AN OXYGEN ABSORBENT PACKAGE

Abstract: An oxygen absorbent package used for preserva­ tion or storage of foodstuff containing large amounts of water, alcohols or oils is partly or wholly formed with an air-permeable sheet (10) obtained by making a sheet from thermo plastic fibres by a papermaking process, drying the sheet, impregnating it with a fluorine resin and pressing the resin-impregnated sheet by heated rolls. The obtained package is proof against water and oils. Use of the air-permeable sheet (10) in various combinations with a woven reinforcing material (50), a porous low softening point resin layer (22), a composite 2-layer film (40) and a porous composite film (42) provides a package with improved properties, e.g. heat resistant. The oxygen absorbent package may be used for storage of a packed foodstuff which is heated for long-time preservation.

Description: This invention is directed to an oxygen absorbent package used for storage of foodstuff of high water content, foodstuff containing alcohols and foodstuff containing a large amount of oil, and is also directed to an oxygen absorbent package used for storage of retort foods. Use of oxygen absorbents is known as a technique for storage of foodstuff. According to this technique, foodstuff to be stored and an air-permeable package packed with an oxygen absorbent are enveloped in a gas-barrier packaging material and the sealed system is brought into a substantially oxygen-free state to thereby prevent oxidation of packed foodstuff and development of bacteria or mold on or in such foodstuff (See US Patents 4,113,652, 4,104,192, 4,119,472, 4,127,503, 4,166,807 and 4,192,773). This technique is used for storage of a variety of foods. There is also known a retort pouching technique for long-time preservation of foodstuff. This technique features heating of the packed foodstuff at 100-150°C for 2-80 min to kill the bacteria causative of putrefaction to thereby allow long-time storage of foodstuff. It has been, however, extremely difficult to exterminate even the spores of thermophilic bacteria in the retort food, and it has been quite probable that the spores of a bacterium germinate and grow in the retort food to cause its deterioration, or that the foodstuff, especially the oil or fat contained in the foodstuff is oxidized by oxygen present in the spaces in the retort pouch to change the taste or quality of foodstuff before the retort food is served to the consumers. If the foodstuff storage technique by use of oxygen absorbents can be applied to said retort foods, it becomes possible not only to inhibit germination and growth of the spores of thermophilic bacteria in retort foods but also prevent the oxidation of oil and fat and accordingly the change of taste of foodstuff, thus realizing an elongation of safe storage period of retort foods. Generally, when air-permeable packages used as oxygen absorbent packages are applied to the field of foodstuff, it is essential that such packages satisfy the requirements for waterproofness, resistance to breakage and hygienic safety in addition to air permeability. Further, in case such packages are used for storage of foodstuff of high water content, foodstuff containing alcohols or foodstuff containing a large amount of oil, they are required to have proofness against alcohols and oils beside waterproofness. Heat resistance is also required for the packages when they are used for storage of retort foods. As materials used for oxygen absorbent packages, there are known microporous films made of porous plastic materials, such as Celgard (Celanese Corp.), Nitto Flon NTF (Nitto Electric Industrial Co., Ltd). and NF sheet (Tokuyama Soda Chemical Co.) and nonwoven fabrics made of polyethylene fibre, polyamide fibre, polyester fibre, polypropylene fibre or the like, such as Tyvek (E.I. Du Pont) and Spubond (Asahi Kasei Co.,Ltd). These packaging materials, however, have the following defects. (1) When contacted with foodstuff containing oil or alcohol, said packaging materials are impregnated with oil or alcohol from the portion contacted therewith. (2) Since the sealing material is polyethylene having a low softening point, the sealed portion may peel off to break the package when the package is applied to retort foods and subjected to heat sterilization. (3) Oil or alcohol can penetrate into the packed substance to obstruct absorption of oxygen, making the oxygen absorbent unable to perform its food preserving activity. For the improvement of resistance to oils and alcohols, Japanese Patent Application Kokai (Laid-Open) No. 64959/83 and Abstract Bulletin of the Institute of Paper Chemistry 3794, Vol 54, No. 4, October 1983, disclose a method in which the packaging material composed of a microporous film or nonwoven fabric is impregnated or coated with a fluorine resin. This method, however, has the disadvantage that since the microporous film or nonwoven fabric used for the packaging material is hydrophobic, the fluorine resin applied does not spread all over the packaging material but merely stays in the surface of the packaging material, thus imposing a limitation to the degree of proofness against oils and alcohols that the packaging material can provide. Further, since there locally exist parts with large openings in said microporous film or nonwoven fabric, even if it is impregnated or coated with a fluorine resin, oil or alcohol enters from the parts with large openings and can infiltrate into the side of the packaging material or even into the oxygen absorbent packed therein. When an oxygen absorbent package using paper made from fluorine resin-added natural pulp as a part of packaging material is contacted with an oil-containing moist food such as doughnut or fried fish ball, oil soaks into the packaging material along with water in a relatively short time. Thus, it is difficult to provide such oxygen absorbent packages with satisfactory oil resistance in practical use. Japanese Utility Model Publication No. 22048/84 discloses an oxygen absorbent package comprising a laminate package made by bonding a microporous film and a sheet having a higher softening point than said film. This package, however, cannot be applied to retort foods containing an oil or alcohol because the microporous film is not provided with resistance to oils and alcohols. EP-A-151489 discloses an oxygen absorbent package which is partly formed with an air permeable sheet made from plastic fibre produced by a wet processing and drying which sheet can be treated with a water or oil repellent. It is the object of the present invention to provide an oxygen absorbent package which does not permit penetration of oil or alcohol into the package and can fulfill its function even if it is packed with retort foods containing an oil or alcohol along with water and subjected to heat sterilization. Said object is achieved by an oxygen absorbent package according to claims 1 to 18 and a retort food according to claims 19 and 20. It has been found that a sheet material obtained by wet processing plastic fibre such as polyethylene or polypropylene followed by drying has no water resistance and sucks up water when contacted with water. When this sheet material after drying is impregnated with a fluorine resin in the form of an emulsion or solution and dried, the fluorine resin is adhered to the whole of fibre, and when this sheet material is hot pressed, it is provided with not only air permeability and water resistance but also a high degree of resistance to alcohols and oils. The thus obtained sheet is