WO2023149476A1

WO2023149476A1 - Multilayer sheet for food, packaging material for food, container for food, and manufacturing method for multilayer sheet for food

Info

Publication number: WO2023149476A1
Application number: PCT/JP2023/003229
Authority: WO
Inventors: 直紀田矢; 壮宮田
Original assignee: リンテック株式会社
Priority date: 2022-02-07
Filing date: 2023-02-01
Publication date: 2023-08-10
Also published as: TW202346166A

Abstract

The present invention involves a multilayer sheet for food having a paper base material and a resin layer provided on one or both sides of the paper base material, the resin layer containing polylactic acid. In a temperature–displacement curve, from −20°C to the temperature at completion of indenter penetration, of the resin layer obtained by thermomechanical analysis according to JIS K 7196 (2012), the slope of the curve at each temperature is determined, and the number of peaks of a temperature–slope curve obtained by plotting the slope against temperature is two or less. The present invention further involves a manufacturing method for the multilayer sheet for food, and a packaging material for food and a container for food that uses the multilayer sheet for food.

Description

Food multilayer sheet, food packaging material, food container, and method for producing food multilayer sheet

The present invention relates to a multilayer food sheet, a packaging material for food, a container for food, and a method for producing a multilayer food sheet.

In recent years, due to growing awareness of environmental issues, efforts have been made to positively use paper, which is a renewable resource, for food containers, food packaging materials, and the like.
For these applications, multi-layer food sheets obtained by laminating a resin layer on the surface of paper are used, for example, for the purpose of imparting water resistance, oil resistance, gas barrier properties, and the like.

In addition, food containers, etc. are required to have heat resistance because they may come into direct or indirect contact with heated food. For example, containers used for foods that are heated by a microwave oven or the like are required to maintain their intended functions without being deformed or degraded even under heating. Against this background, synthetic resins with good heat resistance, such as polypropylene, have conventionally been used for the resin layers of multilayer food sheets.

However, synthetic resins such as polypropylene are derived from fossil resources, and environmental pollution at the time of disposal is regarded as a problem. Therefore, the use of materials with less environmental load for the resin layers of multi-layer sheets for food is being studied.
In Patent Document 1, as a microwave paper container having a resin layer having excellent oil resistance when heated, a laminate having a resin layer made of a kneaded product of biomass resin and synthetic resin on at least one side of a paper substrate is molded. Disclosed is a microwave paper container characterized by:

JP 2008-189341 A

However, the technique of Patent Document 1 is a technique that requires the blending of a synthetic resin that may affect the environment in addition to the biomass resin, so it sufficiently solves the problem of reducing the environmental load. I couldn't say. Therefore, there is a demand for a multi-layer food sheet that uses materials with less environmental impact and that has excellent heat resistance.

The present invention has been made in view of the above problems, and has a small environmental load and excellent heat resistance. The object is to provide a food container.

The present inventors have found that the above problems can be solved by applying a resin layer containing polylactic acid and exhibiting specific behavior in thermomechanical analysis to a multi-layer food sheet. Arrived.

That is, the present invention relates to the following [1] to [12].
[1] having a paper substrate and a resin layer provided on one or both sides of the paper substrate,
The resin layer contains polylactic acid,
In the temperature-displacement curve from -20 ° C. to the indentation completion temperature of the resin layer obtained by thermomechanical analysis according to JIS K 7196 (2012), the slope of the curve at each temperature was obtained, and the slope was measured against the temperature. A multi-layer sheet for foods, wherein the temperature-slope curve obtained by plotting has two or less peaks.
[2] When the number of peaks in the temperature-slope curve is 1, the peak, and when the number of peaks in the temperature-slope curve is 2, the peak among the 2 peaks When the peak with the larger height is the maximum peak of the displacement slope, the softening point temperature obtained by analysis according to JIS K 7196 (2012) from the temperature-displacement curve corresponding to the maximum peak of the displacement slope. is 140° C. or higher, the food multilayer sheet according to [1] above.
[3] The multi-layer sheet for food according to [1] or [2] above, wherein the resin layer has a melting point of 140 to 180°C.
[4] The multi-layer sheet for food according to any one of [1] to [3] above, wherein the polylactic acid content in the resin layer is 60 to 100% by mass.
[5] The food product according to any one of the above [1] to [4], wherein the content of the L-form-derived structural unit in the total polylactic acid contained in the resin layer is 94 to 100% by mass. multilayer sheet.
[6] The food multilayer sheet according to any one of [1] to [5] above, wherein the resin layer further contains a nucleating agent.
[7] Any of the above [1] to [6], wherein the ratio of the thickness of the resin layer to the thickness of the paper base [resin layer/paper base] is 0.01 to 1.5. The multi-layered sheet for foods according to
[8] The multi-layer sheet for food according to any one of [1] to [7] above, wherein the resin layer has a thickness of 1 to 120 μm.
[9] Any one of the above [1] to [8], wherein the displacement rate of the resin layer between 0 ° C. and 140 ° C. measured by thermomechanical analysis according to JIS K 7196 (2012) is 20% or less. 3. The multi-layer sheet for food according to .
[10] A food packaging material formed using the multi-layer food sheet according to any one of [1] to [9] above.
[11] A food container formed using the multi-layer food sheet according to any one of [1] to [9] above.
[12] A method for producing a multilayer food sheet according to any one of [1] to [9] above,
A method for producing a multi-layer sheet for food, comprising a step of forming a resin layer by forming a layer of a resin layer-forming material containing polylactic acid, followed by annealing treatment.

