JPH0346490B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for improving the gas barrier properties of a packaging container, and more specifically, the present invention relates to a method for improving the gas barrier properties of a packaging container, and more specifically, the present invention relates to a method for improving the gas barrier properties of a packaging container, and more specifically, the present invention relates to a method for improving the gas barrier properties of a packaging container. Concerning methods for improving barrier properties. (Prior art) Ethylene-vinyl alcohol copolymer can be melt-molded and has gas permeation resistance (gas barrier property).
As a thermoplastic resin with excellent properties, it is widely used in the field of packaging plastic containers. However, metal materials used for canned goods and glass used for bottling have almost zero permeability to gases such as oxygen, whereas ethylene-vinyl alcohol copolymers have almost zero permeability to gases such as oxygen. It still permeates a non-negligible amount of gas, and there is a strong desire to improve its gas barrier properties. As seen in Japanese Patent Publication No. 57-48459, the gas barrier properties of ethylene-vinyl alcohol copolymers can be improved by heat-treating them so that they have a main endothermic peak and a sub-endothermic peak within a specific temperature range. It is known to do. (Problems to be Solved by the Invention) The present invention is intended to improve the gas barrier properties of packaging materials using the above-mentioned ethylene-vinyl alcohol copolymer, and to improve the gas barrier properties in a simple manner. The technical problem is to provide a method that can be carried out by means. (Means for Solving the Problems) According to the present invention, by irradiating a plastic packaging material provided with a gas barrier layer containing an ethylene-vinyl alcohol copolymer with ionizing radiation, Improve barrier properties. (Function) The present invention is based on the novel finding that when an ethylene-vinyl alcohol copolymer is irradiated with ionizing radiation, gas permeability to oxygen and the like can be significantly reduced. FIG. 1 of the attached drawings is based on Example 1, which will be described later.
A laminated tube consisting of a 43 micron thick middle layer of ethylene-vinyl alcohol copolymer with an ethylene content of 30 mol% and a saponification degree of 99.5%, and a 386 micron thick inner and outer layer of low density polyethylene with a density of 0.926 g/cc. Containers that have not been irradiated,
The standing time and oxygen permeability (Qo ₂ , cc/m ²
atm.day). In Figure 1, the oxygen permeability (Qo ₂ ) increases with time even in the unirradiated container because the ethylene-vinyl alcohol copolymer absorbs moisture over time, and this moisture absorption increases the oxygen permeability. It is. According to the results shown in Figure 1, the oxygen permeability in the unirradiated container was approximately 1.3 cc/m ² . The oxygen permeability in the electron beam irradiation container is about 0.2 cc/m ² . The surprising fact becomes clear that the oxygen permeability is on the order of atm.day, and the oxygen permeability is on the order of an order of magnitude lower. In electron beam irradiated containers, the oxygen permeability tends to increase over time, but within 3 months, which is the shelf life of food packed in plastic containers, the oxygen permeability is suppressed to a lower level than in non-irradiated containers. That is clear. Figure 2 shows the relationship between the cumulative amount of oxygen permeation and the elapsed time for the unirradiated containers and electron beam irradiated containers used for the measurements in Figure 1. It can be seen that during the storage period, the amount of oxygen permeation in the electron beam irradiated container was suppressed to a lower value than in the unirradiated container. In the present invention, the oxygen permeability of the ethylene-vinyl alcohol copolymer is improved by irradiation with ionizing radiation without significantly reducing the mechanical properties of the copolymer. Although the reason for this has not yet been elucidated, it is presumed to be as follows. It is generally known that the behavior of polymeric materials when irradiated with ionizing radiation is broadly classified into collapsed polymers and crosslinked polymers. A typical example of the former is polyvinyl alcohol, and a typical example of the latter is polyethylene. Whether these polymers are collapsed or cross-linked, polymer radicals are formed by the abstraction of hydrogen atoms, and when the polymer radicals recombine, they form a cross-linked structure, allowing molecular chain scission without recombination. Some things occur. In the ethylene-vinyl alcohol copolymer that is the subject of the present invention, the coexistence of crosslinked ethylene repeating units and collapsible vinyl alcohol repeating units suppresses the collapse of the polymer chain. It is believed that oxygen permeation is suppressed by the radicals being stored relatively stably and the polymer radicals reacting with permeating oxygen. This estimation agrees well with the fact that when an ethylene-vinyl alcohol copolymer is heated to 100°C after irradiation, its oxygen permeability becomes the same level as that of an unirradiated copolymer. (Operations and Effects of the Invention) According to the present invention, the gas permeability to oxygen and the like can be significantly improved by a simple operation of irradiating a packaging material containing an ethylene-vinyl alcohol copolymer with ionizing radiation. It becomes possible. In addition, the method of the present invention improves oxygen barrier properties and sterilizes the packaging material at the same time.
