JP7530546B2 - Manufacturing method of polyolefin foam, manufacturing method of packaging material, polyolefin foam, and packaging material for food - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies (1) Name of the exhibition: Exhibition and Business Negotiations 2020 Date held: February 12, 2020 (2) Name of publication: TOTAL FOOD PACKAGE GUIDE 2020 Chuo Kagaku Plastic Food Packaging Containers General Catalog Date of publication: March 30, 2020

The present invention relates to a method for producing a polyolefin foam using a resin composition suitable for use in, for example, food packaging, a method for producing a packaging material using the produced foam, a polyolefin foam made from the resin composition, and a food packaging material made from the foam.

Traditionally, in the field of building materials, foams made from resin compositions containing synthetic resins and other materials have been used as heat insulation, cushioning, soundproofing, and cold insulation materials. However, for example, heat insulation materials have problems such as the tendency to generate soot and toxic gases when incinerated at the time of disposal, high fuel calories, inability to mold to the desired width due to insufficient foaming, and insufficient insulation.

Patent Document 1 discloses that when expandable polypropylene is used as the raw material for the foam, by blending recycled resin that is a mixture of polystyrene resin and polyolefin resin (polypropylene resin and polyethylene resin), the foam is less likely to produce soot when burned, has low combustion calories, and has the desired expandability and heat insulation properties. The specific components of the foam are paper pellets in a specified blend, the recycled resin, the expandable polypropylene, and water.

JP 2011-213966 A

On the other hand, in the food packaging field, foam packaging made from a resin composition containing a specific synthetic resin is used in the hope of absorbing shocks applied to the packaged food, preventing the food from slipping out of position, and reducing the total weight including the food. In other words, since food is placed inside the packaging, hygiene of the foam is also essential, and therefore the foam of Patent Document 1, which has little track record, is difficult to adopt in the food packaging field.

In recent years, inorganic fillers have been blended into the resin composition of foams used in food packaging to improve the strength, rigidity, and other resistance properties of the packaging material, as well as to reduce raw material costs. There are many types of inorganic fillers, and the type and ratio of fillers to be blended are determined appropriately depending on the function and use of the packaging material. In addition, as interest in environmental issues grows, efforts to reduce the amount of synthetic resin used are becoming more active.

Generally, resin compositions kneaded with synthetic resins and inorganic fillers are sometimes distributed as pellets. Pellets of resin compositions used as raw materials for foams are sometimes made porous in order to increase the water absorption rate of the water added during production and thereby increase the expansion ratio of the foam. However, if the pellets are porous and water-absorbent, it can be difficult to control the quality in response to seasonal changes in temperature and humidity.

The object of the present invention is to provide a method for producing a polyolefin foam that is easy to use as a packaging material for food products, that takes environmental measures into consideration, and that avoids the trouble of quality control before production, a method for producing a packaging material for food products, a polyolefin foam, and a packaging material for food products.

That is, the present invention is a method for producing a polyolefin foam used as a packaging material for food, comprising the steps of heating a resin composition kneaded with at least a polyolefin resin and an inorganic filler to produce a molten material, adding 0.5% by weight to 3% by weight of water relative to 100% by weight of the molten material and further mixing to produce a hydrous molten material, and vaporizing the water contained in the hydrous molten material to produce a sheet-like foam that is corrugated when viewed from the end .

The resin composition does not contain a chemical foaming agent, and it is desirable that the foaming ratio of the foam to the resin composition is 4 times or more.

It is desirable that the weight ratio of the polyolefin resin to the inorganic filler is 85:15 to 30:70.

In addition, the method for producing a packaging material for food according to the present invention is characterized by comprising the steps of heating a resin composition obtained by kneading at least a polyolefin resin and an inorganic filler to produce a molten material, adding 0.5% by weight to 3% by weight of water relative to 100% by weight of the molten material and further mixing to produce a hydrous molten material, evaporating the water contained in the hydrous molten material to produce a sheet-like foam having a thickness of 500 μm to 3000 μm and a corrugated shape when viewed from the end , and molding the sheet-like foam.

