JP2020037602A - Resin foam sheet - Google Patents

Abstract

To provide a resin foam sheet having an excellent flexibility that allows its use as a cushioning material and being hard to cause damage and performance degradation by repeated bending, etc. even after being subjected to a high temperature of about 60°C.SOLUTION: The resin foam sheet according to the present invention is a flexible resin foam sheet that has a restoration rate of 80% or more after being tensioned 200% at 60°C.SELECTED DRAWING: None

Description

  The present invention relates to a resin foam sheet, for example, to a resin foam sheet used as a cushioning material such as a cushioning material for a display.

  2. Description of the Related Art In electronic devices such as a notebook personal computer, a mobile phone, a smartphone, a tablet terminal, and a television, a cushioning material is used to cushion an impact applied to a display or the like. In these electronic devices, a polyolefin-based resin foam sheet is widely used as a cushioning material because it is capable of exhibiting high cushioning performance even though it is thin. Polyolefin-based resin foam sheets generally use polyethylene-based resins and polypropylene-based resins, and various resins have been used for the purpose of further improving performance.

  For example, in Patent Document 1, an olefin block copolymer (A) having a melting point of 115 ° C. or more and a melt index (MI) of 0.1 to 40 g / 10 min (190 ° C.), and an MI of 0.1 to 25 g / A polyolefin resin foam sheet containing a polypropylene resin (B) for 10 minutes (230 ° C.) is disclosed. Patent Document 1 states that by using such a resin composition, it is possible to obtain a molded article having an excellent balance of physical properties such as mechanical strength, heat resistance, flexibility, and elongation, and a complicated shape.

JP 2015-187232 A

  By the way, in recent years, the development of foldable, rollable, and the like in the above electronic devices, particularly in smartphones, tablet terminals, and thin televisions, has been advanced. In this application, a strong stress is generated in the resin foam sheet used as the cushioning material due to the bending of the electronic device, and the resin foam sheet may be damaged or its performance may be deteriorated. In particular, a smartphone, a tablet terminal, or the like may be placed in a high temperature in an automobile or the like, and in this case, there is a problem that the damage and the performance decrease are more likely to occur.

  Therefore, the present invention provides a resin foam sheet having good flexibility that can be used as a cushioning material or the like even at a high temperature of about 60 ° C., and is less likely to be damaged or deteriorate in performance even when bending and the like are repeated. The task is to

As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by setting the restoration rate of the sheet after being stretched by 200% at 60 ° C. within a predetermined range, and have completed the present invention described below. That is, the present invention provides the following [1] to [8].
[1] A resin foam sheet having elasticity, wherein the resin foam sheet after the 200% tension at 60 ° C. has a recovery rate of 80% or more.
[2] The resin foam sheet according to [1], wherein the 20% compressive strength is 500 kPa or less.
[3] The resin foam sheet according to the above [1] or [2], wherein the change rate between the tensile storage elastic modulus at 23 ° C and the tensile storage elastic modulus at 60 ° C is 50% or less.
[4] The resin foam sheet according to any one of the above [1] to [3], wherein a change rate between a tensile storage elastic modulus at 23 ° C and a tensile storage elastic modulus at 80 ° C is 50% or less.
[5] The resin foam sheet according to any one of [1] to [4], wherein the degree of crosslinking is 20% by mass or more.
[6] The resin foam sheet according to any one of the above [1] to [5], wherein a change rate between a tensile storage elastic modulus at 23 ° C and a tensile storage elastic modulus at 120 ° C is 70% or less.
[7] The resin foam sheet according to any one of the above [1] to [6], wherein the resin constituting the resin foam sheet contains at least one kind of elastomer.
[8] An adhesive tape, comprising: the resin foam sheet according to any one of [1] to [7]; and an adhesive provided on at least one surface of the resin foam sheet.

  According to the present invention, there is provided a resin foam sheet having good flexibility that can be used as a cushioning material or the like even at a high temperature of about 60 ° C., and is less likely to be damaged or deteriorate in performance even when bending and the like are repeated. it can.

[Resin foam sheet]
The resin foam sheet of the present invention has elasticity, and the resilience of the resin foam sheet after being stretched by 200% at 60 ° C. is 80% or more. Since the resin foam sheet of the present invention has elasticity and a high restoration rate after 200% tension at 60 ° C. (that is, the resin foam sheet has a high restoration property), the resin foam sheet at a high temperature of about 60 ° C. Can be suitably used for foldables and the like since the resin foam sheet is unlikely to be damaged or its performance is not deteriorated even if bending and the like are repeated. From the viewpoint of enhancing the resilience when the film is locally stretched at a high temperature of about 60 ° C., the sheet resilience after stretching 200% at 60 ° C. is preferably 82% or more, and 85% or more. Is more preferable, and the closer to 100%, the better, and the better, 100% or less.
In the present invention, “200% tension” means that the resin foam sheet is stretched until the length of the stretched sheet becomes 200% of the original length, and the resin foam sheet is stretched at 60 ° C. The sheet restoration rate after 200% tension can be calculated from the following equation.
Sheet restoration rate (%) after 200% tension at 60 ° C. = {1− (xy) / y} × 100
(However, x is the length of the resin foam sheet measured after pulling the resin foam sheet at 200% and curing for 1 minute from the pull release, and y is the original length of the resin foam sheet before tension. It is a length. A more specific measuring method is described in Examples.)

  In addition, the resin foam sheet of the present invention preferably has a sheet restoration rate of 80% or more after 200% tension at 23 ° C. When the sheet restoration ratio after 200% tension at 23 ° C. is 80% or more, high restoration is obtained even when local large stretching is applied at room temperature of about 23 ° C. (hereinafter sometimes simply referred to as “room temperature”). The properties are maintained, and the resin foam sheet is less likely to be damaged or deteriorate in performance. From such a viewpoint, the sheet restoration ratio after 200% tension at 23 ° C. is preferably 83% or more, more preferably 90% or more, still more preferably 95% or more, and the closer to 100%, the better. % Or less.

