JP2015042708A - Expanded sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an expanded sheet which exhibits excellent shock absorbability even when the thickness thereof is very thin.SOLUTION: The expanded sheet comprises an expanded body having 30-200 μm thickness, 0.2-0.7 g/cmdensity and 10-100 μm average cell diameter. When such a state is kept that the expanded sheet is held between a supporting member and a strain detecting member, impact force is exerted on the supporting member in an impact testing device, and the strain of the strain detecting member is measured by a strain gauge, the strain restraint rate shown by the expression:(the strain restraint rate (%))={(ε-ε)/ε}×100 is 20% or higher (in which εis the maximum strain of the strain detecting member when the impact force is exerted on the supporting member in such a state that the expanded sheet is not held between the supporting member and the strain detecting member, and εis the maximum strain of the strain detecting member when the impact force is exerted on the supporting member in such a state that the expanded sheet is held between the supporting member and the strain detecting member).

Description

  The present invention relates to a foam sheet excellent in impact absorption 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 organic electroluminescence display (hereinafter referred to as “OLE display”), a plasma display, a so-called “mobile phone”, “smart phone”, or “portable information terminal” When an optical member such as an image display member, a camera, or a lens attached to a camera is fixed to a predetermined part (for example, a housing or the like), a foam material is used as an impact absorbing material.

As such a foam material, a polyethylene foam having a low foaming and an closed cell structure and a foaming ratio of about 30 times has been 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 or an electric / electronic product made of a foamed structure having an average cell diameter of 1 to 50 μm. A sealing material for equipment (see Patent Document 2) or the like is used.

JP 2001-100216 A JP 2002-309196 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. For example, as shown in FIG. 21, the image display member 102 of the smartphone 101 is configured by laminating a cover glass 103, an adhesive 104, a panel glass 105, an OLE display 106, and a foam sheet 107, and is housed in a housing 108. The The foam sheet 107 is attached to the entire back surface of the OLE display 106.

  For this reason, with the decrease in the clearance that can accommodate the foam sheet 107, the conventional foam sheet 107 does not exhibit sufficient shock absorption when the thickness is reduced. Therefore, even if the area of the image display member 102 is increased, when the smartphone 101 is dropped on the ground or the like, the impact at the time of collision is absorbed and the OLE display 106 constituting the image display member 102 is deformed. There is a need for foam sheets that prevent breakage due to corrosion.

  Then, this invention is made | formed in order to solve the problem mentioned above, and it aims at providing the foam sheet which exhibits the outstanding impact absorption, even if thickness is very thin. It is another object of the present invention to provide an electric / electronic device that is less likely to damage an image display member or the like due to an impact when dropped, even if it is reduced in size and thickness.

In order to achieve the object, the foam sheet according to claim 1 has a thickness of 30 to 200 μm, a density of 0.2 to 0.7 g / cm ³ , and an average cell diameter of 10 to 100 μm. The foam sheet is configured to be able to hold the foam sheet sandwiched between a support member and a strain detection member, and strains the strain detection member when an impact force is applied to the support member. When an impact test apparatus that measures with a gauge is used, the strain suppression rate represented by the following formula is 20% or more.
Strain suppression rate (%) = {(ε ₀ −ε ₁ ) / ε ₀ } × 100
(In the above equation, ε ₀ is the maximum strain of the strain detection member when an impact force is applied to the support member in a state where the foam sheet is not mounted between the support member and the strain detection member. And ε ₁ is the maximum strain of the strain detection member when an impact force is applied to the support member in a state where the foam sheet is mounted between the support member and the strain detection member.

  The foam sheet according to claim 2 is the foam sheet according to claim 1, wherein the impact test device holds the foam sheet in a state of being sandwiched between the support member and the strain detection member. A holding means for holding the foamed sheet at a compression rate in an arbitrary thickness direction, an impact load means for applying an impact force to the support member held by the holding means, and the support member by the impact load means. A strain detecting means for detecting strain generated in the strain detecting member by the impact force applied to the foam sheet via the strain gauge, and the strain suppression rate based on the strain detected by the strain detecting means. Output means for calculating and outputting.

  The foam sheet according to claim 3 is the foam sheet according to claim 2, wherein the holding means is fixed perpendicularly to the table so that one surface side of the strain detection member is in close contact, and the strain is detected. A fixing jig in which a concave portion is formed in the central portion so that the portion receiving the impact force of the detection member can be deformed in the load direction, and an opening is formed in the central portion and slides in the direction toward the fixing jig. The support member, which is provided so as to cover the entire surface of the foam sheet fixed to the other surface side of the strain detection unit, is opposite to the surface of the support member that is in close contact with the foam sheet. A foaming sheet which is in contact with a surface and is fixed to the table in a state where the supporting member is pressed toward the foaming sheet; and the foaming sheet by adjusting a pressing pressure with which the pressing jig presses the supporting member Thickness of Pressure adjusting means for arbitrarily setting the compression rate in the direction, and the impact load means applies an impact force to the support member that enters the opening of the pressing jig and covers the foam sheet, The strain detecting means detects strain generated by deformation of the strain detecting member into the concave portion by the strain gauge.

  The foam sheet according to claim 4 is the foam sheet according to claim 2 or claim 3, wherein the impact load means includes a pendulum-type hammer whose one end is pivotally supported, and the hammer. And a hammer holding means for lifting and holding at a predetermined angle, and the impact force is applied by swinging down the hammer held by the hammer holding means.

  Further, the foam sheet according to claim 5 is the foam sheet according to any one of claims 1 to 4, wherein the strain detection member is configured by closely contacting two plate members, and the strain gauge is Of the two plate members, the first plate member to which the foam sheet is in close contact is attached to a surface opposite to the surface to which the foam sheet is in close contact. It is characterized by being laminated in between.

  Further, the foam sheet according to claim 6 is the loss tangent (tan δ) which is a ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement in the foam sheet according to any one of claims 1 to 5. Has a maximum value in a range from 500 Hz to 50000 Hz, and the loss tangent (tan δ) from 500 Hz to 3000 Hz is 0.2 or more.

  Furthermore, a foamed sheet according to a seventh aspect is characterized in that the foamed sheet according to any one of the first to sixth aspects is used as an impact absorbing sheet for electric / electronic devices.

  An electric / electronic device according to claim 8 is characterized in that the foamed sheet according to any one of claims 1 to 7 is used.

  An electrical / electronic device according to claim 9 is the electrical / electronic device according to claim 8, wherein the electrical / electronic device includes an image display member, and the electrical / electronic device according to any one of claims 1 to 7 is provided. The foamed sheet described above has a structure sandwiched between a casing of the electric / electronic device and the image display member.

