Foam sheet

Abstract

To provide a foam sheet that is highly impact-resistant, lightweight, and resistant to tearing. [Solution] The foam sheet 1A of the present invention has a surface density of 0.02 g / cm³ 2 The following conditions are met: the tensile modulus is 7 MPa or higher, the tear strength is 50 N / cm or higher, and the foaming ratio of at least one of the first surface 11 and the second surface 12 opposite to the first surface 11 is lower than the foaming ratio of the interior 13.

Description

[Technical Field]

[0001] This invention relates to a foamed sheet. [Background technology]

[0002] Foams have traditionally been widely used as cushioning materials for packaging. When foams are used as cushioning materials for packaging, resins with high modulus of elasticity, such as polypropylene, are often used to ensure ease of handling and strength (see, for example, Patent Document 1). Such foams exhibit high rigidity, which enhances impact resistance, and can also be made lighter. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2009-84304 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, with conventional foams, lowering the surface density to reduce weight presents a problem: it becomes difficult to prevent cracking and splitting while maintaining high rigidity.

[0005] Therefore, the object of the present invention is to provide a foam sheet that is highly rigid, lightweight, and resistant to tearing and cracking. [Means for solving the problem]

[0006] As a result of diligent research, the inventors have found that the above problems can be solved by setting the surface density of the foam to below a predetermined value, setting the tensile modulus and tear strength of the foam to above a predetermined value, and setting the foaming ratio of the surface of the foam lower than the foaming ratio of the interior, and have completed the present invention as described below. In other words, the present invention provides the following [1] to [8].

[0007] [1] Surface density is 0.02 g / cm³ 2 The following conditions apply: the tensile modulus is 7 MPa or higher, and the tear strength is 50 N / cm or higher. A foam sheet in which the foaming ratio of at least one of the first surface and the second surface opposite to the first surface is lower than the foaming ratio of the interior. [2] The foam sheet described in [1] above, in which the foaming ratio of the first surface and the foaming ratio of the second surface are lower than the foaming ratio of the interior. [3] The foam sheet described in [1] or [2] above, which contains a polyolefin resin. [4] The foam sheet according to [3] above, wherein the polyolefin resin comprises a polypropylene resin. [5] The foaming ratio of at least one of the surfaces is 5 to 30 times, A foam sheet according to any one of the above [1] to [4], wherein the internal foaming ratio is 20 to 50 times. [6] comprising an intermediate layer and a surface layer provided on at least one of the surfaces of the intermediate layer and located on at least one of the first surface and the second surface, A foam sheet according to any one of the above [1] to [5], wherein the foaming ratio of the surface layer is lower than the foaming ratio of the middle layer. [7] comprising a first surface layer on the first surface, a second surface layer on the second surface, and an intermediate layer provided between the first surface layer and the second surface layer, A foam sheet according to any one of the above [1] to [5], wherein the foaming ratio of the first surface layer and the second surface layer is lower than the foaming ratio of the middle layer. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a foam sheet that is highly impact-resistant, lightweight, and resistant to tearing. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view showing one embodiment of the foam sheet of the present invention. [Figure 2] It is a schematic cross-sectional view showing an embodiment of the foam sheet of the present invention.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described with reference to embodiments. The foam sheet of the present invention has a surface density of 0.02 g / cm 2 or less, a tensile elastic modulus of 7 MPa or more, a tear strength of 50 N / cm or more, and a foaming ratio of at least one of the first surface and the second surface opposite to the first surface is lower than the internal foaming ratio. Thereby, the rigidity of the foam sheet can be increased, the foam sheet can be made lighter, and tearing and cracking of the foam sheet can be made less likely to occur.

[0011] (Surface density) The surface density of the foam sheet of the present invention is 0.02 g / cm 2 or less. When the surface density of the foam sheet is greater than 0.02 g / cm 2 ​​​​​​​​​​​​​​​​The density of the foam sheet of the present invention is preferably 0.2 g / cm³. 3 The density of the foam sheet of the present invention is 0.2 g / cm³. 3 The following conditions make it easier to achieve weight reduction when using the foam sheet of the present invention in applications such as cushioning material, gap filler, and artificial feathers. Furthermore, the density of the foam sheet of the present invention is 0.2 g / cm³. 3 The following conditions make it easier to improve flexibility. From these viewpoints, the density of the foam sheet of the present invention is more preferably 0.15 g / cm³. 3 The following, and more preferably 0.1 g / cm³ 3 The following, and more preferably 0.05 g / cm³ 3 The following applies: The lower limit of the density range of the foam sheet of the present invention is not particularly limited, but the surface density of the foam sheet of the present invention is typically 0.01 g / cm³. 2 Therefore, from the viewpoint of the strength of the foam sheet, a preferred strength is 0.02 g / cm². 2 This concludes the explanation. The density of the foam sheet can be measured by the method described in the examples below. Furthermore, the density of the foam sheet can be adjusted by the type of resin constituting the foam sheet and the foaming ratio of the foam sheet.

[0013] (Tensile modulus of elasticity) The tensile modulus of the foam sheet of the present invention is 7 MPa or higher. If the tensile modulus of the foam sheet is less than 7 MPa, the rigidity of the foam sheet may be low, and its impact resistance may also be low. Furthermore, if the tensile modulus of the foam sheet is less than 7 MPa, the handling of the foam sheet may be poor. From this viewpoint, the tensile modulus of the foam sheet of the present invention is preferably 8 MPa or higher, and more preferably 9 MPa or higher. There is no particular upper limit to the range of the tensile modulus of the foam sheet of the present invention, but the tensile modulus of the foam sheet of the present invention is usually 15 MPa or lower, and from the viewpoint of the flexibility of the foam sheet, it is preferably 12 MPa or lower. The tensile modulus of the foam sheet can be measured by the method described in the examples below. Furthermore, the tensile modulus of the foam sheet can be adjusted by the type of resin constituting the foam sheet and the foaming ratio of the inside and surface of the foam sheet.