The polyolefin foam of the present invention is used as a packaging material for food products, and is a polyolefin foam in a state in which the water in a hydrous molten material containing water and a molten material obtained by heating a resin composition in which at least a polyolefin-based resin and an inorganic filler are kneaded is vaporized, the hydrous molten material containing 0.5 to 3% by weight of water relative to 100% by weight of the molten material, the polyolefin-based resin containing a polypropylene-based resin, the inorganic filler containing talc, a weight ratio of the polypropylene-based resin to the inorganic filler being 85:15 to 30:70, and being in the form of a corrugated sheet when viewed from an end .

The food packaging material of the present invention is a food packaging material formed from a polyolefin foam sheet made by forming the polyolefin foam into a sheet shape, and is characterized in that the polyolefin foam sheet has a thickness of 500 μm to 3000 μm and has a bottom on which the food is placed and a side wall portion that is continuous with the outer peripheral edge of the bottom and extends upward.

The present invention is expected to be easy to use as a packaging material for food products, to take environmental measures into consideration, and to avoid the trouble of quality control before production.

1 is an example of an extruder used in a method for producing a polyolefin foam in one embodiment of the present invention. 1 is an example of a foam sheet used in a method for producing food packaging material in one embodiment of the present invention.

The following describes a method for producing a polyolefin foam in one embodiment of the present invention, a method for producing a packaging material for food products, a polyolefin foam, and a packaging material for food products.

<Outline of the method for producing polyolefin foam>
The method for producing a polyolefin foam includes a first step of heating a resin composition obtained by kneading at least a polyolefin resin and an inorganic filler to produce a molten product, a second step of adding 0.5 to 3 wt % of water to 100 wt % of the molten product and further mixing the mixture to produce a water-containing molten product, and a third step of vaporizing the water contained in the water-containing molten product to produce a foam.

These processes do not require the resin composition to be porous (water-absorbent), which avoids the need for quality control in response to seasonal fluctuations in temperature and humidity, and because moisture is uniformly dispersed, uniform bubbles are obtained. By mixing inorganic fillers into the resin composition, it is possible to reduce the amount of synthetic resin used and to improve the rigidity and strength of the foam produced.

The term "melt" refers to the resin composition that is heated in the first step to melt it and become fluid. The term "water-containing melt" refers to the melt in which the water supplied in the second step is dispersed. The term "foam" refers to the water-containing melt that is released to atmospheric pressure in the third step, causing all or part of the water in the melt to evaporate, forming bubbles, and then solidifying when cooled. Foams are used as packaging materials for food products, cushioning materials, and heat insulating materials, but are not limited to any particular application as long as they are usable.

<Details of Resin Composition>
The resin composition is not particularly limited as long as it is a mixture of at least a polyolefin-based resin among synthetic resins and an inorganic filler as a filler. In addition to the polyolefin-based resin, the synthetic resin to be kneaded may also contain, as necessary, a polystyrene-based resin, for example.

The resin composition may be supplied as a compound that has been produced in advance in a separate process. Here, the compound refers to a compound that is made by kneading at least a polyolefin resin among synthetic resins and an inorganic filler as a filler while heating, extruding, and forming into pellets. In the present invention, the water content of the compound is preferably 1% by weight or less, more preferably 0.5% by weight or less, and even more preferably 0.1% by weight or less.

Polyolefin resins include polyethylene resins (PE) and polypropylene resins (PP), and may be a mixture of polyethylene resins and polypropylene resins, but preferably are polypropylene resins alone.

Examples of polypropylene-based resins include homopolypropylene resins, propylene-ethylene random copolymers, propylene-ethylene block copolymers, and propylene-α-olefin copolymers.

The inorganic filler is, for example, particulate with a size of 10 nm to 100 μm, such as talc, calcium carbonate, silica, clay, wollastonite, potassium titanate, xonotlite, gypsum fiber, aluminum borate, fibrous magnesium compound (MOS), aramid fiber, carbon fiber, glass fiber, mica, glass flakes, and polyoxybenzoyl whiskers. It may be one type or a mixture of two or more types, and is preferably talc, which has a proven track record in the food packaging field, and the average particle size may be 5 μm to 15 μm, 6 μm to 10 μm, or 7 μm to 9 μm.

<Water ratio>
The amount of water may be 0.5% by weight to 3% by weight, based on 100% by weight of the molten material, but is preferably 0.6% by weight to 1.5% by weight, and more preferably 0.8% by weight to 1.2% by weight. If the amount is less than 0.5% by weight, foaming will not occur and the desired physical properties will not be obtained, while if the amount is more than 3% by weight, uniform bubbles will not be obtained and the physical properties of the foam will become unstable.

