Method for manufacturing cross-linked polyolefin resin foam, and cross-linked polyolefin resin foam

Abstract

To provide a cross-linked polyolefin resin foam having excellent performance, and a method for manufacturing the cross-linked polyolefin resin foam having excellent performance.SOLUTION: A method for manufacturing a cross-linked polyolefin resin foam includes: making a cross-linked material 30 by irradiating electron beam to a composition 20 containing a polyolefin resin, an antioxidant, and a foaming agent; and foaming the cross-linked material 30 in a mold 40 so that a maximum foaming pressure in the mold 40 reaches 2.0 kPa or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a crosslinked polyolefin resin foam, and a crosslinked polyolefin resin foam. [Background technology]

[0002] Patent Document 1 discloses a foam obtained by expanding physically crosslinked expandable particles in a mold. The physically crosslinked expandable particles are obtained by crosslinking expandable particles such as polyolefin resin particles using a physical crosslinking method such as electron beam irradiation. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2018-521182 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for crosslinked polyolefin resin foams with excellent performance. The present disclosure has an object to provide a cross-linked polyolefin resin foam having excellent performance and a method for producing the cross-linked polyolefin resin foam having excellent performance. The present disclosure can be realized in the following aspects. [Means for solving the problem]

[0005] [1] A method for producing a crosslinked polyolefin resin foam, comprising irradiating a composition containing a polyolefin resin, an antioxidant, and a foaming agent with an electron beam to form a crosslinking raw material, and foaming the crosslinking raw material in a mold so that the maximum foaming pressure in the mold is 2.0 kPa or more. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide a crosslinked polyolefin resin foam having excellent performance and a method for producing a crosslinked polyolefin resin foam having excellent performance. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is an explanatory diagram illustrating a step of irradiating a sheet-shaped composition with an electron beam. [Figure 2] FIG. 10 is an explanatory diagram illustrating a step of putting a cross-linking raw material into a mold. [Figure 3] FIG. 2 is a schematic diagram illustrating a step of foaming a cross-linking raw material in a mold. DETAILED DESCRIPTION OF THE INVENTION

[0008] Here, a preferred example of the present disclosure will be described. [2] A sample obtained by mixing the polyolefin resin and the antioxidant in the mass ratio of the composition was irradiated with the electron beam used in forming the crosslinked polyolefin resin foam. The shear rate of the sample measured in accordance with JIS K 7199:1999 was 122 sec -1 The method for producing a crosslinked polyolefin resin foam according to [1], wherein the melt viscosity at 200°C is 10,000 Pa·s or less. [3] The method for producing a crosslinked polyolefin resin foam according to [1] or [2], wherein the crosslinked polyolefin resin foam has a gel fraction of 1% or more and 70% or less, measured in accordance with JIS K 6796:2015. [4] The method for producing a crosslinked polyolefin resin foam according to any one of [1] to [3], wherein the antioxidant is a hindered phenol-based antioxidant. [5] The maximum tensile strength measured in accordance with JIS K 6767:2015 is 0.50 MPa or more, A cross-linked polyolefin resin foam having a maximum elongation of 30.0% or more as measured in accordance with JIS K 6767:2015.

[0009] The present disclosure will be described in detail below. In this specification, when a numerical range is indicated using "-", it is intended to include both the lower limit and the upper limit unless otherwise specified. For example, the expression "10-20" includes both the lower limit "10" and the upper limit "20". In other words, "10-20" has the same meaning as "10 or more and 20 or less".

[0010] 1. First embodiment The cross-linked polyolefin resin foam 10 of the first embodiment (see Figure 3, hereinafter also simply referred to as resin foam 10) is a foam obtained by irradiating a composition 20 (see Figure 1) containing a polyolefin resin, an antioxidant, and a foaming agent with an electron beam to form a cross-linking raw material 30 (see Figure 2), and then foaming the cross-linking raw material 30 in a mold 40 (see Figure 2) so that the maximum foaming pressure in the mold 40 is 2 kPa or more.

[0011] 1-1. Raw materials for resin foam 10 1-1-1. Polyolefin resin A polyolefin resin is a resin whose main component is an olefin component unit. A resin whose main component is an olefin component unit is a resin containing 50% by mass or more of an olefin component unit. The content of the olefin component unit in the resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and it is particularly preferable that the resin component is composed only of a polyolefin resin.

[0012] The polyolefin resin is preferably at least one selected from the group consisting of polyethylene resins, polypropylene resins, polybutene, polypentene, and copolymers of olefin monomers and monomers copolymerizable with the olefin monomers.

[0013] The content of the polyolefin resin can be freely set as long as it does not impair the purpose and effect of the present technology.

