JP7496678B2 - Polystyrene resin foam sheet and container - Google Patents

Description

The present invention relates to a polystyrene resin foam sheet (PSP sheet) that is excellent in productivity and moldability, and a container obtained by molding the PSP sheet.

PSP sheets are widely used as various food packaging materials and simple containers because they are lightweight and have excellent thermoformability. PSP sheets are generally manufactured by extruding molten polystyrene resin in an extruder into a sheet while pressing in a physical foaming agent made of aliphatic hydrocarbons such as butane and pentane.

The foaming agent contained in the foam sheet gradually dissipates after production. It is known that the remaining amount of foaming agent affects the moldability of the foam sheet. For example, if the amount of remaining foaming agent is too small, the molded product is likely to have insufficient strength and reduced formability, while if the amount of remaining foaming agent is too large, it is known to cause molding abnormalities such as shrinkage, warping, and surface roughness of the molded product (see Patent Documents 1 and 2). Therefore, it is known that an aging period is necessary after production of the PSP sheet in order to ensure that the remaining amount of foaming agent is appropriate.

Japanese Patent No. 4360609 Japanese Patent Application Laid-Open No. 11-147968

It is known that in general PSP sheets, the higher the content of normal butane, a physical foaming agent, the faster the foaming agent dissipates (Patent Document 1). On the other hand, in the case of heat-resistant PSP sheets that contain a styrene copolymer copolymerized with methacrylic acid or the like as a polystyrene resin, the decrease in the remaining amount of foaming agent is slow even if the normal butane content is high. Therefore, the aging period is longer than that of general PSP sheets, resulting in a problem of low productivity.

The present invention aims to provide a PSP sheet that has a short aging period, excellent productivity, and excellent moldability, and a container obtained by molding the sheet.

The PSP sheet of the present invention is a polystyrene-based resin foam sheet obtained by adding a physical foaming agent and a chemical foaming agent containing normal butane to a base resin containing a styrene-methacrylic acid copolymer, a styrene-acrylic acid copolymer, or a styrene-maleic anhydride copolymer, and foaming the base resin, and is characterized in that the amount of the chemical foaming agent added is 1% by mass or more and 5% by mass or less, and the amount of gas remaining in the polystyrene-based resin foam sheet is 0.161 g or less.

The container of the present invention is characterized in that it is a container formed by thermoforming the PSP sheet of the present invention.

The PSP sheet of the present invention has a shorter aging period than conventional heat-resistant PSP sheets, and therefore has excellent productivity. In addition, when the PSP sheet of the present invention is used, it is possible to suppress delamination (peeling of the laminate film) during container molding, and it has superior moldability compared to conventional heat-resistant PSP sheets.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

The PSP sheet according to this embodiment (hereinafter referred to as "this PSP sheet") is obtained by adding a physical foaming agent and a chemical foaming agent containing normal butane to a base resin containing a styrene-methacrylic acid copolymer, a styrene-acrylic acid copolymer, or a styrene-maleic anhydride copolymer, and foaming the base resin, the amount of the chemical foaming agent added being 1% by mass or more, and the amount of gas remaining in the polystyrene resin foam sheet being 0.161 g or less.

The base resin contains a styrene-methacrylic acid copolymer, a styrene-acrylic acid copolymer, or a styrene-maleic anhydride copolymer (hereinafter referred to as "styrene-methacrylic acid copolymer, etc."). Therefore, the PSP sheet has excellent heat resistance. The base resin may contain only one of styrene-methacrylic acid copolymer, styrene-acrylic acid copolymer, and styrene-maleic anhydride copolymer, or may contain two or more of them. The base resin may be composed of only the styrene-methacrylic acid copolymer, etc., or may be a mixed resin in which the styrene-methacrylic acid copolymer, etc. is the main component and one or more of other polymers, resins, and rubbers are mixed as secondary components. An example of a resin that can be mixed as the secondary component is a polyphenylene ether resin. The ratio of the styrene-methacrylic acid copolymer, etc. in the base resin is usually 70% by mass or more, preferably 80% by mass or more.