<Another feature of the method for producing polyolefin foam>
The resin composition does not contain a chemical foaming agent, and the foam has an expansion ratio of 4 times or more relative to the resin composition.

This avoids the environmental burden associated with the use of chemical foaming agents, and allows foams with the desired expansion ratio to be obtained without kneading a chemical foaming agent into the resin composition. In other words, foams with an expansion ratio of less than 4 times are hard and do not have the physical properties such as strength, rigidity, and elasticity that are suitable for food packaging materials, and are therefore unsuitable as food packaging materials.

Chemical foaming agents include organic and inorganic foaming agents as well as thermal decomposition and reaction systems, such as dinitropentamethylenetetramine (DPT), azodicarbonamide (ADCA), 4,4'-oxybisbenzosulfonylhydrazide (OBSH), hydrogen carbonate, carbonate, sodium hydrogen carbonate (baking soda), and a combination of sodium hydrogen carbonate and citric acid, and include all foaming agents except for naturally occurring foaming agents such as water.

Furthermore, the weight ratio of the polyolefin resin to the inorganic filler is 85:15 to 30:70, preferably 70:30 to 35:65, and more preferably 60:40 to 40:60.

According to this, if the weight ratio of polyolefin resin is higher than 85:15, the foam becomes too soft and does not have the strength, rigidity, elasticity, and other physical properties suitable for food packaging, and if the weight ratio of inorganic filler is higher than 30:70, a sheet cannot be obtained. And more preferably, the polyolefin resin is a polypropylene resin and the inorganic filler is talc.

<Outline of manufacturing method for food packaging materials>
The method for producing a packaging material for food includes a third step of extruding the polyolefin foam produced by the method for producing a polyolefin foam to obtain a sheet-like foam having a thickness of 500 μm to 3000 μm, and a fourth step of molding the sheet-like foam.

These processes are expected to produce food packaging materials with the desired physical properties, such as strength, rigidity, and elasticity. If the sheet is thinner than 500 μm, it will be prone to breakage during molding or use, and if it is thicker than 3,000 μm, there is a risk that the manufacturing costs will be high and the cost-effectiveness will be reduced.

"Food packaging material" refers to a package that at least comprises a bottom on which food is placed and a side wall portion extending upward from the outer periphery of the bottom, preferably a flange portion extending outward from the upper edge of the side wall portion, and more preferably a lid portion extending outward from the outer periphery of the flange portion to cover an opening formed by the upper edge of the side wall portion. The package may or may not be a so-called container, and may be rectangular or circular in plan view, and is not limited thereto. "Food" refers to fresh produce such as meat and fish, and includes all products that are relatively dense and heavy compared to products that create a fluffy texture such as salads and cakes.

<Relationship between each process>
The first to fourth steps are performed in chronological order, and from the viewpoint of improving production efficiency, the intervals between each step may be any interval as long as they do not affect the production of the desired foam and food packaging material. Preferably, the next step is started immediately after the previous step is completed, and each step may be performed in a separate device. However, it is preferable that the first to third steps are performed in one designated device and the fourth step in another device, and more preferably, the first to third steps are performed in an extruder and the fourth step in a molding machine.

<Details of the extruder>
As shown in FIG. 1 , an extruder M used in the method for producing a polyolefin foam includes a heating/cooling type cylinder 101 having a single-shaft screw 111 therein, an inlet 121 provided at the upstream end of the cylinder 101 through which a resin composition is introduced, a die 131 provided at the downstream end of the cylinder 101 through which a water-containing molten material is extruded, and a water injection section 141 provided between the upstream and downstream ends of the cylinder 101 for injecting water therein.

The die 131 is a tool with an outlet hole that extrudes the raw material from the outlet of the cylinder 101 to produce a foam in the desired shape, and may be, for example, a T-die or a circular die, but is preferably a T-die with an outlet hole that makes it easy to produce the foam S into a sheet. There are no particular limitations on the size or shape of the outlet hole, but for example, in the case of a T-die, it may be a rectangular shape with a thickness (clearance) of 0.01 to 0.3 mm and a width of 500 mm to 2000 mm. In the case of a circular die, it may be a ring shape with a diameter of 150 to 650 mm and a thickness (clearance) of 0.01 to 0.3 mm.