In the foam sheet according to claim 1, the foam sheet is formed of a foam having a density of 0.2 to 0.7 g / cm ³ , an average cell diameter of 10 to 100 μm, and a strain suppression rate of 20% or more. By forming the sheet, even if the thickness is as small as 30 to 200 μm, excellent shock absorption can be exhibited. Strain suppression rate is configured to be able to hold a foam sheet sandwiched between a support member and a strain detection member, and measures the strain of the strain member when an impact force is applied to the support member by an impact test. It can be obtained using an apparatus.

In the foam sheet according to claim 2, the strain of the strain detection member when the impact force is applied to the foam sheet by holding the thin foam sheet with an arbitrary holding force by an impact test device using a strain gauge. It becomes possible to measure. In addition, the maximum distortion ε ₀ of the detection member when the foam sheet is not mounted between the support member and the strain detection member, and the detection when the foam sheet is mounted between the support member and the strain detection member By measuring the maximum strain ε _{1 of the} member with a strain gauge, it becomes possible to obtain the strain suppression rate of the foamed sheet.

  Further, in the foam sheet according to claim 3, in a state where the thin foam sheet is held at a compressibility in an arbitrary thickness direction, the impact sheet is used to open the foam sheet from the opening formed in the center portion of the pressing jig. An impact force can be applied to the entire surface of the thin foam sheet through the support plate covering the entire surface. In addition, the fixing jig fixed perpendicularly to the table is in close contact with one surface side of the strain detection member, and the central portion so that the portion receiving the impact force of the strain detection member can be deformed in the load direction. A recess is formed. As a result, the strain detection member can be freely deformed by an impact force, and the strain detection means reliably ensures the maximum strain generated by the deformation of the strain detection member inside the recess by the impact force via the strain gauge. Can be detected.

  Further, in the foam sheet according to claim 4, the impact force is applied to the thin foam sheet through the support plate by swinging down the pendulum type hammer held at a predetermined angle by the hammer holding means. It becomes possible to apply a constant impact force to the entire surface of the thin foam sheet to be held. For this reason, it becomes possible to easily perform comparative studies such as the strain suppression rate for each foam sheet.

  In the foam sheet according to claim 5, of the two plate members constituting the strain detection member, the first plate member to which the foam sheet is in close contact with the surface on the opposite side of the surface to which the foam sheet is in close contact. A strain gauge is attached to the surface. Thereby, in the image display member of a smart phone, the alternative evaluation of the real machine test dropped on the ground etc. is possible in a state where the strain gauge is attached to the OLE display to which the foam sheet is adhered.

  In the foamed sheet according to claim 6, the loss tangent (tan δ), which is the ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement, has a maximum value in the range from 500 Hz to 50000 Hz, and 500 Hz By forming the foamed sheet so that the loss tangent (tan δ) at 3000 Hz is 0.2 or more, even when the thickness is as thin as 30 to 200 μm, it exhibits even better shock absorption during dropping. be able to.

  Furthermore, the foam sheet according to claim 7 is used as an impact absorbing sheet for electric and electronic devices, so that even when the electric and electronic devices are reduced in size and thickness, the impact at the time of dropping is absorbed and damage is prevented. It becomes possible to do.

  Moreover, in the electrical / electronic device according to claim 8, by using the foam sheet according to any one of claims 1 to 8, it is possible to absorb the impact at the time of dropping and suppress the breakage. Therefore, it is possible to reduce the size and thickness of the electric / electronic device.

  In the electrical / electronic device according to claim 9, the foam sheet according to any one of claims 1 to 8 is sandwiched between the casing of the electrical / electronic device and the image display member. Therefore, it is possible to absorb the impact at the time of dropping, and to prevent the image display member from being damaged, and to further reduce the size and thickness.

It is a front view which shows schematic structure of the impact test apparatus which concerns on this embodiment. It is a front view explaining the state at the time of the impact force load of an impact test apparatus. It is a figure which shows schematic structure of an impact test apparatus with a top view, a front view, a left side view, and a right side view. It is a figure which shows schematic structure of the output device part which detects distortion of the 1st board member of an impact test apparatus, and outputs a distortion suppression rate. It is a front view which shows schematic structure of the drop test apparatus which performed the drop test of the smart phone with which the test piece was mounted | worn. It is a figure which shows an example of the standard distortion characteristic at the time of not mounting | wearing with the test piece obtained by the impact test apparatus, and the real machine reference | standard distortion characteristic of the OLE display obtained by the drop test of the smart phone which is not mounting | wearing with a test piece. is there. It is an evaluation result table which shows an example of the evaluation result obtained by the Example and the comparative example. It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing the impact test apparatus with the foam sheet obtained in Example 1, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship between the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foamed sheet obtained in Example 1, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing the impact test apparatus with the foam sheet obtained in Example 2, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship of the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foam sheet obtained in Example 2, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing an impact test apparatus with the foam sheet obtained in Example 3, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship between the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foam sheet obtained in Example 3, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing the impact test apparatus with the foam sheet obtained in Example 4, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship of the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foam sheet obtained in Example 4, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing the impact test apparatus with the foam sheet obtained in the comparative example 1, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship between the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foamed sheet obtained by the comparative example 1, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows an example of the distortion characteristic at the time of mounting | wearing the impact test apparatus with the foam sheet obtained by the comparative example 2, and the actual machine distortion characteristic of the OLE display obtained by the drop test of the smart phone equipped with this foam sheet. . It is a figure which shows the relationship between the storage elastic modulus G 'by the dynamic viscoelasticity measurement of the foamed sheet obtained by the comparative example 2, loss elastic modulus G ", and loss tangent (tan-delta). It is a figure which shows the presence or absence of the fall crack of an OLE display by the actual machine drop test of an Example and a comparative example by the correlation with a distortion suppression rate and loss tangent (tan-delta). It is explanatory drawing which shows schematic structure of the image display member of the conventional smart phone.

  Hereinafter, based on one embodiment which materialized the foam sheet concerning the present invention, it explains in detail, referring to drawings. First, a schematic configuration of the impact test apparatus 1 according to the present embodiment will be described with reference to FIGS.

[Schematic configuration of impact test equipment]
As shown in FIGS. 1 to 4, the impact test apparatus 1 according to the present embodiment includes a holding member 3 as a holding unit that holds a test piece 2 formed of a foam sheet with an arbitrary holding force, and a test piece 2. An impact load member 4 serving as an impact load means for applying an impact force to the load, a strain gauge 6 for measuring the strain of the strain detection member 5 to which the test piece 2 is closely attached, and a strain of the strain detection member 5 from a signal of the strain gauge 6 And a personal computer (hereinafter referred to as “PC” hereinafter) that calculates and outputs a distortion suppression rate [%] based on the distortion data input from the distortion detector 7. 8).

