Resin foam sheet

Abstract

This resin foam sheet has a degree of sheet restoration after 200% stretching at 23°C of 90% or higher and a surface tack of 1,200 gf / cm2 or less. The present invention can provide a resin foam sheet having stretchability, low tack, and excellent restorability.

Description

[Technical field]

[0001] The present invention relates to a resin foam sheet, and more particularly to a resin foam sheet suitable for use in, for example, electronic devices having a flexible function. [Background technology]

[0002] In electronic devices such as notebook personal computers, mobile phones, smartphones, tablet terminals, and televisions, cushioning materials are used to cushion the impacts applied to displays, etc. In these electronic devices, resin foam sheets are widely used as cushioning materials because they are thin yet can exhibit high cushioning performance. In resin foam sheets, polyethylene-based resins and polypropylene-based resins are generally used.

[0003] In recent years, electronic devices having flexible functions such as foldable and rollable, particularly smartphones, tablet terminals, and flat-screen televisions, have been developed. When electronic devices having flexible functions are bent, a strong stress is generated in the resin foam sheet used as a cushioning material, and conventional resin foam sheets may be damaged or have their performance deteriorated. In order to cope with the stress generated in the resin foam sheet, a resin foam sheet with high elasticity is desired. As a resin foam sheet with high elasticity, one having an acrylic foam obtained by foaming an acrylic resin has been proposed (for example, see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2012-519750 Summary of the Invention [Problem to be solved by the invention]

[0005] However, acrylic foam has not only high elasticity but also high tackiness. Due to its high tackiness, acrylic foam is difficult to handle in processes in which the surface protective film is removed, such as a taping process, and is difficult to withstand mass production. Although it is possible to apply a means for suppressing the tackiness of acrylic foam, applying such a means impairs the restoring ability due to the structural framework. In other words, a resin foam sheet that has good stretchability and recovery properties, low tackiness, and can be used as a cushioning material for electronic devices having a flexible function has not yet been developed.

[0006] The present invention has been made in consideration of the above-mentioned problems in the conventional art, and an object of the present invention is to provide a resin foam sheet that has stretchability, low tackiness, and excellent restoring properties. [Means for solving the problem]

[0007] As a result of intensive research, the present inventors have found that the above-mentioned problems can be solved by making the recovery rate of a sheet after 200% stretching at 23° C. fall within a predetermined range and making the tackiness equal to or less than a predetermined value, and have completed the present invention as described below. That is, the present invention provides the following items [1] to

[10] . [1] It has elasticity, a recovery rate of 90% or more after 200% tension at 23°C, and a surface tackiness of 1,200gf / cm 2 The following is a resin foam sheet. [2] The resin foam sheet according to the above [1], having a 25% compressive strength of 500 kPa or less. [3] The resin foam sheet according to the above [1] or [2], which contains a thermoplastic elastomer and a polyolefin-based resin. [4] The resin foam sheet according to the above [3], wherein a content ratio of the thermoplastic elastomer to the polyolefin resin is 90:10 to 70:30 by mass. [5] The resin foam sheet according to the above [3] or [4], wherein the polyolefin resin is at least one selected from a polyethylene resin and a polypropylene resin. [6] The resin foam sheet according to any one of the above [1] to [5], which contains a thermoplastic elastomer and an inorganic filler. [7] The resin foam sheet according to the above [6], wherein a content ratio of the thermoplastic elastomer to the inorganic filler is 100:30 to 100:150 by mass. [8] The resin foam sheet according to any one of the above [1] to [7], which has a closed cell rate of 80% or more. [9] The resin foam sheet according to any one of the above [1] to [8], which has a thickness of 0.03 to 2 mm.

[10] An adhesive tape comprising the resin foam sheet according to any one of the above [1] to [9] and an adhesive material provided on at least one surface of the resin foam sheet. Effect of the Invention

[0008] According to the present invention, it is possible to provide a resin foam sheet that has stretchability, low tackiness, and excellent recovery property. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Resin foam sheet] The resin foam sheet of the present invention has elasticity, a recovery rate after 200% tension at 23° C. of 90% or more, and a surface tackiness of 1,200 gf / cm 2 The result is as follows.

[0010] (Recovery rate after 200% tension at 23℃) The resin foam sheet of the present invention has elasticity and a high recovery rate after 200% stretching at 23°C (i.e., the resin foam sheet has high recovery), and therefore is unlikely to be damaged or have its performance deteriorated even if it is repeatedly folded at room temperature of about 23°C (hereinafter, sometimes simply referred to as "room temperature"), and can be suitably used in electronic devices and the like having a flexible function. From the viewpoint of increasing the recovery rate when locally stretched at room temperature of about 23°C, the sheet recovery rate after 200% stretching at 23°C is preferably 92% or more, more preferably 94% or more, and even more preferably 96% or more, and the closer to 100%, the better. The upper limit of the sheet recovery rate of the resin foam sheet after 200% stretching at 23°C is 100%. In the present invention, "200% stretching" means stretching the resin foam sheet until the length of the sheet after stretching is 200% of its original length, and the sheet recovery rate after 200% stretching at 23°C can be calculated by the following formula. Sheet recovery rate after 200% tension at 23℃ (%) = {1-(xy) / y}×100 In the formula, x is the length of the resin foam sheet measured after stretching the resin foam sheet by 200% and curing for 1 minute after releasing the stretch, and y is the original length of the resin foam sheet before stretching. A more specific measurement method will be described in the Examples.