The physical foaming agent is not particularly limited as long as it contains normal butane and can foam the base resin. Specific examples of the physical foaming agent include aliphatic hydrocarbons such as propane, n-butane, i-butane, a mixture of n-butane and i-butane, pentane, and hexane; cyclic aliphatic hydrocarbons such as cyclobutane and cyclopentane; and halogenated hydrocarbons such as trichlorofluoromethane, dichlorodifluoromethane, 1,1-difluoroethane, 1,1-difluoro-1-chloroethane, 1,1,1,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride; and low-boiling organic compounds such as mixtures thereof. As the foaming agent, butane and a mixture of butane and other aliphatic hydrocarbons (e.g., isobutane) are preferred, and a mixture of n-butane:i-butane=50-100:0-50 (by mass) is particularly preferred.

The amount of the physical foaming agent added can be appropriately selected as needed. The amount can usually be 0.05 to 5.0 parts by mass per 100 parts by mass of the total of the base resin and the chemical foaming agent (amount added). More specifically, it is preferable to use a mixture of butane and other aliphatic hydrocarbons (e.g., isobutane), particularly a mixture of n-butane:i-butane = 50-100:0-50 (by mass), as the physical foaming agent, in an amount of 0.05 to 5.0 parts by mass per 100 parts by mass of the total of the base resin and the chemical foaming agent (amount added).

The chemical foaming agent is not particularly limited as long as it decomposes to generate a gas such as carbon dioxide gas. The chemical foaming agent may be one type alone or two or more types in combination. The chemical foaming agent may be either an organic or inorganic chemical foaming agent. Specific examples of the organic chemical foaming agent include organic acid salts such as sodium citrate, azo compounds such as azodicarbonamide, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, hydrazine derivatives such as toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonylsemicarbazide. Specific examples of the inorganic chemical foaming agent include carbonates such as sodium carbonate and bicarbonates such as sodium bicarbonate. Furthermore, a mixture of sodium bicarbonate and an organic acid such as citric acid can be used as the chemical foaming agent. The chemical foaming agent is preferably selected from sodium bicarbonate and a mixture of sodium bicarbonate and an organic acid (such as citric acid).

The amount of the chemical foaming agent added is 1% by mass or more, preferably 1.1% by mass or more, more preferably 1.5% by mass or more, and 5% by mass or less, preferably 3% by mass or less, based on the total 100% by mass of the base resin and the chemical foaming agent. If the lower limit of the amount of the chemical foaming agent added is less than the above range, the remaining amount of the foaming agent decreases slowly, and the aging period becomes longer, resulting in poor productivity of the foamed sheet, which is not preferable. More specifically, it is preferable to use an organic or inorganic chemical foaming agent, particularly a chemical foaming agent selected from sodium bicarbonate and a mixture of sodium bicarbonate and an organic acid (such as citric acid), as the chemical foaming agent, in an amount of 1% by mass or more based on the total 100% by mass of the base resin and the chemical foaming agent. The upper limit of the amount of the chemical foaming agent added is merely determined from the perspective of clarifying the claims, and does not acknowledge that the PSP sheet cannot be obtained when the upper limit is exceeded, nor does it actively intend to exclude the PSP sheet obtained when the upper limit is exceeded from the scope of the claims.

There is no particular limitation on the method of adding the chemical foaming agent. For example, the chemical foaming agent can be added by mixing it with the base resin beforehand and then feeding it to an extruder or injection molding machine, adding it to the base resin in an extruder, or creating and adding a master batch based on a polystyrene resin. There is also no limitation on the order of addition of the chemical foaming agent and the physical foaming agent. Both may be added at the same time, or one may be added first and the other added later.

In forming the PSP sheet, a bubble regulator can be used in combination with the physical foaming agent and the chemical foaming agent. Specific examples of the bubble regulator include inorganic powders such as talc and silica; acid salts of polycarboxylic acids; and reaction mixtures of polycarboxylic acids with sodium carbonate or sodium bicarbonate.

