A gas barrier film

Abstract: The present invention provides a gas barrier film including a plastic film and a thin film which is made of a composition containing an oxide as a main component and is formed on at least one surface of the plastic film. A specific gravity of the thin film is 55% or more and 100% or less of a specific gravity of the oxide in the form of a crystal generally present at room temperature and under atmospheric pressure. Thus, the invention provides a gas barrier film having an excellent gas barrier and retort properties and bending resistance; a gas barrier film suitably used in a wrapping material for dried food, liquid food or food containing liquids, frozen food and retort food; a gas barrier film that can be used in an electronic oven, can be sterilized by micro waves and can contain a free-oxygen absorber; a gas barrier film suitably used in a paper carton, a tube, a lid and the like; and a gas barrier film suitably used for cutting off gas for industrial use.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a film used in wrapping materials that is required to be airtight such as wrapping materials for food, medicine and electronic components, the wrapping materials having excellent gas barrier and retorting properties and bending resistance (gelbo characteristics). 2. Description of the Related Art: As a conventional gas barrier film, the following films are known: films comprising a plastic film coated with a vinylidene chloride copolymer or a copolymer of ethylene and vinyl alcohol; and films comprising a plastic film having a thin film of aluminum or an oxide (such as silicon oxide and aluminum oxide) laminated thereon. However, the former films have an insufficient gas barrier property against vapor, oxygen and the like. Particularly, when such films are used at a high temperature, the gas barrier property is markedly low. Moreover, the films comprising the vinylidene chloride copolymer have a problem of affecting the environment since they generate chlorine gas when incinerated. Among the latter films, those having a thin film of aluminum are economical and have an excellent gas barrier property. However, since the film is opaque, the contents therein can not be seen from the outside. Moreover, when the film is used for wrapping retort food, it can not be cooked in an electronic oven since micro waves do not penetrate the film. US-A-3,442,686 describes a transparent flexible gas barrier film comprising an inorganic thin film such as a film of silicon oxide and aluminum oxide evaporated on a surface of a synthetic resin film substrate and a film further comprising another resin film laminated on the gas barrier film. Japanese Patent Publication No. 51-48511 discloses a gas barrier film comprising Si x O y (for example, SiO 2 ) evaporated on a surface of a synthetic resin. When such a silicon oxide type film is used for wrapping, the contents therein can be seen from the outside since it is transparent. Such a film can be used in an electronic oven. But the above described films have only an insufficient gas barrier property. Among such films, one having a particular composition, that is, one having a SiO y (wherein y = 1.3 to 1.8) type thin film has a relatively excellent gas barrier property. However, it has a brownish color. In addition, the gas barrier property and the bending resistance of such a silicon oxide type film are still insufficient. "Having a bending resistance" means not losing a gas barrier property by bending a film in steps after forming a film (for example, steps of laminating, printing and making bags) or by careless handling of the film. The bending resistance is evaluated by measuring the gas barrier property after the gelbo treatment. US-A-3,442,686 discloses plastics coated with for example alumina/silica coatings. The plastics have low oxygen transmission rates. EP-A-0 460 796 discloses wrapping materials consisting of plastic films (e.g. PET) coated with silica and doped silica coatings. In particular, the document discloses silica, silica/calcium oxide and silica/magnesium oxide coatings having low oxygen transmission rates. EP-A-0 460 966 discloses coated plastic films having very low water vapour transmission rates. The coatings include alumina and silica/magnesium oxide coatings. Japanese Laid-Open Patent Publication JP-A-62-101428 discloses a gas barrier film comprising a thin film containing aluminum oxide as a main component. This film is completely transparent and chemically stable; and the material costs are small. However, the gas barrier property against oxygen and bending resistance are not sufficient. Moreover, the film requires careful handling because the thin film itself is brittle. Japanese Laid-Open Patent Publication JP-A-2-194944 discloses an Al 2 O 3 -SiO 2 type film as a gas barrier film with a retort property. "Having a retort property" means having an excellent airtightness and flexibility even after a high temperature treatment. The retort property is evaluated, for example, by measuring the gas barrier property under a condition for producing retort food. The Al 2 O 3 -SiO 2 type film comprises an Al 2 O 