Method for producing foamed molded article

Abstract

To provide a method for manufacturing a foam molded body capable of suppressing the formation of a carbide layer on an extruder or a head.SOLUTION: According to the present invention, there is provided a method for producing a foam-molded article, the method including a melt-kneading step, a parison forming step, and a molding step, wherein 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 foaming agent-containing resin composition in a molten state, in the parison forming step, the foaming agent-containing resin composition is extruded from a head to form a foamed parison, in the molding step, the foamed parison is molded to form a foamed molded article, the raw material 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.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-064932 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0009] According to the present invention, the following inventions are provided. [1] A method for producing a foamed molded article, comprising a melt-kneading step, a parison-forming step, and a molding step, wherein 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, wherein 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 [1], wherein the raw material resin composition contains 30% by mass or more of the low-density polyethylene. [3] The method according to [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 melt tension (mN) × 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 [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. [Effects of the Invention]

[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 conducted a detailed analysis of the production processes for both facilities 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 with an oxidation induction time of 0.5 minutes or more at 180°C, leading to the completion of the present invention. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a configuration diagram of a molding apparatus 100 according to an embodiment of the present invention. [Figure 2] 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. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following describes embodiments of the present invention. The 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.Forming equipment 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 Fig. 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> Hopper 5 communicates with internal space 3b of cylinder 3 through opening 3a provided on the side surface of cylinder 3, and raw resin composition 2 is introduced into internal space 3b from hopper 5. Raw resin composition 2 is heated and melted in internal space 3b, and the molten resin is transported toward resin extrusion port 11 provided at the tip of internal space 3b by rotation of screw 7 disposed in internal space 3b.

[0015] <Raw resin composition 2> The raw 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, the foam moldability of raw resin composition 2 can be improved by blending LDPE.

[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.

[0017] By using LDPE contained in raw resin composition 2 that has an oxidation induction time of 0.5 minutes or more at 180°C, the formation of a char layer on 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 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, specifically, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100% by mass, and may be within a range between any two of the values ​​exemplified here.

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

[0020] MT (mN) at 190°C refers to the tension when a strand is extruded from an orifice with 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 on a roller with a diameter of 80 mm at a take-up rate of 16 rpm. MFR (g / 10 min) at 190°C refers to 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 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, 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 of HDPE (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, and may be in a range between any two of the numerical values ​​exemplified here.

[0024] The HDPE contained in 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 HDPE is, for example, 10 to 120 minutes, 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, and 120 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 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 foam nucleating agents (e.g., baking soda and citric acid), antioxidants, and colorants.

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

[0028] The raw resin composition 2 is preferably a mixture of recycled and virgin materials. The recycled materials are obtained by crushing scraps generated during the production of foamed molded articles. The virgin materials are new materials that are not recycled, and the composition of the virgin materials is the same as that described above for the raw resin composition 2. The proportion of recycled materials 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 are 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 conveying 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] <Foaming 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 position of the end of the internal space 3b of the cylinder 3 on the hopper 5 side is 0 and the position of the end on the resin extrusion port 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 before the molten resin is sufficiently mixed, 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 port 11, if the foaming agent injection section 8 is located closer to the resin extrusion port 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 being injected.

[0031] The foaming agent injected through the foaming agent injection section 8 can 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 foaming agents such as air, carbon dioxide, nitrogen gas, and water, as well as organic foaming agents such as butane, pentane, hexane, dichloromethane, and dichloroethane, and even supercritical fluids thereof. Supercritical fluids are preferably produced using carbon dioxide or nitrogen. Nitrogen can be obtained by adjusting the critical temperature to -149.1°C and the critical pressure to 3.4 MPa or higher, while carbon dioxide can be obtained by adjusting the critical temperature to 31°C and the critical pressure to 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 control 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, an annular or linear (e.g., straight) shape. 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 speed 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., and 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 within 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, or 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, and 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 the range between any two of the numerical values ​​exemplified here.

[0037] 2. Manufacturing method of foam molded product A method for producing a foamed 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 longer, the formation of a char layer on extruder 1 and head 12 is suppressed.

[0038] <Melting and kneading process> 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 formation process> 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] <Forming process> 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. [Example]

[0041] 1. Comparative Example 1 1-1. Mass production of foam molded products A foamed molded article with an expansion ratio of 3.0 was produced using the molding apparatus 100 shown in Figure 1. Raw material resin composition 2 was a mixture of virgin material, 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, and black masterbatch, in a mass ratio of 50 / 50 / 1 / 4 / 1, and recycled material obtained by crushing scraps generated in the previous production of foamed molded articles, in a mass ratio of 1:9.

[0042] The temperature control unit 9 was set so that the temperature of the foamed parison 13 would be 180°C. N2 gas was used as the foaming agent, and was injected through the foaming agent injection unit 8 located at the 0.5 L position. 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 it from the head 12 to form the 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] A foamed molded article was produced by blow molding using the foam parison 13 formed under the above conditions. 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 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, the analysis of carbides was carried out.

[0046] FT-IR analysis M1820, H5840B, and the carbonized product were each analyzed by FT-IR. 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] DSC analysis The carbonized material was analyzed by DSC, and the results are shown in Figure 2B. As shown in the DSC curve in Figure 2B, the carbonized material contains low-melting-point components derived from LDPE.

[0048] ·summary From the above, it was found that the main component of the carbonized material was LDPE.

[0049] 2. Example 1 In Example 1, foam-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 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, mass production of foam-molded articles was carried out 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 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 a 60-minute measurement, so the oxidation induction time was determined to be greater than 60 minutes. In addition, the MT and MFR of LDPE were measured at 190°C. The results are also shown in Table 1.

[0054] [Table 1]

[0055] As shown in Table 1, LF405H, the LDPE used in Examples 1 and 2, had a significantly longer oxidation induction time than M1820 used in Comparative Example 1. Furthermore, B5802-1, the HDPE used in Example 2, had a significantly longer oxidation induction time than H5840B used in Comparative Example 1 and Example 1. Considering that the thickness of the char layer was 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 suppressed 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 suppressed the formation of the char layer.

[0056] Furthermore, both M1820 and LF405H had a high MT×MFR value (mN·g / 10 min) of 250 or more at 190°C, and therefore Comparative Example 1 and Examples 1 and 2 all had good foam moldability. [Explanation of symbols]

[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 unit 11: Resin extrusion port 12: Head 13: Foam parison 14: Mold 14a: Split mold 14b: Split mold 100: Molding equipment

Claims

1. A method for producing a foamed molded article, comprising: The method includes a melt-kneading step, a parison forming step, and a molding step, In the melt-kneading step, the 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, In the molding step, the foam parison is molded to form a foam molded article, the raw material resin composition contains low-density polyethylene, The low-density polyethylene has an oxidation induction time measured at 180°C of 0.5 minutes or more.

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

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

4. 10. The method of claim 1, The low-density polyethylene has a melt tension (mN)×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, the raw material resin composition contains high-density polyethylene, The high-density polyethylene has an oxidation induction time measured at 180°C of 10 minutes or more.

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