Hollow plate and method for manufacturing the same

Abstract

To provide a hollow plate which is friendly to the global environment and has good moldability, and a method for manufacturing the hollow plate.SOLUTION: Provided is a hollow plate and a method for manufacturing the same, which is a hollow plate that has an intermediate layer having a hollow structure, and surface layers on both sides of the intermediate layer, and the hollow plate is obtained by melt-coextruding an intermediate layer resin comprising a polyolefin-based resin and an inorganic substance and having a melt index (ASTM D1238, 230°C) of 0.5 to 2.0 (g / 10 minutes), and a surface layer resin comprising a polyolefin-based resin at 190 to 250°C into a hollow plate-shape, and the contained amount of the inorganic substance is 20 mass% or more for the intermediate layer resin.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a hollow plate and a method for manufacturing the same. More specifically, the present invention relates to a hollow plate that is environmentally friendly and has good formability, and a method for manufacturing the same.

Background Art

[0002] Since polyolefin resins are inexpensive and easy to mold, so-called plastic corrugated board formed using them is excellent in impact resistance and compression resistance, and also has water resistance, light transmittance, recyclability, etc. It is used in a wide range of applications such as containers such as packing cases and shopping bags, surface protection packaging materials, partition boards, building materials, and decorative materials. As an example of plastic corrugated board, Patent Document 1 discloses a hollow plate made of a polypropylene resin formed by coextrusion.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] As a result of diligent experimental research conducted by the inventors of this invention, a hollow plate that is environmentally friendly and has good moldability, as well as a method for manufacturing the same, was discovered, leading to the completion of the present invention.

[0007] In other words, the present invention provides a hollow plate having an intermediate layer with a hollow structure and surface layers on both sides of the intermediate layer, wherein the hollow plate is formed into a hollow plate shape by melt co-extruding at 190 to 250°C an intermediate layer resin consisting of a polyolefin resin and an inorganic substance with a melt index (ASTM D1238, 230°C) of 0.5 to 2.0 (g / 10 min) and a surface layer resin consisting of a polyolefin resin, and the inorganic substance content is 20% by mass or more relative to the intermediate layer resin. In the present invention, the thickness of the surface layer may be twice or more the average particle diameter of the inorganic material. In the present invention, the inorganic substance may be one or more selected from the group consisting of talc, calcium carbonate, and clay.

[0008] Furthermore, the present invention also provides a method for manufacturing a hollow plate having a hollow intermediate layer and surface layers on both sides of the intermediate layer, comprising a co-extrusion step of melt-co-extruding an intermediate layer resin made of a polyolefin resin and an inorganic substance with a melt index (ASTM D1238, 230℃) of 0.5 to 2.0 (g / 10 min) and a surface layer resin made of a polyolefin resin at 190 to 250℃, and a cooling step of cooling using a cooling former after the co-extrusion step, wherein the inorganic substance content is 20% by mass or more relative to the intermediate layer resin. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a hollow plate that is environmentally friendly and has good moldability, as well as a method for manufacturing the same. The effects described herein are not necessarily limited to those described herein and may include any of the effects described herein. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing a cross-section of a hollow plate. [Modes for carrying out the invention]

[0011] 1. Hollow plate 1 The hollow plate 1 according to the present invention has an intermediate layer 11 having a hollow structure and surface layers 12 on both sides of the intermediate layer. Furthermore, it is formed into a hollow plate shape by melt co-extruding at 190 to 250°C an intermediate layer resin consisting of a polyolefin resin and an inorganic substance with a melt index (ASTM D1238, 230°C) of 0.5 to 2.0 (g / 10 min) and a surface layer resin consisting of a polyolefin resin, wherein the inorganic substance content is 20% by mass or more relative to the intermediate layer resin.

[0012] By using a large amount of inorganic material and reducing the amount of polyolefin resin used, it is possible to achieve goals such as reducing the use of petroleum-derived plastics and reducing CO2 emissions during incineration, thereby providing an environmentally friendly hollow plate. However, the inventors of this application conducted diligent experimental studies and found that when inorganic material is added at a rate of 20% by mass or more, the inorganic material bleeds out to the surface, resulting in surface abnormalities and molding defects due to poor adhesion.