According to the present invention, it is possible to provide a multi-layer food sheet with low environmental load and excellent heat resistance, a method for producing the same, and a food packaging material and a food container using the multi-layer food sheet.

BRIEF DESCRIPTION OF THE DRAWINGS It is a cross-sectional schematic diagram which shows an example of a structure of the multi-layered sheet for foodstuffs of this embodiment.

Hereinafter, the present invention will be described in detail using embodiments.

In this specification, for preferred numerical ranges (for example, ranges of content etc.), the lower and upper limits described stepwise can be independently combined. For example, from the statement "preferably 10 to 90, more preferably 30 to 60", combining "preferred lower limit (10)" and "more preferred upper limit (60)" to "10 to 60" can also
In the numerical ranges described herein, the upper or lower limits of the numerical ranges may be replaced with the values shown in the examples.

As used herein, "biomass" means renewable organic resources derived from living organisms, excluding fossil resources.

The mechanism of action described in this specification is speculation and does not limit the mechanism of the effects of the present invention.

[Multilayer sheet for food]
The food multilayer sheet of the present embodiment is
Having a paper substrate and a resin layer provided on one or both surface sides of the paper substrate,
The resin layer contains polylactic acid,
In the temperature-displacement curve from -20 ° C. to the indentation completion temperature of the resin layer obtained by thermomechanical analysis according to JIS K 7196 (2012), the slope of the curve at each temperature was obtained, and the slope was measured against the temperature. The food multilayer sheet has two or less peaks in a plotted temperature-slope curve (hereinafter also referred to as "the number of peaks of displacement slope").

FIG. 1 shows a schematic cross-sectional view of a multilayer food sheet 1 that is an example of the present embodiment. The food multi-layer sheet 1 has a structure in which a resin layer 3 is directly laminated on one surface of a paper substrate 2 .
Each constituent member of the food multilayer sheet of the present embodiment will be described in detail below.

<Paper substrate>
The paper base material of the food multilayer sheet of the present embodiment is not particularly limited, and can be appropriately selected according to the use of the food multilayer sheet.
Examples of paper substrates include woodfree paper, kraft paper, glassine paper, parchment paper, rayon paper, gravure paper, art paper, coated paper, recycled paper, and synthetic paper. One layer of these paper substrates may be used alone, or two or more layers of the same or different paper substrates may be laminated for use.

The paper substrate may be subjected to a surface modification treatment to improve adhesion on the side on which the resin layer is provided. Examples of surface modification treatment include formation of a primer layer, ozone treatment, corona treatment, plasma treatment, and ultraviolet treatment. These surface treatments may be performed singly or in combination. Among these, ozone treatment and corona treatment are preferred.

(Basis weight of paper substrate)
Although the basis weight of the paper base is not particularly limited, it is preferably 20 to 400 g/m ² , more preferably 30 to 350 g/m ² , still more preferably 40 to 300 g/m ² . In particular, when used as a food packaging material, the basis weight of the paper substrate is preferably 20 to 100 g/m ² , more preferably 30 to 80 g/m ² , still more preferably 40 to 60 g/m ² . . When used as a container for food, the basis weight of the paper substrate is preferably 100-400 g/m ² , more preferably 150-350 g/m ² , still more preferably 200-300 g/m ² . When the basis weight of the paper base is within the above range, the balance between lightness and strength of the paper base tends to be better.