[0014] (Tear strength) The tear strength of the foam sheet of the present invention is 50 N / cm or more. If the tear strength of the foam sheet is less than 50 N / cm, the foam sheet may be prone to tearing or cracking. From this viewpoint, the tear strength of the foam sheet of the present invention is preferably 54 N / cm or more, more preferably 57 N / cm or more, and even more preferably 60 N / cm or more. The upper limit of the range of tear strength of the foam sheet of the present invention is not particularly limited, but the tear strength of the foam sheet of the present invention is usually 150 N / cm or less, and from the viewpoint of the flexibility of the foam sheet, it is preferably 120 N / cm or less. The tear strength of the foam sheet can be measured by the method described in the examples below. Furthermore, the tear strength of the foam sheet can be adjusted by the type of resin constituting the foam sheet and the foaming ratio of the surface and interior of the foam sheet.

[0015] (Expansion ratio) The foaming ratio of at least one of the first surface and the second surface opposite the first surface of the foam sheet of the present invention is lower than the foaming ratio of the interior of the foam sheet of the present invention. If the foaming ratios of both the first and second surfaces of the foam sheet are higher than or the same as the foaming ratio of the interior of the foam sheet, it becomes difficult to make a foam sheet with low surface density highly rigid while also increasing its tear strength. From this viewpoint, it is preferable that the foaming ratios of both the first and second surfaces of the foam sheet of the present invention are lower than the foaming ratio of the interior of the foam sheet of the present invention.

[0016] In the foamed sheet of the present invention, at least one surface, preferably both the first and second surfaces, which has a lower foaming ratio than the interior, contains air bubbles and constitutes a part of the foam. In this way, the foamed surface of the foamed sheet makes it lighter and prevents creases from forming on the surface due to buckling stress when winding it into a roll, thus preventing defects in appearance. Furthermore, it is possible to increase the length of the roll obtained by winding the foamed sheet and improve handling. In addition, because the middle layer has a low density, the flexibility in the compression direction is improved, making it easier to improve the performance when used in applications such as cushioning material, gap filler, and artificial feathers.

[0017] The foaming ratio of at least one surface, which is lower than that of the interior, is preferably 5 to 30 times. When the foaming ratio of the surface of the foam sheet of the present invention is 5 to 30 times, the foam sheet of the present invention becomes lighter and, while maintaining high rigidity, is more resistant to tearing and cracking. From this viewpoint, the foaming ratios of the above surfaces are more preferably 10 to 25 times, and even more preferably 12 to 23 times. Furthermore, it is even more preferable that both the foaming ratio of the first surface and the foaming ratio of the second surface are within the above range. Note that the foaming ratios of the first surface and the foaming ratio of the second surface of the foam sheet of the present invention may be the same or different.

[0018] The foaming ratio inside the foam sheet of the present invention is preferably 20 to 50 times. When the foaming ratio inside the foam sheet of the present invention is 20 times or more, the foam sheet of the present invention can be made lighter and its flexibility in the compression direction can be further improved. When the foaming ratio inside the foam sheet of the present invention is 50 times or less, the rigidity of the foam sheet of the present invention can be further increased and the tearing of the foam sheet of the present invention can be further suppressed. From this viewpoint, the foaming ratio inside the foam sheet of the present invention is more preferably 22 to 45 times, and even more preferably 25 to 43 times.

[0019] In the foam sheet of the present invention, the ratio of the foaming ratio of the interior to the foaming ratio of the surface, which has a lower foaming ratio than the interior (interior foaming ratio / surface foaming ratio), is preferably 1.2 to 5. When the foaming ratio ratio is 1.2 to 5, the rigidity of the foam sheet can be further increased, the foam sheet can be made lighter, and the foam sheet can be made even less prone to tearing or cracking. From this viewpoint, the foaming ratio ratio is more preferably 1.5 to 4, and even more preferably 1.8 to 3.

[0020] The overall foaming ratio of the foam sheet of the present invention is preferably 18 to 40 cm². 3 The value is / g. The overall foaming ratio of the foam sheet is 18-40cm². 3 If the foaming ratio is 20-37 cm² / g, the rigidity of the foam sheet can be further increased while making the foam sheet even lighter. From this viewpoint, the overall foaming ratio of the foam sheet of the present invention is more preferably 20-37 cm² / g. 3 The value is / g, and more preferably 22-34cm 3 It is / g.

[0021] As described later, by making the foam sheet multilayered and appropriately adjusting the foaming ratio of the surface layer and the middle layer, the foaming ratio of the surface can be made lower than that of the interior, or the foaming ratio can be kept within the above range. In addition, generally, increasing the degree of crosslinking tends to decrease the foaming ratio. Therefore, even without making it multilayered, for example, by increasing the degree of crosslinking on the surface of the foam sheet and decreasing the degree of crosslinking in the interior, the foaming ratio of the surface can be made lower than that of the interior, or the foaming ratio can be kept within the above range. Alternatively, the foaming ratio can be appropriately adjusted by making it multilayered and adjusting the degree of crosslinking.

[0022] The foaming ratio of the surface of the foam sheet refers to the foaming ratio from the surface of the foam sheet up to a thickness that corresponds to 10% of the total thickness in the thickness direction, while the foaming ratio of the interior of the foam sheet refers to the foaming ratio at the center of the foam sheet in the thickness direction, up to a thickness that corresponds to 10% of the total thickness in the thickness direction.

[0023] (degree of crosslinking) The degree of crosslinking of the foam sheet of the present invention is preferably 20 to 70%. A degree of crosslinking of 20 to 70% makes it easier to adjust the foaming ratio within a desired range while improving the various performance characteristics of the foam sheet. From this viewpoint, the degree of crosslinking of the foam sheet of the present invention is more preferably 30 to 60%, and even more preferably 35 to 55%. The degree of crosslinking of the foam sheet can be measured by the method described in the examples below. The degree of crosslinking of the foam sheet can be adjusted by the crosslinking conditions during the manufacturing of the foam sheet. The degree of crosslinking of the foam sheet may be generally uniform in the thickness direction, or it may vary. If the degree of crosslinking varies in the thickness direction, for example, the degree of crosslinking on the surface may be increased and the degree of crosslinking in the interior may be decreased. By increasing the degree of crosslinking on the surface and decreasing the degree of crosslinking in the interior, as described above, it becomes easier to make the foaming ratio of the surface higher than that of the interior.