3 layer and a SiO 2 layer laminated successively on a plastic film. A large scale machinery is required for the production of such a film. The gas barrier property and bending resistance of such a film are not sufficient either. Further, a gas barrier film comprising a layer made of a composite oxide (an oxide containing two or more kinds of metals) formed on a film substrate is known. For example, Japanese Laid-Open Patent Publication JP-A-1-176069 discloses an M-Si-O type gas barrier film, wherein M is at least one kind selected from the group consisting of Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, In, Sn, Sb and V. Japanese Laid-Open Patent Publication JP-A-61-297134 and JP-A-61-297137 disclose a MgO-SiO 2 type and a MgO-CaO type gas barrier films, respectively. However, none of these films has a sufficient gas barrier property, an excellent bending resistance and a retorting property. The thin film formed on the plastic film must be as thin as possible in order to improve the bending resistance and, the contrary, the thin film must have a certain thickness [for example, 200 nm (2000 angstroms)] so as to improve the retort property. When a film comprising an aluminum oxide thin film laminated on a plastic film is used for wrapping retort food, a range of a preferable thickness of the film is particularly small. Moreover, the film used for wrapping retort food is required to be resistant against conditions of the various steps of production such as the heat sealing step, a printing step and the like when it is made into a bag. As described above, a transparent gas barrier film with satisfactory gas barrier and retort properties and an excellent bending resistance has not been provided. SUMMARY OF THE INVENTION The present invention relates to a gas barrier film comprising a plastic film and a thin film formed on at least one surface of the plastic film which thin film is made of a composition containing a single oxide selected from aluminum oxide and silicon oxide as the main component wherein the specific gravity of the thin film is 55% or more and 83% or less of the specific gravity of silicon oxide (2.65); and 55% or more and 83.1% or less of the specific gravity of aluminum oxide (3.97). Thus, the invention described herein makes possible the advantages of (1) providing a gas barrier film having an excellent gas barrier and retort properties and bending resistance; (2) providing a gas barrier film suitably used in a wrapping material for dried food, liquid food or food containing liquids, frozen food and retort food; (3) providing a gas barrier film that can be used in an electronic oven, and can be sterilized by micro waves; (4) providing a gas barrier film suitably used in a paper carton, a tube, a lid and the like, or in a bag in which a free-oxygen absorber functions effectively; and (5) providing a gas barrier film suitably used for cutting off gas for industrial use. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing results of Example 1 and Comparative Example 1 of the present invention, having a ratio of a specific gravity of a thin film to that of quartz as the axis of ordinates and an oxygen permeability of the thin film as the axis of abscissas. DESCRIPTION OF THE PREFERRED EMBODIMENTS The gas barrier film according to the present invention comprises a plastic film and a thin film formed on at least one surface of the plastic film. The plastic film is mainly made of an organic polymer. Examples of the organic polymer include polyethylene, polypropylene, polyethylene terephthalate, polyethylene-2,6-naphthalate, nylon 6, nylon 4, nylon 66, nylon 12, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyamide imide, polyimide, polyether imide, polysulfone, polyphenyl sulfide, polyphenylene oxide and any kind of aromatic polyamides. Such an organic polymer can be used singly, or a copolymer or a blend of two or more of them can be used. A thickness of the plastic film is preferably 1 to 500 µm, and more preferably 5 to 300 µm. Moreover, the plastic film can contain a known additive such as a UV absorber, an antistatic agent, a plasticizer, a lubricant and a coloring agent. When it is used as a transparent gas barrier film, a content of each of the additives must be adjusted so that the resultant film has a light permeability of 50% or more. The thin film formed on at least one surface of the plastic film is made of a composition containing an oxide as a main component. The oxide herein means a compound of a metal, a non-metal or a semiconductor and oxygen. The oxides used according to this invention are silicon oxide and aluminum oxide. When the thin film comprises a composition containing silicon oxide as a main component, the thin film comprises a composition which can contain various types of silicon oxides in which an extent of oxidation of such silicon oxides is not limited. Examples of the silicon oxide include SiO, SiO 2 and the like, and further include a silicon oxide which is less oxidized than SiO. A proportion of such components depends upon the production condition of the thin film. The composition can contain 10% or less of other components unless the