  Although the resin foam sheet of the present invention has elasticity, having elasticity means that, as described above, tensile strain is applied so that the sheet length becomes about 200% of the original length. This also means that the sheet has elasticity such that the sheet restoration rate after the strain is released is about 80% or more.

(Tensile storage modulus)
The tensile storage modulus at 23 ° C. of the resin foam sheet of the present invention is preferably from 1 MPa to 60 MPa. When the tensile storage elastic modulus at 23 ° C. is 1 MPa or more, high resilience is maintained even when a large elongation is locally applied at ordinary temperature, and when it is 60 MPa or less, the flexibility of the resin foam sheet becomes good. From these viewpoints, the tensile storage modulus at 23 ° C. is preferably from 2 MPa to 30 MPa, more preferably from 3 MPa to 15 MPa, even more preferably from 3 MPa to 10 MPa.

  Further, the tensile storage modulus at 60 ° C. of the resin foam sheet of the present invention is preferably from 1 MPa to 40 MPa. When the tensile storage elastic modulus at 60 ° C. is 1 MPa or more, high resilience is maintained even when a large elongation is locally applied at a high temperature of about 60 ° C., and when the tensile storage elastic modulus is 40 MPa or less, the resin is kept at a high temperature of about 60 ° C. The flexibility of the foam sheet is improved. From these viewpoints, the tensile storage modulus at 60 ° C. is preferably from 1.5 MPa to 20 MPa, more preferably from 2 MPa to 10 MPa, even more preferably from 2.5 MPa to 7 MPa.

Further, the tensile storage modulus at 80 ° C. of the resin foam sheet of the present invention is preferably 1 MPa or more and 40 MPa or less. When the tensile storage elastic modulus at 80 ° C is 1 MPa or more, high resilience is maintained even when a large elongation is locally applied at a high temperature of about 80 ° C. The flexibility of the foam sheet is improved. From these viewpoints, the tensile storage modulus at 80 ° C. is preferably from 1.5 MPa to 18 MPa, more preferably from 1.9 MPa to 9 MPa, even more preferably from 2.4 MPa to 6 MPa.
Furthermore, the tensile storage modulus at 120 ° C. of the resin foam sheet of the present invention is preferably 1 MPa or more and 20 MPa or less. When the tensile storage elastic modulus at 120 ° C. is 1 MPa or more, high resilience is maintained even when a large elongation is locally applied at a high temperature of about 120 ° C., and when the tensile storage elastic modulus is 20 MPa or less, the resin is kept at a high temperature of about 120 ° C. The flexibility of the foam sheet is improved. From these viewpoints, the tensile storage modulus at 120 ° C. is preferably from 1.2 MPa to 10 MPa, more preferably from 1.4 MPa to 5 MPa, and still more preferably from 1.5 MPa to 3 MPa.
The tensile storage modulus can be adjusted by adjusting the degree of crosslinking of the resin foam sheet or by appropriately selecting the resin to be used. In addition, when the resin foam sheet is crosslinked, it is easy to adjust the tensile storage modulus within the above range, but it is possible to satisfy the above range by selecting the resin to be used without crosslinking. There is.

(Change rate of tensile storage modulus)
The rate of change between the tensile storage modulus at 23 ° C. and the tensile storage modulus at 60 ° C. is preferably 50% or less. When the rate of change between the tensile storage modulus at 23 ° C. and the tensile storage modulus at 60 ° C. is 50% or less, the resilience that can withstand practical use is maintained even at a high temperature of about 60 ° C. Indicates that In this respect, the rate of change between the tensile storage modulus at 23 ° C and the tensile storage modulus at 60 ° C is preferably 40% or less, more preferably 30% or less, and even more preferably 25% or less.
In addition, the rate of change between the tensile storage modulus at 23 ° C. and the tensile storage modulus at 60 ° C. is the difference between the tensile storage modulus at 23 ° C. and the tensile storage modulus at 60 ° C. It can be obtained by dividing by the tensile storage modulus, and the rate of change based on each temperature, which will be described later, can also be obtained.

  Further, the rate of change between the tensile storage modulus at 23 ° C. and the tensile storage modulus at 80 ° C. is preferably 50% or less. When the rate of change between the tensile storage elastic modulus at 23 ° C. and the tensile storage elastic modulus at 80 ° C. is 50% or less, even at a high temperature of about 80 ° C., the resilience that can withstand practical use is maintained. Indicates that In this respect, the rate of change between the tensile storage modulus at 23 ° C and the tensile storage modulus at 80 ° C is preferably 40% or less, more preferably 35% or less, and even more preferably 30% or less.

  Further, the change rate between the tensile storage elastic modulus at 23 ° C. and the tensile storage elastic modulus at 120 ° C. is preferably 70% or less. If the rate of change between the tensile storage elastic modulus at 23 ° C. and the tensile storage elastic modulus at 120 ° C. is 70% or less, resilience that can withstand practical use even at a high temperature of about 120 ° C. is maintained. Is shown. In this respect, the rate of change of the tensile storage modulus at 23 ° C. and the change rate of the tensile storage modulus at 120 ° C. are preferably 68% or less, more preferably 65% or less, and even more preferably 60% or less.