[0013] In response to this, the inventors of the present invention conducted further intensive experimental studies and found that by melt-co-extruding an intermediate layer resin containing 20% ​​or more by mass of inorganic material and a surface layer resin made of polyolefin resin at 190-250°C to form a hollow plate, it is possible to prevent inorganic material from bleeding out to the surface, thereby providing a hollow plate that is environmentally friendly and has good moldability.

[0014] (1) Intermediate layer 11 Examples of the structure of the intermediate layer 11 include a sheet-like structure consisting of a pair of substantially parallel liner sections and a plurality of ribs perpendicular and / or diagonal thereto, as described in Japanese Patent Publication No. 38-4185 and Japanese Patent Publication No. 38-17182, that is, a corrugated cardboard-like structure as shown in Figure 1. The ratio of ribs to liner portions is preferably in the range of 0.1 to 10 by weight, more preferably in the range of 0.2 to 5, and even more preferably in the range of 0.5 to 2, considering the physical properties and molding surface.

[0015] The intermediate layer resin is a resin consisting of a polyolefin resin and an inorganic material, with a melt index (ASTM D1238, 230℃) of 0.5 to 2.0 (g / 10 min). Specifically, for example, this includes a resin composed of a polyolefin resin and an inorganic material blended in a predetermined ratio.

[0016] Examples of polyolefin resins include polyethylene such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene, and ultra-low-density polyethylene, as well as polypropylene such as polypropylene homopolymer, polypropylene random copolymer, and polypropylene block copolymer. A polypropylene block copolymer is a block copolymer of propylene and one or more other α-olefins (e.g., ethylene, 1-butene, 4-methyl-1-pentene, 1-pentene, 1-octene, 1-decene, 1-dodecene, etc.). In this invention, polyethylene and / or polypropylene are particularly preferred among these materials. Furthermore, as polypropylene, ethylene-propylene block copolymer containing 5.0 to 10.0% by mass of ethylene is particularly preferred from the viewpoint of mechanical properties such as impact resistance.

[0017] Examples of the inorganic substances include silicates, sulfates, carbonates, phosphates, borates, oxides, or hydrates thereof, such as magnesium, aluminum, calcium, titanium, iron, zinc, etc., and inorganic fibers. Specifically, for example, clays such as talc and kaolin, calcium carbonate, magnesium carbonate, zinc oxide, titanium oxide, silica, alumina, aluminum hydroxide, magnesium hydroxide, aluminum silicate, magnesium silicate, calcium silicate, aluminum sulfate, magnesium sulfate, calcium sulfate, magnesium phosphate, barium sulfate, silica sand, carbon black, zeolite, molybdenum, diatomaceous earth, sericite, shirasu, calcium sulfite, sodium sulfate, potassium titanate, bentonite, wollastonite, dolomite, graphite, glass fiber, micro glass, carbon fiber, ceramic fiber, rock wool, etc. These may be prepared by a synthetic method or may be of natural origin. Further, these may be used alone or in combination of two or more thereof.

[0018] In the present invention, among these, it is particularly preferable to use any one or more selected from the group consisting of talc, calcium carbonate, and clay, and it is more preferable to use talc and / or calcium carbonate. Talc and / or calcium carbonate have a relatively small particle size and good dispersibility, so they are excellent in handleability. In addition, talc and / or calcium carbonate are buried in large quantities all over the world and are considered to be fully compatible with the SDGs. Furthermore, although the price of oil has recently been soaring, they have the advantage of being stably supplied and being able to be used at low cost because they are inexpensive and have a large supply volume.

[0019] The shape of the inorganic substance is not particularly limited and may be any of particulate, flaky, granular, fibrous, etc. Also, as the particulate form, it may be spherical as generally obtained by a synthetic method, or may be irregularly shaped as obtained by pulverizing the collected natural mineral.

[0020] In the present invention, the content of the inorganic substance is 20% by mass or more with respect to the intermediate layer resin, preferably 30% by mass or more, more preferably 40% by mass or more, and particularly preferably 45% by mass or more. Thereby, the amount of petroleum-derived plastic used and the amount of CO2 emissions during combustion can be further reduced. Further, the upper limit value of the content of the inorganic substance is preferably 65% by mass or less with respect to the hollow layer resin. When the content of the inorganic substance exceeds 65% by mass, the dispersibility deteriorates and the connection of the resin deteriorates, so the probability of molding defects and deterioration of physical properties increases.