[0024] (Thickness) The thickness of the foam sheet of the present invention is preferably 450 to 1800 μm. When the thickness of the foam sheet of the present invention is 450 to 1800 μm, the foam sheet can be made even lighter, and the impact resistance of the foam sheet can be easily improved. From this viewpoint, the thickness of the foam sheet of the present invention is more preferably 690 to 1500 μm, and even more preferably 800 to 1240 μm.

[0025] (Multilayer foam sheet) The foam sheet of the present invention may be a single-layer foam sheet or a multi-layer foam sheet, provided that the foaming ratio of at least one of the first and second surfaces is lower than the foaming ratio of the interior. However, a multi-layer foam sheet is preferred. Being a multi-layer foam sheet makes it easier to make the foaming ratio of the surface lower than that of the interior. Furthermore, it makes it easier to increase the tensile modulus and tear strength of the foam sheet even if the surface density of the foam sheet is low.

[0026] In the case of a multilayer foam sheet of the present invention, it has an intermediate layer and a surface layer provided on at least one surface of the intermediate layer and located on at least one of the first and second surfaces, wherein the foaming ratio of the surface layer is lower than that of the intermediate layer. Furthermore, the foam sheet of the present invention may also preferably comprise a first surface layer on a first surface, a second surface layer on a second surface, and an intermediate layer provided between the first and second surface layers. In this case, it is sufficient if the foaming ratio of at least one of the first and second surface layers is lower than that of the intermediate layer, but it is preferable that the foaming ratios of both surface layers are lower than that of the intermediate layer. By forming a multilayer structure in this way and lowering the foaming ratios of both surface layers, it becomes easier to increase the tensile modulus and tear strength of the foam sheet while keeping the surface density low. The foaming ratios of the surface layers, for example, the first surface layer, the second surface layer, and the intermediate layer, can be measured by the method described in the examples below. The foaming ratio of each layer can also be adjusted by the degree of crosslinking in each layer, the type and amount of foaming agent, the type of resin constituting each layer, and so on.

[0027] In the foam sheet of the present invention, the foaming ratio of the surface layer, which is lower than that of the middle layer (for example, the foaming ratios of the first and second surface layers), is preferably 5 to 30 times, more preferably 10 to 25 times, and even more preferably 12 to 23 times, similar to the foaming ratios of at least one of the surfaces and the first and second surfaces described above. The foaming ratios of the first surface layer and the second surface layer in the foam sheet of the present invention may be the same or different. Furthermore, the foaming ratio of the middle layer in the foam sheet of the present invention is preferably 20 to 50 times, more preferably 22 to 45 times, and even more preferably 25 to 43 times, similar to the foaming ratio of the interior of the foam sheet described above.

[0028] In the foam sheet of the present invention, the ratio of the foaming ratio of the middle layer to the foaming ratio of the surface layer, which has a lower foaming ratio than the foaming ratio of the middle layer (foaming ratio of the middle layer / foaming ratio of the surface layer), is preferably 1.2 to 5, more preferably 1.5 to 4, and even more preferably 1.8 to 3, similar to the ratio of the foaming ratio of the interior to the foaming ratio of the surface layer of the foam sheet described above.

[0029] The thickness of the surface layer in the foam sheet of the present invention is preferably 100 to 300 μm. When the thickness of the surface layer is 100 to 300 μm, tearing of the foam sheet can be further suppressed and the foam sheet can be made even lighter. From this viewpoint, the thickness of the surface layer is more preferably 150 to 250 μm, and even more preferably 120 to 220 μm. Note that the thickness of the surface layer refers to the thickness of each surface layer, and if the foam sheet has a first and a second surface layer, it is preferable that each of the first and second surface layers be within the above range. The thicknesses of the first surface layer and the second surface layer in the foam sheet of the present invention may be the same or different.

[0030] The thickness of the middle layer in the foam sheet of the present invention is preferably 350 to 1200 μm. When the thickness of the middle layer is 350 to 1200 μm, the foam sheet can be made lighter, and the flexibility in the compression direction can be further improved, making it easier to increase the impact resistance of the foam sheet of the present invention and making it less prone to cracking and tearing. From this viewpoint, the thickness of the middle layer is more preferably 450 to 1000 μm, and even more preferably 500 to 800 μm.

[0031] In the foam sheet of the present invention, the ratio of the thickness of the middle layer to the thickness of the surface layer (thickness of the middle layer / thickness of the surface layer) is preferably 1.5 to 10. When the above thickness ratio is 1.5 to 10, the rigidity of the foam sheet can be further increased, the foam sheet can be further reduced in weight, and the foam sheet can be made even less prone to tearing or cracking. From this viewpoint, the above thickness ratio is more preferably 2.0 to 7, and even more preferably 2.5 to 5.

[0032] The foam sheet of the present invention may further include one or more other layers between the first surface layer and the middle layer, as long as it achieves the effects of the present invention. In this case, the other layers are preferably foam layers made of foam, and their foaming ratio is preferably higher than that of the first surface layer and lower than that of the middle layer. Similarly, the foam sheet of the present invention may further include one or more other layers between the second surface layer and the middle layer, insofar as it achieves the effects of the present invention. In this case, the other layers are preferably foam layers made of foam, and their foaming ratio is preferably higher than that of the second surface layer and lower than that of the middle layer.

[0033] (resin) The resin constituting the foam sheet of the present invention is not particularly limited, but examples include polyolefin resins, urethane resins, acrylic resins, and elastomer resins. The resin used in the foam sheet may be used alone or in combination of two or more types. Among these, polyolefin resins are preferred for the foam sheet of the present invention. The inclusion of polyolefin resin in the foam sheet further improves the flexibility and mechanical strength of the foam sheet. The polyolefin resin content in the foam sheet is preferably 60 to 100 parts by mass, more preferably 80 to 100 parts by mass, even more preferably 90 to 100 parts by mass, and even more preferably 95 to 100 parts by mass, per 100 parts by mass of the resin component.