suitable for use as a material for forming an oxygen absorbent package. It was further found that the incorporation of a specific reinforcing material for reinforcing the strength of said sheet can provide a sheet improved in strength against stress in the longitudinal direction. Further a package bonded with a porous composite film (b) made by laminating a resin layer having a lower softening point than said sheet and a resin layer having a higher softening point than said sheet and perforating the laminate is provided with high heat resistant, eliminating any fear of breakage even under high temperatures, in addition to air permeability and water resistance, and also has a high degree of resistance to alcohols and oils which cannot be expected from conventional packaging materials. Such a sheet can ideally serve as an oxygen absorbent package for retort foods. The present invention further provides an oxygen absorbent package using said sheet bonded with a porous composite film or films as a laminate, in which said sheet may be bonded to a single piece of porous composite film or may be sandwiched between the porous composite films, said film being a perforated laminate sheet consisting of a resin layer having a lower softening point than the plastic fibre forming said sheet and a resin layer having a higher softening point than said plastic fibre, said porous composite film being bonded at its low softening point resin layer side to said air-permeable sheet, and an oxygen absorbent is packed in the package with the air-permeable sheet side being positioned on the inside. FIG. 1 is a sectional view showing the structure of an embodiment of the oxygen ansorbent package according to this invention. The package is composed of an air-permeable sheet (incorporated with a fluorine resin) 10 and a composite film 40. The composite film 40 consists of a low softening point film layer 20 on the inside and a high softening point film layer 30 on the outside. FIG. 2 is a similar view to FIG. 1, showing a package comprising a laminate of a porous composite film 42 and an air-permeable sheet 10. FIG. 3 is a similar view to FIG. 1, showing a package composed of a laminate of an air-permeable sheet 10 and a reinforcing material 50 on one side and a composite film 40 on the other side. FIG. 4 is a similar view to FIG. 1, showing a package composed of a laminate of a porous composite film 42 and an air-permeable sheet 10 on one side and a composite film 40 on the other side. FIG. 5 is a similar view to FIG. 1, showing a package comprising a laminate of an air-permeable sheet 10 and a reinforcing material layer 50 which forms the inner layer. FIG. 6 is a similar view to FIG. 1, showing an oxygen absorbent package comprising a laminate of an air-permeable sheet 10 and a porous low softening point resin layer 22. FIG. 7 is a similar view to FIG. 1, showing a package comprising a laminate of porous composite film 42/air-permeable sheet 10/porous low softening point resin layer 22. FIG. 8 is a sectional view showing the structure of a package comprising a laminate of an air-permeable sheet 10 and a reinforcing material 50. The reinforcing material 50 is composed of weft yarns 60 and warp yarns 70. FIG. 9 shows an example of the structure of reinforcing material 50. In the drawings, 20 indicates a low softening point resin layer (e.g. polyethylene layer), 30 a high softening point film layer (e.g. polyester film layer), 40 a composite film, 42 a porous composite film (e.g. nylon/high-density polyethylene film (sealed side)), and 90 a powdered oxygen absorbent. The plastic fibre used in this invention can be produced by various methods. For example, a polymer solution is flushed in a non-solvent under high-speed stirring to form fibres; shear is applied to the polymer solution in a polymerizer to directly form fibres; fibres are formed by flush spinning and then beaten (flushing methods); or an extrusion molded sheet is drawn in the longitudinal direction, split and then beaten (drawing method). Flushing methods are preferred. As the plastic fibre forming the sheet (which is hereafter referred to simply as plastic fibre) in this invention, there can be used fibres made of one or more of the resins selected from polyolefins such as polyethylene and polypropylene, polyamides such as nylon, polyesters such as polyethylene terephthalate, preferably fibres made of polyethylene or polypropylene, or composite fibres composed or, for example polyethylene/polypropylene, polyethylene/polyethylene terephthalate and polyethylene/polypropylene/polyethylene terephthalate. Such plastic fibres are preferably added with polyvinyl alcohol so that the fibres are well dispersed in water in the course of the sheet making. Since the pore opening of the sheet is preferably less than 50 µm it is desirable that the fibre width is as small as possible, usually less than 50 µm preferable less than 30 µm. The preferred fibre length is usally 0.1 to 200 mm. The plastic fibres are suspended in water, if necessary with an additive or additive such as a dispersant or defoaming agent. The plastic fibre suspension is introduced into a sheet machine, filtered by wire, suction dehydrated and then passed through a dryer to obtain a dry sheet material. The concentration of the plastic fibres in water suspension is not specifically defined in this invention, but in view of workability in the sheet making process, the fibre concentration is usually set in the range of 0.1 to 10% by weight, preferably 0.5 to 5% by weight. The time and temperature for drying the web are not critical as far as drying is effected so that the water content of the sheet material is preferably less than 0.1%. The basis weight of the dry sheet material is set in the range of 5 to 300 g/m², preferably 20 to 150 g/m², by taking into consideration the air permeability of the sheet material, its strength and economy. The thus obtained sheet material, although having air permeability, has still no waterproofness and is well permeable to water. Then said sheet material is immersed in or contacted with an emulsion formed by dispersing a fluorine resin in an aqueous solution or a solution formed by dissolving a fluorine resin in a solvent, thereby impregnating said sheet material with the fluorine resin. Any fluorine resin can be used. However, it is preferred to use ethylene tetrafluoride resin, ethylene tetrafluoride-propylene hexafluoride copolymer, ethylene tetrafluoride-fluoroalkoxyethylene copolymer resin, ethylene chloride trifluoride resin or derivatives thereof. The amount of fluorine resin used is 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, to 100 parts by weight of plastic fibre. The fluorine resin-impregnated sheet material is then pressed by heated rolls, whereby said sheet material is provided with proofness against water, oils and alcohols and becomes an "air-permeable sheet" referred to in this invention. Pressing by heated rolls is preferably carried out by pressing the sheet material while heating it to a temperature at which the sheet surface is fused so that the plastic fibres therein are fusion bonded at their crossing points. The temperature at which said pressing by heated rolls is conducted is usually 120-140°C, preferably 125-135°C, and the pressure is usually 49 to 588 kPa (0.5-6 kg/cm²), preferably 294 to 490 kPa (3-5 kg/cm²). The thickness of the air-permeable sheet is usually 1 mm or less, preferably 0.2 mm or less. In this invention, a reinforcing material is preferably laminated to the air-permeable sheet for increasing