JP7617389B2 - Plastic containers and their recycling methods - Google Patents

Description

The present invention relates to plastic containers and methods for recycling them.

Patent document 1 discloses a plastic container capable of storing contents such as mayonnaise.

JP 2016-190654 A

Plastic containers such as those described in Patent Document 1 are often discarded after the contents have been used up, but considering the impact on the environment, it is desirable to recycle plastic containers and obtain recycled resin.

However, in plastic containers like those in Patent Document 1, the contents may adhere to the inner surface and may not be easily removed even by washing, and the recycled resin obtained by recycling such plastic containers is unlikely to be of high quality.

The present invention was made in consideration of these circumstances, and aims to provide a plastic container that can be recycled to produce high-quality regenerated resin.

According to the present invention, there is provided a plastic container for accommodating contents, which comprises, in order from the inside surface side of the container, an inner layer, an intermediate layer, and an outer layer, the intermediate layer being water-soluble, and the inner layer and the outer layer having different specific gravities.

When recycling the plastic container of the present invention, the inner and outer layers can be separated by dissolving the intermediate layer containing the water-soluble resin, and then the inner and outer layers can be sorted based on their specific gravity. This makes it easy to extract only the outer layer and obtain high-quality recycled resin.

Various embodiments of the present invention will be described below. The embodiments described below can be combined with each other.
Preferably, the plastic container is as described above, wherein one or both of the inner layer and the outer layer adjacent to the intermediate layer is foamed.
Preferably, in the plastic container described above, one or both of the inner layer and the outer layer, the layer adjacent to the intermediate layer, does not contain an adhesive resin.
Preferably, in the plastic container described above, the inner layer and the outer layer have different tensile moduli.
Preferably, the method for recycling plastic containers described above comprises a crushing step, a dissolving step, and a specific gravity sorting step, in which the plastic container is crushed to form a plurality of crushed pieces in the crushing step, the intermediate layer contained in the crushed pieces is dissolved to separate the inner layer and the outer layer in the dissolving step, and the inner layer and the outer layer are sorted based on specific gravity in the specific gravity sorting step.

FIG. 2 is a front view showing the plastic container 1 and the cap 13. FIG. 2A is a layer structure diagram of a plastic container 1 according to a first embodiment of the present invention, FIG. 2B shows pulverized pieces 1p obtained by crushing the plastic container 1, and FIG. 2C shows the state after the middle layer 3p in the pulverized pieces 1p has been dissolved to separate the inner layer 2p and the outer layer 4p. FIG. 3A is a layer structure diagram of a plastic container 1 according to a second embodiment of the present invention, FIG. 3B shows crushed pieces 1p obtained by crushing the plastic container 1, and FIG. 3C shows the state after the middle layer 3p in the crushed pieces 1p has been dissolved to separate the inner layer 2p and the outer layer 4p.

The following describes the embodiments of the present invention. The various features shown in the following embodiments can be combined with each other. In addition, each feature can be an invention independently.

1. First embodiment 1-1. Configuration of plastic container

Figure 1 shows a schematic diagram of a plastic container 1 according to one embodiment of the present invention. As shown in Figure 1, container 1 is a container that contains contents. The contents are not particularly limited, but include oily contents (e.g., edible oil and mayonnaise) and spicy contents (e.g., sauce). Oily contents are difficult to remove by washing during recycling, and spicy contents leave an odor even after washing, so with conventional containers it is difficult to obtain high-quality recycled resin when the contents are oily or spicy. On the other hand, with container 1 according to this embodiment, high-quality recycled resin can be obtained even when such contents are stored.

The container 1 is designed to squeeze the body 14 etc., through a spout 12 with a screw thread 11, and the spout 12 is usually sealed with a cap 13. The container 1 is a blow molded body formed by blow molding. Details of blow molding will be described later.

Container 1 has a multi-layer structure. Figure 2A shows an example of the layer structure of container 1, which comprises, from the inside surface of container 1, an inner layer 2, an intermediate layer 3, and an outer layer 4. Intermediate layer 3 is water-soluble, and inner layer 2 and outer layer 4 have different specific gravities.

The intermediate layer 3 is preferably composed of a resin composition containing a water-soluble resin. The water-soluble resin is a resin that can be dissolved to 1% by weight (preferably 2, 3, 4, or 5% by weight) or more by immersion and stirring in water at 100°C (preferably 20, 30, 40, 50, 60, 70, 80, or 90°C).