(Thickness of paper substrate)
The thickness of the paper substrate is not particularly limited, but is preferably 20 μm to 2 mm, more preferably 30 μm to 1 mm, still more preferably 40 μm to 500 μm. In particular, when used as a food packaging material, the thickness of the paper substrate is preferably 25-110 μm, more preferably 35-90 μm, and even more preferably 45-80 μm. When used as a container for food, the thickness of the paper substrate is preferably 100-600 μm, more preferably 160-500 μm, still more preferably 220-400 μm.
When the thickness of the paper substrate is at least the above lower limit, the strength of the paper substrate is improved, and it tends to be possible to suppress unintentional tearing. Further, when the thickness of the paper substrate is equal to or less than the above upper limit, the lightness and flexibility tend to be better.
The "thickness of the paper base material" means the thickness of the entire paper base material, and when the paper base material is a base material consisting of multiple layers, the total means the thickness of

<Resin layer>
The resin layer of the food multi-layer sheet of the present embodiment contains polylactic acid.
The resin layer may contain only polylactic acid, but may contain components other than polylactic acid, if necessary. Examples of components other than polylactic acid include nucleating agents.

(polylactic acid)
Lactic acid, which is a raw material for polylactic acid, is a renewable biomass material because it can be produced by allowing lactic acid bacteria to act on glucose, sucrose, etc. extracted from plants. Moreover, since polylactic acid is a resin that is biodegradable under predetermined conditions, the multi-layered sheet for food according to the present embodiment, in which polylactic acid is used for the resin layer, has a small environmental load.
In addition, in this embodiment, "biodegradability" means a property capable of being chemically decomposed by the action of hydrolysis, enzymatic decomposition, microbial decomposition, or the like.

Examples of polylactic acid include, for example, one or more selected from the group consisting of L homopolymers, D homopolymers, L and D copolymers, L and D isomers, and poly(lactic acid) other than lactic acid. Examples thereof include copolymers with monomers.
One type of polylactic acid may be used alone, or two or more types may be used in combination.

Polylactic acid may contain structural units derived from monomers other than lactic acid, but from the viewpoint of further reducing the environmental load, the content of structural units derived from monomers other than lactic acid is Low is preferred. From this point of view, the content of lactic acid-derived structural units contained in polylactic acid is preferably 90 to 100% by mass, more preferably 95 to 100% by mass, and still more preferably 99 to 100% by mass.

From the viewpoint of increasing the heat of fusion of the resin layer and further improving the heat resistance, the content of the structural unit derived from the L-form in the total polylactic acid contained in the resin layer is The content is preferably 94 to 100% by mass, more preferably 95 to 100% by mass, still more preferably 96 to 99.8% by mass.

The mass average molecular weight (Mw) of polylactic acid is not particularly limited, but is preferably 10,000 to 1,000,000, more preferably 50,000 to 800,000, and still more preferably 100,000 to 600,000. be.
When the mass-average molecular weight (Mw) of polylactic acid is at least the above lower limit, heat resistance and mechanical properties tend to be better. Further, when the mass average molecular weight (Mw) of polylactic acid is equal to or less than the above upper limit, the moldability tends to be better.
The mass average molecular weight (Mw) can be measured by the method described in Examples.

The melt flow rate (MFR) of polylactic acid at a temperature of 210° C. and a load of 2.16 kg is not particularly limited, but is preferably 3 to 20 g/10 minutes, more preferably 4 to 18 g/10 minutes, and still more preferably 5 to 18 g/10 minutes. 15 g/10 minutes.
When the melt flow rate (MFR) of polylactic acid is at least the above lower limit, moldability tends to be better. Further, when the melt flow rate (MFR) of polylactic acid is equal to or less than the above upper limit, heat resistance and mechanical properties tend to be better.
The melt flow rate (MFR) of polylactic acid can be measured by the method described in Examples.

Polylactic acid may be obtained by a one-step process of directly polymerizing lactic acid to obtain a polymer, or obtained by a two-step process of ring-opening polymerization of lactide obtained by a cyclization reaction to obtain a polymer. It can be anything.

The content of polylactic acid in the resin layer is not particularly limited, but from the viewpoint of further reducing the environmental load, it is preferably 60 to 100% by mass, more preferably 80 to 100% by mass, based on the resin layer (100% by mass). 100% by mass, more preferably 90 to 100% by mass.

The content of polylactic acid in all the resins contained in the resin layer is not particularly limited, but from the viewpoint of further reducing the environmental load, it is preferably is 90 to 100% by mass, more preferably 95 to 100% by mass, still more preferably 99 to 100% by mass.

(Nucleating agent)
When the resin layer further contains a nucleating agent, the crystallinity of the resin layer is enhanced, and the number of peaks of the displacement slope of the resin layer tends to be easily adjusted to two or less.
A nucleating agent may be used individually by 1 type, and may be used in combination of 2 or more type.