[0021] Further, in the present invention, the content of the inorganic substance with respect to the entire hollow plate 1 is preferably 20% by mass or more, more preferably 30% by mass or more, and still more preferably 40% by mass or more. Here, in the recycling method, manufacturers of container packaging such as plastics are obliged to recycle container packaging waste. One of the means for manufacturers to fulfill their obligation to recycle container packaging waste is to pay a commission fee for recycling to the Japan Container Packaging Recycling Association. For container packaging made of a plurality of materials that are difficult to separate, it is stipulated that the container packaging should be discarded according to the classification of the material with the highest ratio by weight among the materials constituting the container packaging. Therefore, particularly when the content of the inorganic substance with respect to the entire hollow plate 1 exceeds 50% by mass, the hollow plate 1 is not included in the classification of plastics, and as a result, it is possible to suppress the manufacturer of the hollow plate 1 from being obliged to pay the above-mentioned commission fee based on the recycling law.

[0022] The melt index (hereinafter also referred to as "MI"), which is an index of the melt viscosity of the intermediate layer resin, is in the range of 0.5 to 2.0 (g / 10 min) as measured according to ASTM D1238 [at 230°C with a load of 21.18 N (2.16 kg)]. If the MI is less than 0.5 (g / 10 min), problems such as peeling of the surface layer and cracking due to powder falling occur during coextrusion. Also, if it exceeds 2.0 (g / 10 min), problems of molding defects occur.

[0023] A commercially available inorganic-containing polyolefin resin may be used for the intermediate layer resin. Furthermore, the intermediate layer resin may contain, as needed, dispersants, antioxidants, UV absorbers, antistatic agents, antibacterial agents, flame retardants, light stabilizers, lubricants, and the like.

[0024] The thickness of the hollow intermediate layer 11 is preferably 3,000 to 10,000 μm, and more preferably 4,000 to 7,000 μm, from the viewpoint of practicality and continuous moldability.

[0025] (2) Surface layer 12 As for the structure of the surface layer 12, for example, if the intermediate layer 11 is the aforementioned corrugated cardboard structure, it is formed in layers on both sides of the pair of liner parts as shown in Figure 1.

[0026] The surface resin is a polyolefin resin, specifically as described above. If a resin other than a polyolefin resin is used as the surface resin, poor fusion with the intermediate resin will occur, making molding defects, interfacial delamination, and cracking more likely. In the present invention, polyethylene and / or polypropylene are particularly preferred among these materials. Furthermore, ethylene-propylene block copolymer containing 5.0 to 10.0% by mass of ethylene is particularly preferred as the polypropylene.

[0027] The melt index, an indicator of the melt viscosity of the surface resin, is measured according to ASTM D1238 [230°C with a load of 21.18N (2.16kg)] and is in the range of 0.5 to 2.0 (g / 10 min). If the MI is less than 0.5 (g / 10 min), problems such as surface peeling and powder shedding cracks occur during co-extrusion. If it exceeds 2.0 (g / 10 min), problems with molding defects occur. Furthermore, in this invention, from the viewpoint of moldability, it is preferable that the MI of the intermediate layer resin is greater than or equal to the MI of the surface resin.

[0028] The thickness of the surface layer 12 is preferably at least twice the average particle diameter of the inorganic material. If it is less than twice the average particle diameter, the inorganic material will escape from the surface layer 12, making it difficult to prevent bleed-out.

[0029] In this invention, the average particle diameter of inorganic matter can be, for example, the average particle diameter of powder calculated from the following formula (1) using the specific surface area value per gram of powder measured by the SS-100 powder specific surface area measuring device manufactured by Shimadzu Corporation.

[0030]

number

[0031] The average particle size of the inorganic material is preferably 5 to 50 μm, and more preferably 5 to 20 μm from the viewpoint of improving dispersibility. If the average particle size is less than 5 μm, the dispersibility of the inorganic material becomes extremely poor, making it easy for extrusion defects and molding defects due to aggregation to occur. Also, if the average particle size exceeds 50 μm, problems such as clogging of the extruder's filter or die are likely to occur.