  Further, the holding member 3 that holds the test piece 2 with an arbitrary holding force includes a fixing jig 11 fixed to the table 10, a strain detection member 5, the test piece 2, and a support member facing the fixing jig 11. 9, and a slidable holding jig 12 so as to be held between them. The holding jig 12 has an opening 12A formed at the center, is slidably installed on a pedestal 13 that can slide on the table 10, and can slide on the table 10 together with the pedestal 13. .

  The pedestal 13 is provided with a lever 14 for sliding the pedestal 13 and a fixing member 15 that is slid by the lever 14 and fixed to the table 10 after the temporary position is determined. Further, the pressing jig 12 is provided with a pressing pressure adjusting means 16. The pressing pressure adjusting means 16 is preferably provided with a digital gauge or the like interlocked with the pressing jig 12. The pressing pressure adjusting means 16 finely adjusts the pressing jig 12 on the pedestal 13 fixed at the temporary position with respect to the test piece 2 in the front-rear direction (the left-right direction in FIG. 1). This makes it possible to adjust the holding force acting on the test piece 2 sandwiched between the strain detection member 5 and the support member 9.

  The impact load member 4 for applying an impact force to the test piece 2 held by the holding member 3 is a shaft having one end 22 pivotally supported with respect to the column 20 and a hammer 24 on the other end side. 23 and an arm 21 for lifting and holding the hammer 24 at a predetermined angle. The arm 21 can be fixedly held at a predetermined angle. Here, the hammer 24 uses a steel ball, so that the hammer 24 can be integrally lifted at a predetermined angle by providing an electromagnet 25 at one end of the arm 21. The hammer 24 is not limited to the electromagnet 25 as long as the hammer 24 can be lifted together with the arm 21 and the hammer 24 and the arm 21 can be easily separated. Further, the hammer 24 is not limited to a spherical shape, and may be a block shape, for example.

  As shown in FIG. 4, the strain detection member 5 that is in close contact with the fixing jig 11 has two first plate members 5 </ b> A and a second plate member 5 </ b> B that are made of resin such as acrylic resin or polycarbonate. It is configured. The strain gauge 6 is a lower end portion of the surface on the fixing jig 11 side of the first member 5A to which the test piece 2 is in close contact, that is, a surface opposite to the surface to which the test piece 2 of the first member 5A is in close contact. Is stuck between the first plate member 5A and the second plate member 5B. Thereby, the strain generated by the impact force of the first plate member 5A can be detected by the strain gauge 6.

  The strain detection member 5 is in close contact with the fixing jig 11 so that the lower end thereof faces the position where the hammer 24 of the support member 9 collides. Further, on the surface of the fixing jig 11 to which the strain detection member 5 is in close contact, a portion from a lower end receiving the impact force of the strain detection member 5 to a predetermined height (for example, a height of 4 cm) is a load direction. A recessed portion 27 that is recessed by a predetermined depth (for example, a depth of about 5 mm) is formed so as to be deformable. In addition, by providing an adhesive or the like on the outer surface of the second plate member 5B of the strain detection member 5, the strain detection member 5 can be attached to the fixing jig 11, and can be reliably fixed to the fixing jig.

  The support member 9 is formed of a resin plate material such as acrylic resin or polycarbonate, is in close contact with the test piece 2 that is in close contact with the first plate member 5 </ b> A of the strain detection member 5, and is pressed by the pressing jig 12. The strain detector 7 is electrically connected to the strain gauge 6 and the PC 8, detects the strain ε of the first plate member 5 </ b> A from the signal from the strain gauge 6, and outputs it to the PC 8. The PC 8 sequentially stores the time data of the strain ε input from the strain detection unit 7, calculates the time change of the strain, the maximum value of the strain amount, the strain suppression rate described later, and the like and displays them on the liquid crystal display or the like.

  Here, the test piece 2 corresponds to the foam sheet 107 of the smartphone 101 shown in FIG. The first plate member 5A of the strain detection member 5 corresponds to the OLE display 106 of the smartphone 101 shown in FIG. The second plate member 5B of the strain detection member 5 corresponds to the cover glass 103, the adhesive 104, and the panel glass 105 of the smartphone 101 shown in FIG. The support member 9 corresponds to the housing 108 of the smartphone 101 shown in FIG.

Next, an impact test method using the impact test apparatus 1 configured as described above will be described. (Holding process)
First, without inserting the test piece 2 between the fixing jig 11 and the holding jig 12 of the holding member 3, only the strain detection member 5 and the support member 9 are inserted in a closely contacted state. Then, the pedestal 13 is slid toward the fixing jig 11 by the lever 14 and is brought into close contact with the support plate 28 to temporarily fix the pedestal 13. Next, the pedestal 13 is finely adjusted by the pressing pressure adjusting means 16, and the fixing jig 11 and the support plate 28 are tightly fixed. At this time, the memory provided in the pressing pressure adjusting means 16 is reset, and the position of the pressing jig 12 is used as a reference point.

  Next, the thickness of the test piece 2 is measured in advance. Then, the holding pressure adjusting means 16 is adjusted, the holding jig 12 is moved from the reference point by the thickness of the test piece 2, and the test piece 2 is set between the strain detection member 5 and the support member 9. At this time, when the test piece 2 formed of an impact absorbing material that relaxes and absorbs the impact during the impact action is mounted, the thickness of the test piece 2 is taken into account while checking the gauge of the pressing pressure adjusting means 16. By adjusting the pressing pressure, it is possible to fix the test piece 2 at a compression rate close to the mounting state of the actual machine. Further, by providing an adhesive or the like on a part or the entire surface of one side of the test piece 2, it can be attached to the strain detection member 5 and can be reliably fixed to the strain detection member 5.

(Shock load process)
After the test piece 2 is set, the electromagnet 25 provided at one end of the arm 21 is switched on, and the hammer 24 is fixedly held on the arm 21. Then, after the arm 21 is swung up to an arbitrary predetermined angle and fixed, the electromagnet 25 is switched off to cause the hammer 24 to collide with the support member 9 held by the holding member 3 (see FIG. 2).

(Detection process)
Further, the strain of the first plate member 5 </ b> A of the detection member 5 at this time is measured by the strain gauge 6.
Here, before inserting the test piece 2, the strain detection member 5 and the support member 9 are in close contact with each other, and are firmly fixed to the fixing jig 11, and the hammer 24 is caused to collide with the support member 9 in the same manner. In this case, the strain suppression rate [%] of the test piece 2 can be calculated by measuring the strain of the first plate member 5A of the detection member 5 and calculating the following equation (1).

Strain suppression rate (%) = {(ε ₀ −ε ₁ ) / ε ₀ } × 100 (1)

In the above equation (1), ε ₀ is the first plate member 5A when an impact force is applied to the support member 9 in a state where the test piece 2 is not mounted between the strain detection member 5 and the support member 9. Ε ₁ is the maximum strain of the first plate member 5A when an impact force is applied to the support member 9 with the test piece 2 mounted between the strain detection member 5 and the support member 9. is there.