characteristics of the thin film are spoiled. When the thin film comprises a composition containing aluminum oxide as a main component, the thin film comprises a composition which can contain various types of aluminum oxides in which an extent of oxidation of such aluminum oxides is not limited. Examples of the aluminum oxide include AlO, Al 2 O 3 and the like, and further include an aluminum oxide which is less oxidized than AlO. A proportion of such components depends upon a production condition of the thin film. The composition can contain 3% or less of other components unless the characteristics of the thin film are spoiled. As described above the specific gravity of the thin film is 55% or more and 83% or less (for silicon oxide) or 83.1% or less (for aluminum oxide) of that of the corresponding naturally occurring oxide or the corresponding oxide having a crystal form generally present at room temperature under atmospheric pressure (hereinafter referred to as the "standard oxide"). A specific gravity herein means a ratio of a mass of a material with a certain volume at a certain temperature to a mass of a reference material (i.e., water at a temperature of 4°C) with the same volume. Some oxides have a plurality of crystal forms. The specific gravity of the standard oxide is herein taken as the specific gravity of that present at room temperature under atmospheric pressure. For example, the specific gravity of silicon oxide is taken as that of quartz, 2.65. The specific gravity of aluminum oxide is taken as that of α-Al 2 O 3 , 3.97. Generally a specific gravity is measured by measuring a mass and a volume of a substance and calculating a ratio of the mass to that of water at a temperature of 4°C with the same volume. But it is difficult to measure the specific gravity of the thin film in this invention because the thin film is laminated onto the plastic film. Therefore, preferably, the thin film is stripped off from the plastic film or the plastic film alone is melted at first. Then the specific gravity of the thin film is measured according to JIS K 7112. For example, in a sink and float test, a sample is sunk in a solution with a known specific gravity, and then the specific gravity of the sample can be determined from the setting and floating state thereof. As such a solution, a mixture of carbon tetrachloride and bromoform or methylene iodide can be used. Alternatively the specific gravity can be measured by a density gradient method in which a sample is sunk in a solution having a continuous density gradient. The oxide thin film formed on a substrate film by evaporation generally has a lower density, i.e., a lower specific gravity than that of a naturally existing oxide. This is because (1) atoms of an atmospheric gas used in the evaporation step such as oxygen and nitrogen are incorporated into the film during the evaporation and (2) an atomic orientation of the thin film is disturbed because the thin film is formed by evaporation. A lower density, i.e., a lower specific gravity degrades the gas barrier property. As described above, a ratio of the specific gravity of the thin film in the gas barrier film of the present invention to the specific gravity of the oxide is higher than the conventional one and is 55% or more, preferably 60% or more and most preferably 63% or more. When the ratio is less than 55%, the thin film can not sufficiently cut off gas because the density of the oxide or the composite oxide is low, i.e., the thin film is coarse. A thin film having a high density of the oxide or the composite oxide therein, i.e., a fine thin film is required to provide a satisfactory gas barrier property. In other words, the thin film must have a large specific gravity. This means that a preferable thin film has a large ratio of the specific gravity of the thin film to that of the oxide or the composite oxide contained therein. When the thin film comprises a composition containing silicon oxide as a main component, the specific gravity of the thin film is preferably 1.80 to 2.20 (i.e., 68 to 83% of the specific gravity of quartz), and more preferably 1.90 to 2.15. When the specific gravity of the thin film is less than 1.80, the structure of the thin film can be too coarse to provide a sufficient gas barrier property to the resultant film. When the specific gravity of the thin film is over 2.20, although the gas barrier property of the resultant film after forming the thin film is excellent, the mechanical property, especially the bending resistance of the resultant film is inferior and the gas barrier property is largely degraded after the gelbo treatment because the thin film is too rigid. Thus such a film is not suitable for use as a gas barrier film. When the thin film comprises a composition containing aluminum oxide as a main component, the specific gravity of the thin film is preferably 2.70 to 3.30 (i.e., 68 to 83% of the specific gravity of α-aluminum oxide), and more preferably 2.80 to 3.20. When the specific gravity of the aluminum oxide thin film is less than 2.70, the structure of the thin film can be too coarse to provide a sufficient gas barrier property to the resultant