[0014] 1-1-1-1.Polyethylene resin Examples of polyethylene resins include low density polyethylene, linear low density polyethylene, linear very low density polyethylene, medium density polyethylene, high density polyethylene, and copolymers containing ethylene as the main component. Examples of copolymers containing ethylene as a main component include copolymers of ethylene and one or more comonomers selected from α-olefins having 3 to 10 carbon atoms, vinyl esters, unsaturated carboxylic acid esters, conjugated dienes, and non-conjugated dienes. Examples of α-olefins having 3 to 10 carbon atoms include propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene. Examples of vinyl esters include vinyl acetate and vinyl propionate. Examples of unsaturated carboxylic acid esters include methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate.

[0015] The polyolefin resin is preferably a polyethylene resin, and low-density polyethylene (LDPE) is preferably used as the polyethylene resin from the viewpoint of crosslinking of molecular chains and melt tension at high temperatures during foaming.

[0016] 1-1-1-2. Polypropylene resin The polypropylene-based resin is preferably at least one selected from the group consisting of propylene homopolymers such as isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene, propylene-ethylene random copolymers, propylene-ethylene block copolymers, propylene-butene random copolymers, propylene-butene block copolymers, propylene-ethylene-butene terpolymers, propylene-acrylic acid copolymers, and propylene-maleic anhydride copolymers.

[0017] 1-1-1-3. Other resins In addition, the raw materials of the resin foam 10 may contain, in addition to the polyolefin resin, other resins, resins other than polyolefin resins such as elastomers, rubber components, etc., as long as the purpose and effects of the present technology are not impaired. The resin other than polyolefin resin is preferably one or more selected from the group consisting of thermoplastic resins such as polystyrene resins, polyamide resins, and polyester resins. The elastomer other than polyolefin resin is preferably one or more selected from the group consisting of olefin thermoplastic elastomers and styrene thermoplastic elastomers. The rubber component is preferably one or more selected from the group consisting of ethylene-propylene-diene rubber (EPDM), natural rubber (NR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), and silicone rubber (SI).

[0018] 1-1-2. Antioxidants Examples of the antioxidant include hindered phenol-based antioxidants, hindered amine-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants, and in particular, those having a molecular weight of preferably 500 or more, more preferably 600 or more, even more preferably 700 or more, and particularly preferably 800 or more. Furthermore, the antioxidant is preferably a hindered phenol-based antioxidant.

[0019] Examples of the hindered phenol antioxidant include pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 4-methyl-2,6-di-tert-butylphenol, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4 Examples of suitable hydroxyphenyl propionates include pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-C9 side chain alkyl ester, 2,4-bis[(octylthio)methyl]-o-cresol, 2,4-bis[(dodecylthio)methyl]-6-methylphenol, and 3,9-bis[1,1-dimethyl-2-[(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxaspiro[5,5]undecane. Among these, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate are more preferred.

[0020] Examples of the hindered amine antioxidant include tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate, bis(1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate, methyl 1,2,2,6,6-pentamethyl-4-piperidylsebacate, bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate, poly((6-(( 1,1,3,3-tetramethylbutyl)amino)-s-tetrazine-2,4-diyl)(2,2,6,6-tetramethyl-4-piperidyl)imino)hexamethylene(2,2,6,6-tetramethyl-4-piperidyl)imino)), bis(1,2,2,6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]butyl malonate, and the like.

[0021] The amount of antioxidant is preferably 0.10 parts by mass or more, more preferably 0.20 parts by mass or more, and even more preferably 0.30 parts by mass or more, per 100 parts by mass of the polyolefin-based resin. The amount of antioxidant is preferably 1.50 parts by mass or less, more preferably 1.00 parts by mass or less, and even more preferably 8.00 parts by mass or less, per 100 parts by mass of the polyolefin-based resin. Therefore, the amount of antioxidant is preferably 0.10 parts by mass or more and 1.50 parts by mass or less, more preferably 0.20 parts by mass or more and 1.00 parts by mass or less, and even more preferably 0.30 parts by mass or more and 8.00 parts by mass or less, per 100 parts by mass of the polyolefin-based resin.

[0022] 1-1-3. Foaming agent The foaming agent is preferably a thermal decomposition type that decomposes upon heating to generate gas, and is not particularly limited. For example, one or more of azodicarbonamide (ADCA), 2,2'-azobisisobutyronitrile, diazoaminobenzene, benzenesulfonylhydrazide, benzene-1,3-sulfonylhydrazide, diphenyloxide-4,4'-disulfonylhydrazide, 4,4'-oxybisbenzenesulfonylhydrazide, paratoluenesulfonylhydrazide, N,N'-dinitrosopentamethylenetetramine, N,N'-dinitroso-N,N'-dimethylphthalamide, terephthalazide, pt-butylbenzazide, sodium bicarbonate, ammonium bicarbonate, etc. are used. Azodicarbonamide and 4,4'-oxybisbenzenesulfonylhydrazide are particularly preferred.