the strength of the package. The reinforcing material usable in this invention includes knits of filaments or strips made of plastic materials, for example, polyolefins such as polyethylene and polypropylene, polyamides such as nylon, polyesters such as polyethylene terephthalate, and fabric-like products obtained by fusion bonding the weft and warp of said filaments or strips or fixing them with a bonding agent. Suitable additives such as pigments may be incorporated in the plastic reinforcing material. These reinforcing materials are commercially available as they are sold under the names of, for example, Nisseki Warifu (Nisseki Synthetic Resin Products Mfd. Co.), Daiyacross (Diatex Corp.), Kurenet (Kuraray Co., Ltd.), Sofucross (Shin Nippon Sheeting Co., Ltd.), and Sofunet (Shin Nippon Sheeting Co., Ltd.). An especially preferred reinforcing material is one using yarns obtained by cutting to a width of 0.3-3 mm a stretched or non-stretched film formed by bonding a low-density polyethylene, ethylene-vinyl acetate copolymer or ethylene-acrylic ester copolymer having a softening point of 120°C or below, preferably 115°C or below, to both sides of a high-density polyethylene film, said yarns being interwoven into a lattice pattern of 3-50 yarns/2,54 cm (inch) widthwise and fusion bonded to each other. The size and number of the yarns are suitably selected according to the required air permeability of the air-permeable sheet. The air permeability of the sheet, as measured in terms of Garley's permeability prescribed in JIS P-8117, is usually in the range of 1-5,000 s/100 ml 6,45 cm² (in²), preferably 5-1,000 s/100 ml 6,45 cm² (in²). In the present invention, lamination of the fluorine resin-impregnated plastic fibre and said reinforcing material is accomplished by hot press using heated rolls. Therefore, in case of using a laminated combination of sheet and reinforcing material, it is expedient to employ a method in which the fluorine resin-impregnated and dried sheet made of plastic fibre is hot pressed by heated rolls while concurrently laminating the reinforcing material to the sheet. In the present invention, a porous low softening point resin film or a composite film may be sealed to the air-permeable sheet. The composite film is preferably porous. Preferred examples of porous low softening point resin film usable in this invention are a perforated polyethylene film, a vinyl acetate copolymer film and a ionomer film. The porous composite film used in this invention is further explained. This film is bonded to the air-permeable sheet for improving the heat resistance and oil resistance of the packaging material. Said porous composite film is a perforated composite film comprising a laminate of a resin layer having a lower softening point than the plastic fibre constituting the air-permeable sheet and a resin layer having a higher softening point than said plastic fibre. The low softening point resin layer of the porous composite film serves as an adhesive layer for bonding the air-permeable sheet and the porous composite film. The material used for the low softening point resin layer is not specifically defined except that such material must have a lower softening point than the plastic fibre of the air-permeable sheet, but it is preferred to use polyethylene, polypropylene, ethylenevinyl acetate copolymer, ethyl acrylate-ethylene copolymer, ethylene-methyl methacrylate copolymer or ethylene-acrylic ester-maleic anhydride copolymer. The high softening point resin layer of the porous composite film is intended to afford heat resistance to the packaging material. The material used for such high softening point resin layer is not specified saving that it must have a higher softening point than the plastic fibre of the air-permeable sheet, but nylon or polyethylene terephthalate is preferably used. Lamination of the low softening point resin layer and the high softening point resin layer for forming said composite film is usually accomplished by dry lamination, extrusion lamination or coextrusion lamination. The thickness of the composite film is not specified, but it is preferably less than 300 µm, more preferably less than 100 µm, in view of cut resistance and economy. A light absorbing pigment or the like may be added to the film. Any suitable method may be used for perforating the composite film and low softening point resin film, but usually a needle thrusting method, a corona discharge method or a laser beam irradiation method is used. In the present invention, the needle thrusting method which is capable of forming small openings or the laser beam irradiation method. is preferred. The latter is especially preferred as this method is capable of uniformIy forming a plurality of openings having small size and providing a regular fracture section. According to this laser beam irradiation method, usually CO₂ laser beams or He-Ne laser beams with a power density of 1 × 10⁻² to 1 × 10⁸ W/cm² are passed intermittently through a prescribed lens to irradiate the composite film for 1 × 10⁻³ to 1 × 10⁻⁷ s so that the irradiated part is perforated by the heat generated on absorbing the laser beams. The diameter and number of the openings formed in the composite film and low softening point resin film are so adjusted that the package obtained by bonding the air-permeable sheet and said porous composite film or low softening point resin film will have a Garley's permeability of 1 to 5,000 s/100 ml 6,45 cm² (in²), preferably 5 to 1,000 s/100 ml 6,45 cm² (in²). More specifically, the opening diameter is 10 mm or less, preferably 1 mm or less, and the number of the openings is more than 100 openings/m², preferably more than 10,000 openings/m², the porosity (open area per unit area) being 0.8 to 40%, preferably 1 to 30%. Bonding of the porous composite film or low softening point resin film and the air-permeable sheet can be accomplished by hot press using heated rolls. For such bonding, therefore, it is expedient to employ a method in which the air-permeable sheet obtained by impregnating a sheet of plastic fibre with an emulsion or solution of a fluorine resin and then drying the same is hot pressed by heated rolls while laminating thereto the low softening point resin layer of the porous composite film. The oxygen absorbent to be encapsulated in the package of this invention is not limited to any specific types but any of those capable of absorbing oxygen is usable. Typically, oxygen absorbents containing sulfites, hydrogen sulfites, dithionites, ferrous salts, hydroquinone, catechol, resorcinol, pyrogallol, gallic acid, Rongalit, ascorbic acid and/or its salts, isoascorbic acid and/or its salts, unsaturated fatty acid compounds, sorbose, glucose, lignin, dibutylhydroxytoluene, or butylhydroxyanisol, or oxygen absorbents containing metal powder such as iron powder are used. Among them, oxygen absorbents containing ascorbic acid and/or its salts, isoascorbic acid and/or its salts, unsaturated fatty acid compounds or iron powder are preferred. Those containing iron powder are especially preferred. Carbon dioxide generating type oxygen absorbents and carbon dioxide absorbing type oxygen absorbents are also usable. As described above, the oxygen absorbent package of this invention is of a structure in which a part or the whole of the package is constituted by an air-permeable sheet obtained by making a sheet from a plastic fibre, impregnating the dried sheet with an emulsion or a solution of a fluorine resin and hot pressing the resin-impregnated sheet, or of a structure in which a