film. When the specific gravity of the thin film is over 3.30, although the gas barrier property of the resultant film after forming the thin film is excellent, the mechanical property, especially the bending resistance of the resultant film is and the gas barrier property is largely degraded after the gelbo treatment because the thin film is too rigid. Thus such a film is not suitable for use as a gas barrier film. The thickness of the thin film is not limited to, but preferably 5 nm - 800 nm (50 to 8000 angstroms) in view of the gas barrier property and flexibility, more preferably 7 nm - 500 nm (70 to 5000 angstroms) and most preferably 10 nm - 300 nm (100 to 3000 angstroms). The gas barrier film of the present invention is produced as follows: The plastic film is obtained as follows: an organic polymer is melt-extruded, and oriented longitudinally and/or perpendicular to the longitudinal direction, if necessary. Then the polymer is cooled down and thermo-set. The surface of the plastic film can be treated by corona discharge, glow discharge or anchor coating before forming the thin film so as to improve adhesiveness to the thin film unless the satisfactory properties of the film of the present invention are lost. The plastic film can be further provided with printing and/or decoration. The thin film is formed on the plastic film by PVD (physical vapor deposition) such as a vacuum evaporation, a sputtering evaporation, and an ion plating evaporation or CVD (chemical vapor deposition). For example, in the vacuum evaporation, evaporation source materials are heated by resistance heating, high frequency heating or electron beam heating. When a thin film comprising a composition containing silicon oxide as a main component is formed, Si, SiO, SiO 2 and the like are used as the evaporation source materials. When a thin film comprising a composition containing aluminum oxide as a main component is formed, Al, Al 2 O 3 and the like are used as the evaporation source materials. When two or more kinds of the evaporation source materials are used, each material is separately loaded into different crucibles or into a hearth divided by a carbon plate. Each source material is spontaneously heated by different heating sources. When the heating source is an electron beam gun (hereinafter referred to as the "EB gun") the ratio of emission time to respective source materials is, for example, preferably 10 : 10 to 60 : 10 in the case of using Si and SiO 2 as the source materials. In the case of using Al and Al 2 O 3 , the ratio is preferably 10 : 30 to 10 : 60. Furthermore, thin films with various specific gravities can be formed by varying conditions of the evaporation. Such conditions include an emission current of the EB gun and a vapor pressure. Generally, the plastic films to be evaporated are conveyed at a determined rate during the evaporation, and the speed at which the films are conveyed (i.e., the line speed) is included as one of the conditions. The emission current of the EB gun is, for example, preferably 0.8 to 1.5 Ampere (A) in the case of using Si and SiO 2 . In the case of using a combination of Al and Al 2 O 3 the current is preferably 1.0 to 2.0 A. The line speed of the plastic film is, for example, preferably 60 to 120 m/minute (min.) in the case of using Si and SiO 2 as the evaporation source material in the electron evaporation. In the case of using Al and Al 2 O 3 , the rate is preferably 40 to 100 m/min. In the case of using Al in the high frequency induction heating evaporation, the line speed is preferably 20 to 150 m/min. The vapor pressure is, for example, preferably 1.33 x 10 -3 Pa to 0.67 Pa (1 x 10 -5 to 5 x 10 -3 Torr) in the case of using Si and SiO 2 as the evaporation source materials in the electron evaporation. In the case of using Al and Al 2 O 3 , the pressure is preferably 1.33 x 10 -3 Pa to 1.07 Pa (1 x 10 -5 to 8 x 10 -3 Torr). When Al is evaporated by the high frequency induction heating evaporation, the vapor pressure is preferably 1.33 x 10 -3 Pa to 1.07 Pa (1 x 10 -5 to 8 x 10 -3 Torr). As a reactive gas used in the vacuum evaporation, oxygen, nitrogen, vapor or the like is used. A reactive evaporation using an ozone addition or ion assist can be performed instead of introducing such gas. In the vacuum evaporation, a bias voltage can be applied to a substrate, or the substrate can be heated or cooled down unless the objectives of the present invention are spoiled. Also in other production methods such as the sputtering evaporation or CVD, the production conditions can be varied. The gas barrier film obtained in the abovementioned manner can be used singly or other organic polymer films or thin layers can be laminated or coated thereon. An example includes but is not limited to the practical use of a lamination structure of an unoriented polypropylene film/an adhesive layer/a silicon oxide type thin layer/a polyethylene terephthalate film/an adhesive layer/a printed layer/a polyethylene terephthalate film. The gas barrier film obtained in the abovementioned manner is suitable for use in wrapping materials which can be used in an electronic oven, sterilized by micro