[0023] The amount of the foaming agent is preferably 10.00 parts by mass or more, more preferably 13.00 parts by mass or more, and even more preferably 15.00 parts by mass or more, per 100 parts by mass of the polyolefin-based resin. The amount of the foaming agent is preferably 30.00 parts by mass or less, more preferably 25.00 parts by mass or less, and even more preferably 20.00 parts by mass or less, per 100 parts by mass of the polyolefin-based resin. Therefore, the amount of the foaming agent is preferably 10.00 parts by mass or more and 30.00 parts by mass or less, more preferably 13.00 parts by mass or more and 25.00 parts by mass or less, and even more preferably 15.00 parts by mass or more and 20.00 parts by mass or less, per 100 parts by mass of the polyolefin-based resin.

[0024] 1-1-4.Foaming aid The foaming aid is not particularly limited. Examples of the foaming aid include metal oxides such as zinc oxide, zinc stearate, and lead oxide, lower or higher fatty acids or their metal salts, urea and its derivatives, etc. These foaming aids can be used alone or in combination.

[0025] The amount of zinc oxide as a foaming aid is preferably 0.10 parts by mass or more, more preferably 0.30 parts by mass or more, and even more preferably 0.50 parts by mass or more, per 100 parts by mass of the polyolefin-based resin. The amount of zinc oxide as a foaming aid is preferably 1.00 parts by mass or less, more preferably 0.80 parts by mass or less, and even more preferably 0.60 parts by mass or less, per 100 parts by mass of the polyolefin-based resin. Therefore, the amount of zinc oxide as a foaming aid is preferably 0.10 parts by mass or more and 1.00 parts by mass or less, more preferably 0.30 parts by mass or more and 0.80 parts by mass or less, and even more preferably 0.50 parts by mass or more and 0.60 parts by mass or less, per 100 parts by mass of the polyolefin-based resin.

[0026] The amount of zinc stearate as a foaming aid is preferably 0.50 parts by mass or more, more preferably 0.80 parts by mass or more, and even more preferably 0.90 parts by mass or more, based on 100 parts by mass of the polyolefin-based resin. The amount of zinc stearate as a foaming aid is preferably 1.50 parts by mass or less, more preferably 1.20 parts by mass or less, and even more preferably 1.10 parts by mass or less, based on 100 parts by mass of the polyolefin-based resin. Therefore, the amount of zinc stearate as a foaming aid is preferably 0.50 parts by mass or more and 1.50 parts by mass or less, more preferably 0.80 parts by mass or more and 1.20 parts by mass or less, and even more preferably 0.90 parts by mass or more and 1.10 parts by mass or less, based on 100 parts by mass of the polyolefin-based resin.

[0027] 1-1-5.Other ingredients The composition may optionally contain a release agent, a surface tension adjuster, a filler (such as calcium carbonate), a pigment, a plasticizer, a function-imparting agent (such as a flame retardant), etc. An example of the release agent is glycerin monostearate.

[0028] 1-2. Configuration of resin foam 10 1-2-1.Crosslinked structure A crosslinked structure is formed in the resin foam 10. When the resin foam 10 is in a sheet form, a crosslinked structure is formed in the surface layer portion of the resin foam 10 or throughout the entire resin foam 10. The resin foam 10 is obtained by foaming the crosslinked resin composition by heating or the like in step C, which is performed after step A, which will be described later, of obtaining a raw material composition and step B, which will be described later, of obtaining a crosslinked resin composition by irradiating with an electron beam. When the raw material composition obtained in step A is in a sheet form, at least one side, preferably both sides, of the sheet are irradiated with an electron beam, and it is preferred that at least one side, preferably both sides, of the sheet are irradiated. By irradiating with an electron beam at an acceleration voltage corresponding to the thickness of the sheet of the raw material composition, an appropriate crosslinked structure is formed in the surface layer portion of the sheet or within the sheet, allowing appropriate foaming in step C, which obtains the resin foam 10.