Examples of nucleating agents include inorganic nucleating agents such as clay, talc, bentonite, silica, calcium sulfate, calcium carbonate, sodium hydrogen carbonate, and ammonium hydrogen carbonate; metal carboxylates, sorbitol compounds, metal phosphate organic nucleating agents such as salts;

When the resin layer contains a nucleating agent, the content of the nucleating agent in the resin layer is not particularly limited. 100% by mass), preferably 0.1 to 40% by mass, more preferably 1 to 25% by mass, and still more preferably 2 to 10% by mass.

(other ingredients)
The resin layer may or may not contain components other than the components described above as long as the effects of the present invention are not impaired.
Other components include, for example, resins other than polylactic acid; additives such as antioxidants, lubricants, ultraviolet absorbers, colorants, and antiblocking agents; and the like.
For each of the other components, one type may be used alone, or two or more types may be used in combination.

When the resin layer contains a resin other than polylactic acid, the resin other than polylactic acid is preferably a biomass-derived resin from the viewpoint of further reducing the environmental load.
From the viewpoint of further reducing the environmental load, the content of the biomass-derived resin (including polylactic acid) contained in the resin layer is preferably based on the total resin (100% by mass) contained in the resin layer. is 90 to 100% by mass, more preferably 95 to 100% by mass, still more preferably 99 to 100% by mass.

When the resin layer contains other components, the content of the other components is not particularly limited, but each is preferably 0.0001 to 40% by mass, more preferably 0.0001 to 40% by mass, based on the resin layer (100% by mass). is 0.0005 to 25% by mass, more preferably 0.001 to 10% by mass.

(Amount of heat of fusion of resin layer)
The heat of fusion of the resin layer in differential scanning calorimetry according to JIS K 7122 (2012) is preferably 36 J/g or more, more preferably 37 J/g or more, still more preferably 38 J/g or more, and even more preferably 39 J/g or more. is.
When the heat of fusion of the resin layer is equal to or higher than the above lower limit, the multi-layer sheet for food of the present embodiment has excellent heat resistance.
The amount of heat of fusion of the resin layer can be adjusted within the above range, for example, by increasing the crystallinity of the resin layer. Examples of the method for enhancing the crystallinity of the resin layer include a method of adding a nucleating agent, a method of performing an annealing treatment after forming a layer of the resin layer-forming material, and the like. It is also effective to select polylactic acid having a high L-form content as the polylactic acid contained in the resin layer.

Although the upper limit of the heat of fusion of the resin layer is not particularly limited, it is preferably 60 J/g or less, more preferably 55 J/g or less, still more preferably 50 J/g or less.
When the heat of fusion of the resin layer is equal to or less than the above upper limit, the resin layer has appropriate flexibility, and the occurrence of cracks, peeling, etc. in the resin layer due to bending, impact, etc. tends to be suppressed.
The heat of fusion of the resin layer can be measured by the method described in Examples.

(Melting point of resin layer)
Although the melting point of the resin layer is not particularly limited, it is preferably 140 to 180°C, more preferably 150 to 175°C, still more preferably 160 to 170°C.
When the melting point of the resin layer is at least the above lower limit, the resin layer is less likely to melt even when it comes into contact with heated food or the like, and there is a tendency to obtain a multi-layered sheet for food with more excellent heat resistance. Further, when the melting point of the resin layer is equal to or lower than the above upper limit, the resin layer has appropriate flexibility, and the resin layer tends to be prevented from cracking, peeling, or the like due to bending, impact, or the like. .
The melting point of the resin layer can be measured by the method described in Examples.

(Thermo-mechanical properties of resin layer)
In the temperature-displacement curve from -20 ° C. to the indentation completion temperature of the resin layer obtained by thermomechanical analysis according to JIS K 7196 (2012), the slope of the curve at each temperature is obtained, and the slope is plotted against the temperature. The number of peaks in the temperature-slope curve obtained by this is two or less.
When the number of peaks of the displacement slope is 2 or less, the multi-layered sheet for food of the present embodiment has excellent heat resistance.
The peak number of the displacement slope of the resin layer can be adjusted to the above range by, for example, increasing the crystallinity of the resin layer. Examples of the method for enhancing the crystallinity of the resin layer include a method of adding a nucleating agent, a method of performing an annealing treatment after forming a layer of the resin layer-forming material, and the like. It is also effective to select polylactic acid having a high L-form content as the polylactic acid contained in the resin layer.
The peak number of the displacement slope can be measured by the method described in the Examples.