It can be advantageously applied to the so-called azeptic filling method in which the contents are sterilized outside the container and then filled into a sterile container. (Description of preferred embodiments of the invention) As the ethylene-vinyl alcohol copolymer, a copolymer having an ethylene content of 20 to 70 mol%, particularly 30 to 50 mol% is advantageously used. If the ethylene content is higher than the above range, the oxygen permeability coefficient of the copolymer itself becomes large, which is not preferable for the purpose of the present invention.On the other hand, if the ethylene content is lower than the above range, the copolymer tends to disintegrate when irradiated with radiation. This is undesirable because the mechanical properties deteriorate significantly. The molecular weight of the ethylene-vinyl alcohol copolymer may be within a film-forming range. The packaging material used in the present invention may be made of ethylene-vinyl alcohol copolymer alone, or may be made of ethylene-vinyl alcohol copolymer and other thermoplastic resins such as polyolefin, polyamide,
It can be a blend with polyester or the like, or a laminate of an ethylene-vinyl alcohol copolymer and another thermoplastic resin. In the present invention, it is preferable to use a moisture-resistant thermoplastic resin as the inner and outer layers in the form of a sandwich sandwiched laminate with respect to the intermediate layer containing the ethylene-vinyl alcohol copolymer. In other words, by creating a structure in which the moisture-resistant resin is sandwiched, direct contact between the ethylene-vinyl alcohol copolymer and oxygen is prevented during radiation irradiation, and the disintegration of the copolymer is suppressed. , it is possible to suppress a decrease in the oxygen permeability coefficient due to moisture absorption of the copolymer. As the moisture-resistant resin, a thermoplastic polymer having a water absorption rate of 3.5% or less, preferably 2.5% or less when left in an atmosphere of 23°C and 50% relative humidity (RH) for 5 days, such as the aforementioned polyethylene or polypropylene, is used. , polyolefins such as ethylene-vinyl acetate copolymers and ethylene-butene 1 copolymers, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polycarbonates, polyamides, acrylonitrile-styrene-butadiene copolymers, and methyl to the above copolymers. It is preferable to use a thermoplastic polymer such as a nitrile resin such as a graft polymer of methacrylic acid, an acrylonitrile-butadiene copolymer, or a graft polymer of methyl methacrylic acid on an acrylonitrile-styrene copolymer. It should be noted that since it is generally difficult to directly bond a layer of ethylene-vinyl alcohol copolymer alone to a layer of a low water absorption thermoplastic polymer such as the polyolefin, both layers may be bonded using an isocyanate-based adhesive or an epoxy-based adhesive. Ethylene-acrylic acid copolymer, polyester ionomer for adhesive, maleic anhydride grafted polyethylene,
extruding a special adhesive layer of a carbonyl group-containing polymer such as maleic anhydride grafted polypropylene, maleic anhydride grafted ethylene-vinyl acetate copolymer, maleic anhydride grafted ethylene-propylene copolymer as an intervening layer, or It is desirable to blend a small amount of the carbonyl group-containing polymer described above in advance into the layer of polyolefin or the like. For the intermediate layer, in addition to the ethylene-vinyl alcohol copolymer alone, a blend of the copolymer and other resins can be used. As such a thermoplastic polymer, ethylene-vinyl alcohol copolymer There are many thermoplastic polymers that can be kneaded with polymers and melt-molded into films, such as polyolefin polymers or those containing at least one polar group such as a carbonyl group, a hydroxyl group, or an ether group. Any suitable thermoplastic polymer or combination thereof may be used. A specific example will be given and explained below. (i) Polyolefin polymer; low density, medium density or high density polyethylene, polypropylene, ethylene-propylene copolymer, polybutene-1, polypentene-1,
Poly-4-methylpentene-1. (ii) A thermoplastic polymer containing at least one of a carbonyl group, a hydroxyl group, or an ether group; examples of carbonyl group-containing polymers include carboxylic acids, carboxylic acid salts, carboxylic acid anhydrides, carboxylic esters, carboxylic acid amides, carbonic acid ester,
Carbonyl group based on urea or urethane, etc.
120-1400meq/100g polymer, especially 150-1400meq/100g polymer
Polymers containing a concentration of 1200 meq/100 g of polymer are preferably used, and these polymers may contain ether groups or hydroxyl groups in addition to carbonyl groups. In the case of a heat-sealable multilayer film, bag, or squeeze container, the inner surface to be heat-sealed is made of low-density polyethylene, while the outer surface is made of a thermoplastic polymer having a higher melting point than the low-density polyethylene, such as polypropylene. It may also be made of polyester, polyamide, etc. Examples of suitable layer combinations for multilayer or laminated structures are as follows. Polyolefin/ethylene-vinyl alcohol copolymer/polyolefin, polyolefin/blend/polyolefin, polyolefin/blend/ethylene-vinyl alcohol copolymer/blend/polyolefin. In addition, for the purpose of adding other performances such as pressure resistance and heat resistance while basically utilizing the above configuration, (i)
Polyesters such as polyethylene terephthalate and polybutylene terephthalate, (ii) polypropylene, (iii) polycarbonate, (iv) acrylonitrile.
