JP7849149B2 - Resin sheets and containers - Google Patents

Description

This invention relates to a resin sheet having a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin, and to a container using this resin sheet.

Resin containers are widely used for food products such as retort foods, seasonings like miso, and confectionery. These resin containers are known to be made by molding resin sheets, for example, using thermoforming. Such resin containers require sufficient heat resistance to prevent deformation during heat sterilization of the contents or heating in a microwave oven. Therefore, the resin sheet constituting the container typically has a resin layer made of propylene-based resin.

Incidentally, in recent years, in the field of synthetic resin materials, there has been a growing demand to shift from fossil fuel-derived synthetic resin materials to biomass-derived synthetic resin materials, driven by the need to reduce environmental impact through the creation of a circular economy. For these reasons, the use of biomass-derived synthetic resin materials in resin sheets constituting food containers is being considered. For example, Patent Document 1 proposes a resin sheet having a first layer containing a propylene-based resin and a second layer containing both a propylene-based resin and a biomass-derived ethylene-based resin.

Japanese Patent Publication No. 2020-163634

However, it was found that when food containers used for retort foods and other products were constructed using resin sheets made from the biomass-derived ethylene resin described above, sufficient heat resistance could not be obtained.

The present invention has been made based on the circumstances described above, and its purpose is to provide a resin sheet that can reduce the environmental burden and has excellent heat resistance, preventing deformation when the contents are heat-sterilized or when heated by a microwave oven or the like.
Another object of the present invention is to provide a resin container that can reduce environmental impact and has excellent heat resistance, preventing deformation when the contents are subjected to heat sterilization or heating by a microwave oven or the like.

The resin sheet of the present invention is a resin sheet having a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin,
The aforementioned resin sheet is intended to be molded by thermoforming.
The resin sheet has two first resin layers, and a gas barrier layer is provided between the two first resin layers.
The two first resin layers consist of the blended resin layer,
The propylene-based resin contains a propylene homopolymer and has a storage modulus of 5 × ^10⁷ Pa or higher at 140°C.
The biomass-derived ethylene resin has a melt flow rate (MFR) of 0.32 g/10 min or more and 0.34 g/10 min or less, measured under conditions of a temperature of 190°C and a load of 2.16 kgf.
The proportion of the propylene homopolymer in the blended resin layer is 50% by mass or more.
The aforementioned resin sheet is characterized by having a shrinkage rate of 3% or less at 140°C .

In the resin sheet of the present invention, the resin sheet has a second resin layer containing a propylene-based resin, and it is preferable that the second resin layer is the blended resin layer.
Furthermore, it is preferable to have an adhesive resin layer.
Furthermore, it is preferable that the mass of the blended resin layer is 50% or more of the total mass of the resin sheet.

The container of the present invention is characterized by being formed from the above-mentioned resin sheet by thermoforming.

The resin sheet of the present invention has a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin, thereby reducing the environmental impact. Furthermore, the propylene-based resin in the blended resin layer contains homopolypropylene and has a storage modulus of 5 × ^10⁷ Pa or higher at 140°C, resulting in a resin container with excellent heat resistance that does not deform when subjected to heat sterilization of the contents or heat treatment using a microwave oven, etc.
Furthermore, because the container of the present invention uses the above-mentioned resin sheet, it is possible to reduce the environmental burden, and it also provides excellent heat resistance, preventing deformation when the contents are subjected to heat sterilization or heating by a microwave oven or the like.

This is an explanatory cross-sectional view showing the configuration of an example of the resin sheet of the present invention.

The embodiments of the present invention will be described in detail below.
[Resin sheet]
The resin sheet of the present invention has a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin (hereinafter also referred to as "bioethylene-based resin"). In addition to the propylene-based resin and the bioethylene-based resin, the blended resin layer may also contain a fossil fuel-derived ethylene-based resin.

