Multilayer resin sheets and molded containers

Abstract

Provided is a multilayer resin sheet which has oxygen barrier properties and which can be molded into a molded article having excellent notch folding ability and / or excellent tear resistance. This multilayer resin sheet has a laminate structure in which a surface shell layer, an oxygen barrier layer, a base material layer, and a lower shell layer are laminated in this order. In order to improve the notch folding ability, the base material layer and the lower shell layer contain a polystyrene-based resin in which rubber particles having an average particle size of less than 6 μm are dispersed. In order to improve the tear strength, the surface shell layer contains a low density polyethylene and a high density polyethylene at a predetermined mix ratio.

Description

[Technical Field]

[0001] The present invention relates to a thermoplastic multilayer resin sheet and a molded container equipped therewith. [Background technology]

[0002] In recent years, food packaging containers have generally been manufactured by molding multilayer resin sheets to suppress the deterioration of food quality due to the permeation of gases such as oxygen, nitrogen, carbon dioxide, and water vapor from inside and outside the packaging container. It is conventionally known that multilayer resin sheets may have an ethylene-vinyl alcohol copolymer resin layer to provide oxygen barrier properties that block oxygen permeation, or a polyolefin resin layer to provide water vapor barrier properties that block water vapor permeation (Patent Documents 1-5). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-58619 [Patent Document 2] Japanese Patent Application Publication No. 11-138705 [Patent Document 3] Japanese Patent Publication No. 2003-112392 [Patent Document 4] Japanese Patent Publication No. 2000-108287 [Patent Document 5] Japanese Patent Publication No. 2018-12263 [Overview of the project] [Problems that the invention aims to solve]

[0004] Some food packaging containers have connecting parts that link multiple container bodies, and these connecting parts have notches (hereinafter referred to as "notches") formed in them to separate each container body. There are also packaging containers called dispensing packaging containers, which have notches formed in the lid to discharge the food packaged inside the packaging container to the outside of the packaging container. However, when notches are formed in molded products made using the multilayer resin sheets disclosed in the above-mentioned Patent Documents 1 to 5, there is a problem that the folding characteristics, which are important when folding the notches, are poor. Furthermore, when the multilayer resin sheets disclosed in the above-mentioned Patent Documents 1 to 5 are molded and used as food packaging containers, there is a problem that when food packaging products are transported in an environment where they are stacked and subjected to loads, a part of the resin layer constituting the packaging container may rupture, causing the packaged food to leak out.

[0005] The present invention has been made in view of the problems of the prior art described above, and in one embodiment, aims to provide a multilayer resin sheet that can be molded into a molded product having oxygen barrier properties and excellent notch-breaking properties that allow the notch to break stably regardless of the speed at which the notch is broken when separating the packaging container into individual container bodies by hand, or when discharging the packaged food from the packaging container through a notch formed in the packaging container. In another embodiment, the present invention aims to provide a multilayer resin sheet that can be molded into a molded product having oxygen barrier properties and excellent break resistance that can suppress the leakage of contents that may occur when the resin layer constituting the packaging container breaks due to loads such as stacking, and a molded container. [Means for solving the problem]

[0006] [1] A multilayer resin sheet having a laminated structure in which an epidermal layer, an oxygen barrier layer, a base layer, and an underlayer are laminated in this order, The oxygen barrier layer is laminated to the epidermal layer and the substrate layer via an adhesive layer. A multilayer resin sheet in which the base layer and the underlayer are layers containing polystyrene resin and multiple rubber particles dispersed in the resin, and the average particle size of the rubber particles contained in the base layer and the underlayer is less than 6 μm. [2] The multilayer resin sheet according to [1], wherein the base layer and the underlayer each contain less than 7% by mass of rubber particles relative to the total mass of polystyrene resin and rubber particles. [3] The surface layer comprises a mixture of low-density polyethylene and high-density polyethylene in the multilayer resin sheet according to [1] or [2]. [4] A multilayer resin sheet according to [3], wherein the ratio of high-density polyethylene to the total mass of low-density polyethylene and high-density polyethylene in the surface layer is 30% by mass or more and 70% by mass or less. [5] The density of the mixture of low-density polyethylene and high-density polyethylene is 0.91-0.97 g / cm³. 3 The multilayer resin sheet described in [3] or [4]. [6] The surface layer is 5 to 40% of the thickness of the entire multilayer resin sheet, as described in any one of the following [1] to [5]. [7] The lower layer is a multilayer resin sheet according to any one of the following [1] to [6], containing 0.04 phr to 0.50 phr of a pigment suitable for printing by laser marking. [8] The base layer is a multilayer resin sheet according to any one of the following [1] to [7], containing 1 phr to 5 phr of white pigment. [9] A multilayer resin sheet as described in any one of the following [1] to [8], wherein the total thickness of the multilayer resin sheet is 100 to 1300 μm.

[10] A molded container comprising a multilayer resin sheet as described in any one of items [1] to [9].

[11] A multilayer resin sheet having a laminated structure in which an epidermal layer, an oxygen barrier layer, a base layer, and an underlayer are laminated in this order, The oxygen barrier layer is laminated to the skin layer and the base material layer via an adhesive layer, The base material layer and the hypodermal layer are layers containing a polystyrene-based resin, The skin layer contains a mixture of low-density polyethylene and high-density polyethylene, and the ratio of high-density polyethylene to the total mass of low-density polyethylene and high-density polyethylene is 30% by mass or more and 70% by mass or less, and the thickness is 5 to 40% with respect to the entire multilayer resin sheet. Multilayer resin sheet.

[12] The density of the mixture of low-density polyethylene and high-density polyethylene is 0.91 to 0.97 g / cm 3 The multilayer resin sheet according to

[11] .

[13] The base material layer and the hypodermal layer are layers containing a polystyrene-based resin and a plurality of rubber particles dispersed in the resin, and the average particle diameter of the rubber particles contained in the base material layer and the hypodermal layer is less than 6 μm, the multilayer resin sheet according to

[11] or

[12] .