sheet prepared by laminating said air-permeable sheet and a reinforcing material is used as a part or the whole of the packaging material. Also, said air-permeable sheet (which may hereinafter be represented by (a)) and a porous composite film (b) obtained by laminating a resin layer having a lower softening point than said plastic fibre and a resin layer having a higher softening point than said plastic fibre are bonded to each other with the low softening point resin layer of said film (b) serving as bonded side to form a packaging material, and this packaging material may be used as a part or the whole of the oxygen absorbent package with said air-permeable sheet (a) being positioned on the inside of the package. The following are typical embodiments of oxygen absorbent packages according to this invention. (1) An oxygen absorbent package comprising an air-permeable sheet (a) and a composite film (b) including a polyethylene layer such as nylon/polyethylene film or polyethylene terephthalate/polyethylene film, the polyethylene layer side of said composite film (b) being laminated to said sheet (a) with an oxygen absorbent disposed therebetween, the laminate being heat sealed edgewise so the oxygen absorbent is encapsulated therein. (See FIG. 1). (2) An oxygen absorbent package comprising two sheets of packaging material obtained by laminating an air-permeable sheet (a) to the low softening point resin layer side of a porous composite film (b) consisting of a resin layer having a higher softening point than the plastic fibre used for said air-permeable sheet and a resin layer having a lower softening point than said plastic fibre, said two sheets of packaging material being laminated with an oxygen absorbent disposed therebetween, said laminate being heat sealed edgewise to encapsulate the oxygen absorbent therein. (See FIG. 2). (3) An oxygen absorbent package comprising a laminate sheet (a) consisting of an air-permeable sheet and a reinforcing material, and a composite film (b) including a polyethylene layer such as a nylon/polyethylene film or a polyethylene terephthalate/polyethylene film, the polyethylene side of (b) being laminated to the reinforcing material side of (a) with an oxygen absorbent disposed therebetween, the laminate being heat sealed edgewise so that the oxygen absorbent is encapsulated therein. (See FIG. 3). (4) An oxygen absorbent package in which the air-permeable sheet side of a packaging material obtained by bonding a porous composite film (b) to an air-permeable sheet (a) is laminated to the high-density polyethylene side or polypropylene side of a composite film (c) such as a nylon/high-density polyethylene film, a polyethylene terephthalate/high-density polyethylene film or a nylon/polypropylene film, with an oxygen absorbent disposed therebetween, and the laminate is heat sealed edgewise so as to encapsulate the oxygen absorbent in the package. (See FIG. 4). (5) An oxygen absorbent package comprising a laminate sheet consisting of an air-permeable sheet and a reinforcing material, said laminate sheet being tucked in with the reinforcing material on the inside and heat sealed edgewise to encapsulate the oxygen absorbent therein. (See FIG. 5). (6) An oxygen absorbent package comprising a packaging material obtained by laminating an air-permeable sheet (a) to the low softening point resin layer side of a porous composite film (b) consisting of a resin layer having a higher softening point than the plastic fibre used for said air-permeable sheet and a resin layer having a lower softening point than said plastic fibre, said packaging material being tucked in with (a) on the inside and heat sealed edgewise to encapsulate the oxygen absorbent therein. (See FIG. 2). (7) An oxygen absorbent package comprising a laminate of an air-permeable sheet and a porous low softening point resin film, said laminate being tucked in with the low softening point resin film on the inside and heat sealed edgewise to encapsulate the oxygen absorbent therein. (See FIG. 6). (8) An oxygen absorbent package comprising a pair of packaging material prepared by laminating a porous plastic film (d) having a lower softening point than the plastic fibre constituting the air-permeable sheet (a) to the low softening point resin layer side of a porous composite film (b) with an air-permeable sheet (a) interposed therebetween, said pair of packaging material being laminated with the porous plastic films (d) opposing to each other and with an oxygen absorbent disposed therebetween, the laminate being then heat sealed edgewise to encapsulate the oxygen absorbent therein. (See FIG. 7). The shape of the package is free to choose, but usually it is square. The oxygen absorbent package of this invention is not only excellent in air permeability, waterproofness, breakage resistance and hygienic safety but is also provided with high oil and alcohol resistance as well as heat resistance, so that the package of this invention is especially useful for storage or retort foods although such a package is also applicable to other types of foodstuff. A retort food can be prepared by encasing the foodstuff and the oxygen absorbent package of the present invention into a gas-barrier type package material (e.g., pouch) used for the retort food, sealing the material completely by an ordinary means, and heating the resultant at a temperature of 90°C to 130°C for 1 to 60 min. Thus, a retort food capable of being stored for a long period of time can be obtained. The invention will be described in greater detail below by reference to the following examples. Example 1 (Production of sheet material and material for air-permeable sheet) Various types of plastic fibres shown in Table 1 were dispersed in water to a concentration of 1% by weight. Each of the thus prepared suspensions of plastic fibres was introduced into a sheet machine to form a sheet. In this sheet making process, the plastic fibre suspension was dehydrated by a wire net to form a web and this web was held between a pair of rolls and compressed under a pressure of 98 kPa (1 kg/cm²) for further dehydration. Thereafter, the web was dried at 100°C for 1 min to a water content of less than 0.1% to produce a sheet material having a basis weight of 90 g/m². The types of plastic fibre and the compositional ratio of each type of plastic fibre used for producing the sheet material are shown in Table 1. Symbol for sheet material Type of plastic fibre used Compositional ratio (%) A Polyethylene fibre (av. fibre length: 0.9-1.3 mm, amorphous) 100 B Polyethylene fibre (av. fibre length: 0.9-1.3 mm, amorphous) 50 Polyethylene-polypropylene composite fibre (diam. 0.2 mm, length 10 mm) 50 C Polyethylene fibre (av. fibre length: 0.9-1.3 mm, amorphous) 50 Polyester fibre (diam. 1.5 mm, length 10 mm) 50 Example 2 (Production of air-permeable sheet) Each of the sheet materials obtained in Example 1 was impregnated with a dilute aqueous solution of a water and oil repellent Asahi guard AG 530 (available from Asahi Glass Co., Ltd.) (fluorine resin concentration: 2% by weight) in an amount of 25 parts by weight and 50 parts by weight to 100 parts by weight of sheet material, and then dried at 60°C for 5 min. The sheet material impregnated with said water and oil repellent was heated to 140°C and passed through a pair of rolls loaded with a pressure of 588 kPa (6 kg/cm²) at a speed of 10 m/min to produce an air-permeable sheet having the plastic fibres fusion bonded to each other. The Garley's permeability of the thus obtained air-permeable sheets was measured