Method for producing foam molded body

Abstract

The present invention provides a method for producing a foam molded body, with which it is possible to suppress the formation of a carbide layer on an extruder or a head.　The present invention provides a method for producing a foam molded body, the method including a melt kneading step, a parison formation step, and a molding step. In the melt kneading step, a starting material resin composition is melt-kneaded in the presence of a foaming agent using an extruder so as to form a foaming agent-containing resin composition in a molten state. In the parison formation step, the foaming agent-containing resin composition is extruded from a head so as to form a foamed parison. In the molding step, the foamed parison is molded so as to form a foam molded body. The starting material resin composition contains a low-density polyethylene. The low-density polyethylene has an oxidation induction time of 0.5 minute or more as measured at 180°C.

Description

Method for producing foamed molded article

[0001] The present invention relates to a method for producing a foamed molded article.

[0002] For example, in an air conditioning system for an automobile or the like, a tubular air conditioning duct is used to ventilate air.

[0003] Foam molded articles made from foamed resins, which are produced by foaming thermoplastic resins with a blowing agent, are known as air conditioning ducts. Demand for foam molded articles is expanding because they can simultaneously achieve high thermal insulation and lightweight construction.

[0004] A widely known method for producing such foamed molded articles is foam molding, in which a molten foaming resin is clamped in a split mold and air is blown into the mold to expand it (see, for example, Patent Document 1).

[0005] Japanese Patent Application Laid-Open No. 2017-064932

[0006] In foam blow molding, a raw material resin and a foaming agent are melt-kneaded in an extruder to produce a molten foaming-agent-containing resin composition, which is then extruded from a head to form a foam parison, which is then molded using a mold to produce a foamed molded article.

[0007] Meanwhile, when the present inventors investigated facilities for mass-producing foam molded articles, they found that a char layer was likely to form on the extruder or head in some cases. If a char layer is formed on the extruder or head, the char will likely adhere to the foam molded article, which is the final product, so it is desirable to prevent the formation of a char layer on the extruder or head.

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a method for producing a foamed molded article that can suppress the formation of a carbonized layer on the extruder or head.

[0009] The present invention provides the following inventions: [1] A method for producing a foam-molded article, comprising a melt-kneading step, a parison-forming step, and a molding step, wherein in the melt-kneading step, a raw resin composition is melt-kneaded in the presence of a foaming agent using an extruder to form a molten foaming-agent-containing resin composition; in the parison-forming step, the foamed parison is extruded from a head to form a foamed parison; and in the molding step, the foamed parison is molded to form a foam-molded article, wherein the raw resin composition contains a low-density polyethylene, and the low-density polyethylene has an oxidation induction time measured at 180°C of 0.5 minutes or more. [2] The method described in [1], wherein the raw resin composition contains 30% by mass or more of the low-density polyethylene. [3] The method described in [1] or [2], wherein the low-density polyethylene has an oxidation induction time measured at 180°C of 10 minutes or more. [4] The method according to any one of [1] to [3], wherein the low-density polyethylene has a value (mN·g / 10 min) of melt tension (mN) × melt flow rate (g / 10 min) at 190°C of 130 or more. [5] The method according to any one of [1] to [4], wherein the raw material resin composition contains high-density polyethylene, and the high-density polyethylene has an oxidation induction time measured at 180°C of 10 minutes or more.

[0010] The present inventors analyzed the components of the charred material and found that it originated from low-density polyethylene. Furthermore, they found that among the facilities mass-producing foamed molded articles using raw resins containing low-density polyethylene, some are prone to forming a charred layer and others are not. They then analyzed the production processes of both facilities in detail and found that the low-density polyethylene used in the mass-production facilities prone to forming a charred layer has a relatively short oxidation induction time at 180°C. This analysis revealed that the formation of a charred layer in the extruder or head can be suppressed by using low-density polyethylene contained in the raw resin that has an oxidation induction time of 0.5 minutes or more at 180°C, leading to the completion of the present invention.