[14] The base material layer contains less than 7% by mass of rubber particles with respect to the total mass of the polystyrene-based resin and the rubber particles, the multilayer resin sheet according to any one of

[11] to

[13] .

[15] The hypodermal layer contains 0.04 phr or more and 0.50 phr or less of a pigment having printability by laser marking, the multilayer resin sheet according to any one of

[11] to

[14] .

[16] The base material layer contains 1 phr or more and 5 phr or less of a white pigment, the multilayer resin sheet according to any one of

[11] to

[15] .

[17] The thickness of the entire multilayer resin sheet is 100 to 1300 μm, the multilayer resin sheet according to any one of

[11] to

[16] .

[18] A formed container including the multilayer resin sheet according to any one of

[11] to

[17] . [Effect of the Invention]

[0007] In a multilayer resin sheet according to one embodiment of the present invention, the base layer and the underlayer contain a polystyrene-based resin in which rubber particles with an average particle size of less than 6 μm are dispersed. The notches formed on the molded product of the multilayer resin sheet have improved crack propagation properties, and thus excellent notch-breaking properties can be provided, allowing the notches to break stably regardless of the speed at which they are broken by hand.

[0008] In another embodiment of the present invention, the multilayer resin sheet contains low-density polyethylene and high-density polyethylene in a predetermined mixing ratio in the surface layer. For example, when a part or all of a molded container is formed using this multilayer resin sheet, the fracture resistance of the surface layer, which is the layer that comes into contact with the contents, is improved, thereby suppressing leakage of contents due to loads during stacking. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view showing an example of a laminated structure of a multilayer resin sheet according to an embodiment of the present invention. [Modes for carrying out the invention]

[0010] As shown in Figure 1, a multilayer resin sheet according to one embodiment of the present invention has a laminated structure in which an epidermal layer 10 / adhesive layer 11a / oxygen barrier layer 12 / adhesive layer 11b / base layer 13 / underlayer 14 are laminated in this order from top to bottom on the paper. In this embodiment, the oxygen barrier layer 12 is laminated to the epidermal layer 10 and base layer 13 via adhesive layers 11a and 11b, while the base layer 13 and underlayer 14 are laminated directly.

[0011] The following describes each layer in the following order: the epidermal layer 10, adhesive layers 11a and 11b, oxygen barrier layer 12, base material layer 13, and subcutaneous layer 14. Then, the multilayer resin sheet itself and the food packaging container formed from it will be described exemplified.

[0012] <Epidermal layer 10> The surface layer 10 of this embodiment preferably contains a polyolefin resin, although this is not limited to it, in order to impart water vapor barrier properties to the multilayer resin sheet. Among polyolefin resins, it is possible to significantly improve the fracture resistance of the surface layer 10 by using a resin mixture of low-density polyethylene and high-density polyethylene. For example, if a part or all of a molded container is formed using a multilayer resin sheet so that the surface layer 10 is located on the inside, the fracture resistance of the surface layer 10, which is the layer that comes into contact with the contents, is improved, thereby suppressing leakage of the contents. This invention is not intended to be limited by theory, but it is presumed that this is because the fracture resistance is improved by creating appropriate molecular entanglement between polyethylenes when low-density polyethylene and high-density polyethylene are mixed in appropriate amounts.

[0013] Examples of polyolefin resins that constitute the surface layer 10 include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear polyethylene, ethylene-α·olefin copolymer polymerized using a metallocene catalyst, ethylene-vinyl acetate copolymer, ethylene-ionomer resin, ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer, ethylene-methacrylic acid copolymer, ethylene-methyl methacrylate copolymer, ethylene-propylene copolymer, homopolypropylene, block polypropylene, random polypropylene, etc. These polyolefin resins may be used individually or in combination of two or more, but among them, a resin mixture of low-density polyethylene and high-density polyethylene is preferred from the viewpoint of improving the fracture resistance of the surface layer 10.

[0014] The method for mixing the above resin mixture is not particularly limited, and general mixing methods can be used. For example, dry blending methods, in which individual pellets are simply mixed using a mixing and stirring device such as a tumbler, and compounding methods, in which heat is applied to the resin pellets using an extruder or the like to melt and knead them, can be used.

[0015] When applying a resin mixture composed of low-density polyethylene and high-density polyethylene, the ratio of high-density polyethylene to the total mass of low-density polyethylene and high-density polyethylene is preferably 30% by mass or more and 70% by mass or less. By setting the ratio of high-density polyethylene to 30% by mass or more, the fracture resistance can be significantly improved. As a result, for example, when a multilayer resin sheet is molded into a packaging container, the surface layer constituting the packaging container can obtain fracture resistance against loads due to stacking. Furthermore, by setting the ratio to 70% by mass or less, the fracture resistance of the multilayer resin sheet can be appropriately suppressed from becoming too high, and phenomena such as punching defects, where the multilayer resin sheet does not tear during the punching process when molding into containers, etc., can be suppressed. The above ratio of high-density polyethylene is more preferably 40% by mass or more and 60% by mass or less.

[0016] When applying a resin mixture combining low-density polyethylene and high-density polyethylene, the density of the low-density polyethylene is not limited to 0.91-0.93 g / cm³. 3 It can be set to 0.915~0.925 g / cm³. 3 It is preferable to do so. The density of high-density polyethylene is not limited to 0.94 to 0.97 g / cm³. 3 It can be expressed as 0.955~0.965 g / cm³. 3 It is preferable that the resin mixture, which is a combination of low-density polyethylene and high-density polyethylene, has a density of 0.91 to 0.97 g / cm³. 3 It can be expressed as 0.93~0.95 g / cm³. 3 It is preferable to do so.

[0017] The epidermal layer 10 may contain resins other than the polyolefin resin described above, as long as they do not hinder the effects of the present invention, and various additive components other than resin components may also be added. Examples of such additive components include colorants such as pigments and dyes, release agents such as silicone oil and alkyl esters, fibrous reinforcing agents such as glass fibers, granular lubricants such as talc, clay, and silica, antistatic agents such as salt compounds of sulfonic acid and alkali metals and polyalkylene glycols, and additives such as ultraviolet absorbers and antibacterial agents. However, generally, the polyolefin resin content in the epidermal layer 10 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass. In preferred embodiments, the total content of low-density polyethylene and high-density polyethylene in the epidermal layer 10 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass.