[0011] (stretchability) The resin foam sheet of the present invention has elasticity, and by "elasticity," we mean that, as described above, even when a tensile strain is applied so that the sheet length becomes approximately 200% of its original length, the sheet has an elasticity such that the sheet recovery rate after the strain is released is approximately 90% or more.

[0012] (tackiness) Since the resin foam sheet of the present invention has low surface tackiness, the resin foam sheet has high handleability in the production process, and productivity can be improved. 2 It is preferable that the resistance is less than 800 gf / cm 2More preferably, it is 600 gf / cm or less. 2 When the tackiness of the surface of the resin foam sheet is a predetermined value or more, the sheet has excellent conformability, and when the resin foam sheet is used for an adhesive tape, for example, the sheet has a tackiness of 25 gf / cm or more from the viewpoint of improving adhesive strength. 2 It is preferable that the resistance is 50 gf / cm or more. 2 More preferably, it is 75 gf / cm or more. 2 More preferably, it is equal to or greater than this. The method for measuring the tackiness of the surface of the resin foam sheet is as described in the Examples section below.

[0013] (25% compressive strength) The resin foam sheet of the present invention preferably has a 25% compressive strength of 500 kPa or less. When the resin foam sheet has a 25% compressive strength of 500 kPa or less, the flexibility required as a buffer material is ensured, and the impact absorption of the resin foam sheet is excellent. In addition, the resin foam sheet has excellent conformability, and for example, when the resin foam sheet is used in an adhesive tape, the adhesive strength is improved. In order to further increase flexibility and further improve shock absorption and conformability, the 25% compressive strength is more preferably 450 kPa or less, further preferably 400 kPa or less, and even further preferably 380 kPa or less. In order to impart appropriate mechanical strength to the resin foam sheet, the 25% compressive strength is preferably 10 kPa or more, more preferably 15 kPa or more, and even more preferably 25 kPa or more.

[0014] (Closed bubble rate) The closed cell ratio of the resin foam sheet is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and even more preferably 95% or more. If the closed cell ratio is 80% or more, the sheet has high restorability even when exposed to a strong impact, and is less likely to be damaged or have a deteriorated performance even after repeated stretching by bending, etc. The higher the closed cell ratio, the better, and it is sufficient if it is 100% or less. The method for measuring the closed cell ratio is as follows. A flat square test piece with a side length of 5 cm is cut out from the resin foam sheet. The thickness of the test piece is then measured to calculate the apparent volume V1 of the test piece, and the weight W1 of the test piece is also measured. Next, the volume V2 occupied by the air bubbles is calculated based on the following formula. The density of the test piece is ρ(g / cm 3 ) Volume occupied by bubbles V2=V1-W1 / ρ Next, the test piece was submerged 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. After that, the test piece was taken out of the water, and the water adhering to the surface of the test piece was removed. The weight W2 of the test piece was then measured, and the closed cell ratio F1 was calculated based on the following formula. Closed bubble rate F1 (%) = 100-100 × (W2-W1) / V2

[0015] (Expansion Ratio) The expansion ratio of the resin foam sheet is preferably 1.1 to 15 times, more preferably 1.4 to 10 times, and even more preferably 1.6 to 5 times. When the expansion ratio is within the above range, the resin foam sheet is appropriately expanded, so that the flexibility is good, and the impact absorption and followability are likely to be good. In addition, a certain level of mechanical strength is imparted to the resin foam sheet, so that the impact resistance and the like are also good.

[0016] (degree of crosslinking) The resin foam sheet of the present invention is preferably crosslinked. The degree of crosslinking of the resin foam sheet of the present invention is preferably 20% by mass or more, and more preferably 20% by mass or more and 60% by mass or less. In the present invention, by setting the degree of crosslinking of the resin foam sheet within the above range, it becomes easier to increase the sheet recovery rate in a room temperature environment, and it becomes easier to adjust the sheet recovery rate within a desired range. In addition, it is possible to obtain a resin foam sheet that has good flexibility even at room temperature and is less likely to be damaged or have its performance deteriorated. From these viewpoints, the degree of crosslinking is more preferably 20% by mass or more and 50% by mass or less, and even more preferably 20% by mass or more and 45% by mass or less. However, when a dynamically crosslinked olefin-based thermoplastic elastomer described later is used as the resin, the sheet recovery rate in a room temperature environment can be increased and adjusted to within the desired range even without crosslinking.

[0017] (average bubble diameter) The resin foam sheet used in the present invention has an average bubble diameter of preferably 5 μm to 500 μm, more preferably 10 μm to 250 μm, and even more preferably 25 μm to 200 μm. By setting the average bubble diameter to be equal to or larger than these lower limits, flexibility can be easily ensured, and by setting the average bubble diameter to be equal to or smaller than these upper limits, restorability can be ensured, and damage and performance degradation are unlikely to occur even when the sheet is repeatedly bent, etc. In this specification, the average cell diameter means the average value of the average cell diameter in MD and the average cell diameter in TD of a resin foam sheet (hereinafter, also simply referred to as "(MD+TD) / 2"). In addition, MD means machine direction and is a direction that coincides with the extrusion direction, etc., and TD means transverse direction, which is a direction perpendicular to MD and is a direction parallel to the sheet surface in the resin foam sheet. In addition, when the extrusion direction is unknown, any one direction perpendicular to the thickness direction and parallel to the sheet surface is taken as MD.