(Output process)
Then, the signal from the strain gauge 6 is input to the strain detector 7. In the strain detector 7, the signal from the strain gauge 6 is converted into strain ε and output to the PC 8. The PC 8 sequentially stores the time data of the strain ε input from the strain detection unit 7, calculates the time change of the strain, the maximum amount of strain, the strain suppression rate, and the like, and displays them on the liquid crystal display or the like. Therefore, the impact test apparatus 1 has a time change of the strain generated in the strain detection member 5 and a maximum amount of strain when the impact force is applied to the test piece 2 formed of a thin foam sheet having a thickness of 30 to 200 μm. A value, a distortion suppression rate, etc. can be detected and output.

[Schematic configuration of drop test equipment]
Next, with the foam sheet 107 of the same material and thickness as the test piece 2 attached to the smartphone 101 shown in FIG. 21, the strain gauge 6 is attached to the OLE display 106 and a drop test (hereinafter referred to as “actual machine test”). A schematic configuration of the drop test apparatus 31 for performing the above will be described with reference to FIG.

  As shown in FIG. 5, the drop test device 31 includes a drop table 32 that is formed of a thick metal plate such as a stainless steel plate and arranged horizontally, and a guide rail 33 that is erected vertically on the upper surface of the drop table 32. The chuck member 35 attached so as to be freely dropped along the guide rail 33 and the lower end of the guide rail 33 at a low height (for example, a height of 5 cm) and free-falling. It comprises a pair of stop members 36 provided so that the chuck member 35 abutted and stopped. The pair of stop members 36 are formed of a hard rubber plate or the like. In addition, a height detection device (not shown) that detects the height of each chuck member 35 from each stop member 36 is provided.

  Further, on the outer surface of the chuck member 35 with respect to the guide rail 33, the chuck member 35 is erected in a right-angled outward direction (the front side in FIG. 5) from both side edges in the horizontal direction perpendicular to the dropping direction P. A pair of plate-like arm portions 35A is provided. A pair of air cylinders 37 are attached to the opposing positions of the tip portions of the pair of arm portions 35A, and the pistons 37A of the air cylinders 37 protrude from the horizontal outer side to the horizontal inner side by a predetermined length. It is attached as possible.

  Thereby, as shown in FIG. 5, the drop test apparatus 31 can support the smartphone 101 horizontally by causing the piston 37 </ b> A of each air cylinder 37 to protrude inward in the horizontal direction. Further, the chuck member 35 can be freely dropped from an arbitrary height while the smartphone 101 is horizontally supported. When the chuck member 35 is freely dropped from an arbitrary height, the chuck member 35 becomes a predetermined height (for example, a height of 5 cm) from each stop member 36 by a height detection device (not shown). In this case, the piston 37A of each air cylinder 37 is pulled in, and only the smartphone 101 is configured to fall freely horizontally.

  A strain gauge 6 is attached to the OLE display 106 of the smartphone 101, and the strain gauge 6 is electrically connected to the strain detector 7. As a result, a signal from the strain gauge 6 when the smartphone 101 is dropped horizontally from an arbitrary height is input to the strain detector 7, and the strain detector 7 detects the strain ε of the OLE display 106 to detect the PC8. Output to. The PC 8 sequentially stores the time data of the strain ε input from the strain detection unit 7, calculates the time change of the strain, the maximum amount of strain, the strain suppression rate, and the like, and displays them on the liquid crystal display or the like.

  Accordingly, the drop test device 31 projects the piston 37A of each air cylinder 37 inward in the horizontal direction, and horizontally supports the smartphone 101 with the same material and the same thickness of the foam sheet 107 as the test piece 2. The smartphone 101 can be freely dropped horizontally with respect to the drop table 32 from any height by freely dropping the chuck member 35 from any height. Further, the PC 8 is the time variation of the distortion of the OLE display 106 and the amount of distortion when the smartphone 101 on which the foam sheet 107 of the same material and thickness as the test piece 2 is mounted is freely dropped horizontally from an arbitrary height. Can be calculated and displayed on a liquid crystal display or the like.

[Strain characteristics without specimen]
Here, in the state where the test piece 2 is not mounted between the strain detection member 5 and the support member 9 by the impact test apparatus 1, the first of the detection member 5 measured by causing the hammer 24 to collide with the support member 9 is measured. An example of the distortion characteristics (hereinafter referred to as “reference distortion characteristics”) of the plate member 5A will be described with reference to FIG. Also, an example of the distortion characteristics of the OLE display 106 (hereinafter referred to as “actual machine reference distortion characteristics”) measured by the drop test apparatus 31 by horizontally dropping the smartphone 101 without the foam sheet 107 attached thereto is shown in FIG. Based on

  In the impact test by the impact test apparatus 1, the weight of the hammer 24 is set to 96 grams, and the angle lifted with respect to the column 20 of the shaft 23 (the swing angle θ in FIG. 1) is set to 47 degrees. went. Moreover, the actual machine test by the drop test apparatus 31 was performed by setting the drop height to 1.5 m. In addition, each impact test and each actual machine test of Examples 1 to 4 and Comparative Examples 1 and 2 which will be described later were also performed with the same settings.

  As shown in FIG. 6, the reference strain characteristic curve 41 representing the reference strain characteristic by the impact test becomes a maximum value of “0.612” after about 0.3 msec after receiving the impact force of the hammer 24, and about 0 The distortion becomes “−0.1” after 49 msec. In addition, an actual machine reference strain characteristic curve 42 representing an actual machine reference strain characteristic by an actual machine test has a maximum strain value of “0.607” about 0.3 msec after contact with the drop table 32, and the distortion after about 0.45 msec. Is “−0.1”.

  Therefore, as shown in FIG. 6, it is considered that the reference distortion characteristic curve 41 and the actual machine reference distortion characteristic curve 42 substantially coincide. That is, an actual machine test by the drop test apparatus 31 in a state where the foam sheet 107 is not attached to the smartphone 101, an impact test in a state where the test piece 2 is not attached between the strain detection member 5 and the support member 9. It can be reproduced by an impact test with the device 1. The frequency (load speed) of the reference distortion characteristic curve 41 and the actual machine reference distortion characteristic curve 42 was approximately the same at about 1000 Hz.

[Foamed sheet]
Next, the foam sheet of the present invention will be described. The foam sheet of the present invention is composed of a foam having a thickness of 30 to 200 μm, a density of 0.2 to 0.7 g / cm ³ , an average cell diameter of 10 to 100 μm, and a strain suppression rate of 20% or more. ing. Therefore, a desired impact absorbability can be exhibited.

  The thickness of the foamed sheet of the present invention is 30 to 200 μm. The lower limit is preferably 40 μm, more preferably 50 μm, and the upper limit is preferably 170 μm, more preferably 150 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 200 μm or less, it can easily follow a minute clearance. The foamed sheet of the present invention is excellent in impact absorbency despite the thinness of 30 to 200 μm.