[0034] (Polyolefin resin) Examples of polyolefin resins include polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer. Among these, polyethylene resin and polypropylene resin are preferred, and polypropylene resin is more preferred. By including polypropylene resin in the foam sheet of the present invention, the rigidity of the foam sheet of the present invention can be increased. From this viewpoint, the content of polypropylene resin in the foam sheet of the present invention is preferably 30 to 90 parts by mass, more preferably 40 to 80 parts by mass, and even more preferably 45 to 75 parts by mass, per 100 parts by mass of the resin component of the foam sheet.

[0035] The polyolefin resin contained in the foam sheet of the present invention preferably contains polyethylene resin in addition to polypropylene resin. By containing polyethylene resin in addition to polypropylene resin, it is possible to suppress the occurrence of cracks in the foam sheet of the present invention. From this viewpoint, the polyethylene resin content in the foam sheet of the present invention is preferably 10 to 70 parts by mass, more preferably 20 to 60 parts by mass, and even more preferably 25 to 55 parts by mass, per 100 parts by mass of the resin component of the foam sheet.

[0036] The polypropylene resin used as a polyolefin resin is not particularly limited, and examples include propylene homopolymer (homopolypropylene) and copolymers of propylene and other olefins. The copolymer of propylene and other olefins may be a block copolymer, a random copolymer, or a random block copolymer, but propylene homopolymer (homopolypropylene) and random copolymer (random polypropylene) are preferred, and random copolymer (random polypropylene) is more preferred. Furthermore, two or more polypropylene resins may be used in combination, for example, homopolypropylene and random polypropylene may be used in combination.

[0037] Examples of copolymers of propylene and other olefins include propylene-α-olefins that preferably contain 75% by mass or more, and more preferably 90% by mass or more, of propylene. Other olefins copolymerized with propylene include, for example, α-olefins such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, and 1-decene, with ethylene being preferred among these. Therefore, ethylene-propylene random copolymer is more preferred as random polypropylene.

[0038] As a polyethylene resin used as a polyolefin resin, low-density polyethylene (density: 0.930 g / cm³) is used. 3 (less than), medium-density polyethylene (density: 0.930 g / cm³) 3 More than 0.942g / cm 3 (less than), high-density polyethylene (density: 0.942 g / cm³) 3 Examples include linear low-density polyethylene, and among these, linear low-density polyethylene is preferred.

[0039] Linear low-density polyethylene is more preferably obtained by copolymerizing ethylene (for example, 75% or more by mass, preferably 90% or more by mass, relative to the total amount of monomer) with a small amount of α-olefin as needed. Specific examples of α-olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene, among which α-olefins having 4 to 10 carbon atoms are preferred. The density of linear low-density polyethylene is 0.870 to 0.910 g / cm³. 3 Preferably, 0.875~0.907 g / cm³ 3 More preferably, 0.880~0.905 g / cm³ 3 This is even more preferable. Multiple polyethylene resins may be used as the polyethylene resin, and polyethylene resins outside the density range described above may also be used.

[0040] Examples of ethylene-vinyl acetate copolymers used as polyolefin resins include those containing 50% by mass or more of ethylene.

[0041] The foamed sheet of the present invention may further contain a resin other than a polyolefin resin in addition to the polyolefin resin. Examples of resins other than polyolefin resins include elastomer resins. Examples of elastomer resins include various rubber components such as polybutadiene rubber, polyisoprene rubber, styrene-butadiene copolymer (SBR), or hydrogenated styrene rubber (HSBR). Other examples include thermoplastic elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

[0042] When the foam sheet of the present invention comprises the above-described surface layer (preferably a first and second surface layer) and middle layer, the types of resins constituting the surface layer (for example, a first surface layer and a second surface layer) and middle layer may be the same or different. However, it is preferable that the types of resins constituting the surface layer and middle layer are the same. Therefore, when the foam sheet of the present invention comprises the surface layer (for example, a first surface layer and a second surface layer) and middle layer, the resins constituting the surface layer and middle layer are preferably polyolefin resins, and the polyolefin resin preferably contains polypropylene resin, and more preferably contains polypropylene resin and polyethylene resin. The content of polyolefin resin, polypropylene resin, and polyethylene resin in each layer and their details are as described above. Furthermore, the content of polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer may be the same or different. However, it is preferable that the content of polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer be the same. Furthermore, the content of polypropylene resin in the polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer may be the same or different. However, it is preferable that the content of polypropylene resin in the polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer be the same. In addition, the content of polyethylene resin in the polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer may be the same or different. However, it is preferable that the content of polyethylene resin in the polyolefin resin in the surface layer (e.g., the first surface layer and the second surface layer) and the middle layer be the same.

[0043] (Foaming agent) The foamed sheet of the present invention is preferably a foamed sheet obtained by foaming a foaming composition containing the above-mentioned resin and foaming agent. Examples of foaming agents include thermal decomposition foaming agents, and as thermal decomposition foaming agents, organic foaming agents and inorganic foaming agents can be used. Thermal decomposition foaming agents are usually those that have a decomposition temperature higher than the melting temperature of the resin, for example, those with a decomposition temperature of 140 to 270°C should be used. Specific examples of organic blowing agents include azodicarbonamide, azodicarboxylic acid metal salts (such as barium azodicarboxylic acid), azobisisobutyronitrile and other azo compounds, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, hydrazodicarbonamide, hydrazine derivatives such as 4,4'-oxybis(benzenesulfonyl hydrazide) and toluenesulfonyl hydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide. Examples of inorganic blowing agents include ammonium carbonate, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate. Among these, azo compounds are preferred from the viewpoint of obtaining fine bubbles, as well as from the viewpoints of economy and safety, and azodicarbonamide is particularly preferred. These pyrolysis-type blowing agents can be used individually or in combination of two or more. The amount of thermal decomposition type foaming agent in the foaming composition can be adjusted according to the foaming ratio of the foam, but is preferably 1 to 25 parts by mass, more preferably 3 to 20 parts by mass, and even more preferably 4 to 15 parts by mass per 100 parts by mass of resin component.