Examples of water-soluble resins include saponified polyvinyl acetate, polyvinylpyrrolidone, polyethylene glycol, polyacrylamide, polymethacrylamide, polyhydroxyethyl methacrylate, polypentaerythritol triacrylate, polypentaerythritol tetraacrylate, polydiethylene glycol diacrylate, copolymers of the monomers that constitute these, and copolymers of 2-methacryloyloxyethyl phosphorylcholine and other monomers (e.g., butyl methacrylate, etc.). Among these, a structure consisting of one or more selected from saponified polyvinyl acetate, polyvinylpyrrolidone, and polyethylene glycol and a functional group described below is preferred.

Examples of saponified polyvinyl acetate include polyvinyl alcohol or copolymers of vinyl alcohol with other compounds, and saponified vinyl acetate modified with hydrophilic groups, hydrophobic groups, anions, cations, amide groups, or reactive groups such as acetoacetyl groups, and vinyl alcohol. The degree of saponification of the saponified polyvinyl acetate is not particularly limited, but is preferably 20 to 100 mol % of the total polyvinyl acetate, and more preferably 50 to 95 mol %.

The water-soluble resin is preferably butenediol vinyl alcohol copolymer (BVOH). This is because BVOH has excellent gas barrier properties and water solubility. An example of a commercially available BVOH product is Nichigo G Polymer (manufactured by Mitsubishi Chemical Corporation).

The resin composition constituting the intermediate layer 3 may contain additives or resins other than water-soluble resins to the extent that the water-solubility of the intermediate layer 3 is not impaired.

The inner layer 2 is closer to the inner surface of the container 1 than the intermediate layer 3, and may be a single layer or a multilayer structure. In this embodiment, the inner layer 2 includes, in order from the inner surface of the container 1, an innermost layer 2a and an adhesive resin layer 2b. The outer layer 4 is closer to the outer surface of the container 1 than the intermediate layer 3, and may be a single layer or a multilayer structure. In this embodiment, the outer layer 4 includes, in order from the inner surface of the container 1, an adhesive resin layer 4a, a repro layer 4b, and an outermost layer 4c. The inner layer 2 and the outer layer 4 may include layers other than those shown here, or may not include some of the layers shown here. For example, when the adhesion between the inner layer 2 or the outer layer 4 and the intermediate layer 3 is high, the adhesive resin layers 2b and 4a can be omitted.

The innermost layer 2a and the outermost layer 4c can be composed of a resin composition containing a resin (preferably a thermoplastic resin). Examples of the resin include polyolefins (e.g., polyethylene, polypropylene), polyesters (e.g., PET), EVOH, and nylon. The resin composition may contain only a resin, or may contain a resin and an additive. Examples of the additive include a lubricant and a filler.

The adhesive resin layers 2b and 4a are composed of an adhesive resin. Examples of adhesive resins include acid-modified polyolefin resins (e.g., maleic anhydride-modified polyethylene and maleic anhydride-modified polypropylene). By providing the adhesive resin layers 2b and 4a, the adhesion between the layers adjacent to the adhesive resin layers 2b and 4a is improved.

In one or both of the inner layer 2 and the outer layer 4, the layer adjacent to the intermediate layer 3 (hereinafter, the "adjacent layer") is preferably foamed. In this embodiment, the adjacent layer of the inner layer 2 is the adhesive resin layer 2b, and the adjacent layer of the outer layer 4 is the adhesive resin layer 4a. When the adjacent layer is foamed, water is more likely to penetrate into the intermediate layer 3 through the adjacent layer in the dissolution process described below, and dissolution of the intermediate layer 3 is promoted. The adjacent layer (adhesive resin layer 2b, 4a) can be foamed by mixing a foaming agent with the resin (adhesive resin). An example of a commercially available foaming agent is PEX-CF40E-J (manufactured by Tokyo Ink Co., Ltd.). The above effect is achieved even when only one of the adjacent layers of the inner layer 2 and the outer layer 4 is foamed, but the above effect is more pronounced if both the adjacent layers of the inner layer 2 and the outer layer 4 are foamed.

The reproductive layer 4b is composed of a resin composition containing a reproductive material obtained by recycling scrap generated during the molding of the container 1. The scrap contains all layers of the container 1, so the reproductive material is a mixture of the resin compositions that make up each of the layers of the container 1.

The specific gravity of the inner layer 2 may be greater or less than that of the outer layer 4. The difference in specific gravity between the inner layer 2 and the outer layer 4 can be achieved by varying the specific gravity of the resin used or by varying the type or amount of additives. It is preferable that the specific gravity of one of the inner layer 2 and the outer layer 4 is less than 1 and the specific gravity of the other is greater than 1. In this case, one of the inner layer 2 and the outer layer 4 floats on the water surface and the other sinks in the water, making specific gravity selection easy. The difference in specific gravity between the inner layer 2 and the outer layer 4 is, for example, 0.05 to 1.2, and preferably 0.1 to 0.4. Specifically, the difference in specific gravity may be, for example, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, or 1.2, and may be within a range between any two of the numerical values exemplified here.