(20% compressive strength)
The resin foam sheet of the present invention preferably has a 20% compressive strength of 500 kPa or less. When the 20% compressive strength of the resin foam sheet is 500 kPa or less, the flexibility required as a cushioning material is secured, and the resin foam sheet has excellent shock absorption. In addition, it has excellent followability, and for example, when a resin foam sheet is used for an adhesive tape, the adhesive strength is improved.
The 20% compressive strength is more preferably 450 kPa or less, further preferably 400 kPa or less, and still more preferably 380 kPa or less, in order to further increase the flexibility and further improve the shock absorption and the followability.
Further, the 20% compressive strength is preferably 10 kPa or more, more preferably 15 kPa or more, and further preferably 25 kPa or more. By setting the 20% compressive strength to be equal to or more than these lower limits, appropriate mechanical strength can be imparted to the resin foam sheet.

The resin foam sheet of the present invention has excellent heat resistance to a high temperature of about 60 ° C. It is preferable that the 20% compressive strength is kept low even after strain is applied at a high temperature of about 60 ° C. Specifically, the 20% compressive strength (hereinafter, also referred to as “compressive strength after 60 ° C. tensile”) of the resin foam sheet after 200% tensile at 60 ° C. is preferably 550 kPa or less, more preferably 500 kPa or less. , 450 kPa or less, more preferably 10 kPa or more, more preferably 15 kPa or more, even more preferably 25 kPa or more. As long as the 20% compressive strength of the resin foam sheet after the 200% tension at 60 ° C. is within the above range, the resin foam sheet has appropriate mechanical strength even after being used at a high temperature and maintains flexibility. Can be.
In addition, from the same viewpoint, the change rate of the compressive strength after pulling at 60 ° C. from the 20% compressive strength before pulling 200% at 60 ° C. is preferably 30% or less, and more preferably 20% or less. Preferably, it is 10% or less.
The compressive strength after 60 ° C. pulling means the 20% compressive strength measured after the foam sheet is pulled 200% in a 60 ° C. environment, as in the measurement of the sheet restoration rate described above. The method for measuring the 20% compressive strength is as described in Examples described later.

(Closed cell rate)
The closed cell rate of the resin foam sheet is preferably 80% or more, more preferably 85% or more, still more preferably 90% or more, and even more preferably 95% or more. When the closed cell ratio is 80% or more, the resilience becomes high even when exposed to a strong impact, and damage and performance deterioration hardly occur even after repeated expansion and contraction due to bending or the like. The higher the closed cell rate is, the better, and it may be 100% or less.
The measuring method of the closed cell rate is as follows.
A square test piece having a side of 5 cm is cut out from the resin foam sheet. Then, the thickness of the test piece is measured to calculate the apparent volume V1 of the test piece, and the weight W1 of the test piece is measured. Next, the volume V2 occupied by the bubbles is calculated based on the following equation. In addition, let the density of a test piece be ρ (g / cm ³ ).
Volume occupied by bubbles V2 = V1-W1 / ρ
Subsequently, the test piece was immersed in distilled water at 23 ° C. to a depth of 100 mm from the water surface, and a pressure of 15 kPa was applied to the test piece for 3 minutes. Thereafter, the test piece is taken out of the water to remove moisture adhering to the surface of the test piece, the weight W2 of the test piece is measured, and the closed cell rate F1 is calculated based on the following equation.
Closed cell rate F1 (%) = 100−100 × (W2−W1) / V2

(Apparent density)
The apparent density of the resin foam sheet is preferably 0.08 g / cm ³ or more and 0.8 g / cm ³ or less, more preferably 0.12 g / cm ³ or more and 0.7 g / cm ³ or less, More preferably, it is 0.30 g / cm ³ or more and 0.6 g / cm ³ or less. When the apparent density is within the above range, the flexibility of the resin foam sheet becomes good, and the impact absorption property, follow-up property, and the like are easily improved. Further, a certain mechanical strength is given to the resin foam sheet, and the impact resistance and the like are also improved.

(Expansion ratio)
The expansion ratio of the resin foam sheet is preferably from 1.1 times to 15 times, more preferably from 1.4 times to 10 times, even more preferably from 1.6 times to 3 times. When the expansion ratio is within the above range, the resin foam sheet is appropriately foamed, so that the flexibility is improved, and the impact absorption and the follow-up properties are easily improved. Further, a certain mechanical strength is given to the resin foam sheet, and the impact resistance and the like are also improved.

(Degree of crosslinking)
The resin foam sheet of the present invention is preferably crosslinked. Further, the degree of crosslinking of the resin foam sheet of the present invention is preferably 20% by mass or more, more preferably 20% by mass or more and 60% by mass or less. In the present invention, by setting the degree of cross-linking of the resin foam sheet within the above range, the sheet restoration rate under a high-temperature environment is increased, and the rate of change of the storage elastic modulus is also easily reduced. It becomes easy to adjust the rate of change of the tensile storage modulus within a desired range. Further, it is possible to obtain a resin foam sheet having good flexibility even at a high temperature and hardly causing damage or performance deterioration. From these viewpoints, the crosslinking degree is more preferably 25% by mass or more, still more preferably 30% by mass or more, more preferably 60% by mass or less, and still more preferably 50% by mass or less.
However, as a resin, for example, when using a dynamically cross-linked olefin-based thermoplastic elastomer described below, even without cross-linking, to increase the sheet restoration rate in a high-temperature environment, or to lower the rate of change in storage modulus. In some cases, these can be adjusted within a desired range.

(thickness)
The thickness of the resin foam sheet is preferably 0.03 mm or more and 1.5 mm or less. When the thickness is 0.03 mm or more, it is easy to ensure the impact resistance and the impact absorption of the resin foam sheet. When the thickness is 1.5 mm or less, the thickness can be reduced, and the device can be suitably used for thin electronic devices such as a television, a smartphone, and a tablet. Further, the flexibility of the resin foam sheet can be easily secured.
From these viewpoints, the thickness of the resin foam sheet is more preferably 0.08 mm or more and 1.0 mm or less, and still more preferably 0.10 mm or more and 0.80 mm or less.