The T-die 131 may have a round discharge hole, or may have multiple such round holes adjacent to each other. With this configuration, the extruded rod-shaped foam (also called rod-shaped foam) adheres to adjacent rod-shaped foams in a molten state, resulting in an aggregate of rod-shaped foams. Depending on the arrangement of the round holes, a sheet-shaped aggregate or a cylindrical aggregate can be obtained. Here, the size of the round hole serving as the discharge hole may be 0.01 to 0.3 mm in diameter.

Although not shown in detail, the water injection section 141 is a check valve provided in a hole leading from the outside to the inside of the cylinder 101, and supplies water from a separate metering pump through the check valve toward the vicinity of the screw 111. In other words, the water injection section 141 supplies water to the molten material that is melted by heating the resin composition in the cylinder 101 and pushing it toward the downstream end with the screw 111, making it easier to generate a water-containing molten material, and the check valve is preferably provided at a location in the cylinder 101 where the resin composition pushed toward the downstream end by the screw 111 is completely melted into the molten material.

<Details of molding machine>
The molding machine used in the manufacturing method of food packaging material is a device that performs, for example, press molding, vacuum molding, hot plate pressure molding, vacuum pressure molding, double-sided vacuum molding, and deep drawing molding.

<Relationship between each process and equipment>
When the first to third steps are performed by the extruder, the first step is from introducing the resin composition into the cylinder 101 from the inlet 121 to melting the resin composition by heating the inside of the cylinder 101 and rotating the screw 111 to generate a molten material, the second step is from supplying water to the molten material generated in the first step by injecting water from the water injection section 141 to dispersing the molten material and water by heating the inside of the cylinder 101 and rotating the screw 111 to generate a water-containing molten material, and the third step is from sending the water-containing molten material generated in the second step from the inside of the cylinder 101 to the die 131 to generating a foam by vaporizing the water in the water-containing molten material.

In the third step, a T-die having a rectangular discharge hole can be used as the die 131. That is, the water-containing molten material sent to the die 131 in the third step passes through the discharge hole of the die 131 and is released to atmospheric pressure, thereby obtaining a sheet-like foam.

In the third step, the discharge hole of the T-die 131 may be a round hole, or multiple such round holes may be formed adjacent to each other. With this configuration, the extruded rod-shaped foams are intimately bonded to adjacent rod-shaped foams in a molten state, and an aggregate of rod-shaped foams can be obtained.

When the fourth step is carried out in the molding machine, the fourth step corresponds to the process from feeding the sheet-like foam obtained in the third step from the extruder to the molding machine to molding the sheet-like foam.

<Details of the sheet-shaped foam>
As shown in FIG. 2, the sheet-like foam S (hereinafter also referred to as "sheet S") is wavy when viewed from the end. As shown in the plan view of FIG. 2(a), the straight thin lines are ridgelines indicating the apexes of the crests and valleys of the wave shape, and the horizontal width between the ridgelines (the distance between adjacent crests) is irregular but is 200 μm to 500 μm on average, and as shown in the end view of FIG. 2(b) at the X-X portion of the plan view, the vertical width of each crest and valley of the wave shape (the vertical distance between the apex of the crest and the apex of the adjacent valley) is irregular but is 500 μm to 1500 μm on average. The thickness of the sheet S is not particularly limited, but may be 200 μm to 1500 μm or 300 μm to 800 μm. The shape of the sheet S is based on the method for producing a polyolefin foam and the method for producing a packaging material for food, and is particularly based on one or a combination of two or more of the following: the expansion ratio of the foam, the size and shape of the die outlet hole during extrusion, and the water content in the water-containing melt. Because the sheet S is prone to tearing in the ridge line direction due to the structure of the foam, it is prone to stretching parallel to the ridge line direction and is easy to bend or roll, or in other words, it is difficult to stretch along the ridge line direction and is difficult to bend or roll.

The following describes evaluation tests of a polyolefin foam produced by the method for producing a polyolefin foam in accordance with the present invention and a food packaging material produced by the method for producing a food packaging material using the polyolefin foam.