The amount of residual gas in the PSP sheet is 0.161 g or less. It is preferable that the amount of residual gas is within the above range, since it is possible to improve moldability by suppressing delamination during container molding. The amount of residual gas is defined as the amount of volatilization (g) calculated by the method described in the Examples. The amount of residual gas is not limited to the amount at the time of manufacturing the PSP sheet. The amount of residual gas may be the amount after a certain aging period (e.g., 70 days, 65 days, 60 days, 50 days, or 45 days after manufacturing) has passed after manufacturing. In addition, as described later, the PSP sheet may have a laminate film laminated on at least one side. In this case, the amount of residual gas is calculated based on the foam sheet alone without the laminate film laminated thereon.

There is no particular limitation on the bubble structure contained in the PSP sheet. The bubble structure may be either a closed bubble structure in which no air layers are exchanged between the bubbles, or a continuous bubble structure in which holes are opened in the resin film and air layers move back and forth. It is preferable that the PSP sheet contains a closed bubble structure. Having a closed bubble structure is preferable because it can improve the moldability of the PSP sheet, such as suppressing delamination. Specifically, it is preferable that the closed bubble rate of the PSP sheet is at least 90% by volume or more of the total volume of the bubbles, more preferably 95% by volume or more, and even more preferably 97.0% by volume or more. The closed bubble rate is calculated by the method described in the examples. If the closed bubble rate of the PSP sheet is less than 90% by volume, delamination is likely to occur even with an amount of evaporation of 0.161 g or less, and the variation in secondary thickness increases. If the closed bubble rate of the PSP sheet is 90% by volume or more, it is preferable because it can suppress the occurrence of delamination and the variation in secondary thickness when the amount of evaporation is 0.161 g or less.

There is no particular limitation on the specific shape of the PSP sheet. The specific shape of the PSP sheet can be appropriately determined as needed. Therefore, the term "sheet" also includes a film-like shape. The thickness of the sheet can be, for example, 0.1 to 5 mm, or 0.5 to 3 mm.

There is no particular limitation on the manufacturing method of the PSP sheet. The PSP sheet can usually be obtained by extruding a foamable molten resin, which is a mixture of the base resin, the physical foaming agent, and the chemical foaming agent, with an extruder to foam it. The amount of the chemical foaming agent added is 1% by mass or more, preferably 1.1% by mass or more, and more preferably 1.5% by mass or more, based on 100% by mass of the total of the base resin and the chemical foaming agent. The above explanation applies to the type and upper limit of the amount of the chemical foaming agent and the type and amount of the physical foaming agent in the manufacturing method of the PSP sheet. In addition, in the manufacturing method of the PSP sheet, the manufactured sheet may be stored at room temperature (e.g., 10 to 25°C) for a certain period (aging period; e.g., 70 days, 65 days, 60 days, 50 days, or 45 days after manufacturing) in order to set the amount of residual gas in the sheet to an appropriate range.

The PSP sheet may have a laminate film laminated on at least one side, preferably both sides. When a laminate film is laminated on the PSP sheet, it is preferable because it can impart oil resistance to the inside of the container and can impart design to the inside and outside of the container. There is no particular limitation on the method for laminating the laminate film. As the method, a known lamination method, such as thermal lamination and extrusion lamination, can be used.

There is no particular limitation on the material of the laminate film. Specific examples of the laminate film include polystyrene (PS) film and polyolefin film. As the polyolefin film, a non-stretched polyolefin film can be used. The non-stretched polyolefin film refers to a film made of polyolefins whose main component is ethylene or propylene. Specific examples of the non-stretched polyolefin film include films made of linear low-density polyethylene containing 1-butene, polypropylene, ethylene-vinyl acetate random copolymer, ethylene-acrylic acid ester random copolymer, propylene-ethylene random copolymer, or propylene-ethylene-1-butene random copolymer. The non-stretched polyolefin film is preferably a non-stretched polypropylene film (CPP film).

The laminate film may be a single layer film or a multilayer film having two or more layers laminated together, such as a CPP film/PS film.

When laminating films on both sides, the materials and structures of the laminate films on both sides may be the same or different. For example, a single-layer film may be laminated on one side and a multi-layer film may be laminated on the other side. Also, a PS film may be laminated on one side and a polyolefin film such as those exemplified above may be laminated on the other side.