according to the method prescribed in JIS P-8117. These air-permeable sheets were subjected to an oil resistance test by immersing the sheets in soybean oil and to an alcohol resistance test by immersing the sheet in 100% ethanol. The test results are shown in Table 2. In the oil and alcohol resistance tests, the external appearance of each sheet was visually observed on a daily basis. The evaluations of visual appearance of each sheet as observed at the 14th day and 30th day of immersion are shown in Table 2. As seen from Table 2, each of the sheets showed a Garley's permeability less than 50 s/100 ml of air per 6,45 cm² (in²) and thus had very excellent air permeability. In the immersion tests, each of the sheets retained good external appearance even after 30 days of immersion. Comparative Example 1 An air-permeable sheet was produced by following the same procedure as Example 2 except that the sheet was not impregnated with a fluorine resin emulsion of water/oil repellent Asahi guard AG 530 (available from Asahi Glass Co., Ltd.). The Garley's permeability of this air-permeable sheet was measured according to JIS P-8117. This sheet was also subjected to the same oil resistance test and alcohol resistance test as conducted in Example 2. The test results are shown in Table 2. In said tests, the external appearance of the sheet was visually observed on a daily basis, and the evaluations of visual appearance as observed at the 14th day and 30th day of immersion are shown in Table 2. Comparative Example 2 An air-permeable sheet was produced in the same way as in Example 2 except that a microporous film Celgard (available from Celanese Corp. of the U.S.) was used in place of the sheet material obtained in Example 1. The Garley's permeability of this air-permeable sheet was measured according to JIS P-8117. This sheet was also subjected to said oil resistance test and alcohol resistance test, the test results being shown in Table 2. In said tests, the external appearance of the sheet was visually observed on a daily basis, and the evaluations of visual appearance as observed at the 14th day and 30th day of immersion are shown in Table 2. Comparative Example 3 An air-permeable sheet was produced by following the process of Example 2 except that a nonwoven fabric Tyvek (available from E.I. Du Pont) was used in place of the sheet material obtained in Example 1. The Garley's permeability of this sheet was measured according to JIS P-8117, and the sheet was also subjected to said oil resistance test and alcohol resistance test, obtaining the results shown in Table 2. In said tests, the external appearance of the sheet was visually observed on a daily basis, and the evaluations of visual appearance as observed at the 14th day and 30th day of immersion are shown in Table 2. Comparative Example 4 Each of the sheet materials obtained in Example 1 was impregnated with a dilute aqueous solution of a water and oil repellent Asahi guard AG 530 (available from Asahi Glass Co., Ltd.) in an amount of 25 parts by weight or 50 parts by weight to 100 parts by weight of sheet material, and then dried at 60°C for 5 min. The Garley's permeability of the thus obtained air-permeable sheets was measured by the method prescribed in JIS P-8117. Said sheets were also subjected to an oil resistance test by immersing them in soybean oil and to an alcohol resistance test by immersing the sheets in 100% ethanol. The test results are shown in Table 2. In said oil resistance test and alcohol resistance test, the external appearance of the sheet was visually observed on a daily basis, and the evaluations of visual appearance of the sheets as observed at the 14th day and 30th day of immersion are shown in Table 2. Example 3 (Production of oxygen absorbent package) Each of the air-permeable sheets obtained in Example 2 and cut to a size of 40 × 40 mm and an equally sized composite film consisting of 12 µm thick polyester and 45 µm thick polyethylene were placed one on the other with 3 g of an iron-based oxygen absorbent (oxygen absorbing capacity: 600 ml) disposed therebetween, and said sheet and film were heat sealed edgewise by heating under pressure at 130°C for 3 s to encapsulate the oxygen absorbent. Each of the thus produced oxygen absorbent packages was put into a pouch made of a vinylidene chloride-coated stretched nylon/polyethylene laminate film in which 70 g of fried fish balls and 250 ml of air were also contained, and the pouch was sealed and stored at 25°C. The oxygen removal time (the time required till the oxygen concentration in the pouch became 0.1% after start of storage) was measured. Also, the external appearance of the oxygen absorbent package was visually observed to examine the state of oil impregnation. The state of fried fish balls in the pouch was also observed. The results are shown in Table 3. Comparative Example 5 The oxygen absorbent packages were produced in the same way as in Example 3 except that the air-permeable sheet obtained in Comparative Example 1 was used in place of the air-permeable sheet obtained in Example 2, and these packages were subjected to the same tests as conducted in Example 3. The results are shown in Table 3 along with the results of Example 3. Comparative Example 6 The oxygen absorbent packages were produced by following the procedure of Example 3 except that the air-permeable sheet obtained in Comparative Example 2 was used in place of the air-permeable sheet obtained in Example 2, and the produced packages were subjected to the same tests as conducted in Example 3. The results are shown in Table 3. Comparative Example 7 The oxygen absorbent packages were produced according to Example 3 but by using the air-permeable sheet obtained in Comparative Example 3 in place of the air-permeable sheet obtained in Example 2, and the obtained packages were subjected to the tests of Example 3. The results are shown in Table 3. Comparative Example 8 The oxygen absorbent packages were produced according to Example 3 by using the air-permeable sheet obtained in Comparative Example 4 in place of the air-permeable sheet obtained in Example 2, and these packages were subjected to the tests of Example 3. The results are shown in Table 3. As seen from Table 3, all of the oxygen absorbents of Comparative Examples 5-8 have lost their oxygen absorbing capacity in 7 days of storage. Example 4 An air-permeable sheet was produced according to the method of Example 2 by using the sheet material A obtained in Example 1 (see Table 1). As the fluorine resin source, Asahi guard AG-530 (fluorine resin concentration: 2%) was used in an amount of 50 parts by weight to 100 parts by weight of sheet material (the amount of fluorine resin added: 1 part by weight to 100 parts by weight of sheet material). The air-permeable sheet thus obtained and each of the reinforcing materials made by plain weaving the yarns obtained by cutting an LDPE (low-density polyethylene, m.p. 105°C)/HDPE (high-density polyethylene)/LDPE film to various sizes and numbers were bonded to each other by passing them through the heated rolls of 120°C under a pressure of 49 kPa (0.5 kg/cm²) at a rate of 40 m/min. The tear strength and Garley's permeability of the obtained laminate sheets were measured. The results are shown in Table 4. By way of comparison, the tear strength and Garley's permeability of said air-permeable sheet without said reinforcing material laminated thereto were also measured and shown in Table 4. Run No. Reinforcing material Permeability Tear strength (g) Yarn size (mm) Number of yarns