HEAT-RESISTANT MOISTURE-PROOF FILM

Abstract: A heat-resistant moisture-proof film consisting of respective layers defined in the following, wherein a second layer is jointed to one surface of a first layer and a third layer is jointed to the outer surface of the first layer; (a) the first layer (A) comprises: a single layer member of a laminated layer member of a transparent laminated layer film constituted by a substrate film formed of PVA having a saponification degree of 99 mol% or thereabove and a transparent silicon oxide thin film (SO thin film) formed on at least one of the surfaces of the said substrate film; (b) the second layer (B) comprises: a single layer member or a laminated layer member of a film or films formed of transparent plastics other than PVA, wherein a transparent SO thin film (4) of 100 - 5000Å is formed on at least one of the surfaces of at least one layer film, and moreover, there is included at least one layer of a heat-resistant transparent film having a sum of absolute values of heat shrinkage factors in the lengthwise and widthwise directions is less than 1 % when heated at 150°C for 30 min and having a light transmission of 85 % or thereabove; (c) the third layer (C) comprises a heat-sealable resin layer.

Description: The present invention relates to a heat-resistant moistureproof film and, more particularly, to a transparent heat-resistant moistureproof film which has a high heat-resistance and a high moistureproofness such that almost no gas such as steam can permeate The heat-resistant moistureproof film according to the present invention is composed of a laminated plastic film and is suitable for use as a packaging film for an electroluminescence device which is used for a back light of a liquid crystal display. Liquid crystal display devices are capable of being driven at a low electric power comsumption. Because of this advantage, liquid crystal devices are used in various fields including OA machines. Organic dispersing electroluminescent devices (hereinunder referred to as "EL devices"), which are thin and light-weight, are coming into widespread use as cheap back lights (auxiliary light sources) of plane-luminescence used for liquid crystal devices. Since EL devices include fluorecent materials such as Zns:Mn and Zns:Cu, the EL devices are packaged in a transparent film which has a high moistureproofness before they are used because their luminance is greatly reduced by moisture. Two-laminated moistureproof films, each having a sealant layer on one side thereof which can be heat-sealed, are generally used as a packaging film for EL devices: An EL device is inserted between the sealant layers of the two-laminated moistureproof film and is passed through a vertical pair of heated rollers at an appropriate nip pressure to heat-seal the sealant layers surrounding the EL device. A laminated film having a fluorinated resin film, especially a film containing polychlorotrifluoroethylene (hereinunder referred to as "PCTFE") as the main ingredient, is often used as a packaging film due to its excellent moistureproofness and transparency. Such a laminated film is generally produced by laminating a polyolefin having a thickness of 20 to 100 µm as a heat-sealant on a PCTFE film having a thickness of 70 to 300 µm. This film is said to have the best moistureproofness of all the existent transparent films because steam hardly permeates. A transparent laminated plastic film produced by forming a transparent metal oxide, especially a silicon or aluminum oxide, thin film on the surface of a transparent plastic base film has recently been commercialized as a packaging material having a gas barrier property. However, a laminated film containing PCTFE as the main ingredient is so expensive that the manufacturing cost of a back light is high. In addition, since the moistureproofness of such a laminated film is reduced when the ambient temperature exceeds 50°C, the life of the EL device packaged therein is extremely shortened when it is used at a high temperature. On the other hand, the moistureproofness of a transparent laminated plastic film having a transparent metal oxide thin film on the surfaces thereof is insufficient, so that it has not yet been used as a packaging film for an EL device. In addition, the present inventors have found that since the packaging films are treated by heat-rollers at 110 to 150 °C in the EL device sealing process, the moistureproofness of some of the base films used for these packaging films deteriorates during the heat-treatment. Development of a transparent plastic film which is suitable for use as a packaging film and which is cheaper and has better moistureproofness than a PCTFE film has therefore been demanded. The present invention seeks to overcome the above-described problems in the prior art and to provide a heat-resistant moistureproof laminated plastic film having excellent strength, heat-resistance and economical efficiency as well as excellent transparency and moistureproofness. As a result of studies undertaken by the present inventors, the following have been found: (1) A film produced by forming a silicon oxide thin film on the surface of a specific polyvinyl alcohol base film to provide a film having excellent transparency and moistureproofness, and then protecting the thus-obtained film with another transparent plastic film coated with a silicon oxide thin film formed thereon, has a stable moistureproofness which can be maintained for a long time even in a high-temperature and high-humidity atomosphere. (2) By using a film having specific physical properties as another transparent plastic film, the inherent gas barrier property thereof is maintained in spite of heating and pressurizing the thus-obtained film during the EL device sealing process, so that the deterioration of the moistureproofness thereof is effectively prevented. The present