The PSP sheet may contain other components as necessary, provided that the productivity and moldability are not significantly impaired. Specific examples of the other components include additives used in known resin sheets, such as flame retardants, ultraviolet absorbers, fluorescent brighteners, antistatic agents, antifogging agents, lubricants, antiblocking agents, flow improvers, plasticizers, dispersants, and antibacterial agents.

The container according to this embodiment (hereinafter referred to as the "container") is obtained by thermoforming the PSP sheet. There is no particular limitation on this molding method. The molding method includes known molding methods generally used for polystyrene-based foamed resins, specifically, for example, vacuum molding (plug-assist molding, etc.), injection molding, ultra-high-speed injection molding, injection compression molding, two-color molding, gas-assist and other hollow molding methods, molding using an insulated mold, molding using a rapidly heated mold, foam molding (including supercritical fluids), insert molding, IMC (in-mold coating molding) molding, extrusion molding, sheet molding, thermoforming, rotational molding, lamination molding, press molding, and blow molding. There is also no particular limitation on the molding conditions. The molding conditions can be appropriately determined as needed.

There are no particular limitations on the shape, dimensions, or specific uses of the container. Uses of the container include containers for packaging, such as food and beverage packaging containers. The term "container" includes not only the entire container, but also a portion of the container. For example, the term "container" includes not only the main body of the container, but also the lid of the container.

The present invention will be specifically explained below with reference to examples. Note that the present invention is not limited to the forms shown in the examples. The embodiment of the present invention can be modified in various ways within the scope of the present invention depending on the purpose, application, etc.

1. Production of PSP Sheet The following components were used as raw materials.
(A) Heat-resistant PS resin (base resin); "TEP220" manufactured by Toyo Styrene Co., Ltd. (polyphenylene ether resin: 9% by mass, methacrylic acid copolymerized PS resin: 86% by mass, PS resin: 5% by mass)
(B) Physical foaming agent: butane gas (n-butane/i-butane=7/3)
(C) Chemical foaming agent: "ES405" (a mixture of sodium bicarbonate and citric acid) manufactured by Eiwa Chemical Industry Co., Ltd.
(D) Cell regulator; talc ("ML110" manufactured by Fuji Talc Co., Ltd.)

The heat-resistant PS resin, chemical foaming agent, and cell regulator were mixed, then charged into an extruder and heated to melt. Next, the physical foaming agent was pressed into the molten resin in the extruder. The molten resin was then extruded to obtain a foamed sheet (thickness: 1.7 mm, basis weight: 180 g/m ² ). A CPP/PS film was attached to one side of the foamed sheet and a PS film was attached to the other side by thermal lamination to produce PSP sheets 1 to 3. PSP sheets 4 to 6 were produced in the same manner as PSP sheets 1 to 3, except that no chemical foaming agent was used. Table 1 shows the blending ratio (by mass) of each raw material of PSP sheets 1 to 6 and the cylinder temperature during extrusion.

2. Performance evaluation (1) Aging period Sheet pieces (size: 5 cm length × 8 cm width) were cut from PSP sheets 1 to 6, and 13 of the sheet pieces were bundled to obtain a sheet bundle, whose weight was measured (n = 2). The sheet bundle was then placed in an aluminum cup and left to stand in an oven set at 150 ° C for 12 minutes, and the weight of the sheet bundle after heating was measured.

The amount of volatilization (g) was calculated based on the following formula. The calculated amount of volatilization was defined as the amount of remaining gas. The number of days required for the amount of remaining gas in the PSP sheet to reach 0.161 g was defined as the aging period. The results are shown in Table 1.
Volatilization amount (g) = W1 - W2
W1: weight of sheet stack before heating (g), W2: weight of sheet stack after heating (g)

(2) Closed cell ratio Nine samples (size: 2 cm x 2 cm) were cut from PSP sheets 1 to 6, and the weight of each was measured. The samples were immersed in water and left to stand for 3 minutes under a vacuum of 0.02 MPa or less. Thereafter, the samples were taken out, the surface and cross section were thoroughly wiped to remove moisture, and the weight of each sample was measured.