The 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 0.25 g / cm ³ , more preferably 0.3 g / cm ³ , and the upper limit is preferably 0.6 g / cm ³ , more preferably 0.5 g / cm ³ . When the density of the foam is 0.2 g / cm ³ or more, the strength can be maintained, and when the density is 0.7 g / cm ³ or less, high impact absorbability is exhibited.

  The average cell diameter of the foam is 10 to 100 μm. The lower limit is preferably 15 μm, more preferably 20 μm, and the upper limit is preferably 90 μm, more preferably 80 μm. When the average cell diameter is 10 μm or more, excellent impact absorbability is exhibited. Further, since the average cell diameter is 100 μm or less, the compression recovery property is also excellent.

  The distortion suppression rate of the foamed sheet of the present invention is 20% or more. Even if it is preferably 30% or more, more preferably 40% or more, excellent impact absorbability is exhibited. Since the distortion suppression rate of the foamed sheet is 20% or more, even when the thickness is as thin as 30 to 200 μm, the distortion can be suppressed and excellent shock absorption can be exhibited.

  The foamed sheet of the present invention has a loss tangent (tan δ), which is a ratio of loss elastic modulus to storage elastic modulus by dynamic viscoelasticity measurement, having a maximum value in a range from 500 Hz to 50000 Hz, and loss at 500 Hz to 3000 Hz. The tangent (tan δ) is formed to be 0.2 or more. For this reason, even when the thickness is as small as 30 to 200 μm, it is possible to exhibit even more excellent impact absorbability during dropping.

  The foamed sheet of the present invention is formed so that the loss tangent (tan δ) from 500 Hz to 3000 Hz is 0.2 or more. It is desirable that the loss tangent (tan δ) at 500 Hz to 2000 Hz, more preferably 500 Hz to 1500 Hz, be 0.2 or more. That is, it is considered that the foam sheet having a loss tangent (tan δ) at a frequency close to the load speed (frequency) at the actual machine test of the smartphone 101 has a higher shock absorption at the time of dropping.

  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. For this reason, a foam sheet having a large loss tangent (tan δ), which is a ratio of the storage elastic modulus and the loss elastic modulus, has a higher impact absorbability (distortion suppression rate). Presumed to be high.

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. It is as follows. 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 an open cell structure, a closed cell structure, or a semi-continuous semi-independent structure.

  When the thickness of the foamed 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 foamed sheet is very small (for example, a thickness of 30 to 200 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 loss tangent (tan δ), which is the ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement of the foam, has a maximum value in the range from 500 Hz to 50000 Hz, Since the loss tangent (tan δ) from 500 Hz to 3000 Hz is 0.2 or more, even after the bubbles are crushed, the constituent material of the foam exerts the function of buffering the impact when dropped.

  The foam can be constituted by a resin composition containing a resin material (polymer). The loss tangent (tan δ), which is the ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement of the resin composition in an unfoamed state [resin composition when not foamed (solid)], is from 500 Hz. It has a maximum value in the range up to 50000 Hz, and the loss tangent (tan δ) from 500 Hz to 3000 Hz is preferably 0.2 or more. In the case of a material having two or more maximum values of the loss tangent, it is desirable that at least one of them falls within a range from 500 Hz to 50000 Hz. The maximum value of the loss tangent (tan δ) within the range from 500 Hz to 50000 Hz of the resin composition is preferably higher from the viewpoint of shock absorption.

In addition, the initial elastic modulus (23 ° C., tensile speed 300 mm / min) of the resin composition (solid material) in an unfoamed state is desirably low, 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 loss tangent (tan δ), which is the ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement of the foam, has a maximum value in the range from 500 Hz to 50000 Hz, and the loss tangent at 500 Hz to 3000 Hz. In order to form such that (tan δ) is 0.2 or more, Tg of the resin material (polymer) can be used as an index or a standard. 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. By using such an acrylic polymer and adjusting the amount ratio of the former monomer and the latter monomer, the loss tangent (tan δ), which is the ratio of the loss elastic modulus to the storage elastic modulus by dynamic viscoelasticity measurement, is 500 Hz. To 50,000 Hz, a foam having a loss tangent (tan δ) from 500 Hz to 3000 Hz of 0.2 or more 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 maximum value (peak top) temperature of tan δ is defined as the 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 as the homopolymer, the maximum value (peak top) of the loss tangent (tan δ) of the foam may be due to strong intermolecular interaction. The strength can be increased.

  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 C2-8 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 the like. Among these, nitrile rubber (NBR) and methyl methacrylate-butadiene rubber (MBR) 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, 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 anionic, cationic, and nonionic surfactants may be used. From the viewpoint of finer bubble diameter and stability of foamed foam, anionic surfactants may be used. Particularly preferred is ammonium stearate.

  The addition amount of the surfactant is, for example, 0 to 10 parts by weight with respect to 100 parts by weight of the resin material (polymer), the lower limit is preferably 0.5 parts by weight, and 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.

  The addition amount 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), the lower limit is preferably 0.01 parts by weight, and the upper limit is preferably 5 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.

  The addition amount of the thickener is, for example, 0 to 10 parts by weight with respect to 100 parts by weight of the resin material (polymer), the lower limit is preferably 0.1 parts by weight, and the upper limit is preferably 5 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, rust inhibitors, gelling agents, curing agents, plasticizers, fillers, reinforcing agents, foaming agents, Examples include flame retardants, light stabilizers, ultraviolet absorbers, colorants (such as pigments and dyes), pH adjusters, solvents (organic solvents), thermal polymerization initiators, and photopolymerization initiators.

  Examples of the filler include silica, clay (mica, talc, smectite, etc.), alumina, titania, zinc oxide, zeolite, graphite, inorganic fiber (carbon fiber, glass fiber, etc.), organic fiber, and metal powder. It is done.

  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.

  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 160 ° 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 60 ° 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 155 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 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. 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 0.7 g / cm ^3. .

  The foam sheet of the present invention may have an 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 foam sheet of this invention has a fine tack property, a member etc. can be fixed even if it does not provide an adhesive layer.

  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 foamed sheet of the present invention, for example, an image display member attached to an image display device such as a liquid crystal display, an OLE display, or a plasma display (particularly, a small image display member) ), 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), and the like. It is done.

  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 an image display member, and the foam sheet is sandwiched between the casing of the electric or electronic device and the image display member. Electrical and electronic equipment having a specific structure. Examples of the electric / electronic devices include mobile communication devices such as so-called “mobile phones”, “smartphones”, and “portable information terminals”.