[0029] 1-2-2. Melt viscosity of the measurement sample The polyolefin resin and the antioxidant were mixed in the mass ratio of the composition (a composition containing a polyolefin resin, an antioxidant, and a foaming agent) to prepare a sample. The sample was irradiated with the electron beam used in forming the resin foam 10, and the resultant was used as a measurement sample. The measurement sample was subjected to a shear rate of 122 sec -1 The melt viscosity at 200°C is preferably 10,000 Pa·s or less, more preferably 5,000 Pa·s or less, and even more preferably 2,000 Pa·s or less, from the viewpoint of preventing cracks and fractures from occurring in the resin foam 10. -1 From the viewpoint of preventing bubbles from breaking, the melt viscosity at 200°C is preferably 400 Pa·s or more, more preferably 600 Pa·s or more, and even more preferably 800 Pa·s or more. -1 The melt viscosity at 200°C is preferably 400 Pa·s or more and 10,000 Pa·s or less, more preferably 600 Pa·s or more and 5,000 Pa·s or less, and even more preferably 800 Pa·s or more and 2,000 Pa·s or less. The melt viscosity can be measured in accordance with JIS K 7199:1999 "Plastics - Testing method for flow characteristics of plastics using a capillary rheometer and a slit die rheometer."

[0030] 1-2-3. Density of cross-linking raw material 30 The density of the cross-linking raw material 30 before foaming is 600 kg / m 3 More than 800 kg / m 3 More preferably, 1000 kg / m 3 The density of the cross-linking raw material 30 before foaming is 1600 kg / m 3 Preferably less than 1400 kg / m 3 Less than 1200 kg / m is more preferable. 3 Therefore, the density of the cross-linking raw material 30 before foaming is more preferably 600 kg / m or less. 3 More than 1600kg / m 3 Preferably less than 800 kg / m 3 More than 1400kg / m 3 Less than 1000kg / m is more preferable.3 More than 1200kg / m 3 The following is more preferable: The density of the cross-linking raw material 30 can be measured based on JIS K 7222;2005.

[0031] 1-2-4. Gel fraction of cross-linking raw material 30 The gel fraction of the crosslinking raw material 30 before foaming is preferably 1% or more, more preferably 3% or more, and even more preferably 5% or more, from the viewpoint of preventing cell rupture. The gel fraction of the crosslinking raw material 30 before foaming is preferably 70% or less, more preferably 40% or less, and even more preferably 10% or less, from the viewpoint of preventing cracks and fissures from occurring in the resin foam 10. From these viewpoints, the gel fraction of the crosslinking raw material 30 before foaming is preferably 1% or more and 70% or less, more preferably 3% or more and 40% or less, and even more preferably 5% or more and 10% or less. The gel fraction of the crosslinking raw material 30 before foaming can be measured in accordance with JIS K 6796:2015.

[0032] 1-2-5. Density of resin foam 10 The density of the resin foam 10 is set to 20 kg / m from the viewpoint of moldability. 3 More than 30 kg / m is preferable. 3 More preferably, 40 kg / m 3 The density of the resin foam 10 is more preferably 100 kg / m from the viewpoint of light weight. 3 Preferably less than 80 kg / m 3 Less than 60 kg / m is more preferable. 3 From these viewpoints, the density of the resin foam 10 is more preferably 20 kg / m 3 More than 100kg / m 3 Less than 30 kg / m is preferable. 3 More than 80kg / m 3 Less than 40 kg / m is more preferable. 3 More than 60kg / m 3 The following is more preferable: The density of the resin foam 10 can be measured based on JIS K 7222;2005.

[0033] 1-2-6. Gel fraction of resin foam 10 The gel fraction of resin foam 10 is preferably 1% or more, more preferably 3% or more, and even more preferably 5% or more, from the viewpoint of making the cells less likely to burst. The gel fraction of resin foam 10 is preferably 70% or less, more preferably 40% or less, and even more preferably 10% or less, from the viewpoint of making the resin foam 10 less likely to crack or break. From these viewpoints, the gel fraction of resin foam 10 is preferably 1% or more and 70% or less, more preferably 3% or more and 40% or less, and even more preferably 5% or more and 10% or less. The gel fraction of resin foam 10 referred to here can be measured in accordance with JIS K 6796:2015.

[0034] 1-2-7. 80% compression stress of resin foam 10 The 80% compressive stress of the resin foam 10 is preferably 280 kPa or less, more preferably 250 kPa or less, and even more preferably 230 kPa or less. There is no particular lower limit to the 80% compressive stress of the resin foam 10, but it is usually 0.1 kPa or more. The 80% compressive stress of the resin foam 10 can be measured in accordance with JIS K 6767:2015.

[0035] 1-2-8. Compression set of resin foam 10 From the viewpoint of ensuring an appropriate restoring force, the compression set of the resin foam 10 is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less. The lower limit of the compression set of the resin foam 10 is not particularly limited, but is usually 0.1% or more. The compression set of the resin foam 10 referred to here can be measured in accordance with JIS K 6767:2015. The thickness of the resin foam 10 after release from compression is measured 24 hours after release from compression.