When the number of peaks of the displacement slope is 1, the peak is selected. The maximum peak of the displacement slope", the softening point temperature (hereinafter referred to as "softening point temperature T ( 1)”) is not particularly limited, but is preferably 140° C. or higher, more preferably 143° C. or higher, and still more preferably 145° C. or higher.
When the softening point temperature T(1) of the resin layer is equal to or higher than the above lower limit, the multilayer food sheet of the present embodiment tends to be more excellent in heat resistance.
The softening point temperature T(1) of the resin layer is not particularly limited, but is preferably 180° C. or lower, more preferably 170° C. or lower, and even more preferably 165° C. or lower.
When the softening point temperature T(1) of the resin layer is equal to or lower than the above upper limit, the resin layer has appropriate flexibility, and the occurrence of cracks, peeling, or the like in the resin layer due to bending, impact, or the like is suppressed. tend to
The softening point temperature T(1) can be measured by the method described in Examples.

The displacement rate between 0 ° C. and 140 ° C. of the resin layer measured by thermomechanical analysis according to JIS K 7196 (2012) (absolute value of displacement at 140 ° C. × 100/thickness of the resin layer at 0 ° C.) Although not limited, it is preferably 20% or less, more preferably 15% or less, and still more preferably 10% or less.
When the displacement rate between 0° C. and 140° C. of the resin layer is equal to or less than the above upper limit, the multilayer food sheet of the present embodiment tends to be more excellent in heat resistance.
The displacement rate between 0° C. and 140° C. of the resin layer is preferably as small as possible, and may be 0%. good.
The displacement rate between 0° C. and 140° C. of the resin layer can be measured by the method described in Examples.

(thickness of resin layer)
Although the thickness of the resin layer is not particularly limited, it is preferably 1 to 120 μm, more preferably 5 to 50 μm, still more preferably 10 to 40 μm.
When the thickness of the resin layer is at least the above lower limit, the multilayer food sheet of the present embodiment tends to have better water resistance, oil resistance, and the like. Further, when the thickness of the resin layer is equal to or less than the above upper limit, the environmental load can be further reduced, and the economic efficiency tends to be excellent.

The ratio of the thickness of the resin layer to the thickness of the paper base [resin layer/paper base] is not particularly limited, but is preferably 0.01 to 1.5, more preferably 0.03 to 0.8. , more preferably 0.05 to 0.5.
When the thickness ratio [resin layer/paper substrate] is at least the above lower limit, the multi-layered sheet for food of the present embodiment tends to have better water resistance, oil resistance, and the like. Further, when the thickness ratio [resin layer/paper substrate] is equal to or less than the above upper limit, the environmental load tends to be further reduced.

<Other layers>
The food multi-layer sheet of the present embodiment may or may not have one or more layers other than the paper substrate and the resin layer.
Other layers can be provided, for example, between the paper substrate and the resin layer, on the side opposite to the resin layer of the paper substrate, and when there are two or more other layers, each has the same composition or may have different compositions.
Other layers include, for example, a barrier layer provided between the paper substrate and the resin layer, an adhesive layer for improving the adhesion between any of the layers, and a layer for imparting design properties to the paper substrate. The printing layer etc. which are provided are mentioned.
When the multilayer food sheet of the present embodiment has other layers, the components contained in the other layers are preferably biomass-derived components from the viewpoint of further reducing the environmental load.

The barrier layer includes, for example, a gas barrier layer that blocks permeation of oxygen gas and water vapor, and a light-shielding barrier layer that imparts a light-shielding property that blocks permeation of visible light, ultraviolet light, and the like. These can be formed by, for example, vapor deposition films of inorganic substances, inorganic oxides, etc.; metal foils; and the like.
Vapor-deposited films of inorganic substances, inorganic oxides, etc. can be formed by using conventionally known inorganic substances, inorganic oxides, etc., for example, by a vacuum deposition method, a sputtering method, an ion plating method, or the like.
As the metal foil, conventionally known metal foils can be used, but aluminum foil is preferable from the viewpoint of gas barrier properties and light shielding properties.

As the adhesive layer, a curable or non-curing type adhesive can be used, for example, acrylic adhesive, polyester adhesive, polyether adhesive, polyurethane adhesive, epoxy adhesive, rubber adhesive. Examples include adhesives and the like.

The printed layer is a layer provided for imparting design properties such as characters, information, patterns, and pictures.
The printed layer can be formed using conventionally known coloring agents such as pigments and dyes, and the formation method is not particularly limited. Inorganic pigments such as titanium yellow and chrome; organic pigments such as isoindolinone, Hansa Yellow A, quinacridone, permanent red 4R, phthalocyanine blue, and induslenbly RS; metal pigments made of metal powder such as aluminum and brass; titanium dioxide It can be formed with an ink using a colorant such as a pearlescent pigment made of foil powder such as coated mica or basic lead carbonate; a fluorescent pigment; or the like.