  Next, this invention is demonstrated in detail about Example 1 thru | or Example 4, the comparative example 1, and the comparative example 2 of the foam sheet obtained by the said manufacturing method. First, Example 1 will be described. In addition, this invention is not restrict | limited at all by these Examples. Further, unless otherwise specified, “%” representing the content means% by weight.

Example 1
Acrylic emulsion (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid Disperse ("Robomix") 5 parts by weight of polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20 wt%), solid content 28.7%) 5 parts by weight The mixture was stirred and mixed to produce foam. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 130 μm, a density of 0.38 g / cm ³ , and an average cell diameter of 68 μm was obtained.

(Example 2)
Acrylic emulsion (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid 33%) 5 parts by weight, 4 parts by weight of oxazoline-based crosslinking agent (Epocross WS-700, manufactured by Nippon Shokubai Co., Ltd., solid content 24.9%), polyacrylic acid thickener (ethyl acrylate-acrylic acid co 1.5 parts by weight of a polymer (acrylic acid 20 wt%) and a solid content of 28.7% were stirred and mixed with a disper (“Robomix” manufactured by Primics) to foam. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 150 μm, a density of 0.33 g / cm ³ , and an average cell diameter of 87 μm was obtained.

Example 3
Acrylic emulsion (solid content 55%, ethyl acrylate-butyl acrylate-acrylonitrile copolymer (weight ratio 45: 48: 7)) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid 33%) 5 parts by weight, epoxy crosslinking agent (sorbitol polyglycidyl ether) 4 parts by weight, polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20 wt%), solid content 28. 7%) Part by weight was stirred and mixed with a disper (manufactured by “Robomix” Primex) to make foam. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 150 μm, a density of 0.35 g / cm ³ , and an average cell diameter of 61 μm was obtained.

Example 4
100 parts by weight of a synthetic rubber emulsion (“Lackstar 1570” manufactured by DIC, solid content: 42.4%, NBR), 5 parts by weight of a fatty acid ammonium-based surfactant (aqueous dispersion of ammonium stearate, solid content: 33%), 1.3 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 generate foam. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 150 μm, a density of 0.26 g / cm ³ , and an average cell diameter of 55 μm was obtained.

(Comparative Example 1)
Acrylic emulsion (solid content 60%, butyl acrylate-methyl methacrylate-acrylonitrile copolymer (acrylonitrile 5% by weight)) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid content 33%) 5 parts by weight, 1.5 parts by weight of a polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20 wt%), solid content 28.7%) made by Disper ("Robomix" Primex) The mixture was stirred and mixed to form foam. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 140 μm, a density of 0.38 g / cm ³ , and an average cell diameter of 75 μm was obtained.

(Comparative Example 2)
Acrylic emulsion (solid content 60%, butyl acrylate-methyl methacrylate-acrylonitrile copolymer (acrylonitrile 5% by weight)) 100 parts by weight, fatty acid ammonium surfactant (aqueous dispersion of ammonium stearate, solid content 33%) 5 parts by weight, 1 part by weight of polyacrylic acid thickener (ethyl acrylate-acrylic acid copolymer (acrylic acid 20 wt%), solid content 28.7%) with Disper (manufactured by “Robomix” Primex) The mixture was stirred and foamed. This foam was applied onto a peeled PET film (thickness: 38 μm, trade name “MRF # 38” manufactured by Mitsubishi Plastics) and dried at 70 ° C. for 4.5 minutes and at 140 ° C. for 4.5 minutes. Thus, a foam (foamed sheet) having a thickness of 150 μm, a density of 0.45 g / cm ³ , and an average cell diameter of 69 μm was obtained.

<Evaluation>
The following evaluation was performed about the foam (foamed sheet) obtained by the Example and the comparative example. The results are shown in FIGS.

(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.

(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.

(Impact test / actual machine test)
Using the impact test apparatus 1, each foam (foamed sheet) was sandwiched between the strain detection member 5 and the support member 9 to perform an impact test. The strain characteristic of the strain detection member 5 was measured by setting the weight of the hammer 24 to 96 grams and the swing angle θ of the shaft 23 to the support column 20 to be 47 degrees. Moreover, using the drop test apparatus 31, each foam (foamed sheet) was attached to the smartphone 101, and a drop test (actual machine test) was performed. The drop height was set to 1.5 m, and the actual machine distortion characteristics of the OLE display 106 and the presence or absence of drop cracks in the OLE display 106 were measured.

  In FIG. 8, the distortion characteristic curve 47 showing the distortion characteristic of the distortion | strain detection member 5 measured by mounting | wearing the impact test apparatus 1 with the foam sheet of Example 1 is shown. The strain characteristic curve 47 of the foamed sheet of Example 1 shows that the strain becomes the maximum value of “0.252” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 59% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 8 shows an actual machine distortion characteristic curve 48 representing the actual machine distortion characteristic of the OLE display 106 measured by mounting the foam sheet of Example 1 on the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 48 of the foamed sheet of Example 1 shows that the strain becomes the maximum value of “0.240” after about 0.3 msec after contact with the drop table 32, and the strain becomes “−0. 1 ”.

  Therefore, the strain suppression rate when mounted on an actual machine was about 60%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 47 and the actual machine distortion characteristic curve 48 was substantially the same at about 1000 Hz. Further, no cracks due to dropping occurred in the OLE display 106, and a “◯” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

  FIG. 10 shows a strain characteristic curve 49 representing the strain characteristics of the strain detection member 5 measured by attaching the foam sheet of Example 2 to the impact test apparatus 1. The strain characteristic curve 49 of the foamed sheet of Example 2 shows that the strain becomes the maximum value of “0.263” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 57% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 10 shows an actual machine distortion characteristic curve 50 representing the actual machine distortion characteristic of the OLE display 106 measured by attaching the foam sheet of Example 2 to the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 50 of the foamed sheet of Example 2 shows that the strain becomes the maximum value of “0.266” after about 0.3 msec from the contact with the drop table 32, and the strain becomes “−0. 1 ”.

  Therefore, the strain suppression rate when mounted on an actual machine was about 56%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 49 and the actual machine distortion characteristic curve 50 was substantially the same at about 1000 Hz. Further, no cracks due to dropping occurred in the OLE display 106, and a “◯” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

  In FIG. 12, the distortion characteristic curve 51 showing the distortion characteristic of the distortion | strain detection member 5 measured by mounting the foam sheet of Example 3 in the impact test apparatus 1 is shown. The strain characteristic curve 51 of the foamed sheet of Example 3 shows that the strain becomes the maximum value of “0.324” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 47% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 12 shows an actual machine distortion characteristic curve 52 representing the actual machine distortion characteristics of the OLE display 106 measured by attaching the foam sheet of Example 3 to the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 52 of the foamed sheet of Example 3 has a maximum strain value of “0.316” about 0.3 msec after contact with the drop table 32, and the strain becomes “−0. 09 ".