[0011] 2A shows the configuration of a molding apparatus 100 according to one embodiment of the present invention, and FIG. 2A shows the FT-IR spectra of LDPE, HDPE, and the carbonized product, and FIG. 2B shows the DSC curve of the carbonized product.

[0012] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently. Furthermore, in the following embodiments, elements not specified in the claims are optional elements and can be omitted. Any number of "0"s (for example, one or two) may be added to the end of numerical values ​​disclosed in the following description. For example, one or two "0"s may be added after "1.4" to make it "1.40" or "1.400".

[0013] 1. Molding Apparatus 100 A method for producing a foamed molded article according to one embodiment of the present invention can be carried out, for example, using a molding apparatus 100 including an extruder 1, a head 12, and a mold 14, as shown in Figure 1. The extruder 1 includes a cylinder 3, a hopper 5, a screw 7, a foaming agent injection section 8, a temperature control section 9, and a resin extrusion port 11. Each component will be described in detail below.

[0014] <Hopper 5> The hopper 5 is in communication with the internal space 3b of the cylinder 3 through an opening 3a provided on the side surface of the cylinder 3, and the raw resin composition 2 is introduced into the internal space 3b from the hopper 5. The raw resin composition 2 is heated in the internal space 3b and melted into a molten state. Furthermore, by rotation of the screw 7 arranged in the internal space 3b, the molten resin composition is transported toward a resin extrusion port 11 provided at the tip of the internal space 3b.

[0015] <Raw Material Resin Composition 2> Raw material resin composition 2 contains low-density polyethylene (LDPE). LDPE is a polyethylene having a long-chain branched structure. The density (g / cm ３ ) is 0.910 or more and 0.940 or less, and preferably 0.915 or more and 0.925 or less. ３) is, for example, 0.910, 0.915, 0.920, 0.925, 0.930, 0.935, or 0.940, and may be within a range between any two of the values ​​exemplified here. LDPE can be produced, for example, by polymerizing ethylene using oxygen in the air or a radical initiator such as peroxide as a catalyst under an environment of 1,000 to 4,000 atmospheres and 100 to 350°C using a multi-stage gas compressor. Because LDPE has a long-chain branched structure, blending LDPE can improve the foam moldability of raw resin composition 2.

[0016] The LDPE contained in raw resin composition 2 has an oxidation induction time (OIT) of 0.5 minutes or more measured at 180°C. In a DSC measurement device, the temperature is raised to 180°C in nitrogen, and after the target temperature is reached, the atmospheric gas is switched from nitrogen to air. The time from the time of switching to the rise of the exothermic peak due to oxygen absorption is measured, and this time is defined as the oxidation induction time. In this specification, "oxidation induction time" means the time measured at 180°C, unless otherwise specified.

[0017] By using LDPE contained in raw resin composition 2 having an oxidation induction time of 0.5 minutes or more at 180°C, the formation of a char layer on the extruder 1 and head 12 can be suppressed. The oxidation induction time of LDPE is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 15 minutes or more, and even more preferably 18 minutes or more. The upper limit of the oxidation induction time of LDPE is not particularly specified, but is, for example, 60 minutes. Specific examples of this oxidation induction time include 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, and 60 minutes, and may be in a range between any two of the values ​​exemplified here or greater than any one of them.

[0018] The proportion of LDPE in raw resin composition 2 is not particularly limited, but is, for example, 30% by mass or more. In this case, when LDPE with a short oxidation induction time is used, a char layer is particularly likely to form on the extruder 1 and head 12, so the technical significance of applying the present invention to suppress the formation of a char layer is significant. The proportion of LDPE in raw resin composition 2 is, for example, 30 to 100% by mass, and specifically, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100% by mass, and may be in a range between any two of the values ​​exemplified here.

[0019] The LDPE preferably has a melt tension (mN, hereinafter referred to as "MT") x melt flow rate (g / 10 min, hereinafter referred to as "MFR") value (mN·g / 10 min) at 190°C of 130 or more, more preferably 150 or more, and even more preferably 200 or more. In this case, foam moldability is particularly improved. In order to achieve an expansion ratio of 2.8 or more for the foam molded article, from the viewpoint of improving foam moldability, MT x MFR is preferably 150 or more.