The specific gravity of one of the inner layer 2 and the outer layer 4 is, for example, 0.8 to 0.99, specifically, for example, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, and may be within a range between any two of the numerical values exemplified here. The specific gravity of the other of the inner layer 2 and the outer layer 4 is, for example, 1.01 to 2, specifically, for example, 1.01, 1.05, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, and may be within a range between any two of the numerical values exemplified here.

To make the specific gravity of the inner layer 2 greater than that of the outer layer 4, the resin for the inner layer 2 can be one with a greater specific gravity than that of the outer layer 4. EVOH and nylon usually have a greater specific gravity than polyolefins (e.g., polyethylene and polypropylene), so when the outer layer 4 contains polyolefin and the inner layer 2 contains EVOH or nylon, the specific gravity of the inner layer 2 can be greater than that of the outer layer 4. For example, the resin for the innermost layer 2a can be EVOH or nylon, and the resin for the outermost layer 4c can be polyolefin.

In addition, even if the specific gravity of the resin in the inner layer 2 is the same as that of the resin in the outer layer 4, the specific gravity of the inner layer 2 can be made greater than that of the outer layer 4 by increasing the content of additives (e.g., talc, calcium carbonate) that have a greater specific gravity than the resin in the inner layer 2 than in the outer layer 4.

The resin composition constituting the inner layer 2 is preferably one that contains a biodegradable resin, a material mainly made of cellulose nanofiber (e.g., "Mapuka" (manufactured by Environmental Management Research Institute Co., Ltd.), treated as paper), or a material mainly made of calcium carbonate (e.g., "LIMEX" (manufactured by TBM Co., Ltd.), treated as limestone paper). Biodegradable resins have a small environmental impact because they can be composted, and materials mainly made of cellulose nanofiber or calcium carbonate are treated as non-plastic, making them easy to dispose of and therefore less of a burden on the environment.

To make the specific gravity of the inner layer 2 smaller than that of the outer layer 4, the resin for the inner layer 2 can be one with a smaller specific gravity than that of the outer layer 4. Since polyesters such as PET usually have a larger specific gravity than polyolefins (e.g., polyethylene, polypropylene), when the outer layer 4 contains polyester and the inner layer 2 contains polyolefin, the specific gravity of the inner layer 2 can be made larger than that of the outer layer 4. For example, the resin for the innermost layer 2a can be polyolefin, and the resin for the outermost layer 4c can be polyester.

1-2. Manufacturing method of plastic container 1 The container 1 can be formed by blow molding of a parison. The blow molding may be direct blow molding or injection blow molding. In direct blow molding, a molten cylindrical parison extruded from an extruder is sandwiched between a pair of split dies and air is blown into the parison to manufacture the container 1. In injection blow molding, a test tube-shaped parison with a bottom, called a preform, is formed by injection molding, and blow molding is performed using this parison.

In either blow molding method, the layer structure of the parison is the same as that of the container 1. A multi-layer parison can be formed by co-extrusion molding, multi-layer injection molding, etc.

When container 1 is formed by direct blow molding, the portion of the cylindrical parison within the cavity formed by a pair of split molds becomes container 1, and the portion other than container 1 becomes scrap. This scrap contains the constituent materials of all the layers that make up container 1. Reproduction material is obtained by subjecting this scrap to recycling processes such as crushing.

1-3. Method for Recycling Plastic Containers 1 The method for recycling plastic containers 1 according to one embodiment of the present invention includes a crushing step, a dissolving step, and a gravity separation step.

In the crushing step, as shown in FIG. 2B, the container 1 is crushed to form a plurality of crushed pieces 1p. Each crushed piece 1p has an inner layer 2p, an intermediate layer 3p, and an outer layer 4p. The inner layer 2p, the intermediate layer 3p, and the outer layer 4p correspond to the inner layer 2, the intermediate layer 3, and the outer layer 4 of the container 1. The size of the crushed pieces 1p is not particularly specified, and it is preferable that the size is small enough that the intermediate layer 3 can be dissolved in the dissolution step.

At the beginning of the crushing process, the container 1 is crushed with the inner layer 2 and the outer layer 4 still attached to each other, forming crushed pieces 1p. Then, due to shear force, a part of the crushed pieces 1p is peeled off and separated into the inner layer 2p and the outer layer 4p. At this time, the middle layer 3p usually adheres to the inner layer 2p or the outer layer 4p.