<Total thickness of food multilayer sheet>
The total thickness of the food multilayer sheet of the present embodiment is not particularly limited, and may be determined according to the application of the food multilayer sheet of the present embodiment. It may be in the range of 180-530 μm, or it may be in the range of 240-430 μm.

<Method for producing multi-layer sheet for food>
The method for producing the food multi-layer sheet of the present embodiment is not particularly limited, and for example, it can be produced by laminating a resin layer-forming material containing polylactic acid on one or both sides of a paper substrate. .
From the viewpoint of increasing the heat of fusion of the resin layer and further improving the heat resistance, the resin layer-forming material may contain the above-described nucleating agent in addition to polylactic acid.
Further, the other layer described above may be provided on the surface of the paper substrate on which the resin layer is laminated. In the following description, "on the paper base material" means one surface of the paper base material, both surfaces of the paper base material, surfaces of other layers provided on one side of the paper base material, paper It is intended to include all the surfaces of other layers provided on both sides of the substrate.

Methods for laminating the resin layer-forming material on the paper substrate include, for example, a method of melt extruding the resin layer-forming material onto the paper substrate, and a method of applying the resin layer-forming material as a coating liquid onto the paper substrate. Examples thereof include a method of coating and drying, and a method of laminating a resin layer-forming material formed into a sheet on a paper substrate. Among these, the method of melt extruding the resin layer-forming material is preferable from the viewpoint of productivity.

As a method of melt extruding the resin layer-forming material onto the paper substrate, for example, there is a method of extruding the melted resin layer-forming material onto the paper substrate to form a layer using an extruder and a T-die. mentioned.
The melt extrusion temperature of the resin layer-forming material is not particularly limited, and may be appropriately set according to the type of resin constituting the resin layer. is 190-240°C.
After the resin layer-forming material is melt-extruded, the formed resin layer may be cooled and solidified.

As a method of applying the resin layer-forming material as a coating liquid onto the paper substrate, for example, a coating liquid obtained by dissolving and dispersing the resin layer-forming material in a solvent is coated on the paper substrate and then dried. There is a method to make
Examples of the method of applying the coating liquid include roll coating, spin coating, spray coating, bar coating, knife coating, roll knife coating, blade coating, die coating, gravure coating, and the like. .

As a method for laminating the resin layer-forming material molded into a sheet on a paper substrate, for example, a coating liquid of the resin layer-forming material is applied onto the release film in the same manner as described above, and then dried. A method of forming a resin layer on a release film and laminating the resin layer on the release film to a paper substrate can be mentioned.

In the method for producing a multi-layer sheet for food according to the present embodiment, from the viewpoint of increasing the heat of fusion of the resin layer and further improving the heat resistance, annealing treatment is performed after layer formation of the resin layer-forming material containing polylactic acid. A step of applying to form a resin layer may be included.
Annealing treatment can be carried out, for example, by laminating the resin layer-forming material on one or both sides of the paper substrate and then heating.

The heating temperature in the annealing treatment is not particularly limited, and is preferably determined according to the type of polylactic acid. It may be 120°C.
The heating temperature in the annealing treatment may be determined based on the crystallization temperature of the resin layer before the annealing treatment (hereinafter also simply referred to as the “crystallization temperature”), and is preferably between the crystallization temperature of −40° C. and the crystal. The crystallization temperature is +50°C, more preferably crystallization temperature -35°C to crystallization temperature +40°C, more preferably crystallization temperature -30°C to crystallization temperature +30°C.
When the heating temperature in the annealing treatment is equal to or higher than the above lower limit, the heat of fusion of the resin layer tends to be sufficiently increased. Moreover, when the heating temperature in the annealing treatment is equal to or lower than the above upper limit, it tends to be easy to suppress unintended deterioration of the resin layer due to the annealing treatment.
The crystallization temperature of the resin layer before annealing can be measured by the method described in Examples.

The heating time in the annealing treatment is not particularly limited, but is preferably 10 seconds to 30 minutes, more preferably 20 seconds to 20 minutes, even more preferably 30 minutes, from the viewpoint of increasing the heat of fusion of the resin layer and from the viewpoint of productivity. seconds to 15 minutes.

<Uses of multi-layer sheets for food>
The food multilayer sheet of the present embodiment is excellent in heat resistance, and is therefore suitable for food packaging materials, food containers, and the like. In addition, the food multilayer sheet of the present embodiment is excellent in heat resistance, and is therefore suitable as a packaging material or container for foods heated in a microwave oven.