Longitudinal Transverse Warp Weft 1 2.5 9 9 40 1700 1700 2 2.5 7 7 25 1400 1400 3 2.5 5 5 20 1100 1100 4 1.5 9 9 20 1000 1000 5 0.5 9 9 10 700 700 6 None 7 60 100 Example 5 Each of the air-permeable sheets obtained in Example 4 (reinforcing material-laminated sheets) cut to a size of 40 x 40 mm and a polyester (12 µm thick)/polyethylene (45 µm thick) composite packaging material cut to the same size as said air-permeable sheet were joined to each other with the reinforcing material side of the air-permeable sheet opposing the polyethylene side of the composite packaging material, then an oxygen absorbent mainly composed of iron and having an oxygen absorbing capacity of 600 ml was disposed therebetween, and the joined sheet and packaging material were heat sealed edgewise by heating under pressure at 130°C for 3 s to encapsulate the oxygen absorbent. Each of the thus obtained oxygen absorbent packages was put into a polyvinylidene chloride-coated stretched nylon/polyethylene laminate pouch (KON/PE pouch) together with 70 g of fried fish balls and 250 ml of air, and the pouch was sealed and stored at 25°C. The oxygen removal time (the time required till the oxygen concentration became 0.1% after start of storage) was measured. Also, the external appearance of each oxygen absorbent package was visually observed to examine the state of oil impregnation. The state of fried fish balls in the pouch was also observed. Observation of external appearance of the packages and the state of fried fish balls was conducted on a daily basis. The results are shown in Table 5. Run No. Reinforcing material Oxygen removal time (h) Visual appearance of packaging material State of food Yarn size (mm) Number of yarns 1st day 3rd day 7th day 3rd day 7th day Warp Weft 1 2.5 9 9 20 5 5 5 ○ ○ 2 2.5 7 7 16 5 5 5 ○ ○ 3 2.5 5 5 10 5 5 5 ○ ○ 4 1.5 9 9 14 5 5 5 ○ ○ 5 0.5 9 9 10 5 5 5 ○ ○ Notes: (1) The number of yarns in the reinforcing material shown in Table 5 is that per 6,45 cm² (in²). (2) Evaluation of visual appearance of packaging material: 5 No impregnation of oil or ethanol in the packaging material. 4 Oil or ethanol was impregnated at a part of the packaging material. 3 Oil or ethanol was impregnated partially in the packaging material. 2 Oil or ethanol was impregnated in most part of the packaging material. 1 Oil or ethanol was impregnated all over the packaging material. (3) Evaluation of the state of food: ○ No abnormality. X Putrefied. Example 6 (Production of air-permeable sheets) The sheet materials (A, B and C) obtained from Example 1 were impregnated with a dilute aqueous solution of water/oil repellent Asahi guard AG-530 (available from Asahi Glass Co., Ltd.) in an amount of 50 parts by weight to 100 parts by weight of sheet material (fluorine resin concentration: 2% by weight) and then dried at 60°C for 5 min to obtain the air-permeable sheets (symbolized as A, B and C in correspondence to Table 1). (preparation of porous composite films) The porous composite films were prepared by laminating various types of high softening point resin films and low softening point resin films and perforating the laminates to a prescribed opening diameter and a prescribed number of openings by laser beam irradiation or needle thrusting. The structural details of the respective porous composite films are shown in Table 6. Film No. Structure of porous composite film High softening point resin Low softening point resin Perforating means Opening diameter (mm) Number of openings per m² I NY 15 µm LDPE 40 µm Laser beams 0.15 1000 x 10³ II NY 15 µm PP 40 µm Laser beams 0.15 1000 x 10³ III NY 15 µm LDPE 40 µm Laser beams 0.3 400 x 10³ IV PET 12 µm LDPE 40 µm Laser beams 0.15 1000 x 10³ V NY 15 µm LDPE 40 µm Laser beams 0.15 10 x 10³ VI NY 15 µm PP 40 µm Needle 0.3 1 x 10³ (Production of oxygen absorbent packaging material) The thus obtained air-permeable sheets (A-C) and porous composite films (I-VI) were combined, with the low softening point resin side of the composite film attached to the air-permeable sheet, and they were passed through a pair of heated rolls of 125°C under a pressure of 294 kPa (3 kg/cm²) at a rate of 40 m/min to produce the packaging materials. The Garley's permeability of each of said air-permeable sheets was measured according to JIS P-8117. The results are shown in Table 7. Comparative Example 9 A packaging material was produced in the same way as Example 6 except that a microporous film Celgard (available from Celanese Corp., U.S.) was used in place of the sheet material obtained in Example 1. The Garley's permeability of this packaging material as measured according to JIS P-8117 is also shown in Table 7. Run No. Packaging material Permeability Air-permeable sheet Porous composite film Example 6 1 A I 22 2 B I 21 3 C I 23 4 B II 22 5 B III 15 6 B IV 21 7 B V 3000 8 B VI 100 Comp. Example 9 I 25 Example 7 Each pair of the packaging materials produced as described above by bonding said porous composite films and air-permeable sheets obtained in Example 6 and cut to a size of 40 × 40 mm were joined to each other with their air-permeable sheet sides opposing each other, and after disposing therebetween an oxygen absorbent mainly composed of iron powder and having an oxygen absorbing capacity of 600 ml, the joined packaging materials were heat sealed edgewise at 170°C for 3 s to encapsulate the oxygen absorbent. The oxygen removal time, oil resistance and heat resistance of the thus obtained oxygen absorbent packages were measured. The results are shown in Table 8. Measurement of oxygen removal time: Each of the oxygen absorbent packages was put into a KON (polyvinylidene chloride-coated stretched nylon, 15 µm thick)/PE (40 µm thick) pouch together with 0.5 l of air, followed by sealing of the pouch, and the time required for the oxygen concentration in the pouch to reach 0.1% after storage at 25°C was measured. Determination of oil resistance: The amount of oil impregnated into the packaging material after one-day immersion in soybean oil was measured. The external appearance of the packaging material was also visually observed. Determination of heat resistance: The time till the pouch was broken after immersed in soybean oil of 120°C was measured. Comparative Example 10 An oxygen absorbent package was produced by following the same procedure as in Example 7 except that the packaging material obtained in Comparative Example 9 was used in place of the packaging material obtained in Example 6, and this package was subjected to the same measurements as described above. The results are shown in Table 8 along with the results of Example 7. Example 8 200 g of meat balls inoculated with a thermophilic bacterium Bacillus coagulans were put into a 15 µm nylon/12 µm aluminum foil/30 µm polypropylene pouch along with each of the oxygen absorbent packages obtained in Example 3, followed by heat sterilization with steam of 143 kPa (1.46 kg/cm²) (110°C) for 15 min. After cooling, the pouch was kept in storage at 25°C. The state of the food and the external appearance of the oxygen absorbent package in the storage period were observed. The results are shown in Table 9. Comparative Example 11 The same process and the same test as conducted in Example 8 were repeated except that the oxygen absorbent package of Comparative Example 10 was used in place of the oxygen absorbent package of Example 7. The results are shown in Table 9. As seen from Table 9, in the case of Comparative Example 11, oil was impregnated in the packaging material and the oxygen absorbent was devoid of the oxygen absorbing ability