The pulverized pieces 1p are discharged from the pulverizer through a mesh. The discharged pulverized pieces 1p can be classified into the following five types.
(a) outer layer + intermediate layer (b) outer layer (c) inner layer + intermediate layer (d) inner layer (e) inner layer + outer layer + intermediate layer

Of these, for (e), it is essential to separate the inner layer 2p and the outer layer 4p in the dissolving process, and for (a) and (c), it is desirable to dissolve the intermediate layer 3p in the dissolving process, but the dissolving process is not essential. On the other hand, the dissolving process is a time-consuming process, and it is desirable to minimize the amount of pulverized pieces 1p to be input into the dissolving process. For this reason, it is desirable to perform preliminary separation on the pulverized pieces 1p discharged from the pulverizer and input only a portion of the pulverized pieces 1p into the dissolving process. The proportion of the pulverized pieces 1p to be input into the dissolving process is, for example, 5 to 95 mass%, and preferably 30 to 90 mass%. Specifically, this proportion is, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95 mass%, and may be within a range between any two of the numerical values exemplified here.

Preliminary separation can be performed, for example, based on the ease with which each layer can be crushed. In general, the lower the tensile modulus of the material to be crushed, the more easily the material is stretched when cut and the harder it is to cut. For this reason, the lower the tensile modulus of the material to be crushed, the longer the time it takes for the material to be crushed and pass through the mesh at the exit of the crusher. Therefore, by making the tensile modulus of the inner layer 2 and the outer layer 4 different from each other, preliminary separation can be performed depending on the timing of discharge from the crusher.

For example, when the tensile modulus of the outer layer is lower than that of the inner layer, the timing of discharge from the pulverizer is generally in the order of (d), (e), and (b). The timing of discharge of (a) and (c) varies depending on the tensile modulus of the intermediate layer. For example, when the tensile modulus of the intermediate layer is a value between that of the inner layer and that of the outer layer, the timing of discharge from the pulverizer is generally in the order of (d), (c), (e), (a), and (b). As described above, (d) and (b) do not need to be sent to the dissolving process, and (c) and (a) do not necessarily need to be sent to the dissolving process. For this reason, the amount of pulverized pieces 1p input to the dissolving process can be reduced by measuring the elapsed time from a predetermined start point (e.g., the start point of pulverization, the point at which the first pulverized pieces 1p pass through the mesh, etc.) and inputting only the pulverized pieces 1p that pass through the mesh between the first threshold value and the second threshold value (the second threshold value > the first threshold value) into the dissolving process. The first threshold value and the second threshold value can be appropriately selected according to the physical properties required for the recycled material. For example, if a slightly lower purity of the recycled material is acceptable, the time between the first and second thresholds can be shortened to reduce the amount of pulverized pieces 1p fed into the dissolving process. Even if the relationship between the tensile modulus of each layer is different from the above example, the amount of pulverized pieces 1p fed into the dissolving process can be reduced by feeding only the pulverized pieces 1p that have passed through the mesh during a specific time.

The tensile modulus can be measured in accordance with JIS K 7139 1A. The tensile modulus may be higher for either the inner layer 2 or the outer layer 4, but it is preferable for the inner layer to have a higher modulus. This is because it is desirable to have more outer layers than inner layers since the contents adhere to the inner layer, but materials with high specific gravity are generally expensive, and increasing the specific gravity of the outer layer increases the amount of expensive material used.

The difference in tensile modulus is, for example, 700 MPa or more, and preferably 700 to 3000 MPa. This difference is, for example, 700, 800, 900, 1000, 1500, 2000, 2500, 3000 MPa, and may be within the range between any two of the numerical values exemplified here or any more. The tensile modulus of the inner layer 2 or the outer layer 4, which has a lower tensile modulus, is, for example, 300 MPa or less, and preferably 50 to 300 MPa. This bending modulus is, for example, 50, 100, 150, 200, 250, 300 MPa, and may be within the range between any two of the numerical values exemplified here or any more. The tensile modulus of the inner layer 2 or the outer layer 4, which has a higher tensile modulus, is, for example, 1000 MPa or more, and preferably 1000 to 3300 MPa. Specifically, the tensile modulus is, for example, 1000, 1500, 2000, 2500, 3000, or 3300 MPa, and may be within a range between any two of the values exemplified here, or any value equal to or greater than any of them.