  The resin foam sheet is preferably composed of a single layer, in which case the resin foam sheet is composed of a single foam layer. However, the resin foam sheet may be a multilayer as long as the resin foam sheet as a whole has the various physical properties described above. In the case of a multi-layer, it usually comprises two or more foamed layers, but may be a laminate comprising one or more foamed layers and one or more non-foamed layers as long as the effects of the present invention are not impaired. Even if the resin foam sheet includes a non-foam layer, it can have the above-mentioned various physical properties by having the foam layer.

  In the multilayer resin foam sheet, adjacent layers are different layers. Therefore, for example, adjacent layers differ from each other in at least one of physical properties such as apparent density, degree of cross-linking, and expansion ratio, or composition. Further, the multilayer resin foam sheet typically includes a sheet composed of two foam layers and a sheet composed of one foam layer and one non-foam layer. In the multilayer resin foam sheet, a primer layer, an adhesive layer, and the like may be appropriately provided between adjacent layers.

  The foamed layer has a large number of air bubbles therein by, for example, foaming of a foaming agent described later, and has a foaming ratio of 1.1 times or more. On the other hand, the non-foamed layer is a resin layer having substantially no air bubbles inside, and has a foaming ratio of less than 1.1 times. However, the foaming ratio of the non-foamed layer is usually 1 time. is there. Even if the resin foam sheet has a non-foamed layer, as described above, the expansion ratio of the entire resin foam sheet is preferably 1.1 times or more.

(resin)
The resin constituting the resin foam sheet of the present invention preferably contains at least one kind of elastomer. Since the resin foam sheet contains an elastomer, the resin foam sheet expands when bent or the like, thereby suppressing an increase in internal pressure and reducing the stress applied to the resin foam sheet. Therefore, it is easy to adjust the sheet restoration rate within the above-described predetermined range. In addition, the storage elastic modulus can be easily adjusted within a desired range.

Examples of the elastomer include acrylonitrile butadiene rubber, ethylene-propylene-diene rubber (EPDM), ethylene-propylene rubber (EPM), natural rubber, polybutadiene rubber, polyisoprene rubber, styrene rubber, silicone rubber, and acrylic rubber. As the styrene rubber, a random copolymer or a hydrogenated product thereof may be used as long as it is a polymer of styrene and a conjugated diene compound. Specifically, a styrene-butadiene copolymer (SBR), a hydrogenated product thereof (HSBR), and the like can be given.
Elastomers also include thermoplastic elastomers. Examples of the thermoplastic elastomer include an olefin-based thermoplastic elastomer, a styrene-based thermoplastic elastomer, a vinyl chloride-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer resin, a polyester-based thermoplastic elastomer, and a polyamide-based thermoplastic elastomer.
As the elastomer, one of the above components may be used alone, or two or more thereof may be used in combination. The elastomer is preferably a thermoplastic elastomer. In addition, silicone rubber, olefin-based thermoplastic elastomers, and styrene-based thermoplastic elastomers are more preferable from the viewpoint of easily adjusting the tensile storage elastic modulus, the change rate of the tensile storage elastic modulus, the sheet restoration rate, and the like within the above ranges. The rubber and the elastomer may be mixed with each other, or a thermoplastic resin having good compatibility may be mixed and used as described later.

Examples of the olefin-based thermoplastic elastomer include a blend type, a dynamically crosslinked type, and a polymerized type.More specifically, a thermoplastic crystalline polyolefin such as polypropylene or polyethylene is used for a hard segment, and a soft segment is used for a soft segment. Thermoplastic elastomers using fully or partially vulcanized rubber can be used.
Examples of the thermoplastic crystalline polyolefin include a homopolymer of an α-olefin having 1 to 4 carbon atoms or a copolymer of two or more α-olefins, and polyethylene or polypropylene is preferable. Examples of the soft segment component include butyl rubber, halobutyl rubber, EPDM, EPM, acrylonitrile / butadiene rubber, NBR, and natural rubber. EPDM is preferable among these.
The olefin-based thermoplastic elastomer used in the present invention is an olefin-based thermoplastic elastomer that can be produced by dynamic crosslinking, and is preferably an olefin-based thermoplastic elastomer in which EPDM forms a fine island phase in a polypropylene phase. . When such an olefin-based thermoplastic elastomer is used, although the reason is not clear, the sheet restoration rate described above is obtained regardless of whether the resin foam sheet is crosslinked or not crosslinked, and further, the tensile storage elasticity. The rate of change of the rate can be adjusted within the above-mentioned predetermined range. Examples of commercially available thermoplastic elastomers include "Prime TPO" manufactured by Prime Polymer Co., Ltd. and "EXCELLINK 4700P" manufactured by JSR Corporation.

  In addition, examples of the olefin-based thermoplastic elastomer include a block copolymer type. Examples of the block copolymer type include those having a crystalline block and a soft segment block, and more specifically, a crystalline olefin block-ethylene / butylene copolymer-crystalline olefin block copolymer (CEBC) is exemplified. Is done. In CEBC, the crystalline olefin block is preferably a crystalline ethylene block. As a commercially available product of such a CEBC, “DYNARON 6200P” manufactured by JSR Corporation and the like can be mentioned.

Examples of the styrene-based thermoplastic elastomer include a block copolymer having a styrene polymer or copolymer block and a conjugated diene compound polymer or copolymer block. Examples of the conjugated diene compound include isoprene and butadiene.
The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated. When hydrogenation is performed, the hydrogenation can be performed by a known method.