Styrene copolymer, acrylonitrile-butadiene copolymer or acrylonitrile-styrene-
A layer containing a thermoplastic resin such as a graft polymer of methyl methacrylic acid to a butadiene copolymer (ABS), (v) ABS, and (vi) polymethyl methacrylate may be provided by a coextrusion method. The packaging material can take the form of, for example, a single-layer or multi-layer film, a bag, a squeeze container, a bottle, a tube, a tank or other container. For example, packaging films are manufactured by extrusion, press molding, kander molding, cast molding, or any other method known per se. Bottles, tubes or other containers are manufactured by blow molding, injection molding, extrusion or cast molding or any other means known per se.
Further, a squeezed container can be obtained, for example, by vacuum forming a film or sheet that has been formed, and a bag-shaped container can be obtained by forming a once-formed film into a bag shape by heat sealing or adhesion. It is advantageous to use an electron beam as the ionizing radiation. As the electron beam source, an electron beam accelerator with an accelerating voltage of 100 to 3000 KV is used, and any device such as a scanning type device such as a Van de Graaf type, a curtain type device, or an electrocurtain can be used. can. The irradiation dose varies depending on the type and thickness of the copolymer, but generally speaking, the dose that produces the desired improvement is within the range of 0.5 to 30 megarads, particularly 1 to 15 megarads. All you have to do is decide. If it is lower than the above range, the improvement in gas barrier properties will be insufficient, while if it is higher than the above range, the physical properties will be greatly deteriorated. Other irradiation conditions may be varied over a wide range. For example, the dose rate can be varied, for example, in the range of 0.01 to 50 Mrad/sec, and the temperature during irradiation can be varied as appropriate, but room temperature is generally sufficient. Generally, irradiation time of 1 second or less is sufficient. The atmosphere is
Although it is desirable to carry out the irradiation in an inert atmosphere such as nitrogen, there is no problem even if the irradiation atmosphere is in air. Polymerization curing by electron beam irradiation is desirable from the viewpoint of workability, but gamma rays from radioactive isotopes such as cobalt-60 and cesium-137 can also be used as a radiation source. (Example) Ethylene content 30 as oxygen barrier resin
An ethylene-vinyl alcohol copolymer (A) with an ethylene content of 44 mol% and a vinyl alcohol content of 56 mol% was used. , as a moisture-resistant resin, melting point 105℃, melt index (MI) 0.5g/10min, density 0.91g/cm ³
(20℃) low-density polyethylene (C) is used as an adhesive layer between the oxygen barrier resin and moisture-resistant resin, melting point 102℃, MI 2.0g/10min, density 0.926g/
cm ³ (20°C) acid-modified low-density polyethylene manufactured by Mitsubishi Yuka was molded using the molding equipment shown below. Extruder for oxygen barrier resin layer with built-in screw with diameter 40mm and effective length 800mm, diameter 35mm,
Adhesive interlayer extruder equipped with a built-in screw with an effective length of 700 mm and an adapter with two branched melt channels, and an extruder with a diameter of 65 mm.
Multilayer extrusion machine consisting of an extruder for polyfin resin layers (inner and outer layers) equipped with a built-in screw with an effective length of 700 mm and an adapter with a melt channel branched into two, and a die for symmetrical five-layer extrusion. Using a machine, a molten multilayer parison having the layer structure shown in Table 1 is formed, and this parison is sandwiched between molds cooled to 10℃ and blow-molded at a blow pressure of 6 kg/cm ² to obtain the contents. A multilayer tube container containing 120 g of material was obtained. The obtained container was irradiated with an electron beam of 15 Mrad at an accelerating voltage of 300 KV. Table 1 shows the measurement results of changes in oxygen permeability over time. Example 1 uses ethylene vinyl alcohol copolymer (A), Example 2 uses ethylene vinyl alcohol copolymer (B),
Those that were not irradiated were described as Comparative Examples 1 and 2, respectively.

[Table] Measurement method Oxygen permeability 100% RH inside the container, 20% RH outside the container at 37℃
Oxygen permeability by GC method

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between oxygen permeation and time for unirradiated and irradiated packaging materials.
FIG. 2 is a diagram showing the relationship between cumulative oxygen permeation value and time.

Claims (1)

[Scope of Claims] 1. A method for improving the gas barrier properties of packaging containers, which comprises irradiating ionizing radiation to a plastic packaging material provided with a gas barrier layer containing an ethylene-vinyl alcohol copolymer. 2. The method according to claim 1, wherein the ionizing radiation is an electron beam. 3. The method according to claim 1, wherein ionizing radiation is irradiated at a dose of 0.5 to 30 Mrad. 4. The method of claim 1, wherein the plastic packaging material is a laminate comprising an intermediate layer containing an ethylene-vinyl alcohol copolymer and inner and outer layers of moisture-resistant thermoplastic resin sandwiching the intermediate layer. 5. The method according to claim 1, wherein the ethylene-vinyl alcohol copolymer is an ethylene-vinyl alcohol copolymer having an ethylene content of 20 to 70 mol%.