[Food packaging materials, food containers]
The food packaging material of the present embodiment is a food packaging material formed using the food multilayer sheet of the present embodiment, and the food container of the present embodiment uses the food multilayer sheet of the present embodiment. It is a food container formed by

As one aspect of the food packaging material of the present embodiment, for example, the multi-layered food sheet of the present embodiment is formed into a bag shape, a cup shape, a tray shape, etc., and a material used for packaging food is provided. mentioned. These can be used, for example, when transporting, storing, selling, etc. sweets, lunch boxes, frozen foods, beverages, and the like.
One aspect of the food container of the present embodiment includes, for example, a material that is used to store food by molding the multilayer food sheet of the present embodiment into a cup shape, tray shape, dish shape, or the like. . These can be used, for example, when transporting, storing, or selling sweets, lunch boxes, frozen foods, beverages, etc., or disposable containers such as disposable cups and disposable plates.
Incidentally, on the side opposite to the resin layer of the paper base material of the food packaging material and the food container, matters related to the contents, various designs, etc. may be applied by printing or the like as appropriate.

Food packaging materials and food containers can be produced, for example, by molding the multi-layer food sheet of the present embodiment into a desired shape. Examples of the method for molding the multilayer food sheet of the present embodiment include vacuum molding, air pressure molding, match mold molding, and the like. Alternatively, the food multi-layer sheet of the present embodiment may be heat-sealed and assembled by heat-sealing.

The present invention will be described in more detail below based on examples, but the present invention is not limited by these examples. Measurement methods and evaluation methods for various physical properties are as follows.

[Measurement of thickness of each layer]
It was measured using a constant-pressure thickness measuring instrument manufactured by Teclock Co., Ltd. (model number: "PG-02J", standard specifications: JIS K 6783, Z1702, Z1709).

[Melt flow rate (MFR) of polylactic acid]
The MFR of polylactic acid was measured according to JIS K 7210-1:2014 using a descending flow tester (manufactured by Shimadzu Corporation, model number "CFT-100D") at a temperature of 210°C and a load of 2.16 kg. It was measured.

[Measurement of Heat of Fusion, Crystallization Temperature, and Melting Point of Resin Layer]
The crystallization temperature and melting point of the resin layer were measured according to JIS K 7121 (2012), and the heat of fusion of the resin layer was measured according to JIS K 7122 (2012) by the following procedure.
The resin layer of the multi-layer food sheet having a thickness of 100 μm prepared in each example (however, in the measurement of the crystallization temperature in Examples 3 to 5, the resin layer before annealing treatment) was used as a measurement sample, About 5 mg of the sample is put into an aluminum pan, and a differential scanning calorimeter (DSC) (manufactured by TA Instruments, product name "Q2000") is used to measure the temperature range from -120 to 200 ° C. under a nitrogen atmosphere. , and a temperature increase rate of 10° C./min.
The obtained DSC curve was analyzed in accordance with JIS K 7121 (2012) to determine the crystallization temperature and melting point, and the analysis was performed in accordance with JIS K 7122 (2012) to determine the heat of fusion.

[Measurement of Displacement Rate and Softening Point Temperature of Resin Layer]
The displacement rate and softening point temperature of the resin layer were measured according to JIS K 7196 (2012) by the following procedure.
A thermomechanical analysis (TMA) device (manufactured by Netch Japan Co., Ltd., trade name "TMA 4000SE") was applied to the multi-layer food sheet having a resin layer thickness of 100 μm prepared in each example so that the measurement surface was the resin layer. ”), using a cylindrical indenter with a tip diameter of 1 mm and a length of 1 mm, from the measurement temperature range of -20 ° C to the indenter penetration completion temperature, in an air atmosphere, at a temperature increase rate of 5 ° C / min. A temperature-displacement curve was obtained. In the temperature-displacement curve, the displacement rate between 0° C. and 140° C. (absolute value of displacement at 140° C.×100/thickness of resin layer at 0° C.) was confirmed.
Furthermore, in the temperature-displacement curve obtained above, the slope of the curve at each temperature was determined, and the temperature-slope curve was obtained by plotting the slope against the temperature.
In the obtained temperature-slope curve, the number of peaks (number of peaks of displacement slope) from −20° C. to the indentation completion temperature was confirmed. In the above temperature range, when only one peak was confirmed, the peak was identified as the "maximum displacement slope peak" when two or more peaks were confirmed. . From the temperature-displacement curve corresponding to the maximum peak of the displacement slope, the softening point temperature was determined by analysis according to JIS K 7196 (2012), and this was defined as "softening point temperature T(1)".