[0044] When the foamed sheet of the present invention comprises the above-described surface layer (for example, a first surface layer and a second surface layer) and middle layer, it is preferable that the surface layer (for example, a first surface layer and a second surface layer) and middle layer are layers formed by foaming a foaming composition containing the above-described resin and foaming agent. The amount of foaming agent in the surface layer (for example, a first surface layer and a second surface layer) can be adjusted according to the foaming ratio of the foam, but is preferably 1 to 20 parts by mass, more preferably 3 to 15 parts by mass, and even more preferably 4 to 10 parts by mass, per 100 parts by mass of the resin component. Similarly, the amount of foaming agent in the foaming composition of the middle layer can be adjusted according to the foaming ratio of the foam, but is preferably 3 to 25 parts by mass, more preferably 4 to 20 parts by mass, and even more preferably 5 to 15 parts by mass, per 100 parts by mass of the resin component. Typically, the amount of foaming agent in the foaming composition constituting the surface layer (for example, a first surface layer and a second surface layer) is less than the amount of foaming agent in the foaming composition constituting the middle layer. Furthermore, the amounts of the foaming composition in the first surface layer and the foaming agent in the second surface layer may be the same or different.

[0045] When using a polyolefin resin as the resin, it is preferable to use the above-mentioned pyrolysis blowing agent as the blowing agent. However, a blowing agent other than a pyrolysis blowing agent may be used, for example, a physical blowing agent may be used. As a physical blowing agent, it is preferable to use a high-pressure inert gas. The inert gas is not particularly limited as long as it is inert to the resin composition and can impregnate it, and examples include carbon dioxide, butane gas, nitrogen gas, and air. These gases may be used in mixtures. Of these, carbon dioxide and butane gas are preferred from the viewpoint of easily increasing the foaming ratio of the foam. The inert gas used for impregnation is preferably in a supercritical or subcritical state.

[0046] (Other additives) The foamed sheet or foamed composition may contain additives commonly used in foams, such as crosslinking agents, crosslinking aids, antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, fillers, and decomposition temperature regulators, as needed. Among these, antioxidants and crosslinking aids are preferred.

[0047] Examples of antioxidants include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol and pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], sulfur-based antioxidants such as dilauryl thiodipropionate, phosphorus-based antioxidants, and amine-based antioxidants. The antioxidant is added in an amount of, for example, 0.01 to 5 parts by mass per 100 parts by mass of the resin component.

[0048] A polyfunctional monomer can be used as a crosslinking aid. By adding the crosslinking aid to the polyolefin resin, the electron dose irradiated in step (2) described later is reduced, thereby preventing the severance and degradation of resin molecules associated with electron beam irradiation. Specific examples of crosslinking aids include compounds with three functional groups in one molecule, such as trimethylolpropane trimethacrylate, trimellilic acid trialyl ester, 1,2,4-benzenetricarboxylic acid trialyl ester, and triallyl isocyanurate; compounds with two functional groups in one molecule, such as 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, and divinylbenzene; and diallyl phthalate, diallyl terephthalate, diallyl isophthalate, ethylvinylbenzene, neopentyl glycol dimethacrylate, lauryl methacrylate, and stearyl methacrylate. These crosslinking agents can be used individually or in combination of two or more.

[0049] The amount of crosslinking aid added is preferably 0.5 to 10 parts by mass, more preferably 1.0 to 8 parts by mass, and even more preferably 1.5 to 5 parts by mass, per 100 parts by mass of resin component. By adding 0.5 parts by mass or more, it is possible to stably obtain the desired degree of crosslinking in the foam layer, and by adding 10 parts by mass or less, it becomes easier to control the degree of crosslinking in the foam layer.

[0050] [Layer composition] The foam sheet of the present invention may be a single layer, for example, as shown in Figure 1A, where the foaming ratio of at least one of the first and second surfaces is lower than the foaming ratio of the interior. In the foam sheet 1A shown in Figure 1, the foaming ratio of at least one of the first surface 11 and second surface 12 is lower than the foaming ratio of the interior 13. Furthermore, the foam sheet of the present invention may be a multilayer foam sheet, for example, as long as the foaming ratio of at least one of the first and second surfaces is lower than the foaming ratio of the interior. For example, the foam sheet 1B shown in Figure 2 comprises a first surface layer 20, a second surface layer 30, and an intermediate layer 40. In the foam sheet 1B shown in Figure 2, the foaming ratio of at least one of the first surface layer 20 and the second surface layer 30 is lower than the foaming ratio of the intermediate layer 40, but it is preferable that the foaming ratios of both layers are lower than the foaming ratio of the intermediate layer 40. Also, one of the first surface layer 20 and the second surface layer 30 may be omitted.

[0051] [Method for manufacturing foam sheets] There are no particular limitations on the manufacturing method for producing the foamed sheet of the present invention, but it is preferable to produce it by foaming a foamed composition containing a resin component and a foaming agent with a foaming agent. In this case, foaming with the foaming agent may be carried out by heating or the like. Furthermore, it is preferable to obtain the foam by crosslinking the foamed composition and foaming the crosslinked foamed composition. In addition to the resin component and foaming agent, the foamed composition may contain additives as needed.

[0052] Specifically, it is industrially advantageous to manufacture the foam by a method comprising the following steps (1) to (3). Step (1): A step in which the raw materials of the foaming composition are supplied to a kneading device and kneaded to obtain a sheet-like foaming composition (foaming sheet). Step (2): A step of crosslinking the foamed composition obtained in step (1) by irradiating it with ionizing radiation. Step (3): A step in which the foamed composition crosslinked in step (2) is heated to a temperature above the decomposition temperature of the foaming agent to foam it and obtain a sheet-like foam.

[0053] In step (1), the raw materials constituting the foamed composition are kneaded using a kneader such as a Banbury mixer or a pressure kneader, and then a sheet-like foamed composition (foamed sheet) can be manufactured by continuously extruding it using an extruder, calender, conveyor belt casting, etc. If the foamed sheet is multilayered, the foamed composition constituting each layer is manufactured, and each foamed composition is extruded, for example, by co-extrusion, to manufacture a multilayered foamed sheet. However, a multilayered foamed sheet may also be manufactured by manufacturing single foam layers constituting each layer and stacking these foam layers.