[0018] (Thickness) The thickness of the resin foam sheet is preferably 0.03 to 2.0 mm. When the thickness is 0.03 mm or more, it is easy to ensure the impact resistance and impact absorption properties of the resin foam sheet. When the thickness is 2.0 mm or less, it is possible to make the resin foam sheet thinner, and it is suitable for use in thin electronic devices such as televisions, smartphones, and tablets. Furthermore, it is easy to ensure the flexibility of the resin foam sheet. From these viewpoints, the thickness of the resin foam sheet is more preferably 0.08 to 1.5 mm, and further preferably 0.10 to 1.0 mm.

[0019] The resin foam sheet is preferably a single layer, in which case the resin foam sheet is made of one foam layer. However, the resin foam sheet may be multi-layered as long as the resin foam sheet as a whole has the above-mentioned various physical properties. When the resin foam sheet is multi-layered, it usually consists of two or more foam layers, but may be a laminate including one or more foam layers and one or more non-foam layers as long as the effect of the present invention is not impaired.

[0020] (resin) The resin contained in the resin foam sheet of the present invention preferably contains at least one thermoplastic elastomer. By containing a thermoplastic elastomer, the resin foam sheet can be stretched when folded, etc., to suppress an increase in internal pressure, thereby reducing the stress acting on the resin foam sheet. Therefore, the sheet recovery rate can be easily adjusted to the above-mentioned predetermined range. In addition, as described later, the resin foam of the present invention more preferably contains a thermoplastic elastomer and a polyolefin resin. By using these resins in combination, it is easy to obtain a resin foam sheet with a high recovery rate and reduced surface tackiness.

[0021] (Thermoplastic elastomer) 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, a polyester-based thermoplastic elastomer, and a polyamide-based thermoplastic elastomer. Among the thermoplastic elastomers listed above, at least one selected from an olefin-based thermoplastic elastomer and a styrene-based thermoplastic elastomer is preferred from the viewpoint of easily adjusting the restoration rate of the resin foam sheet within the above range. The thermoplastic elastomer may be used alone or in combination of two or more kinds.

[0022] Olefin-based thermoplastic elastomer Examples of olefin-based thermoplastic elastomers include blend-type, dynamically crosslinked-type, and polymerization-type elastomers. More specifically, examples of such elastomers include thermoplastic elastomers that use thermoplastic crystalline polyolefins such as polypropylene and polyethylene for the hard segments and fully vulcanized or partially vulcanized rubber for the soft segments. Examples of the thermoplastic crystalline polyolefin include homopolymers of α-olefins having 1 to 4 carbon atoms or copolymers of two or more α-olefins, with polyethylene or polypropylene being preferred. Examples of the soft segment component include butyl rubber, halobutyl rubber, ethylene-propylene-diene rubber (EPDM), ethylene-propylene rubber (EPM), acrylonitrile butadiene rubber (NBR), and natural rubber, with EPDM being preferred. Further, the olefin-based thermoplastic elastomer may be a block copolymer type. The resin foam sheet of the present invention is preferably an olefin-based thermoplastic elastomer having a crystalline structure, and more preferably a crystalline olefin block-ethylene-butylene copolymer-crystalline olefin block copolymer (CEBC). By using such an olefin-based thermoplastic elastomer having a crystalline structure, the restorability of the resin foam sheet is improved. In addition, since the olefin-based thermoplastic elastomer having a crystalline structure is also excellent in compatibility with the polyolefin-based resin described later, the resin foam sheet containing the olefin-based thermoplastic elastomer having a crystalline structure and the polyolefin-based resin can reduce tackiness while maintaining a high restoration rate. Moreover, the polyolefin-based resin used in combination with the olefin-based thermoplastic elastomer having a crystalline structure is preferably a polyethylene resin. In the above CEBC, the crystalline olefin block is preferably a crystalline ethylene block, and examples of commercially available products of such CEBC include "DYNARON 6200P" manufactured by JSR Corporation.

[0023] <Styrene-based thermoplastic elastomer> Examples of the styrene-based thermoplastic elastomer include block copolymers having a polymer or copolymer block of styrene and a polymer or copolymer block of a conjugated diene compound, such as isoprene and butadiene. The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated. When hydrogenated, the hydrogenation can be carried out by a known method. The styrene-based thermoplastic elastomer is usually a block copolymer, and examples thereof include styrene-isoprene block copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene block copolymer, styrene-butadiene-styrene block 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), etc. Hydrogenated ones such as hydrogenated styrene-butadiene copolymer (HSBR) can also be used. Among the above-mentioned styrene-based thermoplastic elastomers, SEBS, SEBC and HSBR are more preferable. In the case where the styrene-based thermoplastic elastomer is used in combination with a polyolefin-based resin described below, it is preferable to use a polypropylene resin as the polyolefin-based resin. Commercially available styrene-based thermoplastic elastomers include those manufactured by JSR Corporation under the trade name "DYNARON 8600P" (styrene content: 15% by mass), the trade name "DYNARON 4600P" (styrene content: 20% by mass), and the trade name "DYNARON 1321P" (styrene content: 10% by mass).

