nitto Denko Corporation) was used. The thickness of the touch panel 62 is 0.8 mm. The thickness of the liquid crystal display 63 is 1.7 mm. The thickness of the substrate 65 is 5 mm. As the pressure-sensitive paper 64, a stress measurement film (trade name “Prescale” (two sheets, for fine pressure (4LW), manufactured by FUJIFILM Corporation, sheet having a surface on which a colored portion is colored, thickness 0.16 mm) was used. .
Further, in the stress relaxation tester 6, the casing 67 can be replaced, and the height of the foam insertion gap 68 can be adjusted by adjusting the height of the casing 67.

The height of the gap for foam insertion was set to 200 μm, and the sample to be evaluated and the PET film (thickness: 38 μm, trade name “MRF # 38”, manufactured by Mitsubishi Plastics) for the foam insertion gap of the stress relaxation tester Is inserted so that the total thickness is about 200 μm, a 12N load is applied to the center of the touch panel using a presser using a stress relaxation tester, and the discoloration of the pressure sensitive paper is confirmed, thereby reducing the stress. Sex was evaluated.
In Comparative Example 3, five PET films (thickness: 38 μm, trade name “MRF # 38”, manufactured by Mitsubishi Plastics Co., Ltd.) were laminated in the gap for foam insertion.

  The evaluation results of the stress relaxation test are shown in FIGS. 5 to 11 are pressure images obtained by digitally processing the discoloration of the pressure-sensitive papers of Examples 1 to 5 and Comparative Examples 1 and 3, respectively, and visualizing the pressure acting on the pressure-sensitive papers.

  As shown in Table 2, in the foam sheets of the examples, the R is 0.15 or more in the low impact conditions (a) and (b), 0.10 or more in the high impact conditions, and high in the low impact conditions. In addition to exhibiting impact absorption, it exhibited excellent impact absorption regardless of the magnitude of impact. On the other hand, the foam sheet of the comparative example did not satisfy all the criteria at the same time. Moreover, the foam sheet of the Example showed a high value of the shear adhesive strength to the SUS304BA plate. Therefore, when using by laminating | stacking with another member, even if it does not provide an adhesive layer in the said foaming sheet, the said other member does not peel and position shift can be prevented.

  As shown in FIGS. 5 to 11, the foamed sheet of the example has a smaller pressure acting on the pressure-sensitive paper and is excellent in stress relaxation than the comparative example. Therefore, when the foam sheet of the embodiment is used for a touch panel mounted device, it is possible to highly suppress the occurrence of display unevenness in the display unit due to the user's touch operation.

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 6 Stress Relaxation Tester 61 Pusher 62 Window Lens 63 Liquid Crystal Display 64 Pressure Sensitive Paper 65 Substrate 661 Adhesive Tape 662 Adhesive Tape 67 Housing 68 Space for Foam Insertion

Claims (15)

Storage modulus and loss elasticity at a thickness of 30 to 500 μm, a density of 0.2 to 0.4 g / cm ³ , an average cell diameter of 10 to 150 μm, and an angular frequency of 1 rad / s in dynamic viscoelasticity measurement. The loss tangent (tan δ), which is the ratio of the rate, has a peak top in the range of −30 ° C. or more and 30 ° C. or less, and is composed of a foam formed through Step A for mechanically foaming the emulsion resin composition. Foam sheet.   2. The foamed sheet according to claim 1, wherein a maximum value of loss tangent (tan δ) in the foam in the range of −30 ° C. to 30 ° C. is 0.2 or more. The foamed sheet according to claim 1 or 2, 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 1 N / mm ² or less. 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. The foamed sheet according to any one of claims 1 to 3, which is 0.15 or more at an angle of 30 °.
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 sheet according to any one of claims 1 to 4, wherein at least one surface of the foam has a shear adhesive force (23 ° C, tensile speed 50 mm / min) to a SUS304BA plate of 0.5 N / 100 mm ² or more. .   6. The foam according to claim 1, wherein the foam is formed of at least one resin material selected from the group consisting of acrylic polymers, rubber, urethane polymers, and ethylene-vinyl acetate copolymers. The foam sheet according to 1.   The foam sheet according to any one of claims 1 to 6, wherein the foam is further formed through a step B in which a mechanically foamed emulsion resin composition is applied onto a substrate and dried.   The step B includes a pre-drying step B1 for drying the foam-containing emulsion resin composition applied on the substrate at 50 ° C. or higher and lower than 125 ° C., and then a main drying step B2 for further drying at 125 ° C. or higher and 200 ° C. or lower. The foam sheet according to claim 7.   The foam sheet according to any one of claims 1 to 8, which has an adhesive layer on one side or both sides of the foam.   The foam sheet according to any one of claims 1 to 9, which is used as an impact absorbing sheet for electric / electronic devices.   The foam sheet according to any one of claims 1 to 9, which is used for a touch panel-mounted device.   The electrical / electronic device in which the foam sheet of any one of Claims 1-9 is used.   An electric / electronic device provided with a display member, wherein the foam sheet according to any one of claims 1 to 9 is sandwiched between a housing of the electric or electronic device and the display member. The electric / electronic device according to claim 12.   A touch panel-mounted device in which the foam sheet according to any one of claims 1 to 9 is used.   The touch panel mounting of Claim 14 which has a foam sheet, a display panel, and a touch panel of any one of Claims 1-9, and the said foam sheet is arrange | positioned in the space of the back side of the said display panel. machine.