[0032] The thickness of the surface layer 12 is preferably 20 to 200 μm, more preferably 30 to 150 μm, and even more preferably 40 to 125 μm. If the thickness of the surface layer 12 is less than 20 μm, in addition to bleed-out, processability is poor, such as cracking during processing and poor thermal adhesion. If the thickness of the surface layer 12 exceeds 200 μm, the inorganic content decreases, which is undesirable from the viewpoint of reducing the use of petroleum-derived plastics and reducing CO2 emissions during incineration.

[0033] A commercially available polyolefin resin may be used for the surface resin. In addition, antioxidants, UV absorbers, antistatic agents, antibacterial agents, flame retardants, light stabilizers, lubricants, etc. may be appropriately added to the surface resin as needed.

[0034] 2. Manufacturing method of hollow plate 1 The method for manufacturing a hollow plate 1 according to the present invention is a method for manufacturing a hollow plate 1 having a hollow intermediate layer 11 and surface layers 12 on both sides of the intermediate layer 11, comprising a co-extrusion step of melt-co-extruding an intermediate layer resin made of a polyolefin resin and an inorganic substance with a melt index (ASTM D1238, 230℃) of 0.5 to 2.0 (g / 10 min) and a surface layer resin made of a polyolefin resin at 190 to 250℃, and a cooling step of cooling using a cooling former after the co-extrusion step, wherein the content of the inorganic substance is 20% by mass or more relative to the intermediate layer resin. Furthermore, the method for manufacturing a hollow plate 1 according to the present invention may include other steps as necessary.

[0035] (1) Co-extrusion process In the co-extrusion process, for example, resins are introduced from a main extruder for extruding the intermediate layer resin and a secondary extruder for extruding the surface layer resin, and these are combined in a co-extrusion block equipped with dies that form the hollow intermediate layer and surface layer. The temperature of the co-extrusion block is controlled to 190-250°C for melt extrusion. As the main extruder and secondary extruder, for example, single-screw extruders with a predetermined discharge capacity used in normal melt extrusion can be used.

[0036] More specifically, for example, an intermediate layer resin containing 20% ​​or more by mass of inorganic material relative to the intermediate layer resin is melt-kneaded in a main extruder, and the surface layer resin is melt-kneaded in a secondary extruder, and then joined together in a co-extrusion block equipped with dies. A die having a hollow structure, which is sheet-like, i.e., corrugated, consisting of a pair of parallel liner sections and a plurality of ribs perpendicular and / or oblique to them, may have, for example, a predetermined thickness for the liner sections and a comb-like arrangement of core metals forming the hollow sections, with a structure that allows air to be supplied from the center of each core metal. The surface layer resin is placed on the upper and / or lower surfaces of the intermediate layer, guided to form the surface layer, and joined together in a laminated manner in the die.

[0037] The temperature of the co-extrusion block including the die is set to a range of 190 to 250°C, during which the intermediate layer resin and the surface layer resin are co-extruded. In this invention, the intermediate layer resin contains 20% or more by mass of inorganic material, which is thought to make it difficult to fuse with the surface layer resin. However, it has been found that by setting the temperature within the above range, the fusion between the intermediate layer resin and the surface layer resin can be improved, and the bleed-out of inorganic material can be effectively prevented. In particular, if the temperature of the co-extrusion block including the die falls below 190°C, the resin viscosity becomes too high, resulting in poor fusion between the surface layer resin and the intermediate layer resin. In the cooling process described later, inorganic material adheres to the grooves and surfaces of the cooling former, making molding defects more likely. Furthermore, if the temperature of the co-extrusion block including the die exceeds 250°C, the difference in behavior between the surface layer resin and the intermediate layer resin during cooling and solidification becomes large, resulting in molding defects and a decrease in the physical properties of the manufactured hollow plate.

[0038] (2) Cooling process In the cooling process, for example, the intermediate layer and surface layer, which are merged and integrated in a co-extrusion block and then melt-extruded from a die, are guided to a cooling former to produce a hollow plate with a hollow structure. As a cooling former, for example, one can be used in which a cooling medium is circulated between two upper and lower metal blocks having a gap corresponding to the thickness of the hollow plate, and a vacuum is used to draw in the cooling medium through pores provided on the contact surface side of the metal blocks with the hollow plate, while exchanging heat and cooling by contacting the hollow plate. In addition, the cooling effect may be enhanced by using air cooling near the entrance of the cooling former and by using water mist inside the former.