waves and in which a free-oxygen absorber functions effectively for wrapping dried food, liquid food or food containing a liquid, frozen food and retort food. Examples of the dried food are not limited to but include snacks such as potato chips and popcorn; quickly prepared noodles; table luxuries such as tea and coffee; grains such as flour, rice and wheat. As a material for wrapping the dried food, the gas barrier film of the present invention can be laminated on various films and/or paper. The kinds of gas barrier film and the films and/or paper depend upon characteristics desired by the content to be wrapped. Typical examples include a laminate comprising a polyethylene film laminated on a thin film (i.e., the oxide film) in a gas barrier film of the present invention comprising a polyethyleneterephthalate (PET) film as a base film; one comprising an unoriented polypropylene film laminated on the thin film of the gas barrier film of the present invention comprising a PET film as a base film; one comprising a nylon film and a polyethylene film successively laminated on the thin film in a gas barrier film of the present invention comprising a PET film as a base film; and one comprising a polyethylene film laminated on the thin film in a gas barrier film of the present invention comprising a nylon film as a base film. The method for laminating these films is not limited, but a dry laminate and an extrusion laminate are preferable. Moreover, the gas barrier film can bear a printing, for example, instructions for the use of the contents and the like or can be adhered to a film for decoration or as a reinforcing film. Examples of liquid food or food containing a liquid are not limited to but include various pickles; miso; food contained together with sause; and seasonings such as sause, ketchup and mayonnaise. As a material for wrapping the liquid food or food containing a liquid, various films can be laminated on the gas barrier film depending upon characteristics desired by the content to be wrapped. Typical examples include a laminate comprising a polyethylene film laminated on the thin film in a gas barrier film of the present invention comprising a nylon film as a base film; one comprising an unoriented polypropylene film laminated on the thin film in a gas barrier film of the present invention comprising a nylon film as a base film; one comprising a polyethylene film laminated on the thin film in a gas barrier film of the present invention comprising a PET film as a base film; and one comprising an unoriented polypropylene film laminated on the thin film in a gas barrier film of the present invention comprising a PET film as a base film. The method for laminating these films is not limited, but a dry laminate and an extrusion laminate are preferable. Moreover, the gas barrier film can bear a printing, for example, instructions for the use of the contents and the like or can be adhered to a film for decoration or as a reinforcing film. Furthermore, wrapping bodies can be produced by sticking the gas barrier film onto paper and the like and shaping it into a desired form. The wrapping body is not limited in its shape. Examples include a bag (including a pillow type bag, a paper carton, a three-sided seal bag and a four-sided seal bag), a lid material, a cup, a tube, a standing pack and a lidded container. The gas barrier film of the present invention is used as a whole or a part of such a wrapping material and a wrapping body. The paper carton means a container made of composite paper. The composite paper can be a laminate of the gas barrier film of the present invention on a plastic film and the like. The shape of the paper carton is not limited to but includes a gable top type, a brick type, a rectangular parallelepiped and a cup-like shape. Such a container may have an inner bag or an inlet. Examples of contents which can be contained in such a paper carton include drinks such as milk, yogurt, juice, and soda drinks; alcohols such as sake and whisky; and seasonings such as soy sauce and other sauses. Further, the gas barrier film can be used for cutting off gas for industrial use. Examples The present invention will now be described by way of examples. Each thin film in each gas barrier film obtained in examples and comparative examples described below was evaluated as follows: (1) A method for measuring an oxygen permeability: The amount of oxygen permeated through a film was measured by an oxygen permeability measuring apparatus (produced by MODERN CONTROLS, INC., OX-TRAN 100 (a trade name)) to calculate the amount of oxygen permeated per 1 m 2 and 24 hours under 1 atmospheric pressure (101325 Pa) as an oxygen permeability. (2) A method for retorting: A sample film was allowed to stand at a temperature of 120°C for 30 minutes. (3) A method for the gelbo treatment: A sample film with a size of 28.4 cm x 20.3 cm (11.2 inches x 8 inches) was shaped into the form of a hollow cylinder having a diameter of 8.9 cm (3.5 inches). The cylinder was grasped at two positions near the respective ends with an interval of 17.8 cm (7 inches) therebetween by using a Gelbo Flex Tester (MIL-B131H) (produced by