[0018] The thickness of the surface layer 10 is preferably 5 to 40% of the total thickness of the multilayer resin sheet, and more preferably 10% to 30%. The ratio of the thickness of the surface layer 10 to the total thickness of the multilayer resin sheet is expressed as a percentage of the value obtained by dividing the thickness of the surface layer 10 by the total thickness of the multilayer resin sheet. By setting this ratio of the thickness of the surface layer 10 to 5%, sufficient thickness can be ensured for the surface layer 10, which is advantageous for the development of water vapor barrier properties and fracture resistance. Furthermore, by setting this ratio of the thickness of the surface layer 10 to 40% or less, problems such as punching defects, where the multilayer resin sheet does not tear during the punching process when the multilayer resin sheet is molded into containers, etc., and appearance defects called "whiskers" that occur when the resin stretches can be further suppressed.

[0019] <Adhesive layer 11a, 11b> The adhesive layers 11a and 11b of this embodiment contain an adhesive. The adhesive is not limited, but from the viewpoint of laminating different resin layers, a polyolefin adhesive is preferred. The polyolefin adhesive preferably contains a modified polyolefin polymer. Typical examples include modified homopolymers of olefins having about 2 to 8 carbon atoms, such as ethylene, propylene, and butene-1; modified copolymers of these olefins with other olefins (e.g., ethylene, propylene, butene-1, 3-methylbutene-1, pentene-1, 4-methylpentene-1, hexene-1, octene-1, and decene-1, etc., having about 2 to 20 carbon atoms) and / or vinyl compounds (e.g., vinyl acetate, vinyl chloride, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, and polystyrene, etc.); and modified polyolefin rubbers such as ethylene-butene-1 copolymer and propylene-butene-1 copolymer. Methods of modification include acid modification under graft reaction conditions using unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, and tetrahydrophthalic acid, or their acid halides, amides, imides, anhydrides, esters, etc. Specifically, malenyl chloride, maleimide, maleic anhydride, citraconic anhydride, monomethyl maleate, dimethyl maleate, glycidyl maleate, etc. The adhesive may be used alone or in combination of two or more types.

[0020] As the modified polyolefin polymer, it is preferable to use one or more selected from ethylene resins, propylene resins, ethylene-propylene copolymer rubbers, or ethylene-butene-1 copolymer rubbers, which are modified with unsaturated dicarboxylic acids or their anhydrides, particularly maleic acid or its anhydride.

[0021] The thickness of the adhesive layers 11a and 11b is preferably 2 to 30 μm, and more preferably 5 to 20 μm, respectively. By making the thickness of the adhesive layers 11a and 11b 2 μm or more, sufficient interlayer adhesive strength can be obtained in the multilayer resin sheet, and by making the thickness 30 μm or less, the problem of appearance defects called burrs that occur during die-cutting of containers and the like after molding can be suppressed.

[0022] Various additive components other than adhesives may be added to the adhesive layers 11a and 11b, as long as they do not hinder the effects of the present invention. Examples of such additive components include colorants such as pigments and dyes, release agents such as silicone oil and alkyl esters, fibrous reinforcing agents such as glass fibers, granular lubricants such as talc, clay, and silica, antistatic agents such as salt compounds of sulfonic acid and alkali metals and polyalkylene glycols, and additives such as ultraviolet absorbers and antibacterial agents. However, generally, the adhesive content in the adhesive layers 11a and 11b is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass. In preferred embodiments, the modified polyolefin polymer content in the adhesive layers 11a and 11b is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass.

[0023] <Oxygen barrier layer 12> The oxygen barrier layer 12 of this embodiment contains an oxygen barrier resin to impart oxygen barrier properties to the multilayer resin sheet. Typical oxygen barrier resins include, but are not limited to, ethylene-vinyl alcohol copolymers, polyamides, polyvinyl alcohol, and polyvinylidene chloride. The oxygen barrier resin may be used alone or in combination of two or more types. Among these, ethylene-vinyl alcohol copolymer resins are preferred in terms of extrusion moldability.

[0024] Ethylene-vinyl alcohol copolymers are typically obtained by saponifying ethylene-vinyl acetate copolymers. To provide oxygen barrier properties and extrudeability, it is preferable that the ethylene content is 10 to 65 mol%, preferably 20 to 50 mol%, and the degree of saponification is 90 mol% or more, preferably 95 mol% or more.

[0025] Furthermore, examples of polyamides include lactam polymers such as caprolactam and laurolactam, polymers of aminocarboxylic acids such as 6-aminocaproic acid, 11-aminoundecanoic acid, and 12-aminododecanoic acid, aliphatic diamines such as hexamethylenediamine, decamethylenediamine, dodecamethylenediamine, and 2,2,4- or 2,4,4-trimethylhexamethylenediamine, alicyclic diamines such as 1,3- or 1,4-bis(aminomethyl)cyclohexane and bis(p-aminocyclohexylmethane), and aromatic diamines such as m- or p-xylylenediamine, as well as polycondensates of dicarboxylic acid units such as adipic acid, suberic acid, and sebacic acid, alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid, and aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid, and copolymers thereof.

[0026] Examples of polyamide resins include nylon 6, nylon 9, nylon 11, nylon 12, nylon 66, nylon 610, nylon 611, nylon 612, nylon 6T, nylon 6I, nylon MXD6, nylon 6 / 66, nylon 6 / 610, nylon 6 / 6T, nylon 6I / 6T, etc., with nylon 6 and nylon MXD6 being particularly preferred.