[0039] In this invention, although the intermediate layer resin contains 20% or more by mass of inorganic material, the intermediate layer resin is melt-co-extruded with the surface layer resin at 190-250°C. This prevents the inorganic material from bleeding out to the surface, and as a result, prevents the inorganic material from adhering to the metal block and other components that make up the cooling former. Therefore, stable continuous molding can be performed for long periods of time.

[0040] Furthermore, in order to form the hollow structure, air at a predetermined pressure is supplied from the center of the die's core, and the thickness of the hollow plate is maintained by this internal pressure while it is cooled and solidified, thereby defining the internal hollow structure. In the case of a so-called single-wafer product form that is cut to a predetermined length, a process may be included in which the product is cut to a predetermined length in a constant-pressure chamber, which is installed downstream of the cooling former and controlled to a predetermined pressure, so that the pressure inside the hollow part does not decrease or fluctuate after the cutting process, which would result in an open system. [Examples]

[0041] The present invention will be described in more detail below based on examples. The embodiments described below are merely examples of typical embodiments of the present invention, and this should not be interpreted as narrowing the scope of the present invention.

[0042] <Example 1> As the intermediate layer resin, a polypropylene block copolymer (compound or dry blend) containing 60% by mass of talc [MI = 1.0 (g / 10 min), average particle size 5-20 μm] was used. As the surface layer resin, a polypropylene block copolymer [MI = 0.8 (g / 10 min)] was used. A hollow plate with a target of forming a 99 μm thick surface layer resin on both surfaces of a 5000 μm thick intermediate layer was co-extruded at a die (including block) temperature of 230-240°C to produce a hollow plate with the structure shown in Figure 1. As shown in Figure 1, the obtained hollow plate has an intermediate layer with a hollow structure in which 220 μm thick ribs perpendicular to a pair of 300 μm thick liner sections are arranged parallel to each other at a pitch of 5000 μm, and a 99 μm thick surface layer is formed on both surfaces, with an overall basis weight of 1000 g / m². 2 The weight of the intermediate layer is 820g / m². 2 In addition, in this Example 1, the talc content relative to the entire hollow plate was 49% by mass.

[0043] <Example 2> The resulting hollow plate has a 50 μm thick surface layer formed on both sides, and an overall basis weight of 1000 g / m². 2The weight of the intermediate layer is 910g / m². 2 A hollow plate was manufactured that was the same as in Example 1, except for the following: In Example 2, the talc content relative to the total hollow plate was 54% by mass.

[0044] <Example 3> A hollow plate was manufactured in the same manner as in Example 1, except that a polypropylene block copolymer (compound or dry blend) containing 60% by mass of talc [MI = 0.5 (g / 10 min), average particle size 5-20 μm] was used as the intermediate layer resin and co-extruded at a temperature of 250°C. In this Example 3, the talc content relative to the entire hollow plate was 49% by mass.

[0045] <Example 4> A hollow plate was manufactured in the same manner as in Example 1, except that high-density polyethylene [MI = 2.0 (g / 10 min), average particle size 5-20 μm] containing 50% by mass of talc was used as the intermediate layer resin and co-extruded at a temperature of 190-200°C. In this Example 4, the talc content relative to the entire hollow plate was 41% by mass.

[0046] <Comparative Example 1> A hollow plate was manufactured in the same manner as in Example 1, except that a polypropylene block copolymer containing 60% by mass of talc [MI = 3.0 (g / 10 min), average particle size 5-20 μm] was used as the intermediate layer resin and co-extruded at a temperature of 180°C. In Comparative Example 1, the talc content relative to the entire hollow plate was 49% by mass.

[0047] <Comparative Example 2> A hollow plate was manufactured in the same manner as in Example 1, except that a polypropylene block copolymer containing 60% by mass of talc [MI = 0.3 (g / 10 min), average particle size 5-20 μm] was used as the intermediate layer resin and co-extruded at a temperature of 260°C. In Comparative Example 2, the talc content relative to the entire hollow plate was 49% by mass.

[0048] <Comparative Example 3> A hollow plate was manufactured in the same manner as in Example 1, except that a polypropylene block copolymer containing 60% by mass of talc [MI = 0.3 (g / 10 min), average particle size 5-20 μm] was used as the intermediate layer resin, and a polyarylate resin [MI = 3.0 (g / 10 min)] was used as the surface layer resin, and the plates were co-extruded at a temperature of 230°C. In Comparative Example 3, the talc content relative to the entire hollow plate was 49% by mass.