The propylene-based resin constituting the blended resin layer has a storage modulus at 140°C (hereinafter also simply referred to as "storage modulus") of 5 × ^10⁷ Pa or higher, preferably 6 × ^10⁷ Pa or higher. If the storage modulus of the propylene-based resin is less than 5 × ^10⁷ Pa, it becomes difficult to obtain a container with excellent heat resistance. There is no particular upper limit to the storage modulus of the propylene-based resin, but considering processability and other factors, it is preferable to have a storage modulus of 1.5 × ^10⁸ Pa or lower.
Here, we will explain the storage modulus. Synthetic resins are typical viscoelastic materials, possessing both elastic and viscous properties. Dynamic viscoelasticity measurements allow us to measure the storage modulus G' (Pa) and loss modulus G'' (Pa) by applying a stress or strain that changes (vibrates) over time to a sample and measuring the resulting strain or stress. The storage modulus is the component of energy generated in an object by external force and strain that is stored within the object (i.e., elasticity), while the loss modulus is the component that diffuses to the outside (i.e., viscosity).

The proportion of propylene-based resin in the blended resin layer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. If the proportion of propylene-based resin is too low, it may be difficult to obtain a container with excellent heat resistance.

The propylene-based resin used to constitute the blended resin layer contains a propylene homopolymer (hereinafter also referred to as "homopolypropylene"). It is preferable to use a homopolypropylene with a storage modulus of 5 × ^10⁷ Pa or higher, and particularly a homopolypropylene with a storage modulus of 6 × ^10⁷ Pa or higher.
Furthermore, from the viewpoint of moldability, the homopolypropylene is preferably such that the melt flow rate (MFR), measured under conditions of a temperature of 230°C and a load of 2.16 kgf, is 0.3 g/10 min or more and 5 g/10 min or less, and from the viewpoint of rigidity, the tensile modulus (JIS K7162) is 1500 MPa or more.

Furthermore, the proportion of homopolypropylene in the blended resin layer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, in order to reliably obtain excellent heat resistance.

The propylene-based resin constituting the blended resin layer may contain, in addition to homopolypropylene, a copolymer of propylene and another monomer (hereinafter also referred to as "propylene copolymer"). Ethylene-propylene copolymers can be used as the propylene copolymer. The propylene copolymer may be a random copolymer or a block copolymer. The storage modulus of the propylene copolymer is not particularly limited as long as the storage modulus of the entire propylene-based resin satisfies the above value, but it is preferably 5 × ^10⁷ Pa or higher in order to reliably obtain excellent heat resistance.
Furthermore, from the viewpoint of moldability, the propylene copolymer is preferably such that the melt flow rate (MFR), measured under conditions of a temperature of 230°C and a load of 2.16 kgf, is 0.2 g/10 min or more and 5 g/10 min or less, and from the viewpoint of rigidity, the tensile modulus (JIS K7162) is 1500 MPa or more.

Low-density polyethylene (LDPE), high-density polyethylene (HDPE), and linear low-density polyethylene (LLDPE) can be used as the bioethylene resin constituting the blended resin layer. Furthermore, the bioethylene resin may consist entirely of ethylene monomers derived from biomass, or part of it may be derived from fossil fuels.
Furthermore, from the viewpoint of moldability, the bioethylene resin is preferably such that the melt flow rate (MFR), measured under conditions of a temperature of 190°C and a load of 2.16 kgf, is 0.1 g/10 min or more and 3 g/10 min or less, and from the viewpoint of rigidity, the density is preferably 0.91 g/ ^cm³ or more and 0.97 g/ ^cm³ or less.

Furthermore, it is preferable that the bioethylene-based resin is contained in the blended resin layer at a rate such that the biomass content of the blended resin layer is 5% or more, and particularly 10% or more. If the biomass content of the blended resin layer is less than 5%, it may be difficult to sufficiently reduce the environmental impact.

The resin sheet of the present invention may have a single-layer structure consisting only of the above-described blended resin layer, or it may have a laminated structure with other layers. The following describes a laminated resin sheet, but the resin sheet of the present invention is not limited to the laminated structure described below.