[0027] The oxygen barrier layer 12 may contain resins other than the oxygen barrier resin described above, as long as they do not impair the effects of the present invention, and various additive components other than resin components may also be added. Examples of such additive components include colorants such as pigments and dyes, release agents such as silicone oil and alkyl esters, fibrous reinforcing agents such as glass fibers, granular lubricants such as talc, clay, and silica, antistatic agents such as salt compounds of sulfonic acid and alkali metals and polyalkylene glycols, and additives such as ultraviolet absorbers and antibacterial agents. However, generally, the content of the oxygen barrier resin in the oxygen barrier layer 12 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass. In preferred embodiments, the content of the ethylene-vinyl alcohol copolymer resin in the oxygen barrier layer 12 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass.

[0028] The thickness of the oxygen barrier layer 12 is preferably 1 to 50 μm, more preferably 5 to 30 μm. A thickness of 1 μm or more for the oxygen barrier layer 12 is advantageous in terms of enhancing the oxygen barrier properties of the multilayer resin sheet. Furthermore, a thickness of 50 μm or less for the oxygen barrier layer 12 allows for easier heat stretching of the oxygen barrier layer 12 when the multilayer resin sheet is molded into a container or the like, ensuring a smoother molded product thickness and resulting in a molded product with a better appearance.

[0029] <Base material layer 13> The substrate layer 13 of this embodiment contains a polystyrene resin and a plurality of rubber particles dispersed in the resin. Methods for dispersing the plurality of rubber particles in the polystyrene resin include, but are not limited to, graft polymerization in the presence of a styrene monomer and another polymer. This method makes it possible to directly obtain a graft polymer having a structure in which a plurality of rubber particles are dispersed in a polystyrene resin. Examples of styrene monomers include styrene, α-methylstyrene, p-methylstyrene, dimethylstyrene, pt-butylstyrene, and chlorostyrene. Examples of other polymers include diene-based rubber polymers such as polybutadiene, polystyrene-butadiene copolymers (e.g., random copolymers, block copolymers), polyisoprene, and polychloroprene.

[0030] The base layer 13 may contain a polystyrene resin in which rubber particles are not dispersed. Examples of polystyrene resins include styrene monomers alone or copolymers thereof, such as styrene, α-methylstyrene, p-methylstyrene, dimethylstyrene, pt-butylstyrene, and chlorostyrene, or copolymers of these styrene monomers with other monomers. An example of a copolymer of a styrene monomer with other monomers is polystyrene-acrylonitrile copolymer (AS resin). The polystyrene resin may be used alone or in combination of two or more types.

[0031] Polystyrene resins and resins containing multiple rubber particles dispersed within them are commercially available. Examples include high-impact polystyrene (HIPS resin) and polystyrene-acrylonitrile graft polymer (ABS resin). High-impact polystyrene (HIPS resin) is obtained by polymerizing styrene monomer in the presence of a rubbery polymer (typically polybutadiene), and has a sea-island structure in which the styrene polymer is a continuous phase (sea) and the rubbery polymer, in which a portion of the styrene monomer is graft-polymerized, is a dispersed layer (island). ABS resin is obtained by polymerizing styrene monomer and acrylonitrile monomer in the presence of a rubbery polymer (typically polybutadiene), and has a sea-island structure in which the styrene-acrylonitrile copolymer (AS resin) is a continuous phase (sea) and the rubbery polymer, in which a portion of the styrene monomer and acrylonitrile monomer is graft-polymerized, is a dispersed layer (island).

[0032] In particular, using a mixture of general-purpose polystyrene (GPPS resin) and high-impact polystyrene (HIPS resin) in the base layer 13 is preferable from the viewpoint of rigidity and thermoformability of molded products such as molded containers obtained by molding multilayer resin sheets. Mixing with GPPS resin also has the advantage of being able to adjust the rubber particle content. The mixing ratio of GPPS resin can be adjusted according to the desired rubber content. The average particle size of rubber particles in HIPS resin can be adjusted by controlling the shear force by the speed of the rotor blades in the polymerization tank, controlling the polymerization time, or controlling polymerization with additives.

[0033] It is preferable that the average particle size of multiple rubber particles dispersed in the polystyrene resin (also referred to as the "average rubber particle size") is less than 6 μm. Furthermore, it is more preferable that the average particle size of the rubber particles is 1 μm or more and 3 μm or less. By setting the average particle size of the rubber particles to 1 μm or more, appropriate impact resistance can be obtained, making it less likely for molded products such as molded containers obtained by molding multilayer resin sheets to break when dropped. In addition, by setting the average particle size of the rubber particles to less than 6 μm, preferably 3 μm or less, cracks propagate more easily, so when a multilayer resin sheet is molded into a molded product such as a container and the formed notch is folded by hand, it can be folded easily regardless of the folding speed. In this specification, the average particle size of the rubber particles refers to the arithmetic mean particle size based on the volume-based particle size distribution measured by a laser diffraction / scattering particle size distribution analyzer.

[0034] Furthermore, it is preferable that the content of rubber particles (also referred to as "rubber content") relative to the total mass of polystyrene resin and rubber particles in the base layer 13 be less than 7% by mass, and more preferably 3% by mass or more and 6% by mass or less. By setting the rubber content of the polystyrene resin to 3% by mass or more, further impact resistance can be obtained, and molded products such as molded containers obtained by molding multilayer resin sheets can be made less likely to break when dropped. In addition, setting the rubber content to less than 7% by mass, and even more so to 6% by mass or less, is advantageous for further improving crack propagation.

[0035] Furthermore, when printing is performed on multilayer resin sheets or their molded products using laser irradiation or the like, it is preferable that the base layer 13 contains 1 phr to 5 phr of white pigment. More preferably, the amount of white pigment contained in the base layer 13 is 1.5 phr to 4 phr. The unit phr used here refers to the parts by mass of white pigment per 100 parts by mass of the total resin components in the base layer. Including 1 phr or more of white pigment in the base layer 13 provides opacity, improves the color development of the printed material when printing on multilayer resin sheets or their molded products, and also provides light shielding, suppressing discoloration and deterioration of the contents due to light irradiation from outside the molded product. In addition, by keeping the amount of white pigment in the base layer 13 at 5 phr or less, aggregation of the white pigment can be suppressed, suppressing appearance defects such as aggregates on multilayer resin sheets and their molded products. Furthermore, from a cost perspective, it is preferable to use less white pigment.