[0036] 1-2-9. Maximum tensile strength of resin foam 10 The maximum tensile strength of resin foam 10 is preferably 0.50 MPa or more, more preferably 0.60 MPa or more, and even more preferably 0.70 MPa or more. The upper limit of the maximum tensile strength of resin foam 10 is not particularly limited and is, for example, 5.00 MPa or less. The maximum tensile strength of resin foam 10 can be measured in accordance with JIS K 6767:2015.

[0037] 1-2-10. Maximum elongation of resin foam 10 The maximum elongation of resin foam 10 is preferably 30.0% or more, more preferably 50.0% or more, and even more preferably 70.0% or more. There are no particular limitations on the upper limit of the maximum elongation of resin foam 10, and examples include 150.0% or less and 200.0% or less. The maximum elongation of resin foam 10 can be measured based on JIS K 6767:2015.

[0038] 1-3. Manufacturing method of resin foam 10 The method for producing the resin foam 10 involves irradiating a composition 20 containing a polyolefin resin, an antioxidant, and a foaming agent with an electron beam to form a cross-linking raw material 30, and then foaming the cross-linking raw material 30 in a mold 40 so that the maximum foaming pressure in the mold 40 is 2 kPa or more.

[0039] The method for producing the resin foam 10 includes, for example, the following steps AC. Step A: A step of melt-kneading a polyolefin resin, an antioxidant, a foaming agent, and other additives (foaming aids, etc.) that are blended as needed using a heated kneader, twin-screw extruder, or the like to obtain a composition 20 (see FIG. 1). Step B: A step of irradiating the composition 20 obtained in step A with an electron beam to crosslink it and obtain a crosslinking raw material 30. Step C: A step of forming the resin foam 10 by placing the crosslinking raw material 30 obtained in step B in a predetermined shape such as a sheet or pellet in a mold 40 (a molding die) and heating it inside the mold 40 to cause foaming.

[0040] When melt-kneading in step A, kneading is carried out, for example, at 120° C. for 10 minutes.

[0041] When a sheet-shaped composition 20 is obtained in step A, the composition is formed into a sheet by, for example, heat pressing at 120°C.

[0042] In step B, when the composition 20 obtained in step A is in the form of a sheet, it is sufficient to irradiate the composition with an electron beam at least once from at least one side, preferably from both sides.

[0043] The electron beam irradiation dose (absorbed dose) used in forming the resin foam 10 may be any dose sufficient to achieve the desired degree of crosslinking, but is preferably 100 kGy or less, more preferably 50 kGy or less, and even more preferably 30 kGy or less. The electron beam irradiation dose used in forming the resin foam 10 may be any dose sufficient to achieve the desired degree of crosslinking, but is preferably 10 kGy or more, more preferably 15 kGy or more, and even more preferably 20 kGy or more. Therefore, the electron beam irradiation dose used in forming the resin foam 10 is preferably 10 kGy to 100 kGy, more preferably 15 kGy to 50 kGy, and even more preferably 20 kGy to 30 kGy. The acceleration voltage is adjusted according to the thickness of the sheet before foaming. Because the progress of crosslinking due to electron beam irradiation is affected by the composition of the composition 20, the irradiation dose is usually adjusted while measuring the gel fraction (degree of crosslinking).

[0044] When the cross-linking raw material 30 is foamed in the mold 40, it is heated to a temperature (for example, 200° C.) higher than the melting point of the cross-linking raw material 30 to perform foam molding.

[0045] 2, the mold 40 includes a first component part 41 and a second component part 42. The first component part 41 and the second component part 42 are combined to form the mold 40 having an internal space. After the cross-linking raw material 30 is placed on the upper surface of the first component part 41, the first component part 41 and the second component part 42 are combined to accommodate the cross-linking raw material 30 in the mold 40.

[0046] The maximum foaming pressure in the mold 40 when the crosslinking raw material 30 is foamed in the mold 40 is 2.0 kPa or more, preferably 2.5 kPa or more, more preferably 3.0 kPa or more, for example, 3.5 kPa. The maximum foaming pressure in the mold 40 is measured, for example, by a pressure sensor 50 (see FIG. 2). The pressure sensor 50 is embedded, for example, in a wall portion of the mold 40 (for example, in the central portion of the top wall of the second component portion 42) so as to be exposed to the internal space of the mold 40.

[0047] When the cross-linking raw material 30 is foamed in the mold 40, for example, it is heated at 200° C. for 10 minutes, and then the mold 40 is cooled by water cooling to obtain the resin foam 10.