RIGAKU KOGYO CO., LTD.), and then twisted at a temperature of 20°C under a relative humidity of 65% by 400 degrees so as to make the distance between the grasping positions to be 8.9 cm (3.5 inches). This action was repeated at a rate of 40 times/min. (4) A method for measuring a vapor permeability: A vapor permeability was measured in accordance with JIS K 7129. Example 1 Si with purity of 99.99% and SiO 2 with purity of 99.9% both in the shape of particles with a diameter of about 3 to 5 mm were used as evaporation source materials. They were evaporated upon one surface of respective PET films (produced by Toyo Boseki Kabushiki Kaisha; E5007) with a thickness of 12 µm by an electron beam evaporation (hereinafter referred to as the "EB evaporation") to form silicon oxide gas barrier thin films respectively having a thickness of 30 to 100 nm (300 to 1000 angstroms) as shown in Table 1. The evaporation source materials were not mixed but loaded separately into a hearth divided into two portions with a carbon plate. The EB gun (hereinafter called the "EB gun") was used as a heating source, and the loaded Si and SiO 2 were heated separately. The thickness of each thin film was controlled by varying the following conditions within the following ranges: an emission current of the EB gun from 0.8 to 1.5 A; a ratio of emission time to Si to that to SiO 2 from 10 : 10 to 60 : 10; a PET line speed from 60 to 120 m/min.; and a vapor pressure from 1.3 x 10 -3 Pa to 0.67 Pa (1 x 10 -5 to 5 x 10 -3 Torr) by varying a supply amount of oxygen gas. Under such various conditions, six kinds of the thin films as shown in Table 1 were formed on the PET films, respectively, thereby obtaining gas barrier films. After dissolving the PET film of each of the obtained gas barrier films, a specific gravity of each thin film was measured by the sink and float method. A ratio of the measured specific gravity to that of silicon dioxide (quartz), 2.65, was calculated. The amount of permeated oxygen was further measured to calculate the oxygen permeability of each thin film at this point. The results are shown in Table 1. Comparative Example 1 Three kinds of silicon oxide gas barrier thin films were formed in the same manner as in Example 1 except that Si and SiO 2 as the evaporation source materials were mixed and a thickness of each thin film was made to be 100 nm (1000 angstroms). A ratio of the specific gravity of each thin film to that of quartz and the oxygen permeability were obtained in the same manner as in Example 1. The results are shown in Table 1. Each film obtained in Comparative Example 1 was found to be inferior in the barrier property, having a smaller specific gravity. The results of Example 1 and Comparative Example 1 are shown in Figure 1 as a graph showing a ratio of the specific gravity of each of the obtained silicon oxide gas barrier thin films to that of quartz (i.e., a standard oxide) as the axis of ordinates and the oxygen permeability of the thin film as the axis of abscissas. The thin films obtained in Example 1 (indicated by ○) had 55% or more of the above ratio and excellent oxygen gas barrier properties. However, the thin films obtained in Comparative Example 1 (indicated by ×) had less than 55% of the above ratio and inferior oxygen gas barrier properties. Example 2 Al with purity of 99.99% and Al 2 O 3 with purity of 99.9% both in the shape of particles with a diameter of about 3 to 5 mm were used as evaporation source materials. They were evaporated upon one surface of respective PET films (produced by Toyo Boseki Kabushiki Kaisha; E5100) with a thickness of 12 µm by the EB evaporation to form aluminum oxide gas barrier thin films respectively having a thickness of 300 to 5000 angstroms. The evaporation source materials were not mixed but loaded separately into a hearth divided into two portions with a carbon plate. The EB gun was used as a heating source, and the loaded Al and Al 2 O 3 were each heated separately. The thickness of the thin films was controlled by varying the following conditions within the following ranges: an emission current of the EB gun from 1.0 to 2.0 A; a ratio of emission time to Al to that to Al 2 O 3 from 10 : 30 to 10 : 60; a PET line speed from 40 to 100 m/min.; and a vapor pressure from 1.3 x 10 -3 Pa to 1.07 Pa (1 x 10 -5 to 8 x 10 -3 Torr) by varying a supply amount of oxygen gas. Under such various conditions, eleven kinds of the thin films as shown in Table 2 were formed on the PET films, respectively, thereby obtaining gas barrier films. After dissolving the PET film of each of the obtained gas barrier films, a specific gravity of each thin film was measured by the sink and float method. A ratio of the measured specific gravity to that of α-Al 2 O 3 , 3.97, was calculated. The amount of permeated oxygen was further measured to calculate the oxygen permeability of each thin film at this point. The following experiments were conducted on the gas barrier films which were not used for the measurement of the specific gravity. A wrapping film was obtained by dry laminating an unoriented polypropylene film (hereinafter called the "CPP film") with a thickness of 60 