[0036] Examples of the aforementioned white pigments include titanium dioxide (titanium white), zinc oxide (zinc white), lithopone, and lead white, with titanium dioxide being the most preferred among them.

[0037] In the base layer 13, as with the other layers, other resins may be mixed in, and various additive components other than resin components may be added, as long as they do not hinder the effects of the present invention. Examples of such additive components include compatibilizers that make different components compatible, colorants such as pigments and dyes, release agents such as silicone oil and alkyl esters, fibrous reinforcing agents such as glass fibers, granular lubricants such as talc, clay, and silica, antistatic agents such as salt compounds of sulfonic acid and alkali metals and polyalkylene glycols, ultraviolet absorbers, and antibacterial agents. Furthermore, scrap resin generated in the manufacturing process of multilayer resin sheets and food packaging containers according to one embodiment of the present invention can also be mixed and used. However, generally, the total content of polystyrene resin and rubber particles in the base layer 13 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass. In preferred embodiments, the total content of HIPS (including rubber particles) and GPPS in the substrate layer 13 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass.

[0038] The thickness of the base layer 13 is preferably 100 to 800 μm, and more preferably 150 to 700 μm. A thickness of 100 μm or more for the base layer 13 is advantageous for improving the rigidity of molded products such as containers obtained by molding multilayer resin sheets. By setting the thickness of the base layer 13 to 800 μm or less, heat is more easily transferred in the thickness direction of the sheet when thermoforming the multilayer resin sheet, resulting in good thermoformability and thus enabling the production of molded products with a better appearance.

[0039] <Hypodermal layer 14> The underlayer 14 of this embodiment contains a polystyrene resin and a plurality of rubber particles dispersed in the resin. Preferably, the underlayer 14 is made of the same or similar resin as the base layer 13. Therefore, although a detailed explanation will be omitted, the polystyrene resin in which the rubber particles are dispersed used in the underlayer 14 can be a graft polymer as described in the description of the base layer 13, that is, a graft polymer obtained by graft polymerization in the presence of a styrene monomer and another polymer (for example, a diene-based rubber polymer such as polybutadiene, polystyrene-butadiene copolymer, polyisoprene, or polychloroprene), such as high-impact polystyrene (HIPS resin) or polystyrene-acrylonitrile graft polymer (ABS resin). One type of polystyrene resin may be used alone, or two or more types may be used in combination. Also, similar to the base layer 13, a polystyrene resin without dispersed rubber particles may be appropriately blended into the underlayer 14. In particular, using a mixture of general-purpose polystyrene (GPPS resin) and high-impact polystyrene (HIPS resin) for the underlayer 14 is preferable due to the rigidity and thermoformability of molded containers and the like obtained by molding the multilayer resin sheet. The preferred properties of the rubber particles in the underlayer 14, including the average particle size and rubber content, are the same as those of the base layer 13, so a detailed explanation is omitted.

[0040] The base layer 14 preferably contains 0.04 phr to 0.50 phr of a pigment suitable for laser marking, in order to perform laser irradiation or other marking processes on multilayer resin sheets and their molded products. Furthermore, it is more preferable that the amount of pigment contained in the base layer 14 is between 0.07 phr and 0.15 phr. The unit phr used here refers to the amount of pigment per 100 parts by mass of the total resin components in the base layer. A base layer 14 containing 0.04 phr or more of pigment is advantageous for achieving laser printability, and containing 0.50 phr or less can reduce the cost of multilayer resin sheets and their molded products such as thermoformed containers.

[0041] Pigments suitable for laser printing include mica, titanium dioxide, antimony oxide, metal salts such as copper phosphates and sulfates, and black pigments such as carbon black. Among these, antimony oxide is preferred because it can enhance the contrast of laser-processed markings.

[0042] In the subcutaneous layer 14, as with the other layers, other resins may be mixed in, and various additive components other than resin components may be added, as long as they do not hinder the effects of the present invention. Examples of such additive components include compatibilizers that make different components compatible, colorants such as pigments and dyes, release agents such as silicone oil and alkyl esters, fibrous reinforcing agents such as glass fibers, granular lubricants such as talc, clay, and silica, antistatic agents such as salt compounds of sulfonic acid and alkali metals and polyalkylene glycols, ultraviolet absorbers, and antibacterial agents. However, generally, the total content of polystyrene resin and rubber particles in the subcutaneous layer 14 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass. In preferred embodiments, the total content of HIPS (including rubber particles) and GPPS in the subcutaneous layer 14 is 80% by mass or more, typically 90% by mass or more, more typically 95% by mass or more, and can also be 100% by mass.

[0043] The thickness of the underlayer 14 is preferably 5 to 100 μm, more preferably 10 to 60 μm. By setting the thickness of the underlayer 14 to 5 μm or more, even if the base layer 13 has cosmetic defects such as containing scrap resin, such cosmetic defects will not appear on the surface of the multilayer resin sheet, and a multilayer resin sheet with a better appearance can be obtained. By setting the thickness of the underlayer 14 to 100 μm or less, the content of pigments suitable for laser printing in the multilayer resin sheet can be reduced, thereby suppressing costs.

[0044] <Multilayer resin sheet> The layer structure of the multilayer resin sheet according to one embodiment of the present invention is basically as shown in Figure 1, consisting of a surface layer 10, an adhesive layer 11a, an oxygen barrier layer 12, an adhesive layer 11b, a base layer 13, and an underlayer 14, but the layer structure is not limited to this. For example, each layer may consist of two or more layers. Furthermore, a new structure may be provided in which a layer is added in which scrap, which is generated in the process of manufacturing the multilayer resin sheet or molded products such as molded containers, is finely crushed and returned, or a recycled material that has been repelled after thermal melting is returned to the multilayer resin sheet structure.