[0048] 1-4. Effects of the First Embodiment The resin foam 10 of the first embodiment has good foaming properties. Specifically, the resin foam 10 is wrinkle-free and has good transferability. The method for producing resin foam 10 of the first embodiment can produce resin foam 10 in a simple manner without compression molding by controlling the melt viscosity of the measurement sample and the maximum foaming pressure in mold 40. The method for producing the resin foam 10 of the first embodiment can provide a resin foam 10 with excellent performance (strength, weight reduction, and heat shrinkage resistance).

[0049] 2. Second embodiment The cross-linked polyolefin resin foam 10 of the second embodiment (see Figure 3, hereinafter also simply referred to as resin foam 10) has a maximum tensile strength measured in accordance with JIS K 6767:2015 of 0.50 MPa or more, and a maximum elongation measured in accordance with JIS K 6767:2015 of 30.0% or more.

[0050] 2-1. Raw materials for resin foam 10 Regarding the raw materials of the resin foam 10, the explanation in the section "Raw Materials of the Resin Foam 10" in the first embodiment applies as is, and the description thereof will be omitted.

[0051] Regarding the configuration of the resin foam 10, the explanations in the columns "Crosslinked structure," "Melt viscosity of measured sample," "Density of crosslinked raw material 30," "Gel fraction of crosslinked raw material 30," "Density of resin foam 10," "Gel fraction of resin foam 10," "80% compression stress of resin foam 10," and "Compression set of resin foam 10" in the first embodiment apply as is, and the description thereof will be omitted.

[0052] 2-1-1. Maximum tensile strength of resin foam 10 The maximum tensile strength of resin foam 10 is 0.50 MPa or more, preferably 0.60 MPa or more, and more preferably 0.70 MPa or more. The upper limit of the maximum tensile strength of resin foam 10 is not particularly limited and is, for example, 5.00 MPa or less. The maximum tensile strength of resin foam 10 can be measured in accordance with JIS K 6767:2015.

[0053] 2-1-2. Maximum elongation of resin foam 10 The maximum elongation of resin foam 10 is 30.0% or more, preferably 50.0% or more, and more preferably 70.0% or more. There are no particular limitations on the upper limit of the maximum elongation of resin foam 10, and examples include 150.0% or less and 200.0% or less. The maximum elongation of resin foam 10 can be measured based on JIS K 6767:2015.

[0054] 2-2. Manufacturing method of resin foam 10 A preferred method for producing the resin foam 10 is to irradiate a composition 20 containing a polyolefin resin, an antioxidant, and a foaming agent with an electron beam to form a cross-linking raw material 30, and then foam the cross-linking raw material 30 in a mold 40.

[0055] The method for producing the resin foam 10 includes, for example, the following steps AC. Step A: A step of melt-kneading a polyolefin resin, an antioxidant, a foaming agent, and other additives (foaming aids, etc.) blended as needed using a heated kneader, twin-screw extruder, or the like to obtain a composition 20. Step B: A step of irradiating the composition 20 obtained in step A with an electron beam to crosslink it and obtain a crosslinking raw material 30. Step C: A step of forming the resin foam 10 by placing the crosslinking raw material 30 obtained in step B in a predetermined shape such as a sheet or pellet in a mold 40 (a molding die) and heating it inside the mold 40 to cause foaming.

[0056] When melt-kneading in step A, kneading is carried out, for example, at 120° C. for 10 minutes.

[0057] When a sheet-shaped composition 20 is obtained in step A, the composition is formed into a sheet by, for example, heat pressing at 120°C.

[0058] In step B, when the composition 20 obtained in step A is in the form of a sheet, it is sufficient to irradiate the composition with an electron beam at least once from at least one side, preferably from both sides.

[0059] The electron beam irradiation dose (absorbed dose) used in forming the resin foam 10 may be any dose sufficient to achieve the desired degree of crosslinking, but is preferably 100 kGy or less, more preferably 50 kGy or less, and even more preferably 30 kGy or less. The electron beam irradiation dose used in forming the resin foam 10 may be any dose sufficient to achieve the desired degree of crosslinking, but is preferably 10 kGy or more, more preferably 15 kGy or more, and even more preferably 20 kGy or more. Therefore, the electron beam irradiation dose used in forming the resin foam 10 is preferably 10 kGy to 100 kGy, more preferably 15 kGy to 50 kGy, and even more preferably 20 kGy to 30 kGy. The acceleration voltage is adjusted according to the thickness of the sheet before foaming. Because the progress of crosslinking due to electron beam irradiation is affected by the composition of the composition 20, the irradiation dose is usually adjusted while measuring the gel fraction (degree of crosslinking).