Figure 1 is an explanatory cross-sectional view showing the configuration of an example of the resin sheet of the present invention. This resin sheet 10 is used to form a resin container and has two first resin layers 11 and two second resin layers 12 arranged inside each of the first resin layers, with the first resin layers 11 being the blended resin layer described above. Inside each of the second resin layers 12, a gas barrier layer 13 is provided to prevent or suppress the permeation of gases such as oxygen gas. An adhesive layer 14 is formed between each of the second resin layers 12 and the gas barrier layer 13. In the configuration shown in Figure 1, the second resin layers 12 are arranged inside the first resin layers 11, but the present invention is not limited to this. The arrangement positions of the first resin layers 11 and the second resin layers 12 may be swapped, and the first resin layers 11 and the gas barrier layers 13 may be laminated via the adhesive resin layer 14. Furthermore, the first resin layers 11 and the second resin layers 12 may be made of the same resin material and configured as a single resin layer.

The second resin layer 12 contains a propylene-based resin. As the propylene-based resin, homopolypropylene, propylene copolymer, or a mixture thereof can be used. Furthermore, the second resin layer 12 may be composed solely of a propylene-based resin, or it may be composed of a mixture of a propylene-based resin and other olefin-based resins such as ethylene-based resins. However, from the viewpoint of reducing environmental impact, the second resin layer 12 is preferably the blended resin layer described above.

The material constituting the gas barrier layer 13 can be a resin material such as ethylene-vinyl alcohol copolymer, polyamide, polyvinyl alcohol, or polyvinylidene chloride. Among these, ethylene-vinyl alcohol copolymer is preferred from the viewpoint of moldability.

As the adhesive for forming the adhesive layers 14 and 15, urethane-based adhesives, acid-modified polyolefin-based adhesives, polyester-based adhesives, polyether-based adhesives, polyamide-based adhesives, etc., can be used.

In a resin sheet 10 having such a laminated structure, the thickness of the first resin layer 11, which is a blended resin layer, is, for example, 100 to 800 μm, the thickness of the second resin layer 12 is, for example, 100 to 800 μm, and the thickness of the gas barrier layer 13 is, for example, 5 to 200 μm.
Furthermore, conventionally known methods such as co-extrusion and dry lamination can be used to manufacture the resin sheet 10 having a laminated structure, and from the viewpoint of productivity, it is preferable to manufacture it by co-extrusion.

Furthermore, the surfaces of the first and second resin layers may each be provided with a surface layer (not shown). The material constituting the surface layer is not particularly limited, and various resin materials can be used. However, from the viewpoint of reducing environmental impact, it is preferable that the surface layer contains a biomass-derived resin material such as a biopolyethylene resin.

[container]
The container of the present invention is formed by thermoforming the above-mentioned resin sheet. Specific thermoforming methods include conventionally known methods such as vacuum pressure forming. The molding temperature during thermoforming is appropriately set considering the melting point and softening point of the materials constituting the resin sheet; for example, the heater setting temperature is 300 to 450°C, and the sheet surface temperature is 150 to 190°C.
Such containers of the present invention are suitable for use with foods that require heat sterilization of their contents or heat treatment during cooking, such as retort foods, seasonings such as miso, and confectionery.

According to the present invention, since the container has a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin, it is possible to reduce the environmental burden. Moreover, the propylene-based resin in the blended resin layer contains homopolypropylene and has a storage modulus of 5 × ^10⁷ Pa or higher at 140°C, so a resin container with excellent heat resistance can be obtained that does not deform when the contents are heat-sterilized or when heated in a microwave oven or the like.

The following describes specific examples of the present invention, but the present invention is not limited to these. Furthermore, the propylene resin and ethylene resin used in the following examples , reference examples , and comparative examples have the properties shown in Table 1 below.