Styrene-based thermoplastic elastomers are usually block copolymers, such as styrene-isoprene block copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene block copolymer, and styrene-butadiene-styrene block copolymer. Copolymer, styrene-ethylene / butylene-styrene block copolymer (SEBS), styrene-ethylene / propylene-styrene block copolymer (SEPS), styrene-ethylene / butylene block copolymer (SEB), styrene-ethylene / propylene Block copolymer (SEP), styrene-ethylene / butylene-crystalline olefin block copolymer (SEBC), and the like.
As the styrene-based thermoplastic elastomer, a block copolymer is preferable, and among them, SEBS and SEBC are more preferable.

  Commercially available styrene-based thermoplastic elastomers include “DYNARON 8600P” (a styrene content of 15% by mass), “DYNARON 4600P” (a styrene content of 20% by mass) manufactured by JSR Corporation, and a trade name. "DYNARON 1321P" (styrene content 10% by mass) and the like.

  The resin constituting the resin foam sheet may be composed of an elastomer alone, or may be a mixture of an elastomer and another resin component. Such a resin component is not particularly limited and may be appropriately selected in consideration of the compatibility with the elastomer, and the like.Polyolefin resin, polystyrene resin, polyethylene terephthalate resin, nylon resin, and the like are exemplified. Among them, a polyolefin resin is preferable. By using a polyolefin resin, it is easy to ensure flexibility and mechanical strength of the resin foam sheet while improving foamability and the like.

Examples of the polyolefin resin include a polyethylene resin, a polypropylene resin, an ethylene-vinyl acetate copolymer and the like, and among these, a polyethylene resin is preferable.
Examples of the polyethylene resin include a polyethylene resin polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, and a chromium oxide compound. Preferably, a polyethylene resin polymerized with a metallocene compound polymerization catalyst is used.

As the polyethylene resin, a linear low-density polyethylene is preferable. The linear low-density polyethylene is obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more with respect to the total amount of monomers) and, if necessary, a small amount of α-olefin. Chain low density polyethylene is more preferred. Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Among them, α-olefins having 4 to 10 carbon atoms are preferred.
Polyethylene resin, for example the density of the linear low density polyethylene as described above is preferably 0.870~0.925g / cm ^3, more preferably 0.890~0.925g / cm ^3, 0.910~0.925g / Cm ³ is more preferred. As the polyethylene resin, a plurality of polyethylene resins can be used, and a polyethylene resin having a density other than the above-described range may be added.

The ethylene-vinyl acetate copolymer used as the polyolefin resin includes, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.
Examples of the polypropylene resin include homopolypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. These may be used alone or in combination of two or more. As the α-olefin constituting the propylene-α-olefin copolymer, specifically, ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1- Octene and the like can be mentioned, and among these, α-olefins having 6 to 12 carbon atoms are preferable.

  The resin constituting the resin foam sheet of the present invention preferably contains an elastomer as a main component. Specifically, the content of the elastomer in the resin foam sheet is, for example, 50% by mass based on the total amount of the resin. The content is preferably 70% by mass or more, more preferably 90% by mass or more, and most preferably 100% by mass. As described above, by using a large amount of the elastomer component, it becomes easy to adjust the above-mentioned sheet restoration rate, tensile storage elastic modulus, and change rate of the tensile storage elastic modulus within a desired range.

  When the resin foam sheet is composed of multiple layers, each layer (that is, each foam layer and each non-foam layer) preferably contains an elastomer, and in each layer, the elastomer is a main component as described above. It is preferable that the content ratio of the elastomer to the total amount of the resin in each layer is as described above. Even when the resin foam sheet is composed of multiple layers, it is preferable that the elastomer be the main component of the entire resin foam sheet as described above, since each layer contains an elastomer. Specific examples of the elastomer used in each layer are as described above.

(Additive)
The foamed layer of the present invention (when there are two or more foamed layers, each foamed layer) is preferably obtained by foaming a foamable composition containing the resin and a foaming agent. As the foaming agent, a pyrolytic foaming agent is preferable.
As the thermal decomposition type foaming agent, an organic foaming agent and an inorganic foaming agent can be used. Examples of the organic blowing agent include azo compounds such as azodicarbonamide, metal salts of azodicarboxylic acid (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N'-dinitrosopentamethylenetetramine, and hydra. Examples thereof include hydrazine derivatives such as zodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium carbonate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, anhydrous sodium citrate and the like.
Of these, azo compounds are preferred, and azodicarbonamide is more preferred, from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety.
The thermal decomposition type foaming agent may be used alone or in combination of two or more.

  The amount of the foaming agent in the foamable resin composition is preferably from 1 part by mass to 20 parts by mass, more preferably from 1.5 parts by mass to 15 parts by mass, and more preferably from 2 parts by mass to 100 parts by mass of the resin. It is more preferably at most 10 parts by mass. By setting the blending amount of the foaming agent to 1 part by mass or more, the foamable sheet is appropriately foamed, and it is possible to impart appropriate flexibility and impact absorption to the resin foam sheet. By setting the blending amount of the foaming agent to 20 parts by mass or less, the resin foam sheet is prevented from foaming more than necessary, and the mechanical strength and the like of the resin foam sheet can be improved.

  The foaming resin composition (that is, each foam layer) may contain a decomposition temperature regulator. Decomposition temperature regulators are compounded to lower the decomposition temperature of the pyrolytic foaming agent or to increase or control the decomposition rate. Specific compounds include zinc oxide, zinc stearate, and urea. And the like. The decomposition temperature adjuster is blended, for example, in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin in order to adjust the surface state of the resin foam sheet.

An antioxidant may be blended in the foamable resin composition (each foam layer). Examples of the antioxidant include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol, sulfur antioxidants, phosphorus antioxidants, and amine antioxidants. The antioxidant is blended, for example, in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin.
The foaming resin composition (each foaming layer) contains additives generally used for foams such as heat stabilizers, coloring agents, flame retardants, antistatic agents, and fillers in addition to the above. Is also good.