µm on each thin film formed on the PET film by using a two-component polyurethane adhesive (with a thickness of 4 µm). After the retorting or the gelbo treatment (for 5 minutes) was performed, the amount of permeated oxygen was measured to obtain the oxygen permeability. The results are shown in Table 2. Comparative Example 2a Aluminum oxide gas barrier thin films were formed in the same manner as in Example 2 except that the vapor pressure during the evaporation was 1.33 x 10 -3 Pa to 0.067 Pa (1 x 10 -5 to 5 x 10 -4 Torr) and a cooling temperature of the chill roller was -20°C to -5°C and a thickness of each of the obtained thin films was in the range of 300 to 500 nm (3000 to 5000 angstroms). A ratio of the specific gravity of each thin film to that of α-Al 2 O 3 and the oxygen permeability were obtained in the same manner as in Example 2. The results are shown in Table 2. Comparative Example 2 Aluminum oxide gas barrier thin films were formed in the same manner as in Example 2 except that Al and Al 2 O 3 as the evaporation source materials were mixed and a thickness of each of the obtained thin films was in the range of 50 to 100 nm (500 to 1000 angstroms). A ratio of the specific gravity of each thin film to that of α-Al 2 O 3 and the oxygen permeability were obtained in the same manner as in Example 2. The results are shown in Table 2. The gas barrier films obtained in Example 2 showed an excellent oxygen barrier property, having an oxygen permeability of 3.0 cm 3 /m 2 ·atm·day (1 atm = 101325 Pa) or less before the gelbo treatment. After the gelbo treatment for 5 minutes, the films were excellent in the oxygen barrier property, having the oxygen permeability raised by about 2 cm 3 /m 2 ·atm·day or less. Furthermore, the gas barrier films obtained in Example 2 also had satisfactory oxygen barrier properties after retorting. The films obtained in Comparative Example 2a were excellent in the oxygen barrier property as in Example 2 except when the films were subjected to the gelbo treatment. The films obtained in Comparative Example 2 have a lower specific gravity and were inferior in the oxygen barrier property. Example 3 Al with purity of 99.9% in the shape of particles with a diameter of about 3 to 5 mm were used as evaporation source material. The Al was evaporated upon one surface of respective PET films (produced by Toyo Boseki Kabushiki Kaisha; E5100) with a thickness of 12 µm by the radio frequency (R.F.) induction heating evaporation to form aluminum oxide gas barrier thin films respectively having a thickness of 50 to 400 nm (500 to 4000 angstroms) under the following conditions: a high frequency electric power was 5 kW, a line speed was 20 to 150 m/min.; and a vapor pressure was varied from 1.33 x 10 -3 to 1.06 Pa (1 x 10 -5 to 8 x 10 -3 Torr) by varying the amount of oxygen gas supply. Moreover, a voltage of -1 kV was applied from a mesh-like electrode provided in front of a chill roller (cooling roller). Four kinds of gas barrier films were obtained by respectively forming the thin films on the PET films. After dissolving the PET film of each of the obtained gas barrier films, a specific gravity of each thin film was measured by the sink and float method. A ratio of the measured specific gravity to that of α-Al 2 O 3 , 3.97, was calculated. The amount of permeated oxygen permeability was further measured to calculate the oxygen permeability of each thin film at this point. The following experiments were conducted on the gas barrier films which were not used for the measurement of the specific gravity. A wrapping film was obtained in the similar method as in Example 2. After the retorting or the gelbo treatment was performed, the amount of permeated oxygen was measured to obtain the oxygen permeability. The results are shown in Table 3. Comparative Example 3a Aluminum oxide gas barrier thin film was obtained in the similar manner as in Example 3 except that the vapor pressure during the evaporation was 1.33 x 10 -3 to 0.067 Pa (1 x 10 -5 to 5 x 10 -4 ) Torr and a cooling temperature of the chill roller was -20°C to -10°C. A ratio of the specific gravity of the obtained thin film to that of α-Al 2 O 3 and the oxygen permeability was obtained in the same manner as in Example 3. The results are shown in Table 3. Comparative Example 3 Aluminum oxide gas barrier thin films were obtained in the same manner as in Example 4 except that an electric field was not applied during the evaporation by the R.F. induction heating. A ratio of the specific gravity of each of the obtained thin films to that of α-Al 2 O 3 and the oxygen permeability were obtained in the same manner as in Example 3. The results are shown in Table 3. The gas barrier films obtained in Example 4 showed an excellent oxygen barrier property, having an oxygen permeability of 3.0 cm 3 /m 2 ·atm·day (1 atm = 101325 Pa) or less before the gelbo treatment. After the gelbo treatment for 5 minutes, the films were excellent in the oxygen barrier property, having the oxygen permeability raised by about 2 cm 3 /m 2 ·atm·day or less. Furthermore, the gas barrier films obtained in Example 3 also had satisfactory oxygen barrier properties