[0020] MT (mN) at 190°C means the tension when a strand is extruded from an orifice having a diameter of 2.095 mm and a length of 8 mm at a test temperature of 190°C and an extrusion rate of 10 mm / min using a melt tension tester (manufactured by Toyo Seiki Seisakusho, Ltd.), and this strand is taken up at a take-up rate of 16 rpm around a roller having a diameter of 80 mm. MFR (g / 10 min) at 190°C means the value obtained by measurement in accordance with JIS K-7210 at a test temperature of 190°C and a test load of 2.16 kg.

[0021] The MT×MFR value (mN·g / 10 min) at 190°C is, for example, 130 to 400, specifically, for example, 130, 150, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 350, or 400, and may be in a range between any two of the values ​​exemplified here.

[0022] The MT value (mN) at 190°C is, for example, 90 to 160, preferably 100 to 150, and specifically, for example, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, or 160, and may be in a range between any two of the numerical values ​​exemplified here. The MFR value (g / 10 min) at 190°C is, for example, 0.8 to 3.0, preferably 1.5 to 2.5, and specifically, for example, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 3.0, and may be in a range between any two of the numerical values ​​exemplified here.

[0023] The raw resin composition 2 preferably contains high-density polyethylene (HDPE). By blending HDPE, rigidity and heat resistance can be improved. The density (g / cm ３ ) is 0.941 or more, and preferably 0.942 or more. ３ ) is, for example, 0.941 to 0.965, specifically, for example, 0.941, 0.942, 0.945, 0.950, 0.955, 0.960, 0.965, and may be in a range between any two of the numerical values ​​exemplified here.

[0024] The HDPE contained in the raw resin composition 2 preferably has an oxidation induction time at 180°C of 10 minutes or more, more preferably 15 minutes or more, and even more preferably 25 minutes or more. In this case, the formation of a char layer is further suppressed. The oxidation induction time of the HDPE is, for example, 10 to 300 minutes, preferably 10 to 120 minutes, and even more preferably 10 to 80 minutes. Specific examples of this oxidation induction time include 10, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, 120, 150, 200, 250, and 300 minutes, and may be in a range between any two of the values ​​exemplified here or greater than any one of them.

[0025] The proportion of HDPE in the raw resin composition 2 is, for example, 30 to 70 mass%, specifically, for example, 30, 35, 40, 45, 50, 55, 60, 65, or 70 mass%, and may be in a range between any two of the numerical values ​​exemplified here.

[0026] Raw material resin composition 2 may contain other resins besides LDPE and HDPE. Examples of other resins include polyethylene-based resins such as ethylene copolymers, which are copolymers of ethylene and other olefins (e.g., α-olefins such as 1-butene), acid-modified polyethylene, and polypropylene-based resins such as homopolypropylene, random polypropylene, and block polypropylene. Raw material resin composition 2 may contain various additives other than resins, such as a foam nucleating agent (e.g., sodium bicarbonate and citric acid), antioxidants, and colorants.

[0027] The total proportion of LDPE and HDPE in raw material resin composition 2 is preferably 80 to 100% by mass, and specifically, for example, 80, 85, 90, 95, or 100% by mass, and may be in a range between any two of the numerical values ​​exemplified here.

[0028] The raw resin composition 2 is preferably a mixture of recycled and virgin materials. The recycled material is obtained by crushing scrap generated during the production of foamed molded articles. The virgin material is a new material that is not recycled, and the composition of the virgin material is the same as that described above for the raw resin composition 2. The proportion of recycled material relative to the total of recycled and virgin materials is, for example, 50 to 95 mass%, preferably 80 to 95 mass%. Specific examples of this proportion include 50, 55, 60, 65, 70, 75, 80, 85, 90, and 95 mass%, and may be within a range between any two of the values ​​exemplified here. The greater this proportion, the more likely the resin is to undergo oxidative degradation and form a char layer, making the application of the present invention particularly significant.