The non-foamed layer of the present invention may be composed of the above resin, or may be composed of a resin composition containing various additives in addition to the above resin. As the additives, additives used for general resin films can be used, and for example, antioxidants, heat stabilizers, coloring agents, flame retardants, antistatic agents, fillers, and the like can be used. .
The non-foamed layer is formed from a non-foamable resin layer in which a foaming agent is not substantially mixed. Here, that the blowing agent is not substantially blended means that the foaming agent is not intentionally blended in the non-foamable resin layer, except for the blowing agent that is inevitably blended, In step (3) to be described later, no bubbles are formed, and even if bubbles are formed, the amount is small, and the expansion ratio of the non-foamed layer is less than 1.1 times as described above.

  In the resin foam sheet, the resin is a main component, and the content of the resin is, for example, 70% by mass or more, preferably 80% by mass or more, more preferably 90% by mass, based on the total amount of the resin foam sheet. That is all. In each foamed layer and each non-foamed layer, the resin is the main component, and its content is, for example, 70% by mass or more, preferably 80% by mass or more, more preferably, based on the total amount of each foamed layer. Is 90% by mass or more.

[Production method of resin foam sheet]
Although the resin foam sheet of the present invention is not particularly limited, a foamable sheet having a foamable resin layer composed of a foamable composition containing at least a resin and a pyrolytic foaming agent is heated to foam the pyrolytic foaming agent. It can be manufactured by doing. More specifically, the manufacturing method preferably includes the following steps (1) to (3).
Step (1): Forming a foamable sheet having a foamable resin layer made of a foamable composition containing at least a resin and a pyrolytic foaming agent Step (2): Irradiating the foamable sheet with ionizing radiation Step (3) of cross-linking the foamable sheet: a step of heating the cross-linked foamable sheet to foam the pyrolytic foaming agent to obtain a resin foam sheet.

In the step (1), a method for forming the foamable sheet is not particularly limited. When the resin foam sheet is formed of a single layer, for example, the resin and the additive are supplied to an extruder and melt-kneaded. What is necessary is just to shape | mold by extruding a foamable composition into a sheet form from an extruder.
When the resin foam sheet has a multilayer structure, the foamable sheet may have a multilayer structure. In order to form the foamable sheet into a multilayer structure, it is preferable to form the sheet by coextrusion. In co-extrusion, a plurality of extruders are prepared, components constituting each layer are extruded from each extruder, merged, and extruded into a sheet by a T-die or the like to obtain a foamable sheet having a multilayer structure. Can be.
Further, the resin foam sheet may be formed by pressing the foamable composition or the like. At this time, when the resin foam sheet has a multilayer structure, the resin sheets constituting each layer may be overlapped and then pressed or the like.
In the case of a multilayer, the foamable sheet may be a laminate of one or more foamable resin layers and one or more non-foamable resin layers, or may be a laminate of two or more foamable resin layers.
The molding temperature of the foamable sheet (that is, the temperature at the time of extrusion or the temperature at the time of pressing) is preferably from 50 ° C to 250 ° C, more preferably from 80 ° C to 180 ° C.

  In the step (2), as a method of crosslinking the foamable composition, a method of irradiating the foamable sheet with an ionizing radiation such as an electron beam, α-ray, β-ray, or γ-ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of crosslinking of the obtained resin foam sheet falls within the above-described desired range, but is preferably 1 to 12 Mrad, and is preferably 1.5 to 8 Mrad. Is more preferred. When the foamable sheet is composed of multiple layers, each layer may be crosslinked in step (2).

  In the step (3), the heating temperature when the foamable composition is heated to foam the thermal decomposition type foaming agent may be not less than the foaming temperature of the thermal decomposition type foaming agent, but is preferably 200 to 300 ° C. Preferably it is 220-280 degreeC. In the step (3), the foamable resin layer is foamed to form bubbles to form a foamed layer, but the non-foamable resin layer becomes a non-foamed layer without substantially forming bubbles.

  In the present production method, the resin foam sheet may be stretched in one or both of MD and TD. The stretching of the resin foam sheet may be performed after foaming the foamable sheet to obtain the resin foam sheet, or may be performed while foaming the foamable sheet. In addition, when the resin foam sheet is stretched after foaming the foamable sheet to obtain the resin foam sheet, the resin foam sheet is continuously cooled while maintaining the molten state at the time of foaming without cooling the resin foam sheet. The foamed sheet may be stretched, or after cooling the resin foamed sheet, the resin foamed sheet may be heated again to be in a molten or softened state, and then the resin foamed sheet may be stretched. The resin foam sheet can be easily made thin by stretching. Further, the resin foam sheet may be heated to 100 to 280 ° C, preferably 150 to 260 ° C during stretching.

  However, the production method is not limited to the above, and a resin foam sheet may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, an organic peroxide is previously blended with the foaming composition constituting the foaming resin layer, and the resin composition constituting the non-foaming resin layer, and the foaming sheet is prepared. May be crosslinked by a method of decomposing the organic peroxide by heating.

  When the resin foam sheet is not crosslinked, step (2) is omitted. Therefore, in the step (3), the uncrosslinked foam sheet is foamed by heating.

[Use of resin foam sheet]
The use of the resin foam sheet is not particularly limited, but is preferably used for electronic equipment. By reducing the thickness of the resin foam sheet of the present invention, the resin foam sheet can be suitably used inside a thin electronic device having a small space for disposing the resin foam sheet. Examples of the thin electronic device include a mobile phone, a game device, an electronic organizer, a tablet terminal, a notebook personal computer, a liquid crystal television, and a thin television such as an organic EL television. The resin foam sheet can be used as a cushioning material inside the electronic device, and is preferably used as a cushioning material for a display.