  Therefore, the strain suppression rate when mounted on an actual machine was about 48%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 51 and the actual machine distortion characteristic curve 52 was substantially the same at about 1000 Hz. Further, no cracks due to dropping occurred in the OLE display 106, and a “◯” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

  In FIG. 14, the distortion characteristic curve 53 showing the distortion characteristic of the distortion | strain detection member 5 measured by mounting the foam sheet of Example 4 in the impact test apparatus 1 is shown. The strain characteristic curve 53 of the foamed sheet of Example 4 shows that the strain becomes the maximum value of “0.375” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 39% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 14 shows an actual machine distortion characteristic curve 54 representing the actual machine distortion characteristic of the OLE display 106 measured by attaching the foam sheet of Example 4 to the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 54 of the foamed sheet of Example 4 shows that the strain becomes the maximum value of “0.379” after about 0.3 msec from the contact with the drop table 32, and the strain becomes “−0. 03 ".

  Therefore, the strain suppression rate when mounted on an actual machine was about 38%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 53 and the actual machine distortion characteristic curve 54 was substantially the same at about 1000 Hz. Further, a crack due to dropping occurred in the OLE display 106, and a “◯” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

  In FIG. 16, the distortion characteristic curve 55 showing the distortion characteristic of the distortion detection member 5 measured by mounting | wearing the impact test apparatus 1 with the foam sheet of the comparative example 1 is shown. The distortion characteristic curve 55 of the foamed sheet of Comparative Example 1 shows that the strain becomes the maximum value of “0.511” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 16% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 16 shows an actual machine distortion characteristic curve 56 representing the actual machine distortion characteristics of the OLE display 106 measured by attaching the foam sheet of Comparative Example 1 to the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 56 of the foamed sheet of Comparative Example 1 shows that the strain becomes the maximum value of “0.501” after about 0.3 msec after coming into contact with the drop table 32, and the strain becomes “−0. 1 ”.

  Therefore, the strain suppression rate when mounted on an actual machine was about 16%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 55 and the actual machine distortion characteristic curve 56 was substantially the same at about 1000 Hz. Further, a crack due to dropping occurred in the OLE display 106, and an “x” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

  FIG. 18 shows a strain characteristic curve 57 representing the strain characteristics of the strain detection member 5 measured by mounting the foamed sheet of Comparative Example 2 on the impact test apparatus 1. The distortion characteristic curve 57 of the foamed sheet of Comparative Example 2 shows that the strain becomes the maximum value of “0.525” about 0.3 msec after receiving the impact force of the hammer 24, and the strain becomes “−0. 1 ”. Therefore, the distortion suppression rate is about 14% according to the above formula (1), and is described in the column of “distortion suppression rate” in the evaluation result table 45.

  FIG. 18 shows an actual machine distortion characteristic curve 58 representing the actual machine distortion characteristic of the OLE display 106 measured by mounting the foam sheet of Comparative Example 2 on the smartphone 101 using the drop test apparatus 31. The actual machine distortion characteristic curve 58 of the foamed sheet of Comparative Example 2 shows that the strain becomes the maximum value of “0.522” about 0.3 msec after coming into contact with the drop table 32, and the strain becomes “−0. 1 ”.

  Therefore, the strain suppression rate when mounted on an actual machine was about 14%, which was almost the same as the strain suppression rate measured by the impact test apparatus 1. Moreover, the frequency (load speed) of the distortion characteristic curve 57 and the actual machine distortion characteristic curve 58 was substantially the same at about 1000 Hz. Further, a crack due to dropping occurred in the OLE display 106, and an “x” mark was written in the “actual machine falling crack” column of the evaluation result table 45.

(Dynamic viscoelasticity)
Using a viscoelasticity measuring device (“RSA-G2” manufactured by TA Instruments), each foaming was performed under the measurement conditions of a measurement temperature of −60 ° C. to 25 ° C., a temperature increase interval of 5 ° C./sec, and a frequency of 0.1 to 10 Hz. A frequency temperature dispersion test of the body (foamed sheet) was performed. Then, a master curve was created at a reference temperature of 25 ° C. using the measurement results. Further, the loss tangent (tan δ), which is the ratio of the storage elastic modulus G ′ and the loss elastic modulus G ″ at that time, was measured. FIG. 9, FIG. 11, FIG. 13, FIG. 1 to 4, master curves and loss tangent (tan δ) of Comparative Examples 1 and 2 are shown.

  As shown in FIG. 8, the frequency (load speed) of the strain characteristic curve 47 and the actual machine strain characteristic curve 48 of the foamed sheet of Example 1 was about 1000 Hz. As shown in FIG. 9, in the foam sheet of Example 1, the loss tangent (tan δ) at a frequency of 1000 Hz is “1.356”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. Further, the loss tangent (tan δ) at a frequency of 500 Hz was “1.144”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “1.369”. The loss tangent (tan δ) at the frequency of 2686 Hz was the maximum value “1.375”.

  As shown in FIG. 10, the frequency (load speed) of the strain characteristic curve 49 and the actual machine strain characteristic curve 50 of the foamed sheet of Example 2 was about 1000 Hz. As shown in FIG. 11, in the foam sheet of Example 2, the loss tangent (tan δ) at a frequency of 1000 Hz is “1.293”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. The loss tangent (tan δ) at a frequency of 500 Hz was “1.176”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “1.328”. Further, the loss tangent (tan δ) at the frequency of 3216 Hz was the maximum value “1.329”.

  As shown in FIG. 12, the frequency (load speed) of the distortion characteristic curve 51 and the actual machine distortion characteristic curve 52 of the foamed sheet of Example 3 was about 1000 Hz. Further, as shown in FIG. 13, in the foamed sheet of Example 3, the loss tangent (tan δ) at a frequency of 1000 Hz is “0.837”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. The loss tangent (tan δ) at a frequency of 500 Hz was “0.801”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “0.901”. Further, the loss tangent (tan δ) at the frequency of 47406 Hz was the maximum value “1.039”.

  As shown in FIG. 14, the frequency (load speed) of the distortion characteristic curve 53 and the actual machine distortion characteristic curve 54 of the foamed sheet of Example 4 was about 1000 Hz. Further, as shown in FIG. 15, in the foam sheet of Example 4, the loss tangent (tan δ) at a frequency of 1000 Hz is “0.605”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. The loss tangent (tan δ) at a frequency of 500 Hz was “0.524”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “0.768”. Further, the loss tangent (tan δ) at a frequency of 4026 Hz was the maximum value “1.263”.

  As shown in FIG. 16, the frequency (load speed) of the distortion characteristic curve 55 and the actual machine distortion characteristic curve 56 of the foamed sheet of Comparative Example 1 was about 1000 Hz. As shown in FIG. 17, in the foamed sheet of Comparative Example 1, the loss tangent (tan δ) at a frequency of 1000 Hz is “0.101”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. The loss tangent (tan δ) at a frequency of 500 Hz was “0.100”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “0.088”.