[0060] When the cross-linking raw material 30 is foamed in the mold 40, it is heated to a temperature (for example, 200° C.) higher than the melting point of the cross-linking raw material 30 to perform foam molding.

[0061] The maximum foaming pressure in the mold 40 when the crosslinking raw material 30 is foamed in the mold 40 is preferably 2.0 kPa or more, more preferably 2.5 kPa or more, and even more preferably 3.0 kPa or more, for example, 3.5 kPa. The maximum foaming pressure in the mold 40 is measured, for example, by a pressure sensor 50 (see FIG. 2). The pressure sensor 50 is embedded in a wall of the mold 40 (for example, the center part of the top wall) so as to be exposed to the internal space of the mold 40.

[0062] When the cross-linking raw material 30 is foamed in the mold 40, for example, it is heated at 200° C. for 10 minutes, and then the mold 40 is cooled by water cooling to obtain the resin foam 10.

[0063] 2-3. Effects of the second embodiment In addition to the effects achieved by the resin foam 10 of the first embodiment, the resin foam 10 of the second embodiment has the effects that the maximum tensile strength of the resin foam 10 is relatively high and the maximum elongation of the resin foam 10 is relatively large. [Example]

[0064] The present invention will be explained in more detail below with reference to examples.

[0065] 1. Preparation of Resin Foam Resin foams of Examples and Comparative Examples were produced at the blending ratios shown in Table 1. Details of the raw materials of the resin foams in Table 1 are shown below. In Table 1, the blending ratios represent blending ratios (parts by mass) when the polyolefin resin is taken as 100 parts by mass.

[0066] [Table 1]

[0067] 1-1. Raw materials for resin foam Polyolefin resin: Low-density polyethylene (LDPE), 5321 (Hanwha Solutions) Foaming agent: Azodicarbonamide (ADCA), UNIFOAM AZ VI-50L3ST 60MB (Otsuka Chemical Co., Ltd.) Foaming agent 1: Zinc oxide type 1 (Hakusui Chemical Co., Ltd.) Foaming aid 2: Zinc stearate, Zinc Stearate N (manufactured by Tannan Chemical Industry Co., Ltd.) Antioxidant: Pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], Adekastab AO-60 (ADEKA Corporation) Release agent: Glycerin monostearate, Rikemal (Riken Vitamin Co., Ltd.)

[0068] 1-2. Preparation of resin foams in Examples and Comparative Examples Specifically, each resin foam was produced as follows. 1-2-1. Example 1 Composition 20 was obtained by adding a foaming agent, foaming aid 1, foaming aid 2, antioxidant, and mold release agent to a polyolefin resin in the blending ratios shown in Table 1. The raw materials were crosslinked and foamed by the method described in "1-3. Manufacturing method for resin foam 10" in the first embodiment. The raw materials were kneaded in a heated kneader to obtain a composition 20, which was then formed into a sheet-like composition 20 having a thickness of 2 mm, as shown in FIG. 1. Specifically, the raw materials were melt-kneaded at 120°C for 10 minutes using a heated kneader, and then heat-pressed at 120°C to obtain a sheet-like composition 20. The composition 20 was crosslinked by irradiation with an electron beam irradiation device 60 (acceleration voltage: 800 kV, irradiation dose (absorbed dose): 25 kGy) to obtain a crosslinked raw material 30. The composition 20 was irradiated with electron beams from both sides. The crosslinked raw material 30 was placed in a mold 40 (a molding die). The mold 40 had dimensions of 250 mm × 120 mm × 10 mm. The mold 40 was heated at 200°C for 10 minutes to foam and mold the composition as shown in FIG. 3. The maximum foaming pressure in the mold 40 was 3.5 kPa. Thereafter, the mold 40 was cooled by water cooling, and then the mold 40 was opened to obtain a resin foam. The resin foam had a shape of 250 mm × 120 mm × 10 mm (the same shape as the mold 40), filling the cavity volume within the mold 40.

[0069] The maximum foaming pressure inside the mold 40 was measured using a pressure sensor 50. A low-pressure RTM pressure sensor (Model No. 4001A102FA2.0, manufactured by KISTLER) was used as the pressure sensor 50. The pressure sensor 50 was embedded in the center of the top wall of the mold 40 so as to be exposed to the internal space of the mold 40. During foam molding, the pressure values ​​detected by the pressure sensor 50 were recorded using a handheld logger.

[0070] 1-2-2. Comparative Example 1 In Comparative Example 1, a resin foam was produced in the same manner as in Example 1, except that electron beam crosslinking was not performed and the maximum foaming pressure in mold 40 was 0.4 kPa.