In the dissolution process, as shown in Figures 2B to 2C, the middle layer 3p contained in the pulverized pieces 1p is dissolved to separate the inner layer 2p and the outer layer 4p. The dissolution process can be carried out, for example, by putting the pulverized pieces 1p into water and stirring. The dissolution process can be carried out simultaneously with washing the pulverized pieces 1p.

In the gravity sorting process, the inner layer 2p and the outer layer 4p are sorted based on their specific gravities. In gravity sorting, when the inner layer 2p and the outer layer 4p are placed in a liquid having a specific gravity between the inner layer 2p and the outer layer 4p, one of the inner layer 2p and the outer layer 4p floats on the liquid surface and the other sinks in the liquid, so that the inner layer 2p and the outer layer 4p can be sorted and recovered. When the specific gravity of one of the inner layer 2p and the outer layer 4p is less than 1 and the specific gravity of the other is greater than 1, it is preferable to use water as the liquid.

The outer layer 4p is a mono-material, and is free of dirt and odor from the contents, making it a high-quality recycled material suitable for bottle-to-bottle material recycling. Material recycling is a recycling method in which used plastic products are collected, crushed, washed, etc., and re-used into raw materials, which are then remanufactured into new plastic products.

On the other hand, the inner layer 2p can be used for heat recovery (thermal recycling) or cascade recycling (recycling into products with reduced quality). In addition, if the inner layer 2p is made of biodegradable resin, it can be treated as compost. Furthermore, if the inner layer 2p is made of a material mainly made of cellulose nanofiber or calcium carbonate, it can be disposed of as non-plastic.

2. Second embodiment A plastic container 1 according to a second embodiment of the present invention will be described with reference to Fig. 3. The plastic container 1 according to this embodiment has a layer structure different from that of the first embodiment. The following description will focus on the differences.

This embodiment is characterized in that one or both of the inner layer 2 and the outer layer 4 are adjacent to the intermediate layer 3 (hereinafter, "adjacent layer") and do not contain adhesive resin. In this embodiment, the inner layer 2 does not include an adhesive resin layer 2b, and the inner layer 2 that does not contain adhesive resin is in contact with the intermediate layer 3 as an adjacent layer. The inner layer 2 can be composed of the same resin composition as the innermost layer 2a of the first embodiment. The outer layer 4 does not include an adhesive resin layer 4a, and the repro layer 4b that does not contain adhesive resin is in contact with the intermediate layer 3 as an adjacent layer. The description of the repro layer 4b is the same as in the first embodiment.

When the adjacent layer in either or both of the inner layer 2 and the outer layer 4 does not contain an adhesive resin, water easily penetrates between the adjacent layer and the intermediate layer 3 in the dissolution process, and dissolution of the intermediate layer 3 is promoted. The above effect is achieved even when the adjacent layer in only one of the inner layer 2 and the outer layer 4 does not contain an adhesive resin, but the above effect is more pronounced when the adjacent layer in both the inner layer 2 and the outer layer 4 does not contain an adhesive resin. In addition, the adjacent layer in either or both of the inner layer 2 and the outer layer 4 may be foamed as in the first embodiment. In this case, dissolution of the intermediate layer 3 is promoted even more significantly.

1: Plastic container 1p: Crushed pieces 2: Inner layer 2a: Innermost layer 2b: Adhesive resin layer 2p: Inner layer 3: Intermediate layer 3p: Intermediate layer 4: Outer layer 4a: Adhesive resin layer 4b: Reproduction layer 4c: Outermost layer 4p: Outer layer 11: Thread 12: Spout 13: Cap 14: Body

Claims (2)

A method for recycling plastic containers, comprising the steps of:
The container includes, in order from the inner surface side of the container, an inner layer, an intermediate layer, and an outer layer,
The intermediate layer is water-soluble,
The inner layer and the outer layer have different specific gravities,
The method includes a crushing step, a preliminary separation step, a dissolving step, and a gravity separation step,
In the crushing step, the plastic container is crushed by a crusher to form a plurality of crushed pieces,
In the preliminary separation step, the plurality of pulverized pieces are separated depending on the timing at which the plurality of pulverized pieces are discharged from the pulverizer in the pulverization step, and only a portion of the plurality of pulverized pieces is input into the dissolving step;
In the dissolving step, the intermediate layer contained in the pulverized pieces is dissolved to separate the inner layer and the outer layer,
The method, wherein the gravity separation step separates the inner layer and the outer layer based on their specific gravities. The recycling method according to claim 1,
The preliminary separation step comprises making the tensile modulus of elasticity of the inner layer and the outer layer different from each other, thereby making the timing at which the plurality of pulverized pieces are discharged from the pulverizer different from each other, thereby performing preliminary separation.