[0029] <Screw 7> The screw 7 is disposed within the internal space 3b of the cylinder 3, and by its rotation, kneads the molten resin while transporting it toward the resin extrusion port 11. A motor 4 is provided at one end of the screw 7. The motor 4 not only drives the screw 7 to rotate, but is also capable of controlling the rotation speed.

[0030] <Blowing Agent Injection Section 8> The foaming agent injection section 8 is a section for injecting a foaming agent into the cylinder 3. The location of the foaming agent injection section 8 is not particularly limited. However, assuming that the end of the internal space 3b of the cylinder 3 on the hopper 5 side is 0 and the end on the resin extrusion outlet 11 side is L, the foaming agent injection section 8 is preferably located at a position of 0.3 L to 0.7 L (preferably 0.4 to 0.6 L). If the foaming agent injection section 8 is located closer to the hopper 5 than 0.3 L, the foaming agent may be injected without sufficient kneading of the molten resin, resulting in insufficient dispersion of the foaming agent. Furthermore, since the temperature of the molten resin is typically controlled to gradually decrease toward the resin extrusion outlet 11, if the foaming agent injection section 8 is located closer to the resin extrusion outlet 11 than 0.7 L, the temperature of the molten resin at the injection site may be too low, resulting in a reduced amount of foaming agent injection.

[0031] The foaming agent injected through the foaming agent injection section 8 may be a physical foaming agent, a chemical foaming agent, or a mixture thereof, with physical foaming agents being preferred. Examples of physical foaming agents include inorganic physical foaming agents such as air, carbon dioxide, nitrogen gas, and water, as well as organic physical foaming agents such as butane, pentane, hexane, dichloromethane, and dichloroethane, and even supercritical fluids thereof. Supercritical fluids are preferably produced using carbon dioxide, nitrogen, etc., with nitrogen having a critical temperature of -149.1°C and a critical pressure of 3.4 MPa or higher, and carbon dioxide having a critical temperature of 31°C and a critical pressure of 7.4 MPa or higher. Examples of chemical foaming agents include those that generate carbon dioxide gas through a chemical reaction between an acid (e.g., citric acid or its salt) and a base (e.g., baking soda). The foaming agent injection section 8 may also be an opening connected to the hopper 5.

[0032] <Temperature Control Unit 9> The temperature control unit 9 is configured to individually control a plurality of temperature adjustment units provided in the cylinder 3 and the head 12 to control the temperature of each portion.

[0033] <Head 12 / Mold 14> The molten foaming-agent-containing resin composition obtained by melt-kneading the raw resin composition 2 and the foaming agent in the internal space 3b is extruded through the resin extrusion port 11 and injected into the head 12. The head 12 has a slit, and the foaming-agent-containing resin composition is extruded through the slit to form a foamed parison 13. The shape of the slit is not particularly limited, but may be, for example, annular or linear (e.g., straight). When the slit is annular, a cylindrical foamed parison is obtained. When the slit is linear (e.g., when the head 12 is a T-die), a sheet-shaped foamed parison is obtained. Note that the foaming-agent-containing resin extruded through the resin extrusion port 11 may be stored in an accumulator (not shown) and then extruded through the head 12 by operating the plunger of the accumulator to form the foamed parison 13. In this case, the extrusion speed of the foamed parison 13 can be increased, which has the advantage of making it easier to stabilize the foamed state of the foamed parison 13. The extrusion rate of the foaming agent-containing resin is preferably 250 to 1250 g / sec, and specifically, for example, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, or 1250 g / sec, or may be in a range between any two of the numerical values ​​exemplified here.

[0034] The temperature of the foamed parison 13 is, for example, 160 to 200° C., preferably 170 to 190° C. Specific examples of this temperature include 160, 165, 170, 175, 180, 185, 190, 195, and 200° C., and may be in a range between any two of the values ​​exemplified here.

[0035] The foam parison 13 is molded in a mold 14. The mold 14 is preferably a split mold 14a, 14b that can be opened and closed, and the foam parison 13 is introduced between the split molds 14a, 14b. A foam molded article is obtained by molding the foam parison 13 using the mold 14. The foam molded article is preferably hollow. The molding method using the mold 14 is not particularly limited, and may be blow molding, in which air is blown into the cavity of the mold 14 to form the foam parison 13, vacuum molding, in which the pressure inside the cavity of the mold 14 is reduced from the inner surface of the cavity to form the foam parison 13, or a combination of these.