[0045] The overall thickness of the multilayer resin sheet is preferably 100 to 1300 μm. By making the overall thickness of the multilayer resin sheet 100 μm or more, the strength of the molded product obtained by molding the multilayer resin sheet can be ensured. For example, sufficient thickness can be obtained for the sides or bottom of a container obtained by thermoforming, resulting in sufficient strength for the container. By making the overall thickness of the multilayer resin sheet 1300 μm or less, the cost of the multilayer resin sheet and molded products such as thermoformed containers can be reduced.

[0046] The method for manufacturing multilayer resin sheets is not particularly limited, and general methods can be used. For example, they can be manufactured by a molten co-extrusion method in which multiple resins are bonded and laminated in a molten state using multiple extrusion machines. More specifically, methods include using four or more single-screw or twin-screw extruders to melt and extrude the raw materials for each layer, and obtaining a multilayer resin sheet using a feed block with a selector plug and a T-die, or obtaining a multilayer resin sheet using a multi-manifold die.

[0047] <Food packaging containers> The multilayer resin sheet according to the present invention is thermoformable. Accordingly, according to one embodiment of the present invention, a molded article comprising the multilayer resin sheet according to the present invention described above is provided. There are no particular restrictions on the type of molded article, but for example, a molded container can be cited, and a food packaging container is a particularly preferred embodiment. The multilayer resin sheet according to the present invention can constitute part or all of the molded container. In this case, it is preferable that the multilayer resin sheet constitutes part or all of the molded container such that the surface layer 10 is located on the inner surface side of the molded container and the underlayer 14 is located on the outer surface side of the molded container.

[0048] A specific example of a food packaging container is a dispensing packaging. A dispensing packaging is a small food packaging container that allows for easy extraction of liquid, paste, granular, or powdery contents such as seasonings, beverages, cosmetics, and pharmaceuticals by pinching and bending it with the fingers. Such a dispensing packaging generally comprises a lid made of a rigid material with a notch called a half-cut section in the center of the surface, a fold line, and protrusions to facilitate the extraction of contents, and a container body made of a flexible member whose peripheral edge is fixed to the back surface of the lid, forming pocket sections on both sides of the fold line. Exemplarily, a multilayer resin sheet according to the present invention can be molded into the lid of the dispensing packaging. In this case, it is preferable to manufacture the dispensing packaging such that the surface layer 10 is located on the back side of the lid (the side that contacts the food) and the underlayer 14 is located on the front side of the lid.

[0049] Methods for thermoforming multilayer resin sheets include, but are not limited to, general vacuum forming and pressure forming, as well as applications of these, such as the plug-assisted method, in which a plug is brought into contact with one side of the multilayer resin sheet for thermoforming, and the so-called match-mold method, in which a pair of male and female molds are brought into contact with both sides of the multilayer resin sheet for thermoforming. Furthermore, known sheet heating methods, such as radiant heating using infrared heaters, which are non-contact heating methods, can be applied to heat and soften the multilayer resin sheet before thermoforming. [Examples]

[0050] Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to the contents of Examples and the like at all.

[0051] The raw materials used in the Examples and Comparative Examples are as follows. (1) Skin layer Low-density polyethylene resin (LDPE resin): "Q400" (manufactured by Sinopec Shanghai Petrochemical Company Limited, density: 0.923 g / cm 3 , MI: 4.0 g / 10 min. (190 °C, 2.16 kgf)) High-density polyethylene resin (HDPE resin): "8050" (manufactured by Formosa Plastics Corporation, density: 0.959 g / cm 3 , MI: 6.0 g / 10 min. (190 °C, 2.16 kgf)) (2) Oxygen barrier layer Ethylene-vinyl alcohol copolymer (EVOH): "Eval J171B" (manufactured by Kuraray Co., Ltd., MI: 1.7 g / 10 min. (190 °C, 2.16 kgf), ethylene content 32 mol%) <e (3) Adhesive layer Modified polyolefin polymer (modified PO): "Modic F502C" (manufactured by Mitsubishi Chemical Corporation, MI: 1.3 g / 10 min. (190 °C, 2.16 kgf)) (4) Substrate layer and underlayer HIPS resin (resin produced by graft polymerization of styrene and polybutadiene): "6351" (manufactured by Total Petrochemicals, MI: 3.5 g / 10 min. (200 °C, 5.0 kgf), rubber average particle size: 1 μm) HIPS resin (resin produced by graft polymerization of styrene and polybutadiene): "4241" (manufactured by Total Petrochemicals, MI: 3.5 g / 10 min. (200 °C, 5.0 kgf), rubber average particle size: 6 μm) HIPS resin (resin manufactured by graft polymerization of styrene and polybutadiene): "#532P" (Manufactured by SINOPEC, MI: 4.5g / 10min. (200℃, 5.0kgf), average rubber particle size: 5μm) GPPS resin (resin produced by homopolymerization of styrene monomer): "1050" (Manufactured by Total Petrochemicals, MI: 2.8g / 10min. (200℃, 5.0kgf)) (5) Pigments Titanium dioxide-containing masterbatch: "HMM 1HR480A" (Titanium dioxide concentration in masterbatch: 50% by mass) (Manufactured by Shanghai Rare Beauty New Materials Technology Co., Ltd.) Laser marking masterbatch: "HEM 1HR1771" (Pigment concentration in masterbatch: 1.5% by mass) (Manufactured by Shanghai Rare Beauty New Materials Technology Co., Ltd.)