on the 7th day of storage. Example 9 (Production of oxygen absorbent package) A 1 wt% suspension of polyethylene fibres having a length of 0.9 - 1.3 mm and an average width of 0.01 mm was introduced into a sheet machine. First, water was removed from the plastic fibre suspension by a wire net to form a web and this web was held between a pair of rolls and compressed under a pressure of 98 kPa (1 kg/cm²) for further dehydration. Thereafter, the web was dried at 100°C for 1 min to produce a sheet material with a water content of less than 0.1% and a basis weight of 90 g/m². This sheet material was then impregnated with a dilute aqueous solution of Asahi guard AG 530 (a water and oil repellent, mfd. and sold by Asahi Glass Co., Ltd.) (fluorine resin concentration in the solution being 2% by weight) in an amount of 50 parts by weight to 100 parts by weight of said sheet material, and the impregnated sheet material was dried at 60°C for 5 min to obtain an air-permeable sheet. To this air-permeable sheet was attached the LDPE (low-density polyethylene) side of a composite film consisting of a 12 µm thick PET (polyethylene terephthalate) film and a 40 µm thick LDPE film and perforated by the irradiation of CO₂ laser beams with a power density of 30 W/cm² for a period of 1 × 10⁻⁵ s, and the attached air-permeable sheet and composite film were passed between a pair of rolls of 125°C under a pressure of 294 kPa (3 kg/cm²) at a speed of 40 m/min and thereby bonded to each other to produce a packaging material. The Garley's permeability of this packaging material as determined according to the method of JIS P-8117 was 22 s/100 ml 6,45 cm² (in²). Two pieces of said packaging material cut to a size of 40 × 40 mm were out together with the air-permeable sheet side positioned on the inside, and after placing therebetween an oxygen absorbent mainly composed of iron powder and having an oxygen absorbing capacity of 600 ml, said two pieces of packaging material were heat sealed edgewise at 170°C for 3 s to produce an oxygen absorbent package. Food storage test 200 g of meat balls inoculated with a heat-resistant bacterium Bacillus coagulans (10⁴ cells/g) were placed in a pouch of 15 µm nylon/12 µm aluminum foil/30 µm polypropylene along with said oxygen absorbent package and heat sterilized by steam of 143 kPa (1.46 kg/cm²) (110°C) for 15 min. After cooling, the sealed pouch containing said meat balls and oxygen absorbent package was stored at 25°C, and the change with time of bacillus count in the food and the change with time of peroxide value of the oil and fat contained in the food during the period of storage were measured. The results are shown in Table 10. Comparative Example 12 The same test as described above was conducted by replacing said oxygen absorbent package with a commercially available package. The results are shown in Table 10 along with the test results of Example 9. As seen from Table 10, in the case of Comparative Example 12, the food in the pouch was putrefied and inedible on the 21st day of storage. In the present Example, the oxygen concentration in the gas-barrier pouch was less than 0.1% within one day of storage. Example 9 Comp. Example 12 Bacillus count in food cells/g Before heat treatment 10⁴ 10⁴ Immediately after heat treatment 3 3 Change with time during storage 7th day 5.1 x 10 4.5 x 10² 14th day 8.7 x 10² 6.8 x 10⁴ 21st day 1.5 x 10⁴ 1.0 x 10⁷ Change with time of peroxide value of oil & fat contained in food 7th day 10 15 14th day 10 20 21st day 10 30 As apparent from the embodiments described above, the oxygen absorbent package of this invention is characterized by the use of an air-permeable sheet prepared by impregnating a fluorine resin in a sheet of plastic fibre and then hot pressing the sheet and by encapsulating an oxygen absorbent in a packaging material formed from said air-permeable sheet by bonding, if necessary, a reinforcing material thereto, so that the oxygen absorbent package of this invention has its own merits such as stated below which are absent in the conventional oxygen absorbent packages. In the case of the conventional fluorine resin-added packages, when they are contacted with a 10-50% ethanol solution, the ethanol solution is impregnated into the packaging material, but the package of this invention remains free of impregnation of ethanol even if immersed in 100% ethanol. When an oxygen absorbent package using the conventional fluorine resin-added packaging material is immersed in a tempura (frying) oil, the oil is impregnated into the packaging material in one to three days and the packaged foodstuff is seen through the packaging material. The package of this invention shows no change at all in external appearance of the packaging material even when kept immersed in a tempura oil for 14 days. Further, when the conventional oxygen absorbent packages are contacted with an oil-containing food such as doughnut, the oil infiltrates into the package from the contact area, interrupting absorption of oxygen, so that oxygen is not removed perfectly from within the sealed package, causing the packaged food to gather mold. The package of this invention does not allow infiltration of oil into the inside thereof, so that it retains its normal oxygen removing performance and can keep the freshness of the packaged food. The tear strength of the sheet with no reinforcing material is 60 - 140 g crosswise and 30 - 90 g longitudinally, but when a reinforcing material is laminated to the sheet, its tear strength is improved to 600 - 1,500 g both crosswise and longitudinally. In the case of the package of this invention comprising said air-permeable sheet (a) and a porous composite film (b) consisting of a resin layer having a lower softening point than said plastic fibre and a resin layer having a higher softening point than said plastic fibre, in which said air-permeable sheet is bonded to the low softening point resin layer side of said porous composite film to form a packaging material and a pair of such packaging materials are joined with the air-permeable sheet positioned on the inside and are heat sealed edgewise with an oxygen absorbent packed therein, such package has the following advantage over the conventional oxygen absorbent packages. In the case of the conventional oxygen absorbent packages comprising a laminate of a microporous film and a porous low softening point resin film, when they are used in a retort pouch, the oil contained in the retort food is impregnated into the packaging material to deprive the oxygen absorbent therein of its oxygen absorbing or removing ability. The package of this invention causes no impregnation of oil contained in the food into the packaging material, so that it retains its oxygen absorbing capacity and therefore can maintain its food preserving effect unchanged. Further, the oxygen absorbent package of this invention will not be broken even when subjected to a high-temperature heat sterilization treatment, and thus it can be ideally applied to preservation of retort foods. When retort food is prepared by using the oxygen absorbent package of this invention, the oxygen concentration in the gas-barrier pouch can be maintained less than 0.1% during storage, so that by use of the oxygen absorbent package of this invention, it is possible to prevent germination and growth of the spores of thermophilic bacteria in retort food and to keep the food safe from oxidation of oil and fat contained therein, discoloration and change of quality such as taste, thus enabling long-time storage of retort foods.