<Evaluation test overview>
The evaluation tests were carried out as examples and comparative examples by changing the formulation of the resin composition, the weight ratio in the formulation, and the weight percentage of water relative to 100 weight percent of the melt. The common conditions in the evaluation tests and the evaluation method for each evaluation item are as follows.

<Common conditions for evaluation tests>
The method for producing the polyolefin foam is carried out using an extruder manufactured by Toshiba Machine Co., Ltd., with the cylinder heating temperature set to 160-220°C, the T-die discharge hole having a thickness (clearance) of 0.05 mm and a width of 1000 mm, and a sheet-like foam is obtained. The resin composition is supplied as a compound containing at least a polypropylene resin as a synthetic resin and talc with an average particle size of 8 μm as an inorganic filler, containing no chemical foaming agent, and not having water absorption due to its porosity or shape.

The manufacturing method for food packaging materials is carried out using a press molding machine, and the food packaging materials manufactured are for packaging general fresh produce and have at least a bottom and side walls, with the depth of the packaging material, which corresponds to the height from the opening formed by the upper edge of the side walls to the bottom, being 25 mm.

<Evaluation method of foam>
The basis weight is measured on a 100 mm x 100 mm test piece punched out from the sheet with reference to JIS P8124 "Paper and paperboard - Method of measuring basis weight". The specific gravity and expansion ratio are measured by the underwater displacement method with a Shimadzu specific gravity meter with reference to JIS Z8807 "Method of measuring density and specific gravity of solids".

<Evaluation method for sheets and food packaging materials>
The vertical width of the sheet is measured with a thickness gauge manufactured by Mitutoyo Corporation. The elastic modulus is measured using a universal material testing machine manufactured by Instron Corporation. The orientation of the sheet for measuring the elastic modulus is based on the ridge direction of the corrugated shape, with the ridge direction being the MD and the direction perpendicular to the MD being the TD. The tensile elastic modulus is measured on a 140 mm x 15 mm test piece punched out of the sheet according to JIS K7161 "Plastics - Determination of Tensile Properties". The flexural elastic modulus is measured on a 70 mm x 15 mm test piece punched out of the sheet according to JIS K7171 "Plastics - Determination of Bending Properties". The number of peaks is calculated visually on a 50 mm test piece punched out of the sheet in the TD.

Finally, the condition of the sheet is evaluated as follows: if it can be molded and the unevenness of the ridges is uniform, it is evaluated as "○", if it can be molded but the unevenness is not uniform, it is evaluated as "△", and if it cannot be molded, it is evaluated as "×". In addition, the ease of packing the packaging material by workers is evaluated as "○" if there is no problem, and "×" if it is too hard to bend.

The individual conditions for Examples 1 to 4 and Comparative Examples 1 and 2 are as follows:

Example 1
Synthetic resin = PP, inorganic filler = talc, weight ratio = PP:talc = 45:55, water = 1 wt%

Example 2
Synthetic resin = PP + PE, inorganic filler = talc, weight ratio = PP:PE:talc = 35:10:55, water = 1 wt%

Example 3
Synthetic resin = PP, inorganic filler = talc, weight ratio = PP:talc = 85:15, water = 2% by weight

Example 4
Synthetic resin = PP, inorganic filler = talc, weight ratio = PP:talc = 90:10, water = 1.5 wt%

<Comparative Example 1>
Synthetic resin = PP, inorganic filler = talc, weight ratio = PP:talc = 45:55, water = 0.3 wt%

<Comparative Example 2>
Synthetic resin = PP, inorganic filler = talc, weight ratio = PP:talc = 45:55, water = 5% by weight

<Differences in individual conditions between Examples 1 to 4>
Example 1 is used as a reference. Example 2 is obtained by reducing the weight ratio of the polypropylene resin and adding a polyethylene resin compared to Example 1. Examples 3 and 4 are obtained by changing the weight ratio of the polypropylene resin and talc in the resin composition and the weight percentage of water in the water-containing melt compared to Example 1, and are also intended to examine the critical significance of the weight ratio.

<Differences in individual conditions between Example 1 and Comparative Examples 1 and 2>
In Comparative Examples 1 and 2, the weight percentage of water in the hydrous melt was set outside the range of 0.5% by weight to 3% by weight compared to Example 1, and the critical significance of the above range was also examined.