In Table 1, the storage modulus at 140°C is the value measured under the following conditions.
Test specimen: A 10 mm wide, 1 mm thick film made by hot pressing. Measurement device: Dynamic viscoelasticity analyzer "DHR-2" (TA Instruments Co., Ltd.)
Measurement length: 20mm
Temperature conditions: Heating from 20°C to 160°C at a rate of 5°C/min. Measurement mode: Torsion, frequency 1 Hz, strain 0.1%.
Furthermore, the MFR values were measured according to JIS K7210 for propylene-based resins and JIS K6922-1 for ethylene-based resins. Specifically, the values for propylene-based resins were measured under conditions of a temperature of 230°C and a load of 2.16 kgf, and the values for ethylene-based resins were measured under conditions of a temperature of 190°C and a load of 2.16 kgf.

<Example 1>
Using a multilayer extrusion molding machine capable of forming a multilayer structure of seven layers of four types, consisting of a first resin layer, a second resin layer, an adhesive layer, a gas barrier layer, another adhesive layer, a second resin layer, and a first resin layer, a resin sheet with the configuration shown in Figure 1 was manufactured as follows.
A resin material common to both the first and second resin layers was prepared by mixing 85% by mass of propylene resin (1) and 15% by mass of ethylene resin (1). The obtained resin material was supplied to single-screw extruders for the first and second resin layers, acid-modified polypropylene resin was supplied to a single-screw extruder for the adhesive layer, and ethylene-vinyl alcohol copolymer was supplied to a single-screw extruder for the gas barrier layer. Each material was melt-kneaded in each single-screw extruder, extruded from a T-die molding device connected to each single-screw extruder, and the molten resins extruded into a film were cooled by a cooling roll while being laminated to produce a resin sheet with a thickness of 500 μm. In the above, the cylinder temperature of the single-screw extruders for the first and second resin layers was 230°C, the cylinder temperature of the single-screw extruders for the adhesive layer and gas barrier layer was 210°C, the T-die head temperature was 230°C, and the cooling roll temperature was 50°C.
The mass ratios of each layer in the obtained resin sheet are as follows: the first resin layer is 18% by mass in total for both layers, the second resin layer is 74% by mass in total for both layers, the gas barrier layer is 4% by mass, and the adhesive layer is 4% by mass in total for both layers.
The obtained resin sheet was subjected to vacuum compression molding using a vacuum pressure molding machine at a heater temperature of 350°C to produce a container. The resulting container has a rectangular base measuring 150 mm x 115 mm and a depth of 30 mm.

The biomass content of the obtained resin sheets was measured as follows. Furthermore, the shrinkage rate of the obtained containers at 140°C was measured as follows. The results are shown in Table 2 below.
[Biomass content]
Biomass content is an index representing the mixing ratio of fossil fuel-derived raw materials and biomass-derived raw materials, and is determined by measuring the concentration of radioactive carbon ( ^¹⁴C ). Biomass content was measured according to ASTM 6866 Method B. The sample was burned to generate carbon dioxide, which was then purified in a vacuum line. The purified carbon dioxide was reduced with hydrogen using iron as a catalyst to produce graphite. The graphite was mounted in a ^1⁴C -AMS dedicated device (manufactured by NEC Corporation) based on a tandem accelerator, and the coefficient of ^¹⁴C , the ^¹³C concentration ( ^¹³C / ^¹²C ), and the ^¹⁴C concentration ( ^¹⁴C / ^¹²C ) were measured. Oxalic acid provided by the U.S. National Institute of Standards (NIST) was used as the standard sample for the measurements.
[Shrinkage rate at 140°C]
The containers were heated in an oven at 140°C for 30 minutes. The internal volume of each container before and after heating was measured as follows, and the volume shrinkage rate was calculated using the formula below. This value was defined as the shrinkage rate at 140°C. If the shrinkage rate at 140°C is 3% or less, the container is judged to have high heat resistance, as it does not deform when heated.
Measurement of internal volume: The opening of the container was sealed with an acrylic plate having a hole with an inner diameter of 5 mm. Pure water was poured into the container through the hole in the acrylic plate to fill the container with pure water, and the internal volume of the container was calculated from the mass of the injected pure water and the specific gravity of the pure water.
Calculation formula: Volume shrinkage rate (%) = (Vb - Va) / Va × 100
Va = Internal volume of the container before heat treatment
Vb = Internal volume of the container after heat treatment

<Example 2>
Except for changing 85% by mass of propylene resin (1) to 85% by mass of propylene resin (2), resin sheets and containers were manufactured in the same manner as in Example 1, and the biomass content of the resin sheets and the heat shrinkage rate of the containers at 140°C were measured. The results are shown in Table 2 below.

