In order to prevent oxygen oxidation and store various types of articles, represented by foods, beverages, medicinal products, cosmetics, etc., which easily deteriorate or degrade under the effect of oxygen for a long time, oxygen absorbents are used for removing oxygen within packaging bodies storing these articles.
As the oxygen absorbent, an oxygen absorbent containing an iron powder as a reactive main component is generally used in view of oxygen-absorbing ability, handling and safety. However, the iron-based oxygen absorbent is responsive to a metal detector and thus it is difficult to use a metal detector in inspecting foreign matter. Furthermore, packaging bodies containing an iron-based oxygen absorbent have a risk of ignition, and thus, they cannot be heated by a microwave oven. Moreover, the oxidation reaction of an iron powder requires water, and thus, an oxygen-absorbing effect is exerted only on an article to be packaged rich in moisture content.
Packaging containers are developed by making the container of a multilayer material having an oxygen-absorbing layer formed of an oxygen-absorbing resin composition containing a thermoplastic resin and an iron-based oxygen absorbent, thereby improving a gas barrier property of the container and providing an oxygen-absorbing function to the container itself (see, Patent Literature 1). To describe more specifically, the oxygen-absorbing multilayer film is used in the form having an oxygen-absorbing layer, which is optionally a thermoplastic resin layer having an oxygen absorbent dispersed in an intermediate layer formed of a thermoplastic resin, between layers of a conventional gas barrier multilayer film formed by stacking a heat sealing layer and a gas barrier layer, thereby adding a function of absorbing oxygen within the container to a function of preventing oxygen transmission from outside, and is manufactured by use of a conventional manufacturing method known in the art such as extrusion lamination, coextrusion lamination and dry lamination. However, such an oxygen-absorbing multilayer film has the same problems: a metal detector for inspecting foreign matter for foods etc. cannot be used; heating cannot be made by a microwave oven; and the effect is only exerted on an article to be packaged rich in moisture content. In addition, the multilayer film has a problem of opacity, leading to insufficient visibility of content. A multilayer film using an oxygen absorbent such as an iron powder, has problems: the film is detected by a metal detector used in inspection of foreign matter in foods etc.; the film is opaque, leading to insufficient visibility of content; and if an alcohol beverage is contained, iron reacts with alcohol to produce aldehyde, reducing taste and flavor.
In the aforementioned circumstances, it has been desired to develop an oxygen absorbent containing an organic substance as a reactive main component. As the oxygen absorbent containing an organic substance as a reactive main component, an oxygen absorbent containing ascorbic acid as a main component is known (see, Patent Literature 2).
In the meantime, an oxygen-absorbing resin composition composed of a resin and a transition metal catalyst is known. For example, a resin composition composed of a polyamide as an oxidizable organic component (in particular, a xylylene group-containing polyamide) and a transition metal catalyst, is known (see, Patent Literatures 3 and 4). In Patent Literatures 3 and 4, articles obtained by molding such a resin composition, such as an oxygen absorbent, a packaging material and a multilayer laminated film for packaging are further exemplified.
As an oxygen-absorbing resin composition requiring no moisture content for absorbing oxygen, an oxygen-absorbing resin composition composed of a resin having a carbon-carbon unsaturated bond and a transition metal catalyst, is known (see, Patent Literature 5).
As a composition for trapping oxygen, a composition composed of a polymer containing a substituted cyclohexene functional group or a low molecular-weight substance bound with the cyclohexene ring and a transition metal is known (see, Patent Literature 6).
In the meantime, injection molding, by which molded articles having a complicate-shape can be manufactured in a high yield, has been used for manufacturing a wide variety of products including machine parts, automotive parts, electric/electronic parts, containers for foods or medical products, etc. Recently, as packaging containers, variety types of plastic containers have been widely used because they have advantages of light-weight, transparency, moldability, etc. As a typical plastic container for a beverage, an injection-molded article having a screw thread cutting on the bottle neck designed to sufficiently screw the lid, has been frequently used.
As a material for use in injection-molded articles, general thermoplastic resins such as a polyolefin (polyethylene, polypropylene, etc.), a polyester and a polystyrene are mentioned. Particularly, injection-molded articles mainly formed of a polyester such as polyethylene terephthalate (PET) are used in a wide variety of plastic containers for beverages such as tea, fruit juice beverages, carbonated beverages and alcohol beverages. However, although an injection-molded article mainly formed of a thermoplastic resin is excellent as a packaging material, oxygen tends to easily transmit from the outside, unlike glass bottles and metal containers. Thus, even if a content is packed and hermetically closed therein, the storage stability of the content is still questioned. Accordingly, injection-molded articles having a gas barrier layer as an intermediate layer in order to provide a gas barrier property to such injection-molded articles composed of a general resin have been put into practical use.
In the meantime, as medical packaging containers for packaging and storing a drug solution in a sealed condition, glass ampoules, vials, prefilled syringes, etc. have been conventionally used. However, these glass containers have problems: sodium ion etc. elute off from the container to a liquid content stored therein; and micro substances called flakes generate; when a light-blocking glass container colored with a metal is used, the content is contaminated with the coloring metal; and the container is easily broken by drop impact. In addition to these problems, since glass containers have a relatively large specific gravity, medical packaging containers become heavy. For these reasons, development of alternate materials has been desired. To be more specific, materials lighter than glass, such as a polyester, a polycarbonate, a polypropylene and a cycloolefin polymer, have been investigated as glass alternatives.
For example, a medical container formed of a polyester resin material is proposed (see, Patent Literature 7).
In the meantime, a multilayer container having a gas barrier layer as an intermediate layer in order to provide a gas barrier property to a container made of plastic, has been investigated. Specifically, a prefilled syringe improved in oxygen barrier property by constituting the innermost layer and the outermost layer formed of a polyolefin resin and an intermediate layer formed of a resin composition excellent in oxygen barrier property is proposed (see, Patent Literature 8). Other than this, multilayer containers obtained by laminating a gas barrier layer formed of e.g., a polyamide (hereinafter, sometimes referred to as “nylon MXD6”), which is obtained from metaxylylenediamine and adipic acid, an ethylene-vinyl alcohol copolymer, a polyacrylonitrile, a poly(vinylidene chloride), an aluminum foil, a carbon coat or a vapor-deposited inorganic oxide, on a resin layer, have been investigated.
In recent years, it has been proposed that a small amount of transition metal compound is added to nylon MXD6 and mixed to provide an oxygen-absorbing function and the resultant material is used as an oxygen barrier material constituting containers and packaging materials (see, Patent Literature 9).
As a method for improving storage stability of foods etc. and preventing degradation of taste and flavor thereof, a technique of charging a deoxidized nitrogen gas in a package is known. In the technique, a metal can or a glass bottle is filled with e.g., an alcohol beverage such as sake, wine and shochu, a fruit juice, a vegetable juice, a broth or a tea beverage, and then, filled with nitrogen gas and sealed. However, metal cans and glass bottles inevitably have a problem of a non-combustible waste treatment. In addition, reducing weight is still demanded. Particularly when a metal can is used, elution of a metal component into the content is a problem. Because of this, alternation of metal cans and glass bottles to plastic containers such as a gas barrier multilayer container, has been widely investigated also in the food fields.