[0024] Among the above-mentioned thermoplastic elastomers, a thermoplastic elastomer having a crystalline structure is preferred. A thermoplastic elastomer having a crystalline structure has both a crystalline portion and an amorphous portion. The presence of the crystalline portion increases the recovery rate of a thermoplastic elastomer having a crystalline structure, and furthermore, since it has excellent compatibility with a polyolefin-based resin described later, the tackiness is effectively reduced. Therefore, a resin foam sheet containing a thermoplastic elastomer having a crystalline structure and a polyolefin-based resin has a high recovery rate and low tackiness. Examples of the thermoplastic elastomer having a crystalline structure include CEBC and SEBC.

[0025] (Polyolefin resin) The resin foam sheet preferably contains the above-mentioned thermoplastic elastomer and a polyolefin-based resin. By containing a polyolefin-based resin, the flexibility and mechanical strength of the resin foam sheet can be ensured while improving the foamability, and the tackiness of the surface of the resin foam sheet can be reduced. Examples of polyolefin resins include polyethylene resins, polypropylene resins, and ethylene-vinyl acetate copolymers. Among these, one or more resins selected from polyethylene resins and polypropylene resins are preferred.

[0026] 《Polyethylene resin》 Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, or a chromium oxide compound, and preferably, polyethylene resins polymerized with a polymerization catalyst such as a metallocene compound are used. As the polyethylene resin, linear low density polyethylene is preferred. The linear low density polyethylene is more preferably a linear low density polyethylene obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more based on the total amount of monomers) with a small amount of α-olefin as required. Specific examples of α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Among them, α-olefins having 4 to 10 carbon atoms are preferred. The density of the polyethylene resin, for example the linear low density polyethylene described above, is 0.870 to 0.925 g / cm 3 is preferable, and 0.890 to 0.925 g / cm 3 More preferably, 0.910 to 0.925 g / cm 3 As the polyethylene resin, a plurality of polyethylene resins may be used, and a polyethylene resin having a density outside the above-mentioned range may be added. Commercially available polyethylene resins include those manufactured by Japan Polyethylene Co., Ltd. under the trade name "Kernel KF283" and "Novatec LD," those manufactured by Sumitomo Chemical Co., Ltd. under the trade name "Sumikathene EP" and "Sumikathene GMH," and those manufactured by Asahi Kasei Chemicals Corporation under the trade name "Suntech LD."

[0027] <Polypropylene resin> The polypropylene resin may be a homopolypropylene, which is a homopolymer of propylene, or a copolymer of propylene and a small amount of ethylene and an α-olefin other than propylene, with propylene as the main component (preferably 75% by mass or more, more preferably 90% by mass or more of the total monomers). Copolymers of propylene and α-olefins other than ethylene and propylene include block copolymers, random copolymers, random block copolymers, etc., with random copolymers (ie, random polypropylene) being preferred among these. Examples of the α-olefin other than propylene include ethylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, and 1-octene, and among these, α-olefins having 6 to 12 carbon atoms are preferred. In the copolymer, these α-olefins can be used alone or in combination of two or more kinds. The polypropylene resin may be used alone or in combination of two or more kinds. Commercially available polypropylene resins include those manufactured by Japan Polypropylene Corporation under the trade names "EG7F", "MA1B", "MA3", "MA04A", "SA06GA", "MG03B", "BC2E", "BC02NC", "BC03B", and "BC06C". In addition to these, commercially available polypropylene resins include those manufactured by Prime Polymer Corporation under the trade names "J-2003GP", "J-3000GP", "J106MG", and "J708UG".

[0028] Ethylene-vinyl acetate copolymer The ethylene-vinyl acetate copolymer may be, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene-derived structural units. The density of the ethylene-vinyl acetate copolymer is 0.92 g / cm 3 It is preferable that the content is 0.93 g / cm or more. 3 More preferably, it is 0.94 g / cm or more. 3 It is more preferable that the density of the ethylene-vinyl acetate copolymer is 0.97 g / cm or more. 3 It is preferable that the content is 0.96 g / cm or less. 3 By setting the density of the ethylene-vinyl acetate copolymer within this range, it becomes easy to reduce the compressive strength and the like without impairing the flexibility of the resin foam sheet. Commercially available ethylene-vinyl acetate copolymers include "NUC3726" manufactured by Nippon Unicar Co., Ltd.

[0029] (Other resins) The resin of the resin foam sheet may contain other resins besides the above-mentioned thermoplastic elastomers and polyolefin resins, as long as the effects of the present invention are not impaired. Examples of other resins 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.

[0030] (Resin Mixture Ratio) When the resin foam sheet of the present invention contains a thermoplastic elastomer and a polyolefin resin, the content ratio thereof is preferably adjusted as follows. That is, the content ratio of the thermoplastic elastomer to the polyolefin resin (thermoplastic elastomer:polyolefin resin) is preferably 90:10 to 70:30, more preferably 88:12 to 72:28, and even more preferably 85:15 to 75:25, by mass ratio. By adjusting the content ratio of the thermoplastic elastomer to the polyolefin resin to the above ratio, the sheet recovery rate and surface tackiness in a room temperature environment can be easily adjusted to within the desired range. The total amount of the thermoplastic elastomer and the polyolefin resin based on the total amount of resin contained in the resin foam of the present invention is preferably 60 mass % or more, more preferably 80 mass % or more, and even more preferably 100 mass %. In the resin foam sheet, the resin is a main component, and the resin content is, for example, 70 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more, based on the total amount of the resin foam sheet.