The resin foam sheet used as the cushion material for the display may be disposed, for example, on the back side of a display provided in various electronic devices, and may be used to buffer an impact applied to the display. In this case, the resin foam sheet may be disposed on a support member disposed on the back side of the display cushion material.
In addition, the display may be attached to the electronic device by being fitted into a rectangular recess and having its outer peripheral portion supported from the back side by a narrow rectangular frame-shaped support member. In such a case, the resin foam sheet used as the cushion material for the display is preferably arranged in a rectangular frame shape on the support member, and the display is preferably provided on the resin foam sheet. Even in such an embodiment, the resin foam sheet may be used to cushion the impact applied to the display. In this case, the resin foam sheet is processed into a width of, for example, 5 mm or less, preferably 0.1 mm or more and 3 mm or less, and more preferably 0.2 mm or more and 1 mm or less, corresponding to the width of the support portion. Good.

The electronic device using the resin foam sheet is preferably foldable or rollable. The foldable electronic device can be freely folded and unfolded. In addition, the rollable electronic device can be freely wound and spread. Therefore, the resin foam sheet used for the foldable or rollable electronic device is repeatedly bent or wound.
Since the resin foam sheet of the present invention has high resilience, even if it is repeatedly bent or repeatedly wound, it is less likely to be damaged or deteriorated in performance, so that it can be suitably used for folderable or rollable electronic devices. .

  The resin foam sheet of the present invention can be suitably used for electronic equipment requiring heat resistance. Specifically, it is preferably used for mobile electronic devices such as a mobile phone such as a smartphone, a game device, an electronic organizer, a tablet terminal, and a notebook personal computer. In these portable electronic devices, for example, it is often left in a car in summer, for example, and is often exposed to a high-temperature environment. Even after being left in a car in summer, damage and performance degradation are less likely to occur.

[Adhesive tape]
The resin foam sheet of the present invention may be used for an adhesive tape having a resin foam sheet as a base material. The adhesive tape includes, for example, a resin foam sheet and an adhesive material provided on at least one surface of the resin foam sheet. The adhesive tape can be adhered to another member such as a support member via an adhesive. The pressure-sensitive adhesive tape may be a resin foam sheet provided with a pressure-sensitive adhesive on both sides, or may be a sheet provided with a pressure-sensitive adhesive on one side.
The pressure-sensitive adhesive may be at least one provided with a pressure-sensitive adhesive layer, and may be a single pressure-sensitive adhesive layer laminated on the surface of the resin foam sheet, or may be a double-sided affixed to the surface of the resin foam sheet. The pressure-sensitive adhesive sheet may be used, but is preferably a single pressure-sensitive adhesive layer. The double-sided pressure-sensitive adhesive sheet includes a substrate and pressure-sensitive adhesive layers provided on both sides of the substrate. The double-sided pressure-sensitive adhesive sheet is used for bonding one pressure-sensitive adhesive layer to a resin foam sheet and bonding the other pressure-sensitive adhesive layer to another member.

The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is not particularly limited, and for example, an acrylic pressure-sensitive adhesive, a urethane-based pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, a silicone-based pressure-sensitive adhesive, or the like can be used. Further, a release sheet such as release paper may be further attached on the adhesive material.
The thickness of the adhesive is preferably from 5 to 200 μm, more preferably from 7 to 150 μm, even more preferably from 10 to 100 μm.
When an adhesive tape is used, it may be used for electronic devices as described above, as in the case of the resin foam sheet, and is preferably used for a cushion material for a display. When used as a cushion material for a display, similarly to the case of a resin foam sheet, it is arranged on a back side of the display, for example, on a support member, and is used to cushion an impact applied to the display. Good.
Further, the adhesive tape is preferably arranged in a square frame shape on the support member as described above, and the display is provided on the adhesive tape, similarly to the resin foam sheet. The detailed configuration in this case is as described above except that the resin foam sheet is made of an adhesive tape, and thus the description thereof is omitted. Note that the adhesive material of the adhesive tape may be attached to, for example, a support member. As described above, the electronic device is preferably a folderable or a rollable.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.
In addition, the measuring method of each physical property and the evaluation method of a foamed sheet are as follows.

[Apparent density]
The apparent density was measured according to JIS K7222 (2005).
[Expansion ratio]
The expansion ratio was calculated by dividing the density of the foamable sheet before foaming by the density (apparent density) of the resin foam sheet after foaming.

[Closed cell rate]
It carried out by the method of the said specification.

[20% compressive strength]
The 20% compressive strength was measured in a test environment at 23 ° C. by a measuring method based on JIS K6767.
[Crosslinking degree]
About 100 mg of a test piece was sampled from the resin foam sheet, and the weight A (mg) of the test piece was precisely weighed. Next, this test piece was immersed in 30 cm ³ of xylene at 120 ° C. and allowed to stand for 24 hours, and then filtered through a 200-mesh wire net to collect insoluble matter on the wire mesh, dried in vacuum, and weighed for the insoluble matter. B (mg) was precisely weighed. From the obtained values, the degree of crosslinking (% by mass) was calculated by the following equation.
Degree of crosslinking (% by mass) = (B / A) × 100

[Restoration rate of sheet after 200% tension at 23 ° C]
The resin foam sheet was cut into a dumbbell-shaped No. 1 shape specified in JIS K6251 4.1. Using this as a sample, the length of the sheet after tension was adjusted to the original length by a tensile tester (product name: Tensilon RTF235, manufactured by A & D Corporation) at a temperature of 23 ° C. and a humidity of 50 RH%. It was pulled upward from the lower side along the vertical direction at 500 mm / min until it reached 200%, and it was stationary for 30 seconds in that state. The tensile direction is MD. Thereafter, the sheet was removed from the tensile tester, cured for 1 minute in an environment of a temperature of 23 ° C. and a humidity of 50 RH%, the length (x) of the sheet after curing was measured, and this value and the resin foam sheet before tension were measured. Using the value of the original length (y), the sheet restoration rate after 200% tension was obtained from the following formula.
Sheet restoration rate (%) after 200% tension at 23 ° C. = {1- (xy) / y} × 100

[Restoration rate of sheet after 200% tension at 60 ° C]
Except that the sheet was pulled in an environment of a temperature of 60 ° C. and a humidity of 50 RH%, and was removed from the tensile tester and cured for 1 minute in an environment of a temperature of 60 ° C. and a humidity of 50 RH%. The operation was performed in the same manner as in the recovery rate of the sheet after the% pulling, and measured.