[0071] 1-2-2. Comparative Example 2 In Comparative Example 2, a resin foam was produced in the same manner as in Example 1, except that the exposure dose (absorbed dose) during electron beam irradiation was 100 kGy and the maximum foaming pressure in the mold 40 was 0.5 kPa.

[0072] 1-2-3. Comparative Example 3 In Comparative Example 3, a resin foam was produced in the same manner as in Example 1, except that the maximum foaming pressure in mold 40 was 0.5 kPa.

[0073] 2. Evaluation Method A sample was prepared by mixing a polyolefin resin and an antioxidant at the mass ratio of the resin foam composition (a composition containing a polyolefin resin, an antioxidant, and a foaming agent). The sample was irradiated with an electron beam used in forming the resin foam, and the resultant was used as a measurement sample.

[0074] 2-1. Melt viscosity of the measurement sample The melt viscosity of the measurement sample was measured using a capillograph at a measurement temperature of 200°C and a shear rate of 122 sec -1 was measured.

[0075] 2-2.Density of cross-linking raw material The density of the crosslinking raw material before foaming was measured based on JIS K 7222;2005.

[0076] 2-3. Gel fraction of crosslinking raw material The gel fraction of the crosslinking raw material before foaming was measured based on JIS K 6796:2015.

[0077] 2-4. Foaming The foaming property of the resin foam was evaluated by visual inspection according to the following criteria. A: No wrinkles and good transferability B: Wrinkles and poor transferability C: Cannot be foam molded

[0078] 2-5. Density of resin foam The density of the resin foam was measured based on JIS K 7222;2005.

[0079] 2-6. Gel fraction of resin foam The gel fraction of the resin foam was measured based on JIS K 6796:2015.

[0080] 2-7. 80% compression stress of resin foam The 80% compressive stress of the resin foam was measured based on JIS K 6767:2015.

[0081] 2-8. Compression set of resin foam The compression set of the resin foam was measured based on JIS K 6767: 2015. The thickness of the resin foam after release from compression was measured 24 hours after release from compression.

[0082] 2-9. Maximum tensile strength of resin foam The maximum tensile strength of the resin foam was measured based on JIS K 6767:2015.

[0083] 2-10. Maximum elongation of resin foam The maximum elongation of the resin foam was measured based on JIS K 6767:2015.

[0084] 3.Results The results are shown in Table 1. In Example 1, a resin foam with good foamability (no wrinkles and good transferability) was obtained. In contrast, the resin foam of Comparative Example 1 could not be foam-molded. This is thought to be because the foaming step was carried out without crosslinking the composition with electron beams. The resin foam of Comparative Example 2 could not be foam-molded. This is thought to be because the irradiation dose during electron beam crosslinking was too high, which increased the melt viscosity of the measurement sample and made it difficult to foam. In Comparative Example 3, there were wrinkles and transferability was poor. This is thought to be because the maximum foaming pressure in the mold 40 was relatively low. 4. Effects of the Example According to the above examples, by controlling the melt viscosity of the measurement sample and the maximum foaming pressure in the mold 40, it was possible to produce a resin foam 10 with good foaming properties in a simple manner without compression molding.

[0085] The present disclosure is not limited to the above-described embodiments, and various modifications and variations are possible. [Explanation of symbols]

[0086] 10: Resin foam 20: Composition 30: Crosslinking raw material 40: Type 50: Pressure sensor 60: Electron beam irradiation device

Claims

1. A method for producing a crosslinked polyolefin resin foam, comprising: irradiating a composition containing a polyolefin resin, an antioxidant, and a foaming agent with an electron beam to form a crosslinking raw material; and foaming the crosslinking raw material in a mold so that the maximum foaming pressure in the mold is 2.0 kPa or more.

2. A sample obtained by mixing the polyolefin resin and the antioxidant in the mass ratio of the composition was irradiated with the electron beam used in forming the crosslinked polyolefin resin foam. The shear rate of the sample measured in accordance with JIS K 7199:1999 was 122 sec. －１ The method for producing a crosslinked polyolefin resin foam according to claim 1, wherein the melt viscosity at 200°C is 10,000 Pa·s or less.

3. The method for producing a crosslinked polyolefin resin foam according to claim 1, wherein the crosslinked polyolefin resin foam has a gel fraction of 1% or more and 70% or less, measured in accordance with JIS K 6796:2015.

4. The method for producing a crosslinked polyolefin resin foam according to claim 1 , wherein the antioxidant is a hindered phenol-based antioxidant.

5. The maximum tensile strength measured in accordance with JIS K 6767:2015 is 0.50 MPa or more, A cross-linked polyolefin resin foam having a maximum elongation of 30.0% or more as measured in accordance with JIS K 6767:2015.

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