<Evaluation Results>
The evaluation results of Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1. For ease of reference, the above-mentioned individual conditions are also listed in the table. The evaluation results in the above evaluation methods were calculated by sampling any three points located on a predetermined straight line in the above TD and taking the arithmetic mean value.

<Considerations of Examples 1 to 4>
In comparison with Example 1 as the standard, the reduced amount of polypropylene resin was replaced with polyethylene resin in Example 2, and the weight ratio of polypropylene resin in the resin composition was increased in Examples 3 and 4, but the results were roughly equivalent. On the other hand, in Example 4, compared to Example 3, when the weight ratio was outside the range of 85:15 to 30:70, even if the weight percentage of water in the hydrous melt was within the range of 0.5 to 3 weight%, the formability of the sheet was evaluated as "△", meaning that the sheet could be formed but the unevenness of the peaks was uneven.

<Considerations of Comparative Examples 1 and 2>
The allowable range of the weight percent of water in the hydrous melt is considered to be 0.5 weight percent to 3 weight percent. In Comparative Example 1, when the weight percent of water was set below the minimum value of the allowable range, the expansion ratio of the foam was less than 4 times, and the sheet thickness was outside the range of 500 μm to 3000 μm and became too thin. Therefore, the sheet condition was evaluated as "△" for moldable but unevenness, and "×" for ease of packing into packaging material for packaging, for being too hard to be folded. In Comparative Example 2, when the weight percent of water was set above the maximum value of the allowable range, the foam expanded too much and a moldable sheet could not be obtained.

<Reference Example>
For convenience of explanation, the evaluation results of tests conducted under conditions different from those of the comparative example are shown in Table 2.

In Reference Example 1, the composition and weight ratio of the resin composition are the same as in Example 3, but when the layer structure of the sheet is two types and three layers (polypropylene resin and talc resin composition layer/polypropylene foam layer/polypropylene resin and talc resin composition layer), the expansion ratio is less than four times and the sheet is too hard. In Reference Example 2, the composition and weight ratio of the resin composition are the same as in Example 1, but when the expansion ratio of the foam is set to 1 (non-foamed), the sheet becomes hard.

M: extruder, S: foam, 101: cylinder, 111: screw, 121: inlet, 131: die, 141: water injection section

Claims (6)

A method for producing a polyolefin foam used as a food packaging material, comprising the steps of:
A step of heating a resin composition obtained by kneading at least a polyolefin resin and an inorganic filler to generate a molten material;
adding 0.5% to 3% by weight of water to 100% by weight of the melt and further mixing to produce a water-containing melt;
and a step of vaporizing the water contained in the hydrous melt to produce a foam in the form of a sheet having a corrugated shape when viewed from end . The resin composition does not contain a chemical foaming agent,
The method for producing a polyolefin foam according to claim 1, wherein the foam has an expansion ratio of 4 times or more relative to the resin composition. The method for producing a polyolefin foam according to claim 1 or 2, wherein a weight ratio of the polyolefin resin to the inorganic filler is 85:15 to 30:70. A step of heating a resin composition obtained by kneading at least a polyolefin resin and an inorganic filler to generate a molten material;
adding 0.5% to 3% by weight of water to 100% by weight of the melt and further mixing to produce a water-containing melt;
a step of evaporating the water contained in the water-containing melt to produce a sheet-like foam having a thickness of 500 μm to 3000 μm and a corrugated shape when viewed from the end ;
and a step of molding the sheet - like foam. A polyolefin foam used as a packaging material for food, the polyolefin foam being in a state in which water in a water-containing molten material containing water and a molten material obtained by heating a resin composition obtained by kneading at least a polyolefin resin and an inorganic filler has been vaporized,
The water-containing melt contains 0.5 to 3% by weight of water relative to 100% by weight of the melt,
The polyolefin resin includes a polypropylene resin,
The inorganic filler includes talc,
The weight ratio of the polypropylene resin to the inorganic filler is 85:15 to 30:70;
It is a corrugated sheet when viewed from the end.
A polyolefin foam characterized by: A packaging material for packaging food, formed of a polyolefin foam sheet obtained by forming the polyolefin foam according to claim 5 into a sheet shape,
The thickness of the polyolefin foam sheet is 500 μm to 3000 μm,
A bottom portion on which food is placed;
and a side wall portion extending continuously upward from the outer peripheral edge of the bottom portion.

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