< Reference example 1 >
Resin sheets and containers were manufactured in the same manner as in Example 1, except that 85% by mass of propylene resin (1) was replaced with 80% by mass of propylene resin (3), and 15% by mass of ethylene resin (1) was replaced with 20% by mass of ethylene resin (2). The biomass content of the resin sheets and the heat shrinkage rate of the containers at 140°C were measured. The results are shown in Table 2 below.

<Example 3 >
Except for changing the propylene resin (1) content from 85% by mass to 70% by mass and the ethylene resin (1) content from 15% by mass to 30% by mass in the resin material for the second resin layer, a resin sheet and a container were manufactured in the same manner as in Example 1, and the biomass content of the resin sheet and the thermal shrinkage rate of the container at 140°C were measured. The results are shown in Table 2 below.

<Example 4 >
Except for changing 85% by mass of propylene resin (1) to 60% by mass of propylene resin (3) in the resin material for the second resin layer, and changing 15% by mass of ethylene resin (1) to 30% by mass of ethylene resin (1) and 10% by mass of ethylene resin (3), a resin sheet and a container were manufactured in the same manner as in Example 1, and the biomass content of the resin sheet and the heat shrinkage rate of the container at 140°C were measured. The results are shown in Table 2 below.

<Comparative Example 1>
Resin sheets and containers were manufactured in the same manner as in Example 1, except that 85% by mass of propylene resin (1) and 15% by mass of ethylene resin (1) were replaced with 100% by mass of propylene resin (4). The biomass content of the resin sheets and the heat shrinkage rate of the containers at 140°C were measured. The results are shown in Table 2 below.

<Comparative Example 2>
Resin sheets and containers were manufactured in the same manner as in Example 1, except that 85% by mass of propylene resin (1) was changed to 80% by mass of propylene resin (5), and 15% by mass of ethylene resin (1) was changed to 20% by mass of ethylene resin (3). The biomass content of the resin sheets and the heat shrinkage rate of the containers at 140°C were measured. The results are shown in Table 2 below.

As is clear from the results in Table 2, it was confirmed that, according to Examples 1 to 4 , a resin container with a biomass content of 10% or more can be obtained that has superior heat resistance compared to Comparative Example 2.

10 Resin sheet 11 First resin layer 12 Second resin layer 13 Gas barrier layer 14 Adhesive layer

Claims (5)

A resin sheet having a blended resin layer containing a propylene-based resin and a biomass-derived ethylene-based resin,
The aforementioned resin sheet is intended to be molded by thermoforming.
The resin sheet has two first resin layers, and a gas barrier layer is provided between the two first resin layers.
The two first resin layers consist of the blended resin layer,
The propylene-based resin contains a propylene homopolymer and has a storage modulus of 5 × ^10⁷ Pa or higher at 140°C.
The biomass-derived ethylene resin has a melt flow rate (MFR) of 0.32 g/10 min or more and 0.34 g/10 min or less, measured under conditions of a temperature of 190°C and a load of 2.16 kgf.
The proportion of the propylene homopolymer in the blended resin layer is 50% by mass or more.
The aforementioned resin sheet is characterized by having a shrinkage rate of 3% or less at 140°C . The resin sheet according to claim 1, characterized in that the resin sheet has a second resin layer containing a propylene-based resin, and the second resin layer is the blended resin layer. The resin sheet according to claim 2, characterized by having an adhesive layer. The resin sheet according to any one of claims 1 to 3, characterized in that the mass of the blended resin layer is 50% or more of the total mass of the resin sheet. A container characterized by being formed by thermoforming a resin sheet according to claims 1 to 4.