[Tensile storage modulus]
The tensile storage elastic modulus at 23 ° C. was measured using a tensile storage elastic modulus measuring device (trade name “DVA-200 / L2”, manufactured by AIT Measurement Control Co., Ltd.) under the following measurement conditions. In addition, it measured similarly about each temperature of 60 degreeC, 80 degreeC, and 120 degreeC except having changed temperature.
(Measurement condition)
Length between marked lines: 2.5cm
Sample width: 0.5cm
Sample thickness: same as foam sheet thickness Deformation mode: tensile Static / dynamic stress ratio: 1.5
Set distortion: 0.08%
Set heating rate: 6 ° C / min
Measurement frequency 10Hz

The components used in Examples and Comparative Examples are as follows.
Thermoplastic elastomer (1): CEBC, manufactured by JSR Corporation, product name "DYNARON 6200P"
Thermoplastic elastomer (2): SEBS, manufactured by JSR Corporation, product name "DYNARON 8600P"
Thermoplastic elastomer (3): SEBC, manufactured by JSR Corporation, product name "DYNARON 4600P"
Thermoplastic elastomer (4): HSBR, manufactured by JSR Corporation, product name "DYNARON 1321P"
Thermoplastic elastomer (5): olefin-based thermoplastic elastomer, manufactured by JSR Corporation, product name "EXCELLINK 4700P"
Polyolefin resin: linear low-density polyethylene resin obtained with a polymerization catalyst of a metallocene compound, manufactured by Japan Polyethylene Corporation, trade name “Kernel KF283”, density: 0.921 g / cm ³
Thermal decomposition type foaming agent: Azodicarbonamide decomposition temperature regulator: Zinc oxide, trade name "OW-212F" manufactured by Sakai Chemical Industry Co., Ltd.
Phenolic antioxidant: 2,6-di-t-butyl-p-cresol

[Example 1]
After melt-kneading 100 parts by mass of the thermoplastic elastomer (1), 2.0 parts by mass of a pyrolytic foaming agent, 1.0 parts by mass of a decomposition temperature regulator, and 0.5 parts by mass of a phenolic antioxidant, at 120 ° C. By pressing, a foamed resin sheet having a thickness of 0.3 mm was obtained. After pressing, the foamable sheet was cooled to room temperature (23 ° C.).
Both sides of the obtained foamed resin sheet were cross-linked by irradiating 5 Mrad of an electron beam at an acceleration voltage of 500 keV. Next, the foamable resin sheet was foamed by heating the sheet to 250 ° C. to obtain a resin foam sheet having an apparent density of 0.5 g / cm ³ and a thickness of 0.35 mm.

[Examples 2 to 5]
A resin foam sheet was obtained in the same manner as in Example 1 except that the thermoplastic elastomer was changed to one shown in Table 1.

[Example 6]
A resin foam sheet was obtained in the same manner as in Example 5, except that crosslinking was not performed without irradiation with an electron beam.

[Comparative Example 1]
A resin foam sheet was obtained in the same manner as in Example 1 except that the thermoplastic elastomer was changed to one shown in Table 1.
[Comparative Examples 2 to 5]
A resin foam sheet was obtained in the same manner as in Example 1 except that the thermoplastic elastomer shown in Table 1 was used, and that the resin was not irradiated with an electron beam and was not crosslinked.

The properties and evaluation results of the resin foam sheets obtained in the above Examples and Comparative Examples are shown in Table 1 below.

*: The rate of change in the table indicates the rate of change between the tensile storage modulus at 23 ° C. and the tensile storage modulus at each temperature.

  As is clear from the above results, the resin foam sheets of Examples 1 to 6 in which the restoration ratio of the sheet after 200% tension at 60 ° C. is 80% or more are not only normal temperature of about 23 ° C. but also 60 ° C. It can be seen that even at a high temperature of a certain degree, while having good flexibility that can be used as a cushioning material or the like, damage and performance deterioration are unlikely to occur even if bending and the like are repeated.

Claims (8)

  A resin foam sheet having elasticity, wherein the resin foam sheet has a recovery rate of 80% or more after being stretched by 200% at 60 ° C.   The resin foam sheet according to claim 1, wherein the 20% compressive strength is 500 kPa or less.   The resin foam sheet according to claim 1 or 2, wherein a rate of change between the tensile storage modulus at 23 ° C and the tensile storage modulus at 60 ° C is 50% or less.   The resin foam sheet according to any one of claims 1 to 3, wherein a rate of change between the tensile storage modulus at 23 ° C and the tensile storage modulus at 80 ° C is 50% or less.   The resin foam sheet according to any one of claims 1 to 4, wherein a degree of crosslinking is 20% by mass or more.   The resin foam sheet according to any one of claims 1 to 5, wherein a rate of change between a tensile storage modulus at 23 ° C and a tensile storage modulus at 120 ° C is 70% or less.   The resin foam sheet according to any one of claims 1 to 6, wherein a resin constituting the resin foam sheet includes at least one kind of elastomer.   An adhesive tape, comprising: the resin foam sheet according to any one of claims 1 to 7; and an adhesive provided on at least one surface of the resin foam sheet.