[0052] Various evaluations of multilayer resin sheets were performed using the following methods. (A) Thickness of each layer Test specimens were cut at five evenly spaced points along the entire width direction (TD), which is perpendicular to the flow direction (MD) of the multilayer resin sheet. The specimens were cross-sectioned using a single-edged knife, and the thickness of each layer was measured using an electron microscope. The thickness values ​​for each layer were calculated by averaging the thickness of each layer at five points along the width direction of the multilayer resin sheet. Measuring instrument: Electron microscope KH7700 (manufactured by Hirox) (B) Average rubber particle size in polystyrene resin in the base layer and undercoat layer Test specimens were cut from a multilayer resin sheet at arbitrary positions, and the base layer and underlayer were removed from the specimens by scraping off the layers not to be analyzed using a single-edged knife. Next, the rubber particles were separated by dissolving all but the rubber particles with a solvent (N,N-dimethylformamide), and then the average particle size of the rubber particles in the base layer and underlayer was measured using a laser diffraction / scattering particle size distribution analyzer (Horiba, Ltd., model: LA-920). (C) The amount of rubber particles in the base layer and undercoat layer relative to the total mass of polystyrene resin and rubber particles (rubber content) Measurements were performed using pyrolysis gas chromatography. Test specimens were cut from a multilayer resin sheet at arbitrary positions, and the base layer and underlayer were removed by scraping off the non-analyte layers from the specimen using a single-edged knife. Next, the base layer and underlayer were subjected to thermal decomposition in an environment heated to a constant high temperature using a pyrolysis gas chromatograph (gas chromatograph / Shimadzu Corporation: model GC-2010plus, pyrolysis apparatus / Nippon Analytical Industry Co., Ltd.: model JCI-22). The gas peak areas of the generated butadiene monomer and styrene monomer were determined, and the rubber content in the base layer and underlayer was calculated based on calibration curves for other resins whose rubber content is known. (D) Resistance to fracture of the epidermal layer A square-shaped sealant made of ethylene-vinyl acetate copolymer was placed on the surface of the surface layer of a multilayer resin sheet, and heat-sealed in a square frame shape (seal width 1.5 mm, square size 60 mm x 60 mm) along each side of the sealant. After being left for 24 hours in an atmosphere of 23°C and 50% relative humidity, the presence or absence of rupture in the surface layer was checked when the sealant was peeled off diagonally, using one corner of the sealant as the peeling starting point. Heat sealing conditions • Sealing temperature: 175℃ • Seal pressure: 0.1 MPa • Sealing time: 1.6 sec 〇Peeling conditions • Peeling speed: 830-1700 mm / sec (manual peeling) • Peeling angle: 90° Fracture resistance was evaluated on the following four levels based on the results of 30 repetitions of the above test. 1) ◎: No fractures were observed in any of the tests, and no elongation or cracks were seen. 2) ○: No fractures were observed in any of the tests, however, some elongation or cracking was found in at least some of the tests. 3) △: Minor fractures (2mm x 2mm or smaller) were found in at least some tests. 4) ×: Significant fractures (larger than 2mm x 2mm) were found in at least some tests. (E) Flexural resistance A test specimen (15 mm x 100 mm) was cut from a multilayer resin sheet at an arbitrary position, and both ends of the specimen in the longitudinal direction were chucked. A bending strength test was then performed using a bending strength testing machine. Folding resistance tester: FPC folding resistance tester (Model MY-FPC-01, manufactured by Shenzhen Mingyu Instrument and Equipment Co., Ltd.) Measurement conditions: With a load (1600g) applied, the test specimen was bent at a bending angle (135°) at three different bending speeds. The number of bends until the multilayer resin sheet broke (meaning it fractured in two) was measured. Note that while the bending resistance of the multilayer resin sheet before thermoforming was evaluated here, it has been found that the bending resistance of the pre-thermoforming multilayer resin sheet shows a similar trend to the notch breakage of thermoformed products equipped with the multilayer resin sheet. Bending speed level 1) High speed: 135rpm 2) Medium speed: 70rpm 3) Low speed: 10rpm Flexural resistance was evaluated using the following four levels. 1) ◎: Less than 50 flexion repetitions 2) ○: Number of flexions is 50 or more but less than 100 3) △: Number of flexions is between 100 and 1000 4) ×: Number of flexions exceeds 1000 (F) Laser marking properties Using a laser laser marking machine (LS-MFP / 20) manufactured by Suzhou Laisai Laser Co., Ltd., laser irradiation was performed from the underlayer side of a multilayer resin sheet, and the suitability for printing was confirmed visually. • Laser type: YAG ·Wavelength: 1064nm • Frequency: 20KHz Printability was evaluated using the following four levels. 1) ◎: Clearly printed 2) ○: The printing is not smudged, but the color is weak. 3) △: Printed, but partially faded. 4) ×: Not printed

[0053] <Example 1> Using two φ45mm single-screw extruders (for the adhesive layer and oxygen barrier layer), one φ65mm single-screw extruder (for the undercoat layer), one φ75mm single-screw extruder (for the surface layer), and one φ120mm single-screw extruder (for the base layer), the raw materials for each layer were melt-extruded. By the feed-block method, a multilayer resin sheet with a thickness of 330μm and a width of 640mm was obtained, with a layer structure of surface layer 10 (60μm), adhesive layer 11a (10μm), oxygen barrier layer 12 (15μm), adhesive layer 11b (10μm), base layer 13 (185μm), and undercoat layer 14 (50μm).

[0054] Here, the surface layer 10 uses a resin mixture of high-density polyethylene resin and low-density polyethylene resin, specifically a mixture of HDPE resin "8050" and LDPE resin "Q400" in a mass ratio of 50 / 50. The adhesive layers 11a and 11b use a modified polyolefin polymer "Modic F502C". The oxygen barrier layer 12 uses an ethylene-vinyl alcohol copolymer "EVAL J171B". The base layer 13 uses a mixture of HIPS resin "6351" and GPPS resin "1050" in a mass ratio of 80 / 20, with 5 phr of titanium dioxide-containing masterbatch "HMM 1HR480A" added. The underlayer 14 uses HIPS resin "6351" with 7 phr of laser marking masterbatch "HEM 1HR1771" added.

[0055] <Examples 2-12, Comparative Examples 1-3> A multilayer resin sheet was molded in the same manner as in Example 1, except that the raw material mixing ratio of the surface layer 10, the base layer 13, and the underlayer 14, the thickness of each layer, were changed as shown in Table 1, and the HIPS resin type was changed as appropriate.