[0036] The expansion ratio of the foamed molded article is, for example, 1.5 to 6.0 times, preferably 2.0 to 4.0 times, specifically, for example, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0 times, and may be in a range between any two of the numerical values ​​exemplified here.

[0037] 2. Manufacturing Method of Foam Molded Article A manufacturing method of a foam molded article according to one embodiment of the present invention will now be described. The method of this embodiment comprises a melt-kneading step, a parison-forming step, and a molding step. As described above, by using raw material resin composition 2 containing LDPE having an oxidation induction time measured at 180°C of 0.5 minutes or more, the formation of a char layer on extruder 1 and head 12 is suppressed.

[0038] <Melt-Kneading Step> In the melt-kneading step, a raw resin composition 2 is melt-kneaded in the presence of a foaming agent using an extruder 1 to form a molten foaming-agent-containing resin composition. The raw resin composition 2 can be introduced into the internal space 3b of the cylinder 3 of the extruder 1 through a hopper 5. The foaming agent can be injected into the cylinder 3 through a foaming-agent injection section 8. In the cylinder 3, the raw resin composition 2 is melt-kneaded together with the foaming agent as the screw 7 rotates.

[0039] <Parison Forming Step> In the parison forming step, a molten foaming agent-containing resin composition is extruded from the head 12 to form a foamed parison 13 .

[0040] <Molding Step> In the molding step, the foam parison 13 is molded to form a foam molded article. The foam parison 13 can be molded using a mold 14.

[0041] 1. Comparative Example 1 1-1. Mass Production of Foam Molded Articles A foam molded article with an expansion ratio of 3.0 was produced using the molding apparatus 100 shown in Figure 1. The raw resin composition 2 used was a virgin material obtained by blending LDPE (manufactured by Asahi Kasei Corporation, grade: M1820) / HDPE (manufactured by SCG Chemicals Public Company Limited, grade: H5840B) / foam nucleating agent (sodium bicarbonate and citric acid type) / antioxidant / black masterbatch in a mass ratio of 50 / 50 / 1 / 4 / 1, and a recycled material obtained by crushing scrap generated in the previous foam molded article production in a mass ratio of 1:9.

[0042] The temperature control unit 9 was set so that the temperature of the foamed parison 13 was 180° C. The foaming agent was N ２ Gas was used and injected through a foaming agent injection section 8 located at a position of 0.5 L. The expansion ratio was adjusted by changing the amount of injected gas. A certain amount of the foaming agent-containing resin composition extruded from the resin extrusion port 11 of the extruder 1 was stored in an accumulator (not shown), and then the plunger of the accumulator was operated to extrude the composition through the head 12 to form a foamed parison 13. The extrusion rate of the foaming agent-containing resin composition was 554 g / sec. The head 12 used had a die core diameter of 120 mm.

[0043] The foamed parison 13 formed under the above conditions was used to perform blow molding to produce a foamed molded article. The production of foamed molded articles under the above conditions was repeated to produce approximately 6,000 foamed molded articles per month.

[0044] Three months after the previous disassembly and cleaning, the extruder 1 and the head 12 were disassembled, and a carbonized layer was found to have formed in the area that came into contact with the raw resin composition 2, with the thickness of the carbonized layer being approximately 5 mm at its thickest point.

[0045] 1-2. Analysis of carbides Next, analysis of carbides was carried out.

[0046] FT-IR analysis was performed on M1820, H5840B, and the carbonized product. The results are shown in Figure 2A. As shown in Figure 2A, peaks derived from methyl groups appear in the spectra of M1820 and the carbonized product, whereas peaks derived from methyl groups do not appear in the spectrum of H5840B. This result suggests that the carbonized product contains components derived from M1820, an LDPE.

[0047] The carbonized product was analyzed by DSC. The results are shown in Figure 2B. As shown in the DSC curve in Figure 2B, the carbonized product contains low-melting-point components derived from LDPE.