[0031] (Inorganic filler) The resin foam sheet of the present invention preferably contains a thermoplastic elastomer and an inorganic filler. By using these in combination, it becomes easier to obtain a resin foam sheet having an increased recovery rate and reduced surface tackiness. The inorganic filler is not particularly limited, and examples of the inorganic filler that can be used include oxide-based fillers, nitride-based fillers, carbon-based fillers, and metal fillers. Examples of the oxide-based filler include magnesium oxide, silica, aluminum oxide, and composite oxides thereof. The nitride-based filler includes boron nitride, aluminum nitride, and composite nitrides thereof. Examples of the carbon-based filler include graphite, graphene, carbon nanotubes, carbon black, and graphite. Metal fillers include titanium, copper, nickel, tin, silver, and gold, as well as alloys containing these metals. The type of inorganic filler may be appropriately selected depending on the portion in which the resin foam sheet is used. When the resin foam sheet is used in a portion in which insulation is required, oxide-based fillers and carbon-based fillers are preferred.

[0032] The inorganic filler may have any shape, and examples thereof include spherical fillers and plate-like fillers. The spherical filler is a filler whose shape is spherical or close to spherical, and the ratio of the major axis to the minor axis of each filler is 1 or close to 1, for example, 0.6 to 1.7, preferably 0.8 to 1.5. The plate-like filler is a filler whose shape is thin or scaly, and the major axis of each filler is sufficiently larger than the thickness, for example, the ratio of the thickness to the major axis is 2 or more, preferably 3 or more. Among inorganic fillers, it is preferable to use a spherical filler. The average particle size of the inorganic filler is preferably 0.01 to 150 μm, more preferably 0.1 to 100 μm, and further preferably 1 to 80 μm. The average particle size of the inorganic filler can be determined by measuring the sizes of a plurality of particles (for example, 20 particles) using an electron microscope and averaging the measurements.

[0033] In the resin foam sheet of the present invention, the content ratio of the thermoplastic elastomer to the inorganic filler (thermoplastic elastomer:inorganic filler) by mass is preferably 100:30 to 100:150, more preferably 100:40 to 100:125, and even more preferably 100:50 to 100:100. By adjusting the content ratio of the thermoplastic elastomer to the inorganic filler within the above range, the tackiness of the surface of the resin foam sheet can be easily adjusted to within a desired range.

[0034] (Additives) The resin foam sheet of the present invention is preferably obtained by foaming a foamable composition containing the above-mentioned resin and a foaming agent. The foaming agent is preferably a thermally decomposable foaming agent. As the thermal decomposition type blowing agent, organic blowing agents and inorganic blowing agents can be used. Examples of the organic blowing agent include azo compounds such as azodicarbonamide, azodicarboxylate metal salts (e.g., barium azodicarboxylate), azobisisobutyronitrile, etc., nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, etc., hydrazine derivatives such as hydrazodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), toluenesulfonylhydrazide, etc., and semicarbazide compounds such as toluenesulfonylsemicarbazide, etc. Examples of inorganic foaming agents include ammonium carbonate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate. Among these, from the viewpoint of obtaining fine bubbles, and from the viewpoints of economy and safety, azo compounds are preferred, and azodicarbonamide is more preferred. The thermally decomposable foaming agents may be used alone or in combination of two or more kinds.

[0035] The amount of the foaming agent in the foamable resin composition is preferably 1 part by mass or more and 20 parts by mass or less, more preferably 1.5 parts by mass or more and 15 parts by mass or less, and even more preferably 2 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the resin. By making the amount of the foaming agent 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. In addition, by making the amount of the foaming agent 20 parts by mass or less, the resin foam sheet is prevented from being foamed more than necessary, and the mechanical strength of the resin foam sheet can be improved.

[0036] The foamable resin composition may contain a decomposition temperature regulator. The decomposition temperature regulator is used to lower the decomposition temperature of the thermal decomposition type foaming agent or to increase or adjust the decomposition rate, and specific examples of the regulator include zinc oxide, zinc stearate, and urea. The decomposition temperature regulator is used in an amount of, for example, 0.01 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the resin in order to adjust the surface condition of the resin foam sheet.

[0037] The foamable resin composition may contain an antioxidant. Examples of the antioxidant include phenol-based antioxidants such as 2,6-di-t-butyl-p-cresol, sulfur-based antioxidants, phosphorus-based antioxidants, and amine-based antioxidants. The antioxidant is blended in an amount of, for example, 0.01 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the resin. In addition to the above, the foamable resin composition may contain additives that are generally used in foams, such as a heat stabilizer, a colorant, a flame retardant, an antistatic agent, and a filler.

[0038] [Method of manufacturing resin foam sheet] The resin foam sheet of the present invention is not particularly limited, but can be produced by heating a foamable sheet made of a foamable composition containing at least a resin and a thermally decomposable foaming agent to foam the thermally decomposable foaming agent. More specifically, the production method preferably includes the following steps (1) to (3). Step (1): A step of forming a foamable sheet from a foamable composition containing at least a resin and a thermally decomposable foaming agent. Step (2): A step of irradiating the foamable sheet with ionizing radiation to crosslink the foamable sheet. Step (3): Heating the crosslinked foamable sheet to foam the thermally decomposable foaming agent to obtain a resin foam sheet.

[0039] In the step (1), the method for forming the foamable sheet is not particularly limited, and may be formed by extruding the foamable composition from an extruder into a sheet shape. The molding temperature of the foamable sheet (ie, the temperature during extrusion, etc.) is preferably 50°C or higher and 250°C or lower, more preferably 80°C or higher and 180°C or lower.