[0056] The multilayer resin sheets obtained in the examples and comparative examples were evaluated using the multilayer resin sheet evaluation method described above. The results are shown in Tables 1 to 3.

[0057] [Table 1]

[0058] [Table 2]

[0059] [Table 3]

[0060] <Consideration> The multilayer resin sheet of Example 1 contained low-density polyethylene resin and high-density polyethylene resin in an appropriate blending ratio, so no rupture of the surface layer occurred. Furthermore, because the average particle size of the rubber particles dispersed in the polystyrene resin in the base layer was small and the rubber content was appropriate, the bending characteristics, such as the number of folds endured, met the standard at all bending speeds. In addition, the laser marking performance was also good, with clear printing and meeting the standard. Here, an evaluation of ◎ or ○ indicates that the standard is met, and an evaluation of △ or × indicates that the standard is not met. The same applies below.

[0061] In Example 2, compared to Example 1, only the average rubber particle size in the base layer was changed by changing the HIPS resin species constituting the base layer from "6351" to "#532P". The resulting multilayer resin sheet met the criteria for fracture resistance, bending resistance, and laser marking properties of the surface layer.

[0062] In Examples 3 and 4, only the blending ratio of high-density polyethylene resin in the surface layer composition was changed compared to Example 1. The resulting multilayer resin sheets all met the criteria for surface layer fracture resistance, bending resistance, and laser marking properties.

[0063] In Example 5, the only difference from Example 1 was the amount of titanium dioxide-containing masterbatch added to the composition of the substrate layer. The resulting multilayer resin sheet met the criteria for fracture resistance, bending resistance, and laser marking properties of the surface layer.

[0064] In Example 6, the only difference from Example 1 was the amount of laser marking masterbatch added to the composition of the underlayer. The resulting multilayer resin sheet met the criteria for fracture resistance, bending resistance, and laser marking properties of the epidermal layer.

[0065] In Examples 7 and 8, only the thickness of each layer was changed compared to Example 1. The resulting multilayer resin sheets all met the criteria for fracture resistance, bending resistance, and laser marking properties of the surface layer.

[0066] In Example 9, the average rubber particle size in the base layer was changed from "6351" to "4241" compared to Example 1. The resulting multilayer resin sheet met the criteria for both surface layer fracture resistance and laser marking properties. However, it was found that the number of bends before breakage occurred increased due to the worsening of crack propagation as the average rubber particle size increased.

[0067] In Example 10, the HIPS resin species constituting the base layer was changed from "6351" to "4241" compared to Example 1, and GPPS resin was not incorporated into the base layer, thereby changing the rubber content and average rubber particle size in the base layer. The resulting multilayer resin sheet met the standards for both the fracture resistance of the surface layer and the laser marking properties. However, it was found that the number of bends before breakage occurred increased due to the deterioration of crack propagation properties as the rubber content increased.

[0068] In Comparative Examples 1, 2, and 3, the blending ratio of high-density polyethylene resin and low-density polyethylene resin in the surface layer composition was changed compared to Example 1. Furthermore, the average rubber particle size in the base layer was changed from "6351" to "4241" by changing the HIPS resin species constituting the base layer. The resulting multilayer resin sheets showed a decrease in the fracture resistance of the surface layer, and an increase in the number of bends before folds occurred.

[0069] Example 11 differed from Example 1 in that the amount of laser marking masterbatch added to the composition of the underlayer was changed. In addition, the HIPS resin species constituting the base layer was changed from "6351" to "4241," and the rubber content and average rubber particle size in the base layer were changed by not incorporating GPPS resin into the base layer. The resulting multilayer resin sheet met the standard for fracture resistance of the surface layer. However, it was found that the laser marking performance was reduced due to insufficient addition of laser marking masterbatch. It was also found that the number of bends before folds occurred increased.

[0070] In Example 12, the thickness of the surface layer and the base layer were changed compared to Example 1. The resulting multilayer resin sheet met the criteria for both bending resistance and laser marking properties. However, it was observed that the fracture resistance of the surface layer decreased as the thickness ratio of the surface layer in the multilayer resin sheet decreased. [Explanation of Symbols]

[0071] 10 Epidermal layer 11a, 11b adhesive layer 12. Oxygen barrier layer 13 Base material layer 14 Hypodermal layer

Claims

1. A multilayer resin sheet having a laminated structure in which an epidermal layer, an oxygen barrier layer, a base layer, and an underlayer are laminated in this order, The overall thickness of the multilayer resin sheet is 100 to 1300 μm. The oxygen barrier layer is laminated to the epidermal layer and the substrate layer via an adhesive layer. A multilayer resin sheet in which the base layer and the underlayer are layers containing polystyrene resin and multiple rubber particles dispersed in the resin, and the average particle size of the rubber particles contained in the base layer and the underlayer is less than 6 μm.

2. The multilayer resin sheet according to claim 1, wherein the base layer and the underlayer each contain less than 7% by mass of rubber particles relative to the total mass of polystyrene resin and rubber particles.

3. The multilayer resin sheet according to claim 1 or 2, wherein the surface layer comprises a mixture of low-density polyethylene and high-density polyethylene.

4. The multilayer resin sheet according to claim 3, wherein the ratio of high-density polyethylene to the total mass of low-density polyethylene and high-density polyethylene in the surface layer is 30% by mass or more and 70% by mass or less.

5. The density of the mixture of low-density polyethylene and high-density polyethylene is 0.91–0.97 g / cm³. 3 The multilayer resin sheet according to claim 3 or 4.

6. The multilayer resin sheet according to any one of claims 1 to 5, wherein the surface layer has a thickness of 5 to 40% of the entire multilayer resin sheet.

7. The multilayer resin sheet according to any one of claims 1 to 6, wherein the lower layer contains 0.04 phr or more and 0.50 phr or less of a pigment suitable for printing by laser marking.

8. The multilayer resin sheet according to any one of claims 1 to 7, wherein the base layer contains 1 phr or more and 5 phr or less of a white pigment.

9. A molded container comprising a multilayer resin sheet according to any one of claims 1 to 8.

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