[0054] Furthermore, examples of ionizing radiation used in step (2) include alpha rays, beta rays, gamma rays, and electron beams, but electron beams are preferred. The irradiation dose of ionizing radiation should be such that the desired degree of crosslinking can be obtained, but 0.1 to 10 Mrad is preferred, 0.2 to 5 Mrad is more preferred, and 0.5 to 3 Mrad is even more preferred. In addition, it is desirable to irradiate both sides of the foamed sheet with electron radiation. Furthermore, when the foam sheet of the present invention is a single layer, it is preferable to adjust the accelerating voltage and irradiation dose of ionizing radiation so that the degree of crosslinking on the surface of the foam sheet is greater than the degree of crosslinking inside the foam sheet. When the degree of crosslinking of the foam sheet is high, the foaming ratio is kept low, so by making the degree of crosslinking on the surface of the foam sheet greater than the degree of crosslinking inside the foam sheet, it becomes easy to make the foaming ratio on the surface of the foam sheet smaller than the foaming ratio inside the foam sheet. In step (3), the temperature at which the foaming composition is heated and foamed depends, for example, on the decomposition temperature of the pyrolysis-type foaming agent used as the foaming agent, but is usually 140 to 300°C, preferably 150 to 280°C. Furthermore, in step (3), the foam sheet may be stretched in either the MD (Machine Direction) or TD (Transverse Direction) or both after foaming or while foaming.

[0055] [Uses of foam sheets] The foamed sheet of the present invention is highly impact-resistant, lightweight, and resistant to tearing, making it suitable for use as a cushioning material and gap filler. As a cushioning material, it may be used as a packaging cushioning material, or as a cushioning material to prevent impacts from being applied to various components inside automobiles and electronic devices. As a gap filler, it may be used, for example, as a sealing material to fill gaps inside automobiles and electronic devices to prevent dust, moisture, etc., from entering the inside of electronic devices. Furthermore, the foam sheet of the present invention can also be used for the artificial feathers of shuttlecocks used in badminton. The artificial feathers of a shuttlecock consist of a shaft and a thin-film feather portion attached to the shaft, and the foam sheet of the present invention is preferably used for the feather portion. The foam sheet of the present invention is lightweight yet highly rigid and resistant to tearing and cracking, making it suitable for use in the artificial feathers of shuttlecocks. [Examples]

[0056] The present invention will be described in more detail by reference to examples, but the present invention is not limited in any way by examples.

[0057] The measurement and evaluation methods for each physical property are as follows: <Foaming ratio and thickness of the first surface layer, second surface layer, and middle layer> A sheet sample was imaged using an X-ray CT (Computed Tomography) device. The sheet sample region and bubble wall region of the sample, which consists of a first surface layer, a second surface layer, and a middle layer, were extracted by image analysis. The sheet sample volume and bubble wall volume were calculated from the extracted sheet sample region and bubble wall region. The expansion ratio was calculated from the obtained sheet sample volume and bubble wall volume values ​​using the following formula. Expansion ratio (au) = Sheet sample volume (pixels) / Bubble wall volume (pixels) Furthermore, regarding the thickness of the first surface layer, the second surface layer, and the middle layer, the transition of the expansion ratio from the point of change in the expansion ratio towards the surface layer in the thickness direction was evaluated, and the convergence point where the change in the expansion ratio became small was defined as the boundary of each layer, and measured from images of sheet samples acquired by an X-ray CT scanner.

[0058] [X-ray CT device] 3DX-ray microscope (high-resolution 3DX-ray microscope nano3DX, manufactured by Rigaku Corporation) [X-ray CT measurement conditions] X-ray target: Mo Lens: L1080 (1.08μm / pixel) Binning: 2 Exposure time: 4 seconds Number of images: 1000 sample images Shape: 4mm x 4mm x sample thickness

[0059] [Image Analysis] Images obtained from an X-ray CT scanner were analyzed using the image analysis software Avizo 3D Pro 2023.1.1 (manufactured by Thermo Fisher Scientific) following the procedure below. (i) Image cropping: The acquired image was cropped using "Crop Editor" so that the sheet sample occupied the entire image. However, the surface and outer layers of the sample were to be recognizable. The size of the cropped image was, for example, x=1300, y=600, z=1150. The XZ cross section was a cross section parallel to the surface of the sheet sample (i.e., a cross section perpendicular to the thickness direction).

[0060] (ii) Horizontal correction: Using the "Transform Editor," the image from (i) above (i.e., the image cropped in (i) above) was adjusted so that the surface of the sheet sample was horizontal in the image. Then, "Resample Transformed Image" was applied. The conditions for the analysis module were as follows: Resample Transformed Image Mode:extended Other conditions were set to their initial values.

[0061] (iii) Sheet sample region extraction - 1: For the image in (ii), "Interactive Thresholding" was applied and the threshold was adjusted so that the sheet sample region would be extracted. Subsequently, "Closing" and "Remove Small Spots" were applied to remove noise from the extracted region. The conditions for each analysis module were as follows: Interactive Thresholding Intensity Range: 100-255 Closing Size: 2 Other conditions were set to their initial values. Remove Small Spots Interpretation: XY planes Size: 50

[0062] (iv) Sheet sample region extraction - 2: For the image of (iii), "Compute Ambient Occlusion" was applied to calculate the extent to which the (iii) region covered the image in three dimensions. Subsequently, "Interactive Thresholding" was applied to adjust the threshold so that the sheet sample region would be extracted. The conditions for each analysis module were as follows: Compute Ambient Occlusion Maximum Distance: 100 Number of Rays: 50 Interactive Thresholding Intensity Range: 0.68-1

[0063] (v) Sheet sample area extraction - 3: Apply "OR Image" to the image in (iv) and add the extracted area of ​​the image in (iii) to the extracted area of ​​the image in (iv). Then, apply "Fill Holes" and "Opening" to adjust the image so that the sheet sample area is extracted. The conditions for each analysis module were as follows: · Fill Holes Interpretation: XY planes Other conditions were set to their initial values. Opening Size: 3 Other conditions were set to their initial values.

[0064] (vi) Image orientation conversion: The image from (v) was converted by applying "Crop Editor" so that the XY cross-section was parallel to the surface of the sheet sample.