Claims: An oxygen absorbent package comprising an oxygen absorbent (90) in a package made of a packaging material which is partly or wholly formed with an air-permeable sheet (10) obtained from a plastic fibre by a wet processing, drying the sheet and impregnating same characterised in that the sheet has been impregnated with a fluorine resin and after impregnation has been pressed by heated rolls. The package according to claim 1, wherein the size of the openings of said fibre is 50 µm or less, and the fibre length is 0.1 to 200 mm. The package according to claim 1, wherein the amount of fluorine resin impregnated in the sheet is 0.1 to 10 parts by weight to 100 parts by weight of said plastic fibre. The package according to claim 1, wherein pressing of the resin-impregnated sheet by heated rolls is carried out at a temperature of 120 to 140°C under a pressure of 49 to 588 kPa (0.5 to 6kg/cm²). The package according to claim 1, wherein the thickness of said air-permeable sheet (10) is 1 mm or less. The package according to claim 1, wherein said air-permeable sheet (10) is laminated with a reinforcing material (50). The package according to claim 6, wherein said reinforcing material (50) comprises yarns (60, 70) obtained by cutting a stretched or non-stretched film made by bonding a low-density polyethylene having a melting point of 120°C or below, an ethylene-vinyl acetate copolymer or an ethylene-acrylic ester copolymer to both sides of a high-density polyethylene film, said yarns being interwoven into a lattice pattern and fusion bonded to each other. The package according to claim 6, wherein lamination of said air-permeable sheet (10) and said reinforcing material (50) is accomplished by hot pressing using heated rolls. The package according to claim 1, wherein said air-permeable sheet (10) is laminated with a porous low softening point resin film, (20, 22) and the laminate is tucked in with said low softening point resin film positioned on the inside and is heat sealed edgewise. The package according to claim 9, wherein said low softening point resin film (20, 22) is selected from the group consisting of a polyethylene film a vinyl acetate copolymer film and an ionomer film. The package according to claim 1, comprising a packaging material prepared by bonding said air-permeable sheet (10) to the low softening point resin layer (20) side of a composite film (40, 42) formed by laminating a resin layer (20) having a lower softening point that the plastic fibre constituting said air-permeable sheet (10) and a resin layer (30) having a higher softening point that said plastic fibre, said packaging material being worked into a package so that said air-permeable sheet (10) side forms the inner surface. The package according to claim 1, comprising a pair of lamimates each consisting of said air-permeable sheet (10), a composite film (40, 42) made by laminating a resin layer (20), having a lower softening point than the plastic fibre constituting aid air-permeable sheet (10) and a resin layer (30) having a higher softening point than said plastic fibre, and a porous resin film (22) having a lower softening point than said plastic fibre, in which laminate the low softening point resin layer side (20) of said composite film (40, 42) opposes said porous resin film (22) with said air-permeable sheet (10) interposed therebetween, said pair of laminates being joined together so that the low softening point resin sides (20) of the respective laminates face each other. The package according to claim 10 or 11, wherein said composite sheet (40, 42) is porous. The package according to claim 13, wherein the size of openings of said porous composite sheet (40, 42) is 10 mm or less, and the number of openings is 100 openings/m² or more. The package according to any of claims 10 to 13, wherein the porosity of said porous composite sheet (40, 42) is 1 to 30%. The package according to any of claims 10 to 13, wherein the resin constituting the low softening point resin layer (20) of said porous composite sheet (40, 42) is selected from the group consisting of a polyethylene, a polypropylene, an ethylene-vinyl acetate copolymer and an ethyl acrylate-ethylene copolymer, and the resin constituting the high softening point resin layer (30) of said composite sheet (40, 42) is a nylon or a polyethylene terephthalate. The package according to any of claims 10 to 13, wherein the thickness of said composite film (40, 42) is 300 µm or less. The package according to claim 13, wherein said composite sheet (40, 42) is perforated by irradiation of laser beams. A retort food comprising a foodstuff and the oxygen absorbent package according to any of claims 1 to 18, wherein both said foodstuff and said oxygen absorbent package are encased in a gas-barrier type package material used for a retort food. The retort food according to claim 19, which is treated at a temperature of 90°C to 130°C for 1 to 60 min after encasing the foodstuff and the oxygen absorbent package into said gas-barrier type package material used for the retort food.

Inventors: INOUE, YOSHIAKI
Applicants: MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1987
Main classification: B65D81
IPC codes: D21H27, B32B33, B65D81, D21H17, D21H13, A23L3