  As shown in FIG. 18, the frequency (load speed) of the distortion characteristic curve 57 and the actual machine distortion characteristic curve 58 of the foamed sheet of Comparative Example 2 was about 1000 Hz. As shown in FIG. 19, in the foamed sheet of Comparative Example 2, the loss tangent (tan δ) at a frequency of 1000 Hz is “0.167”, which is described in the column “tan δ (1 kHz)” of the evaluation result table 45. did. The loss tangent (tan δ) at a frequency of 500 Hz was “0.192”, and the loss tangent (tan δ) at a frequency of 3000 Hz was “0.136”.

(Correlation between strain suppression rate and loss tangent (tan δ))
Next, as shown in FIG. 20, “tan δ (1 kHz)” is plotted on the horizontal axis, and “distortion suppression rate (%)” is plotted on the vertical axis, indicating a correlation between the distortion suppression rate and the loss tangent (tan δ). FIG. 61 was created. Then, each data of “tan δ (1 kHz)” and “distortion suppression rate (%)” of each of Examples 1 to 4 and Comparative Examples 1 and 2 is read from the evaluation result table 45 shown in FIG. The examples 1 to 4 are plotted with “◯” marks, and the comparative examples 1 and 2 are plotted with “x” marks. As a result, as shown in FIG. 20, there is a correlation between the data of “tan δ (1 kHz)” and “distortion suppression rate (%)” of Examples 1 to 4 and Comparative Examples 1 and 2. it is conceivable that.

Therefore, from the loss tangent (tan δ) of each of Examples 1 to 4 shown in FIGS. 9, 11, 13, and 15 and the correlation diagram 61 shown in FIG. 20, the thickness is 30 to 200 μm, and the density 0.2 to 0.7 g / cm ³ , an average cell diameter of 10 to 100 μm, and the foam sheet measured using the impact test apparatus 1 has a strain suppression rate of “20% or more”. In an actual machine test, it is considered that cracking due to the drop of the OLE display 106 can be effectively suppressed.

  Further, the distortion suppression rate of the foam sheet is “20% or more”, and the loss tangent (tan δ) of the foam sheet has a maximum value in the range of 500 Hz to 50000 Hz, and further, 500 Hz to 3000 Hz, If the loss tangent (tan δ) from 500 Hz to 2000 Hz, more preferably from 500 Hz to 1500 Hz is “0.2 or more”, it is considered that cracks due to falling of the OLE display 106 can be more effectively suppressed in an actual machine test. .

1 Impact test device 2 Test piece (foamed sheet)
DESCRIPTION OF SYMBOLS 3 Holding member 4 Impact load member 5 Strain detection member 5A 1st board member 5B 2nd board member 6 Strain gauge 7 Strain detection part 8 Personal computer (PC)
DESCRIPTION OF SYMBOLS 9 Support member 10 Table 11 Fixing jig 12 Holding jig 12A Opening part 16 Holding pressure adjustment means 24 Hammer 25 Electromagnet 27 Recessed part 31 Drop test apparatus 101 Smartphone 102 Image display member 106 OLE display 107 Foam sheet 108 Case

Claims (9)

A foam sheet having a thickness of 30 to 200 μm, a density of 0.2 to 0.7 g / cm ³ , and an average cell diameter of 10 to 100 μm,
An impact test apparatus configured to be able to hold the foam sheet sandwiched between a support member and a strain detection member, and to measure strain of the strain detection member when an impact force is applied to the support member using a strain gauge. A foam sheet characterized by having a strain suppression rate represented by the following formula of 20% or more.
Strain suppression rate (%) = {(ε ₀ −ε ₁ ) / ε ₀ } × 100
(In the above equation, ε ₀ is the maximum strain of the strain detection member when an impact force is applied to the support member in a state where the foam sheet is not mounted between the support member and the strain detection member. And ε ₁ is the maximum strain of the strain detection member when an impact force is applied to the support member in a state where the foam sheet is mounted between the support member and the strain detection member. The impact test apparatus comprises:
Holding means for holding the foam sheet in a state sandwiched between the support member and the strain detection member, and holding the foam sheet at a compression rate in an arbitrary thickness direction;
An impact loading means for applying an impact force to the support member held by the holding means;
Strain detection means for detecting, by the strain gauge, strain generated in the strain detection member by an impact force applied to the foam sheet via the support member by the impact load means;
Output means for calculating and outputting the distortion suppression rate based on the distortion detected by the distortion detection means;
The foamed sheet according to claim 1, comprising: The holding means is
It is fixed perpendicularly to the table, and one surface side of the strain detection member is in close contact, and a recess is formed in the central portion so that the portion receiving the impact force of the strain detection member can be deformed in the load direction. Fixing jig,
An opening is formed in the center and is slidable in the direction toward the fixing jig, and is in close contact with the foam sheet so as to cover the entire surface of the foam sheet fixed to the other surface side of the strain detection part. A pressing jig fixed to the table in a state where the supporting member is in contact with the surface opposite to the surface closely attached to the foam sheet and pressed the support member toward the foam sheet;
Pressure adjusting means for adjusting the pressing pressure with which the pressing jig presses the support member to arbitrarily set the compression rate in the thickness direction of the foam sheet;
Have
The impact load means applies an impact force to the support member that enters the opening of the pressing jig and covers the foam sheet,
The foam sheet according to claim 2, wherein the strain detection means detects strain generated by deformation of the strain detection member into the concave portion by the strain gauge. The impact load means includes a pendulum type hammer that is pivotally supported at one end, and
Hammer holding means for lifting and holding the hammer at a predetermined angle;
4. The impact test apparatus according to claim 2, wherein the impact force is applied by swinging down the hammer held by the hammer holding means. The strain detection member is configured by closely adhering two plate members,
The strain gauge is affixed to a surface of the two plate members opposite to a surface of the first plate member to which the foam sheet is in close contact with the surface to which the foam sheet is in close contact. The foam sheet according to claim 1, wherein the foam sheet is laminated between the plate members. Loss tangent (tan δ), which is the ratio of loss modulus to storage modulus by dynamic viscoelasticity measurement, has a maximum value in the range from 500 Hz to 50000 Hz,
The foamed sheet according to any one of claims 1 to 5, wherein the loss tangent (tan δ) at 500 Hz to 3000 Hz is 0.2 or more.   The foamed sheet according to any one of claims 1 to 6, wherein the foamed sheet is used as an impact absorbing sheet for electrical and electronic equipment.   An electric / electronic device, wherein the foam sheet according to claim 1 is used.   An electric / electronic device including an image display member, wherein the foam sheet according to any one of claims 1 to 7 is sandwiched between a housing of the electric / electronic device and the image display member. 9. The electric / electronic device according to claim 8, wherein the electric / electronic device has a structure.

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