[0065] (vii) Volume analysis of the sheet sample region: The volume of the sheet sample region in each cross-section in the thickness direction was analyzed by applying "Material Statistics" to the image in (vi). The conditions for the analysis module were as follows: • Material Statistics Select: Volume per slice

[0066] (viii) Bubble Region Extraction - 1: For the image in (ii), "Interactive Thresholding" was applied and the threshold was adjusted so that the bubble wall region was extracted. The extraction range was then applied to the sheet sample extraction region. Subsequently, "Closing" and "Remove Small Spots" were applied to remove noise from the extracted region. The conditions for each analysis module were as follows: Interactive Thresholding Intensity Range: 90-255 Mask:(v) Image Closing Size: 1 Other conditions were set to their initial values. Remove Small Spots Size: 1500

[0067] (ix) Bubble Region Extraction - 2: Apply "AND NOT Image" to the image of (v) to extract the bubble region by excluding the extracted region of (viii) from the extracted region of (v). Next, apply "Separate Objects" to divide the region where multiple bubbles were connected into individual regions. The conditions for the analysis module were as follows: • Separate Objects Marker Extent: 4 Other conditions were set to their initial values.

[0068] (x) Bubble wall region extraction: The "AND NOT Image" operation was applied to the image of (v) to extract the bubble wall region by excluding the extracted region of (ix) from the extracted region of (v).

[0069] (xi) Image orientation transformation: The "Crop Editor" was applied to the image from (x) to transform it so that the XY cross-section was parallel to the surface of the sheet sample.

[0070] (xii) Volume analysis of the bubble wall region: The volume of the bubble wall region in each cross-section in the thickness direction was analyzed by applying "Material Statistics" to the image of (xi). The conditions for the analysis module were as follows: • Material Statistics Select: Volume per slice

[0071] (xiii) Calculation of the overall thickness of the sheet sample and the foaming ratio of the sheet sample: These were derived using the following formulas, using the sample sheet volume data for each cross-section in (vii) and the bubble wall volume data for each cross-section in (xii). Total thickness of sheet sample (pixel) = Number of XY cross-sections (pixel) where the volume of the sheet sample region is greater than 0 Foaming ratio of the entire sheet sample (a.u.) = Total volume of the sheet sample region (pixel) / Total volume of the bubble wall region (pixel)

[0072] (xiv) Calculation of the thickness per division and the foaming ratio for each division interval when the thickness direction is divided into 100 parts: Derived from the following formula using the sample sheet volume data of each cross-section in (vii) and the bubble wall volume data of each cross-section in (xii). Thickness per division at 100 divisions (pixel) = Total thickness of sheet sample (pixel) / 100 Foaming ratio for each division interval (a.u.) = Total volume of sheet sample included in the region of the division interval (pixel) / Total volume of bubble wall included in the region of the division interval (pixel)

[0073] (xv) Analysis of the foaming ratio change points and calculation of the thickness of each layer: Using the foaming ratio calculated in (xiv), the difference in the foaming ratio between adjacent division intervals was calculated as an absolute value. The point where the calculated foaming ratio difference exceeded 1.3 was defined as the foaming ratio change point. Furthermore, when looking at the foaming ratio difference in order from the change point to each surface layer side, the point where it fell below 0.2 was defined as the foaming ratio convergence point. This convergence point was defined as the boundary of the layer. When the two division intervals corresponding to the convergence point were designated as division intervals A and B (A < B), division interval B was defined as the starting point of the layer, and division interval A was defined as the ending point of the layer. For example, when there were two change points, the thickness of each layer was derived from the following formula. First surface layer (μm) = Division interval A of the first convergence point × Thickness per division at 100 divisions (pixel) × Resolution (μm / pixel) Middle layer (μm) = {Division interval A of the second convergence point - Division interval B of the first convergence point + 1} × Thickness per division at 100 divisions (pixel) × Resolution (μm / pixel) Second surface layer (μm) = {101 - Division interval B of the second convergence point} × Thickness per division at 100 divisions (pixel) × Resolution (μm / pixel)

[0074] (xvi) Calculation of the expansion ratio of each layer: The expansion ratio was derived from the boundary positions of the layers calculated in (xv) using the following formula. The foaming ratio of the first surface layer (au) = Total volume (pixels) of the sheet sample region from division interval 1 to division interval A of the first convergence point / Total volume (pixels) of the bubble wall region from division interval 1 to division interval A of the first convergence point The expansion ratio of the middle layer (au) = Total volume (pixels) of the sheet sample region from the division interval B of the first convergence point to the division interval A of the second convergence point / Total volume (pixels) of the bubble wall region from the division interval B of the first convergence point to the division interval A of the second convergence point The foaming ratio of the second surface layer (au) = Total volume (pixels) of the sheet sample region from division interval B of the second convergence point to division interval 100 / Total volume (pixels) of the bubble wall region from division interval B of the second convergence point to division interval 100

[0075] <Density, surface density, expansion ratio, and thickness of foamed sheets> The density of the foam sheet was measured in accordance with JIS K7222. The thickness of the foam sheet was measured using a dial gauge. The surface density of the foam sheet was calculated by multiplying the density of the foam sheet by the thickness of the foam sheet. The reciprocal of the density of the foam sheet was used as the foaming ratio of the foam sheet.

[0076] <Degree of cross-linking of foamed sheet> Approximately 100 mg of test material was taken from the foam sheet, and its weight A (mg) was accurately weighed. The test material was taken evenly along the thickness direction of the foam sheet. Next, this test material was subjected to xylene 30 cm³ at 120°C. 3 After immersion for 24 hours, the material was filtered through a 200-mesh wire mesh, and the insoluble material on the mesh was collected. The material was then vacuum-dried, and the weight B (mg) of the insoluble material was accurately weighed. From the obtained value, the degree of crosslinking (mass %) was calculated using the following formula. Crosslinking degree (mass%) = 100×(B / A)

[0077] <Tear strength of foamed sheet> The tear strength of the foamed sheet was measured in accordance with JIS K7128-3:1998 (Plastics - Test methods for tear strength of films and sheets - Part 3: Right-angle tear method).