invention provides a heat-resistant moistureproof film comprising a first layer (A), a second layer (B) bonded to one side of the first layer, and a third layer (C) bonded to the other side of the first layer, the first layer (A), the second layer (B) and the third layer (C) being defined as follows: (a) the first layer (A) is a single transparent laminated film or a laminate thereof, the transparent laminated film comprising a base film composed of a polyvinyl alcohol having a saponification degree of not less than 99 mol% and a transparent silicon oxide thin film having a thickness of 100 to 5000 Å (10 to 500 nm) formed on at least one side thereof; (b) the second layer (B) is a single transparent plastic film other than a polyvinyl alcohol film or a laminate thereof, at least one transparent silicon oxide thin film having a thickness of 100 to 5000 Å (10 to 500 nm) being formed on at least one side of at least one of said plastic films, and at least one of said plastic films being a transparent heat-resistant film having a sum of the absolute values of the percentages of heat-shrinkage in the machine direction and transverse direction when heated at 150°C for 30 minutes of not more than 1% and a light transmittance of not less than 85% (ASTM D-1003); and (c) the third layer (C) is a heat-sealable resin layer. The present invention now will be explained in more detail. (1) First layer (A) The first layer (A) is a single transparent laminated film or a laminate thereof, the transparent laminated film comprising a base film composed of a polyvinyl alcohol (hereinunder referred to as "PVA" ) having a saponification degree of not less than 99 mol% and a transparent silicon oxide thin film (hereinunder referred to as "SO thin film") having a thickness of 100 to 5000 Å (10 to 500 nm) formed on at least one side thereof. It is necessary that the PVA base film is composed of PVA having a saponification degree of not less than 99 mol%. If a film composed of PVA having a saponification degree of less than 99 mol% is used as a base film, it is difficult to obtain excellent moistureproofness even if an SO thin film is formed on the surface thereof. The PVA film may either be an unstretched film or a stretched film. PVA film which is biaxially stretched to about 3 × 3 times is preferable from the viewpoint of film strength and moistureproofness. The thickness of the PVA film generally 5 to 400 µm, preferably 10 to 200 µm. The SO thin film may be formed either on one side or both sides of the PVA film. In order to maintain the high moistureproofness for a long time, the SO thin film is preferably formed only on the surface on which the second layer (B), which functions as a protective layer, is laminated. The SO thin film may be formed by vacuum deposition, sputtering or ion plating using silicon oxide, silicon dioxide or a mixture thereof as a depositing raw material. Alternatively, a reactive deposition method using silicon, silicon oxide, silicon dioxide or a mixture thereof while supplying oxygen gas may be adopted. It is possible to use an anchor coating agent prior to the formation of the SO thin film in order to enhance the adhesion strength between the SO thin film and the base film. Preferable anchor coating agents are isocyanate adhesion promoters, polyethyleneimine adhesion promoters, organic titanium adhesion promoters, polyurethane adhesives and polyester adhesives. Solvent-free adhesives such as polyethylene adhesives, polyester adhesives and polyamide adhesives may also be used as the anchor coating agent. The SO thin film may contain not more than 10 wt% of impurities such as calcium, magnesium, and oxides thereof, since otherwise the intended moistureproofness badly deteriorates. The thickness of the SO thin film is 100 to 5000 Å (10 to 500 nm). If the thickness of the SO thin film is less than 100 Å (10 nm) the moistureproofness of the laminated film produced is insufficient. On the other hand, if the thickness exceeds 5000 Å (500 nm) curling is produced on the laminated film, or the SO thin film itself is apt to cause cracking or separation. The first layer (A) may be either a single transparent laminated film composed of the PVA base film with the transparent SO thin film formed on a one side thereof, or a laminate of at least two films of such transparent laminated films bonded to each other by a transparent adhesive. Examples of the adhesive are urethane adhesives, acrylic adhesives and polyester adhesives. (2) Second layer (B) The second layer (B) is bonded to one side, preferably on the SO thin film, of the first layer (A). The second layer (B) functions as a protective layer, and is composed of a single or a laminate composed of a transparent plastic film other than a PVA film. A transparent silicon oxide thin film having a thickness of 100 to 5000 Å (10 to 500 nm) is formed on at least one side of at least one plastic film which constitutes the second layer (B). The second layer (B) includes at least one transparent heat-resistant film. The transparent heat-resistant film has a sum of the absolute values of the percentages of heat-shrinkage in the machine direction and transverse direction when heated at 150°C for 30 minutes of not more than 1% and a light transmittance of not less than 85% as measured by the method of ASTM D-1003 (hereinunder the light transmittance measured by ASTM D-1003 will be referred to as light transmittance " ). The percentage of heat-shrinkage is measured after the film is heated for 30 minutes in a hot-air oven in which the temperature is set at 150°C. The transparent plastic film other than a PVA film is not specified. For example, a polyethylene terephthalate film, polyacetate film, polypropylene film and polyamide film are usable. The following heat-resistant transparent films are also usable as the transparent plastic film other than a PVA film. A biaxially oriented