[0049] These results are shown in Tables 1 and 2. The evaluation criteria for each evaluation are described below.

[0050] [Environmental] A: The inorganic content relative to the entire hollow plate was 40% by mass or more. B: The inorganic content relative to the entire hollow plate was 30% by mass or more. C: The inorganic content relative to the entire hollow plate was less than 30% by mass.

[0051] [Moldability] A: Upon visual inspection, no bleed-out was observed during manufacturing, and there were no extrusion defects or die jams. B: Upon visual inspection, some bleed-out was observed during manufacturing, and some extrusion defects and die clogging were also found, but the product was at a level that did not cause any problems. C: Upon visual inspection, significant bleed-out was observed during manufacturing, along with extrusion defects and die clogging.

[0052] [Bending stiffness] A: The bending stiffness in the MD direction was 600 N·cm or more. B: The bending stiffness in the MD direction was 500 N·cm or more. The bending stiffness in the C:MD direction was less than 500 N·cm.

[0053] [Bending strength] A: The bending strength in the MD direction was 50N or more. B: The bending strength in the MD direction was 40N or more. The bending strength in the C:MD direction was less than 40N.

[0054] [Table 1]

[0055] [Table 2]

[0056] These results indicate that the hollow plates in Examples 1-4 have a high inorganic content relative to the total hollow plate, resulting in an environmentally friendly structure, and also exhibit excellent moldability. Furthermore, the hollow plates in Examples 1-4 showed no problems in terms of physical properties such as bending rigidity and bending strength.

[0057] On the other hand, while the hollow plates in Comparative Examples 1 to 3 had a high inorganic content relative to the total hollow plate, their moldability was poor, making them unsuitable for continuous and stable industrial production. Furthermore, in Comparative Example 3, interfacial delamination and cracking were observed. In addition, due to the poor moldability, the hollow plates in Comparative Examples 1 to 3 had a poorer surface appearance compared to the hollow plates in Examples 1 to 4.

[0058] In addition, since the hollow plate of Example 2 contains 50% or more by mass of inorganic matter relative to the entire hollow plate, it is a hollow plate 1 that is not included in the classification of plastics, and thus the obligation to pay the above-mentioned consignment fee under the Recycling Law can be reduced. [Industrial applicability]

[0059] According to the present invention, it is possible to provide a hollow plate and a method for manufacturing the same that is environmentally friendly and has good moldability. Furthermore, because of its excellent physical properties, it is useful in a wide range of applications such as packaging cases, returnable containers, surface protective packaging materials, partition plates, building materials, and decorative materials. [Explanation of Symbols]

[0060] 1: Hollow plate 11: Middle Class 12: Surface layer

Claims

1. A hollow plate having an intermediate layer having a hollow structure and surface layers on both sides of the intermediate layer, The hollow plate is formed into a hollow plate shape by melt-extrusion at 190 to 250°C of an intermediate layer resin having a melt index (ASTM D1238, 230°C) of 0.5 to 2.0 (g / 10 min) made of a polyolefin-based resin and an inorganic substance, and a surface layer resin made of a polyolefin-based resin, A hollow plate, wherein the content of the inorganic substance is 20% by mass or more relative to the resin of the intermediate layer.

2. The hollow plate according to claim 1 , wherein the thickness of the surface layer is at least twice the average particle size of the inorganic material.

3. 3. The hollow plate according to claim 1, wherein the inorganic substance is at least one selected from the group consisting of talc, calcium carbonate, and clay.

4. A method for manufacturing a hollow plate having an intermediate layer with a hollow structure and surface layers on both sides of the intermediate layer, comprising: a co-extrusion step of melt-co-extruding an intermediate layer resin, which is made of a polyolefin-based resin and an inorganic substance and has a melt index (ASTM D1238, 230°C) of 0.5 to 2.0 (g / 10 min), and a surface layer resin, which is made of a polyolefin-based resin, at 190 to 250°C; a cooling step of cooling the mixture using a cooling former after the co-extrusion step; and The method for producing a hollow plate, wherein the content of the inorganic substance is 20% by mass or more relative to the intermediate layer resin.

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