[0078] <Tensile modulus of foam sheet> The tensile modulus of the foam sheet was measured in accordance with the tensile test method of JIS K7113.

[0079] <Lightweight> The lightweight properties of the foam sheets were evaluated using the following criteria. (Evaluation Criteria) ○: Surface density is 0.2 g / cm³ 2 The following applies: ×: Surface density is 0.2 g / cm³ 2 It is larger than that.

[0080] The components used in the examples and comparative examples are as follows: (Polypropylene resin) • HomoPP: Homopolypropylene, manufactured by Prime Polymer, product name "J106G", MFR 15g / 10 min Random PP, manufactured by Sumitomo Chemical Co., Ltd., product name "AD571", MFR 0.5g / 10 min (Polyethylene resin) • LLDPE: Linear low-density polyethylene, manufactured by Prime Polymer, product name "Ultrasex 1020L", MFR 2.0g / 10 min (Additives) • Foaming agent ADCA: Azodicarbonamide • Antioxidant Phenolic antioxidants • Crosslinking agent 1,9-nonanediol dimethacrylate

[0081] [Example 1] 70 parts by mass of random PP, 30 parts by mass of LLDPE, 5.0 parts by mass of ADCA, 1 part by mass of a phenolic antioxidant, and 3 parts by mass of a crosslinking aid were prepared as the raw material for the first surface layer and the material for the second surface layer. In addition, 70 parts by mass of random PP, 30 parts by mass of LLDPE, 10.5 parts by mass of ADCA, 1 part by mass of a phenolic antioxidant, and 3 parts by mass of a crosslinking aid were prepared as the material for the middle layer. The raw materials for the first surface layer, the second surface layer, and the middle layer were supplied to a multilayer extrusion extruder and melt-kneaded at 150°C. The foamed composition obtained by kneading each supplied component was extruded from the extruder to obtain a foamed sheet having a laminated structure of the first surface foamed composition / middle layer foamed composition / second surface foamed composition. Next, the foamed sheet was crosslinked by irradiating it with an electron beam at an accelerating voltage of 1000 kV for 2.0 Mrad to obtain a crosslinked foamed sheet. Then, the crosslinked foamed sheet was continuously fed into a foaming furnace maintained at -250°C by hot air and an infrared heater, and heated and foamed while being stretched so that the thicknesses of the first surface layer, second surface layer, and middle layer were as shown in Table 1, thereby obtaining the foamed sheet of Example 1. The obtained foamed sheet was evaluated according to the evaluation method described above. The results are shown in Table 1.

[0082] [Examples 2-5] Foam sheets for Examples 2 to 5 were obtained in the same manner as in Example 1, except that the composition of the foaming composition and the thicknesses of the first surface layer, second surface layer, and middle layer in the foam sheet were as shown in Table 1. The obtained foam sheets were evaluated according to the evaluation method described above. The results are shown in Table 1.

[0083] [Comparative Example 1] 70 parts by mass of random PP, 30 parts by mass of LLDPE, 8.0 parts by mass of ADCA, 1 part by mass of a phenolic antioxidant, and 3 parts by mass of a crosslinking aid were prepared as materials for the foamed sheet. The raw materials for the foamed sheet were supplied to an extruder for extrusion molding and melt-kneaded at 150°C. The foamed composition obtained by kneading each supplied component was extruded from the extruder to obtain a foamed sheet. Next, the foamed sheet was crosslinked by irradiating it with an electron beam at an accelerating voltage of 1000 kV for 2.0 Mrad to obtain a crosslinked foamed sheet. Then, the crosslinked foamed sheet was continuously fed into a foaming furnace maintained at 250°C by hot air and an infrared heater, and heated and foamed while being stretched to the thickness shown in Table 1 to obtain a foamed sheet. The obtained foamed sheet was evaluated according to the evaluation method described above. The results are shown in Table 1.

[0084] [Comparative Example 2] A foam sheet for Comparative Example 2 was obtained in the same manner as for Comparative Example 1, except that 50 parts by mass of random PP, 50 parts by mass of LLDPE, 8.0 parts by mass of ADCA, 1 part by mass of a phenolic antioxidant, and 3 parts by mass of a crosslinking aid were prepared as materials for the foam sheet. The obtained foam sheet was evaluated according to the evaluation method described above. The results are shown in Table 1.

[0085] [Table 1]

[0086] In each of the above embodiments, the foaming ratio of at least one of the first surface and the second surface opposite the first surface was lower than the foaming ratio of the interior. As a result, while the bending rigidity was high, the tear strength was greater compared to the comparative example. [Explanation of Symbols]

[0087] 1A, 1B Foam Sheet 11 First surface 12 Second surface 13 Inside 20 First surface layer 30 The second surface layer 40 middle class

Claims

1. Surface density is 0.02 g / cm³ ２ The following conditions apply: the tensile modulus is 7 MPa or higher, and the tear strength is 50 N / cm or higher. A foam sheet in which the foaming ratio of at least one of the first surface and the second surface opposite to the first surface is lower than the foaming ratio of the interior.

2. The foam sheet according to claim 1, wherein the foaming ratio of the first surface and the foaming ratio of the second surface are lower than the foaming ratio of the interior.

3. A foamed sheet according to claim 1 or 2, comprising a polyolefin resin.

4. The foamed sheet according to claim 3, wherein the polyolefin resin comprises a polypropylene resin.

5. The foaming ratio of at least one of the surfaces is 5 to 30 times. The foam sheet according to claim 1, wherein the internal foaming ratio is 20 to 50 times.

6. The structure comprises an intermediate layer and a surface layer provided on at least one of the surfaces of the intermediate layer and located on at least one of the first surface and the second surface. The foam sheet according to claim 1 or 2, wherein the foaming ratio of the surface layer is lower than the foaming ratio of the middle layer.

7. The material comprises a first surface layer on the first surface, a second surface layer on the second surface, and an intermediate layer provided between the first surface layer and the second surface layer. The foam sheet according to claim 1 or 2, wherein the foaming ratio of the first surface layer and the second surface layer is lower than the foaming ratio of the middle layer.

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