The closed cell ratio (volume %) was calculated based on the following formulas (I) and (II). The results are shown in Table 1.
F = (V2/V) x 100
= {(M-m) / [m x (f-1) / S0]} x 100 ... (I)
Closed cell ratio=(100-F)...(II)

F: open cell ratio (volume %), V2: open cell volume (cm ³ ), V: actual cell volume of foam (cm ³ ), m: sample mass before immersion in water (g), M: sample mass after evacuation (g), f: expansion ratio, S0: specific gravity of solid component of laminate

(3) Moldability The amount of residual gas in PSP sheets 1 to 6 after the aging period shown in Table 2 had elapsed was calculated by the method described in (1) above. The results are also shown in Table 2.

After the aging period, PSP sheets 1 to 6 were thermoformed by both vacuum forming to produce containers. The containers were visually inspected for delamination (peeling of the laminate film). The results are shown in Table 2.

3. Results As shown in Table 1, the aging period of 105 days or more was required for PSP sheets 4 to 6, which did not use a chemical foaming agent. On the other hand, the aging period of PSP sheets 1 to 3, which used a chemical foaming agent, was shorter than that of PSP sheets 4 to 6, and it is understood that the productivity was improved.

As can be seen from Table 2, Comparative Examples 1 to 3, which used PSP sheets 1 to 3 before the aging period, had residual gas amounts exceeding 0.161 g, and delamination occurred during container molding. On the other hand, Examples 1 to 6, which used PSP sheets 1 to 3 after the aging period, had residual gas amounts of 0.161 g or less, and no delamination occurred during container molding.

In addition, in Comparative Examples 4 and 5, in which the PSP sheet 4, which does not use a chemical foaming agent, has a residual gas amount of 0.161 g or less, no delamination occurs during container molding, but as in Table 1, the aging period is longer than in Examples 1 to 6. Therefore, it can be seen that the storage period of the sheet is longer in Comparative Examples 4 and 5 than in Examples 1 to 6, and the productivity of the container is also inferior.

Table 2 also shows that delamination occurred during container molding in Comparative Examples 7 and 8, which used PSP sheets 5 and 6 that did not use a chemical foaming agent and had a closed cell rate of less than 90% by volume. In particular, in Comparative Example 7, delamination occurred even though the aging period was long and the residual gas amount was 0.161 g or less, and the container productivity was poor.

Claims (6)

A polystyrene-based resin foamed sheet obtained by adding a physical foaming agent and a chemical foaming agent containing normal butane to a base resin containing a styrene-methacrylic acid copolymer, a styrene-acrylic acid-based copolymer, or a styrene-maleic anhydride-based copolymer,
A polystyrene-based resin foamed sheet, characterized in that the amount of the chemical foaming agent added is 1% by mass or more and 5% by mass or less, and the amount of residual gas in the polystyrene-based resin foamed sheet, as measured by the method described below, is 0.161 g or less ;
<Measurement method>
A sheet piece (size: 5 cm length x 8 cm width) is cut from the PSP sheet, 13 pieces of the sheet piece are bundled to obtain a sheet bundle, and the weight of the bundle is measured. The sheet bundle is then placed in an aluminum cup and left to stand in an oven set at 150°C for 12 minutes, and the weight of the sheet bundle after heating is measured. The amount of volatilization (g) is calculated based on the following formula, and the calculated amount of volatilization is defined as the amount of remaining gas.
Volatilization amount (g) = W1 - W2
W1: weight of the sheet stack before heating (g), W2: weight of the sheet stack after heating (g) . The polystyrene-based resin foam sheet according to claim 1, characterized in that the chemical foaming agent is selected from sodium bicarbonate and a mixture of sodium bicarbonate and citric acid. The polystyrene-based resin foam sheet according to claim 1 or 2, characterized in that a laminate film is laminated on at least one surface of the polystyrene-based resin foam sheet. The polystyrene-based resin foam sheet according to claim 1 or 2, characterized in that a laminate film is laminated on both sides of the polystyrene-based resin foam sheet. The polystyrene resin foam sheet according to any one of claims 1 to 4, characterized in that the closed cell rate is 90 volume % or more. A container formed by thermoforming the styrene-based resin foam sheet according to any one of claims 1 to 5.