[Preparation of multi-layer sheet for food]
Examples 1 and 2, Comparative Example 1
A resin layer-forming material composed of polylactic acid described in Table 1 or a resin layer-forming material obtained by mixing polylactic acid and a nucleating agent was melted into the paper described in Table 1 at 200 ° C. using a T-die. Extruded onto a substrate. After that, the resin layer was cooled and solidified by a water-cooled roll adjusted to a water temperature of 23° C. to obtain a multi-layer sheet for food.
Two types of multi-layer sheets for food were produced, one having a resin layer thickness of 100 μm and the other having a resin layer thickness of 20 μm. Also in Examples 3 to 5 below, multi-layer sheets for food having resin layers with the above two types of thicknesses were produced.

Examples 3-5
A resin layer-forming material composed of polylactic acid described in Table 1 or a resin layer-forming material obtained by mixing polylactic acid and a nucleating agent was treated in the same manner as in Example 1 to obtain a paper base material described in Table 1. The resin layer was cooled and solidified by a water-cooled roll adjusted to a water temperature of 23°C. Thereafter, the sheet was annealed under the conditions shown in Table 1 in an air atmosphere, and then naturally cooled in the air to obtain a multi-layered sheet for food. Table 1 shows the crystallization temperatures of the resin layers before the annealing treatment in Examples 3 to 5.

In Table 1, the details of the nucleating agent and the paper substrate are as follows.
<Nucleating agent>
・ Clay: Hojun Co., Ltd., trade name “Esben W”
・ Talc: manufactured by Kami Talc Co., Ltd., trade name “Talc KA-9”
<Paper substrate>
・Paper base material 1: Basis weight 50 g/m ² , thickness 70 μm
・Paper base material 2: Basis weight 260 g/m ² , thickness 320 μm

From the results of Table 1, the food multilayer sheets of Examples 1 to 5 of the present embodiment, in which the number of peaks of the displacement slope of the resin layer is 2 or less, have a small displacement rate between 0 ° C. and 140 ° C., and are heat resistant. It can be seen that it is superior to On the other hand, the food multilayer sheet of Comparative Example 1, in which the number of peaks of the displacement slope of the resin layer exceeded two, had insufficient heat resistance.

1 multi-layer sheet for food 2 paper substrate 3 resin layer

Claims

Having a paper substrate and a resin layer provided on one or both surface sides of the paper substrate,
The resin layer contains polylactic acid,
In the temperature-displacement curve from -20 ° C. to the indentation completion temperature of the resin layer obtained by thermomechanical analysis according to JIS K 7196 (2012), the slope of the curve at each temperature was obtained, and the slope was measured against the temperature. A multi-layer sheet for foods, wherein the temperature-slope curve obtained by plotting has two or less peaks.
When the number of peaks in the temperature-slope curve is 1, the peak, and when the number of peaks in the temperature-slope curve is 2, the height of the peak among the 2 peaks When the larger peak is the maximum peak of the displacement slope, the softening point temperature obtained from the temperature-displacement curve corresponding to the maximum peak of the displacement slope by analysis according to JIS K 7196 (2012) is 140. ° C. or higher, the multi-layer sheet for food according to claim 1.
The food multilayer sheet according to claim 1 or 2, wherein the resin layer has a melting point of 140 to 180°C.
The food multilayer sheet according to claim 1 or 2, wherein the polylactic acid content in the resin layer is 60 to 100% by mass.
The food multilayer sheet according to claim 1 or 2, wherein the content of the L-form-derived structural unit in the total polylactic acid contained in the resin layer is 94 to 100% by mass.
The food multilayer sheet according to claim 1 or 2, wherein the resin layer further contains a nucleating agent.
The food multilayer sheet according to claim 1 or 2, wherein the ratio of the thickness of the resin layer to the thickness of the paper base [resin layer/paper base] is 0.01 to 1.5.
The multilayer food sheet according to claim 1 or 2, wherein the resin layer has a thickness of 1 to 120 µm.
The multilayer food sheet according to claim 1 or 2, wherein the resin layer has a displacement rate between 0°C and 140°C of 20% or less, as measured by thermomechanical analysis according to JIS K 7196 (2012).
A food packaging material formed using the food multilayer sheet according to claim 1 or 2.
A food container formed using the multi-layer food sheet according to claim 1 or 2.
A method for producing the food multilayer sheet according to claim 1 or 2,
A method for producing a multi-layer sheet for food, comprising a step of forming a resin layer by forming a layer of a resin layer-forming material containing polylactic acid, followed by annealing treatment.