after retorting. The film obtained in Comparative Example 3a was excellent in the oxygen barrier property as in Example 3 except when the film was subjected to the gelbo treatment. The films obtained in Comparative Example 3 have a lower specific gravity and were inferior in the oxygen barrier property. Example 4 A silicon oxide gas barrier film with a thickness of 70 nm (700 angstroms) was formed on one surface of a PET film with a thickness of 12 µm by the EB evaporation using Si and SiO 2 used in Example 1 under the conditions used for Sample 1-3. A CPP film with a thickness of 40 µm was dry laminated on the evaporated film using an urethane type adhesive. An oriented polypropylene film (hereinafter referred to as the "OPP film") bearing a printing thereon was dry laminated on the other surface of the PET film. The resultant film was heat sealed to produce a bag with a size of 30 cm x 20 cm. The gelbo treatment was performed for 2.5 minutes by using a gelbo tester. Then potato chips were bagged therein, and the opening was heat sealed. The bag was allowed to stand in a room at a temperature of 25°C with a relative humidity of 65% for 6 months. An eating quality test after that time found that the color, taste and dryness of the potato chips were not changed. Comparative Example 4 An OPP film bearing a printing thereon was dry laminated on the film obtained in Comparative Example 1-2 to produce a bag with a size of 30 cm x 20 cm. As in Example 4, the gelbo treatment was performed for 2.5 minutes by using the gelbo tester, potato chips were bagged therein, and the bag was allowed to stand in a room at a temperature of 25°C with a relative humidity of 65% for 6 months. The eating quality test found that the potato chips had changed color a little, had a slight oxidized flavor, had absorbed moisture and had a less crisp touch. Sample No. Specific Gravity of Thin Film Ratio A (%) Thickness of Thin Film nm (angstroms) Oxygen Permeability (cm 3 /m 2 ·atm·day) Example 1: 1 1.70 64.0 30 (300) 3.0 2 1.80 67.9 30 (300) 2.8 3 1.87 70.6 70 (700) 2.0 4 1.90 71.7 70 (700) 1.8 5 2.05 77.4 100 (1000) 1.4 6 2.12 80.0 100 (1000) 1.0 Comparative Example 1 1 1.40 52.8 100 (1000) 50.0 2 1.35 50.9 100 (1000) 50.0 3 1.31 49.4 100 (1000) 80.0 Sample No. Specific Gravity Ratio A (%) Thickness of Thin Film nm (angstroms) Oxygen Permeability (cm 3 /m 2 ·atm·day) Before Treatment After Retorting After Gelbo Treatment Example 2: 1 2.71 68.3 30 (300) 3.0 4.5 3.5 2 2.87 72.3 50 (500) 2.0 2.6 2.5 3 2.98 75.1 50 (500) 2.0 2.4 2.5 4 2.95 74.3 100 (1000) 1.6 2.0 2.0 5 3.02 76.1 100 (1000) 1.5 2.0 2.0 6 3.15 79.3 100 (1000) 1.5 1.8 2.1 7 3.14 79.1 200 (2000) 1.3 1.5 2.3 8 3.22 81.1 200 (2000) 1.3 1.4 2.3 9 3.00 75.6 300 (3000) 1.2 1.4 2.5 10 3.15 79.3 300 (3000) 1.1 1.5 2.6 11 3.30 83.1 500 (5000) 0.9 1.3 3.0 Comparative Example 2a: 1 3.32 83.6 300 (3000) 1.0 1.7 over 2 3.35 84.4 500 (5000) 0.9 1.4 over Comparative Example 2: 1 2.10 52.9 50 (500) 10.0 over 20.0 2 2.15 54.2 100 (1000) 10.0 over 25.0 Sample No. Specific Gravity Ratio A (%) Thickness of Thin Film nm (angstroms) Oxygen Permeability (cm 3 /m 2 ·atm·day) Before Treatment After Retorting After Gelbo Treatment Example 3: 1 2.72 68.5 50 (500) 2.8 4.3 3.2 2 3.03 76.3 100 (1000) 1.5 2.2 2.0 3 3.18 80.1 200 (2000) 1.2 1.4 2.4 4 3.27 82.4 400 (4000) 0.9 1.4 3.0 Comparative Exampe 3a: - 3.40 85.6 400 (4000) 0.7 1.8 over Comparative Example 3: 1 2.08 52.4 50 (500) 10.0 over 20.0 2 2.16 54.4 100 (1000) 10.0 over 28.0

Claims: A gas barrier film comprising a plastic film and a thin film formed on at least one surface of the plastic film which thin film is made of a composition containing a single oxide selected from aluminum oxide and silicon oxide as the main component wherein the specific gravity of the thin film is 55% or more and 83% or less of the specific gravity of silicon oxide which is 2.65, when the film has silicon oxide as its main component 55% or more and 83.1% or less of the specific gravity of aluminum oxide which is 3.97, when the film has aluminium as its main component. A gas barrier film according to claim 1, wherein the oxide is aluminum oxide and the specific gravity of the thin film is 2.70 to 3.30. A gas barrier film according to claim 1, wherein the oxide is silicon oxide and the specific gravity of the thin film is 1.80 to 2.20. A multilayered gas barrier film comprising the gas barrier of anyone of claims 1 to 3 and a heat seal layer formed on the gas barrier film. A wrapping material comprising the gas barrier film of anyone of claims 1 to 3. A wrapping material comprising the multilayered gas barrier film of claim 4. A shaped body using the wrapping material according to claim 5 or 6.

Inventors: HARIMA TEIZO, ISEKI KIYOSHI, KOBE HIDEOMI, KONTANI TORU, MATSUDA SHUSEI, MORIHARA YOSHIHARU, OYA TOSHIYUKI, UNO TOSHIO, YAMADA YOZO, HARIMA, TEIZO, ISEKI, KIYOSHI, KOBE, HIDEOMI, KONTANI, TORU, MATSUDA, SHUSEI, MORIHARA, YOSHIHARU, OYA, TOSHIYUKI, UNO, TOSHIO, YAMADA, YOZO, Harima, Teizo c/o Toyo Boseki K.K., Res. Center
Applicants: TOYO BOSEKI, TOYO BOSEKI KABUSHIKI KAISHA
Application year: 1992
Main classification: B65D65
IPC codes: B32B15, B32B27, C23C14