[0048] ・Summary From the above, it was found that the main component of the carbonized material is LDPE.

[0049] 2. Example 1 In Example 1, foamed molded articles were mass-produced in the same manner as in Comparative Example 1, except that LDPE (manufactured by Japan Polyethylene Corporation, grade: LF405H) was used as the LDPE.

[0050] Three months after the previous disassembly and cleaning, the extruder 1 and the head 12 were disassembled, and a carbonized layer was found to have formed in the area that came into contact with the raw resin composition 2, with the thickness of the carbonized layer being approximately 2 mm at its thickest point.

[0051] 3. Example 2 In Example 2, foamed molded articles were mass-produced in the same manner as in Example 1, except that HDPE (manufactured by Keiyo Polyethylene Co., Ltd., grade: B5802-1) was used as the HDPE.

[0052] Three months after the previous disassembly and cleaning, the extruder 1 and the head 12 were disassembled, and a carbonized layer was found to have formed in the area that came into contact with the raw resin composition 2, with the thickness of the carbonized layer being approximately 1.5 mm at its thickest point.

[0053] 4. Measurement of Oxidation Induction Time, MT, and MFR The oxidation induction time at 180°C was measured for each of the LDPE and HDPE used in Comparative Example 1 and Examples 1 and 2. The results are shown in Table 1. For B5802-1, no exothermic peak due to oxygen absorption was observed in the 60-minute measurement, so the oxidation induction time was determined to be more than 60 minutes. In addition, the MT and MFR of the LDPE were measured at 190°C. The results are also shown in Table 1.

[0054]

[0055] As shown in Table 1, LF405H, the LDPE used in Examples 1 and 2, was found to have a significantly longer oxidation induction time than M1820 used in Comparative Example 1. Furthermore, B5802-1, the HDPE used in Example 2, was found to have a significantly longer oxidation induction time than H5840B used in Comparative Example 1 and Example 1. Considering that the thickness of the char layer is in the order Comparative Example 1 >> Example 1 > Example 2, it was found that the use of LDPE with an oxidation induction time of 0.5 minutes or more at 180°C significantly suppresses the formation of the char layer, and that the use of HDPE with an oxidation induction time of 10 minutes or more at 180°C further suppresses the formation of the char layer.

[0056] Furthermore, since both M1820 and LF405H had high MT×MFR values ​​(mN·g / 10 min) at 190°C, Comparative Example 1 and Examples 1 and 2 all had good foam moldability.

[0057] 1: Extruder, 2: Raw resin composition, 3: Cylinder, 3a: Opening, 3b: Internal space, 4: Motor, 5: Hopper, 7: Screw, 8: Foaming agent injection section, 9: Temperature control section, 11: Resin extrusion port, 12: Head, 13: Foamed parison, 14: Mold, 14a: Split mold, 14b: Split mold, 100: Molding device

Claims

1. A method for producing a foamed molded article, comprising: a melt-kneading step, a parison-forming step, and a molding step; in the melt-kneading step, a raw material resin composition is melt-kneaded in the presence of a foaming agent using an extruder to form a molten foaming-agent-containing resin composition; in the parison-forming step, the foaming-agent-containing resin composition is extruded from a head to form a foamed parison; and in the molding step, the foamed parison is molded to form a foamed molded article; the raw material resin composition contains low-density polyethylene, and the low-density polyethylene has an oxidation induction time of 0.5 minutes or more measured at 180°C.

2. The method according to claim 1, wherein the raw resin composition contains 30% by mass or more of the low-density polyethylene.

3. The method of claim 1, wherein the low density polyethylene has an oxidation induction time measured at 180°C of 10 minutes or more.

4. The method according to claim 1, wherein the low-density polyethylene has a melt tension (mN) x melt flow rate (g / 10 min) value (mN·g / 10 min) at 190°C of 130 or more.

5. The method according to any one of claims 1 to 4, wherein the raw material resin composition contains high-density polyethylene, and the high-density polyethylene has an oxidation induction time measured at 180°C of 10 minutes or more.

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