[0040] In the step (2), the foamable sheet is crosslinked by irradiating the foamable sheet with ionizing radiation such as electron beams, α-rays, β-rays, γ-rays, etc. The dose of the ionizing radiation may be adjusted so that the crosslinking degree of the resulting resin foam sheet falls within the desired range described above, and is preferably 1 to 12 Mrad, and more preferably 1.5 to 8 Mrad.

[0041] In the step (3), the heating temperature when the expandable sheet is heated to expand the thermally decomposable foaming agent may be equal to or higher than the foaming temperature of the thermally decomposable foaming agent, and is preferably 200 to 300°C, more preferably 220 to 280°C.

[0042] In the present production method, the resin foam sheet may be stretched in either MD or TD or both. The resin foam sheet may be stretched after the foamable sheet is foamed to obtain a resin foam sheet, or may be stretched while the foamable sheet is foamed. When the resin foam sheet is stretched after the foamable sheet is foamed to obtain a resin foam sheet, the resin foam sheet may be stretched while maintaining the molten state at the time of foaming without cooling the resin foam sheet, or the resin foam sheet may be stretched after cooling the resin foam sheet by heating the resin foam sheet again to a molten or softened state. The resin foam sheet is easily thinned by stretching. The resin foam sheet may be heated to, for example, 100 to 280°C, preferably 150 to 260°C during stretching.

[0043] However, the present 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 ionizing radiation, crosslinking may be performed by a method in which an organic peroxide is blended in advance with the foamable composition constituting the foamable resin layer and the resin composition constituting the non-foamable resin layer, and the foamable sheet is heated to decompose the organic peroxide.

[0044] When the resin foam sheet is uncrosslinked, step (2) is omitted. Therefore, in step (3), the uncrosslinked foamable sheet is heated to be foamed.

[0045] [Applications of resin foam sheets] The application of the resin foam sheet is not particularly limited, but it is preferably used for electronic device applications. The resin foam sheet of the present invention can be suitably used inside a thin electronic device where the space for arranging the resin foam sheet is small by making it thin. Examples of the thin electronic device include mobile phones, game devices, electronic organizers, tablet terminals, notebook personal computers, thin televisions such as liquid crystal televisions and organic electroluminescence televisions. The resin foam sheet can be used as a cushioning material inside the electronic device, and is preferably used as a cushioning material for displays.

[0046] The resin foam sheet used as the cushioning material for a display may be placed, for example, on the back side of a display provided in various electronic devices, and used to cushion the display from shocks. In this case, the resin foam sheet may be placed on a support member placed on the back side of the cushioning material for a display. Also, the display may be attached to the electronic device by being fitted into a rectangular recess and having its outer periphery supported from the rear side by a support member having a narrow rectangular frame shape. In such a case, the resin foam sheet used as the cushioning material for the display may be arranged in a rectangular frame shape on the support member, and the display may be provided on the resin foam sheet. In such an embodiment, the resin foam sheet may be used to cushion the impact acting on the display. In this case, the resin foam sheet may be processed to a width of, for example, 5 mm or less, preferably 0.1 mm or more and 3 mm or less, more preferably 0.2 mm or more and 1 mm or less, corresponding to the width of the support part.

[0047] The electronic device in which the resin foam sheet is used is preferably an electronic device having a flexible function, such as a foldable electronic device or a rollable electronic device. A foldable electronic device can be freely folded or unfolded. A rollable electronic device can be freely rolled or unfolded. Therefore, the resin foam sheet used in a foldable or rollable electronic device is repeatedly folded or rolled. The resin foam sheet of the present invention has high resilience and is therefore unlikely to be damaged or have its performance deteriorated even when repeatedly folded or rolled, and can therefore be suitably used in foldable or rollable electronic devices.

[0048] The resin foam sheet of the present invention has low tackiness and is therefore excellent in handleability during the production process. Specifically, the occurrence of problems such as winding around rollers and blocking during the production process can be suppressed, thereby improving productivity.

[0049] [Adhesive tape] The resin foam sheet of the present invention may be used in an adhesive tape having a resin foam sheet as a substrate. 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 other members such as a support member via the adhesive material. The adhesive tape may include an adhesive material provided on both surfaces of the resin foam sheet, or an adhesive material provided on one surface. The adhesive material may be one that includes at least an adhesive layer, and may be a single adhesive layer laminated on the surface of the resin foam sheet, or a double-sided adhesive sheet attached to the surface of the resin foam sheet, but is preferably a single adhesive layer. The double-sided adhesive sheet includes a substrate and adhesive layers provided on both sides of the substrate. The double-sided adhesive sheet is used to adhere one adhesive layer to the resin foam sheet and adhere the other adhesive layer to another member.

[0050] The adhesive constituting the adhesive layer is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, a silicone adhesive, etc. may be used. In addition, a release sheet such as release paper may be further attached onto the adhesive material. The thickness of the adhesive material is preferably from 5 to 200 μm, more preferably from 7 to 150 μm, and further preferably from 10 to 100 μm. When the adhesive tape is used, it is preferably used for electronic device applications as described above, similar to the resin foam sheet, and is preferably used as a cushioning material for displays. When used as a cushioning material for displays, it is preferably placed on a support member, for example, on the back side of a display, similar to the resin foam sheet, and used to cushion the impact acting on the display. In addition, the adhesive tape may be arranged in a rectangular frame shape on the support member as in the case of the resin foam sheet, and a display may be provided on the adhesive tape. In this case, the detailed configuration is the same as described above except that the resin foam sheet is the adhesive tape, so that the description is omitted. The adhesive material of the adhesive tape may be attached to the support member, for example. In addition, the electronic device may be an electronic device having a flexible function such as a foldable electronic device or a rollable electronic device, as described above, so that the characteristics of the resin foam sheet can be more effectively utilized. EXAMPLES

[0051] The present invention will be described below with reference to examples, but the present invention is not limited to these examples in any way. The methods for measuring the various physical properties and the methods for evaluating the resin foam sheet are as follows.

[0052] [Expansion ratio] The expansion ratio was calculated by dividing the density of the expandable sheet before expansion by the density of the resin foam sheet after expansion.

[0053] [Closed bubble rate] The test was carried out according to the method described in the above specification.

[0054] [25% compressive strength] The 25% compressive strength was measured in a test environment of 23°C using a measurement method conforming to JIS K6767.

[0055] [Crosslinking degree] A test piece of about 100 mg was taken 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 of xylene at 120°C. 3 After immersing the sample in the solution for 24 hours, the insoluble matter on the screen was collected by filtration through a 200-mesh wire screen and dried in a vacuum. The weight B (mg) of the insoluble matter was precisely weighed. The degree of crosslinking (mass%) was calculated from the obtained value using the following formula. Degree of crosslinking (mass%) = (B / A) × 100

[0056] [Average bubble diameter] The resin foam sheet was cut in the thickness direction along the MD and TD, and a 200x magnified photograph was taken using a digital microscope (Keyence Corporation, product name "VHX-900"). In the enlarged photograph, the bubble diameters in MD and TD were measured for all bubbles present on the cut surface of 2 mm length in each of the MD and TD, and this operation was repeated five times. The average values ​​of the bubble diameters in MD and TD of all bubbles were taken as the average bubble diameters in MD and TD, and the average value of these was calculated as the average bubble diameter ((MD+TD) / 2).

[0057] [Sheet recovery rate after 200% stretching at 23℃] The resin foam sheet was cut into a dumbbell shape No. 1 as specified in JIS K6251 4.1. This was used as a sample, and pulled vertically from bottom to top at 500 mm / min in an environment of 23°C and 50% RH until the length of the sheet after pulling was 200% of its original length, and then left in that state for 30 seconds. The pulling direction is MD. After that, the sheet was removed from the tensile tester and cured for 5 minutes in an environment of 23°C and 50% RH. The length (x) of the sheet after curing was measured, and the sheet recovery rate after 200% stretching was calculated using this value and the original length (y) of the resin foam sheet before stretching according to the following formula. Sheet recovery rate after 200% tension at 23℃ (%) = {1-(xy) / y}×100

[0058] [Dynamic bending resistance test] A resin foam sheet (40mm x 10mm) was attached to the sample placement area of ​​a testing machine (a small desktop durability testing machine manufactured by Yuasa System Co., Ltd.), and then the testing machine was placed in a 23°C oven and left to stand for 5 minutes. The sheet was then folded by moving the movable part of the testing machine, and the maximum bending position was adjusted so that the distance between the folded sheets was 2mm. This was repeated 100,000 times. After the bending test, samples with a thickness change of 10% or more were rated as "C," samples with thin creases were rated as "B," and samples with no creases were rated as "A."

[0059] [Tackiness] The resin foam sheet was cut into a 10 cm square and measured at a temperature of 23° C. using a tack tester (manufactured by UBM, product name "Tack Tester TA-500"). Cross-sectional area of ​​0.25 cm for the surface of the resin foam sheet 2 The jig (made of metal) is placed under a load of 5000gf / cm 2 After pressing for 5 seconds, the load (gf / cm) when the jig is pulled up at a speed of 1 mm / sec 2 ) was measured, and the obtained value was regarded as the tackiness of the surface of the resin foam sheet.

[0060] The components used in the examples and comparative examples are as follows. Thermoplastic elastomer (1): CEBC, manufactured by JSR Corporation, product name "DYNARON 6200P" Thermoplastic elastomer (2): HSBR, manufactured by JSR Corporation, product name "DYNARON 1321P" Polyolefin resin (1): Linear low-density polyethylene resin obtained by polymerization of a metallocene compound, manufactured by Japan Polyethylene Co., Ltd., product name "Kernel KF283", density: 0.921 g / cm 3 Polyolefin resin (2): Polypropylene resin (ethylene-propylene random copolymer), manufactured by Japan Polypropylene Corporation, product name "Novatec EG7F", ethylene content: 3% by mass Inorganic filler: Magnesium oxide, manufactured by Ube Materials, product name "RF-50-SC", average particle size: 50 μm, shape: spherical Thermal decomposition type foaming agent: Azodicarbonamide Decomposition temperature regulator: Zinc oxide, manufactured by Sakai Chemical Industry Co., Ltd., product name "OW-212F" Phenolic antioxidant: 2,6-di-t-butyl-p-cresol

[0061] [Example 1] 80 parts by mass of thermoplastic elastomer (1), 20 parts by mass of polyolefin resin (1), 2.7 parts by mass of thermally decomposable foaming agent, 1.0 part by mass of decomposition temperature regulator, and 0.5 parts by mass of phenol-based antioxidant were melt-kneaded, and then pressed at 120° C. to obtain a foamable sheet having a thickness of 0.3 mm. After pressing, the foamable sheet was cooled to room temperature (23° C.). Both sides of the obtained foamable sheet were crosslinked by irradiating with 4.5 Mrad of electron beam at an acceleration voltage of 500 keV, and then heated to 250°C to foam the foamable sheet, thereby obtaining a resin foam sheet with an expansion ratio of 2.9 times and a thickness of 0.21 mm.

[0062] [Example 2] A resin foam sheet was obtained in the same manner as in Example 1, except that the compounding ratio of the thermoplastic elastomer (1), the polyolefin resin (1), and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0063] [Example 3] 80 parts by mass of thermoplastic elastomer (2), 20 parts by mass of polyolefin resin (2), 2.3 parts by mass of thermally decomposable foaming agent, 1.0 part by mass of decomposition temperature regulator, and 0.5 part by mass of phenol-based antioxidant were melt-kneaded, and then pressed at 120° C. to obtain a foamable sheet having a thickness of 0.3 mm. After pressing, the foamable sheet was cooled to room temperature (23° C.). Both sides of the obtained foamable sheet were crosslinked by irradiating with 2.5 Mrad of electron beam at an acceleration voltage of 500 keV, and then heated to 250°C to foam the foamable sheet, thereby obtaining a resin foam sheet with an expansion ratio of 2.4 times and a thickness of 0.20 mm.

[0064] [Example 4] A resin foam sheet was obtained in the same manner as in Example 3, except that the compounding ratio of the thermoplastic elastomer (2), the polyolefin resin (2), and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0065] [Example 5] 100 parts by mass of thermoplastic elastomer (1), 30 parts by mass of inorganic filler, 2.2 parts by mass of thermally decomposable foaming agent, 1.0 part by mass of decomposition temperature regulator, and 0.5 parts by mass of phenol-based antioxidant were melt-kneaded and then pressed at 120° C. to obtain a foamable sheet having a thickness of 0.3 mm. After pressing, the foamable sheet was cooled to room temperature (23° C.). Both sides of the obtained foamable sheet were crosslinked by irradiating with 2.5 Mrad of electron beam at an acceleration voltage of 500 keV, and then heated to 250°C to foam the foamable sheet, thereby obtaining a resin foam sheet with an expansion ratio of 2.5 times and a thickness of 0.30 mm.

[0066] [Examples 6 to 8] A resin foam sheet was obtained in the same manner as in Example 5, except that the compounding ratio of the inorganic filler and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0067] [Comparative Example 1] A resin foam sheet was obtained in the same manner as in Example 1, except that the polyolefin resin (1) was not contained.

[0068] [Comparative Example 2] A resin foam sheet was obtained in the same manner as in Example 1, except that the compounding ratio of the thermoplastic elastomer (1), the polyolefin resin (1), and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0069] [Comparative Example 3] A resin foam sheet was obtained in the same manner as in Example 3, except that the compounding ratio of the thermoplastic elastomer (2), the polyolefin resin (2), and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0070] [Comparative Examples 4 to 5] A resin foam sheet was obtained in the same manner as in Example 5, except that the compounding ratio of the inorganic filler and the thermally decomposable foaming agent was changed to that shown in Table 1.

[0071] The physical properties and evaluation results of the resin foam sheets obtained in the above Examples and Comparative Examples are shown in Table 1 below. [Table 1]

[0072] As is clear from the above results, the resin foam sheets of the examples have a recovery rate of 90% or more after 200% tension at 23°C, and a surface tackiness of 1,200 gf / cm 2 Since the recovery rate is less than 90% or the surface tackiness is less than 1,200 gf / cm, it is understood that the resin foam sheets are unlikely to be damaged or have a deterioration in performance even when repeatedly folded, have good handleability, and are excellent in productivity. In contrast, the resin foam sheets of each comparative example have a recovery rate of less than 90% or a surface tackiness of less than 1,200 gf / cm 2 Since the thickness exceeds 1 mm, repeated bending or the like easily causes damage or reduces performance, and it is therefore inferior in terms of handleability and productivity.

Claims

1. It has elasticity, a recovery rate of 90% or more after 200% tension at 23°C, and a surface tackiness of 1,200 gf / cm ２ The resin foam sheet comprises a thermoplastic elastomer and an inorganic filler, the content ratio of the thermoplastic elastomer to the inorganic filler being 100:30 to 100:150 in terms of mass ratio, and the inorganic filler being a spherical filler.

2. It has elasticity, a recovery rate of 90% or more after 200% tension at 23°C, and a surface tackiness of 1,200 gf / cm ２ a content ratio of the styrene-based thermoplastic elastomer to the polyolefin-based resin is 85:15 to 70:30 in terms of mass ratio, and the resin content in the resin foam sheet is 70 mass% or more based on the total amount of the resin foam sheet.

3. The resin foam sheet according to claim 1 or 2, having a 25% compressive strength of 500 kPa or less.

4. The resin foam sheet according to claim 1 , comprising a thermoplastic elastomer and a polyolefin resin.

5. The resin foam sheet according to claim 4 , wherein the polyolefin resin is at least one selected from a polyethylene resin and a polypropylene resin.

6. The resin foam sheet according to any one of claims 1 to 5, having a closed cell rate of 80% or more.

7. The resin foam sheet according to any one of claims 1 to 6, having a thickness of 0.03 to 2 mm.

8. An adhesive tape comprising the resin foam sheet according to any one of claims 1 to 7 and an adhesive material provided on at least one surface of the resin foam sheet.

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