polyethylene terephthalate film is preferable as the transparent plastic film other than a PVA film. The resin of the transparent heat-resistant film used for the second layer (B) is not limited so long as it has the above-described properties. Polysulfones, polyarylates and polyimides, for example, are usable as the resin of the transparent heat-resistant film. As the polysulfones, aromatic polysulfones having a sulfone bond and an ether bond in the main chain, and having an aromatic residue of following formulae (a) to (d) between these bonds, are preferably used from the viewpoint of heat resistance: Examples of such aromatic polysulfone resins are resins having repeating units of formulae (1) to (10): Among these, an aromatic polysulfone resin having the repeating unit (1) or (2) is preferably used because they are easily commercially available. The polymerization degrees (n) in the above repeating units of formulae (1) to (10) are not specified, so long as the resins allow the formation of a film. However, it is preferred that the polymerization degree (n) in the resin having the repeating unit (2) is 100 to 160 and that the polymerization degree (n) in the resin having the repeating unit (1) is 50 to 80. As the polyarylate, polycondensates of a bivalent phenol and an aromatic dicarboxylic acid are usable. For example, resins having repeating units of formulae (11) to (16) are usable. In the following chemical formulae, (BIS A) represents a bisphenol residue. The polymerization degrees (n) in the above repeating units of formulae (11) to (16) are not specified, so long as the resins allow the formation of a film. However, it is preferred that the polymerization degree (n) in a resin having the repeating unit (11) is 50 to 110 Among these, a polyarylate resin having the repeating unit (11) is preferably used because it is easily commercially available. As the polyimide, in addition to a polyimide in a narrow sense, that is a polyimide having only an imide bond in the main chain, a polyether imide having an imide bond and an ether bond in the main chain, and a polyamideimide having an imide bond and amide bond in the main chain are usable. Among these, aromatic polyimides having an aromatic residue of formulae (e) to (g) which is bonded between the above-described bonds are preferably used from the viewpoint of heat-resistance: As the polyimide, resins having repeating units of formulae (17) to (19) are usable. A resin having a repeating unit of formula (17) is a polyimide, a resin having a repeating unit of formula (18) is a polyether imide and a resin having a repeating unit of formula (19) is a polyamideimide. Some of the commercially available polyimide films are coloured films, but any film having a light transmittance of not less than 85% is usable in the present invention. In the present invention, a film composed of a resin having the repeating unit (1) (generally called a polyether sulfone) is the most preferable. Since the solvent resistance of a film composed of a resin which is generally called a polysulfone and has the repeating unit (2) or a polyarylate is not always sufficient, when the film is bonded to another film by a dry-lamination method as described later, the surface of the film is easily eroded. A film composed of the polyether sulfone is free from such problem. The thickness of the transparent heat-resistant plastic film which constitutes the second layer (B) is not specified, but it is preferably 10 to 100 µm. The heat-resistant moistureproof film of the present invention has in the second layer (B) thereof at least one layer of a transparent heat-resistant film in which the sum of the absolute values of the percentages of heat-shrinkage in the machine direction and transverse direction when heated at 150°C for 30 minutes is not more than 1%. It is believed that it is probably due to the following function of this transparent heat-resistant film that the heat-resistant moistureproof film of the present invention has an excellent moistureproofness. That is, even if the second layer (B) is a laminate, the percentage of heat-shrinkage of the film as a whole is lowered. As a result, when the heat-resistant moistureproof film is heated during the EL device sealing process, not only is the generation of cracking on the SO thin film of the second layer (B) prevented, but also the shrinkage of the base film of the first layer (A) which is bonded to the SO thin film is suppressed, so that generation of cracking on the SO thin film on the surface of the base film is effectively prevented. The transparent heat-resistant film of the second layer (B) has a light transmittance of not less than 85%. Therefore, even if the transparent heat-resistant film of the present invention is laminated on another transparent plastic film, it is possible to maintain a good transparency and to sufficiently transmit the light emitted from the EL device. The second layer (B) is composed of a single-layer or a laminate of the above-described plastic film(s). If it is a single-layer film, it is necessary that the film itself is constituted the above-described transparent heat-resistant film. On the other hand, if the second layer (B) is a laminate, the transparent heat-resistant film is preferably disposed on the outermost layer. In producing the laminate, an adhesive such as a urethane adhesive, acrylic adhesive or polyester adhesive is usable. A transparent SO thin film is formed on at least one side of at least one layer of a transparent plastic film which constitutes a second layer (B), and the second layer (B) is used as a transparent laminated film. If the second layer (B) is a single-layer film, it is necessary that the SO thin film is formed on one side of the film itself (transparent heat-resistant film) so as to constitute a transparent laminated film. The SO thin film is formed by the above-described method to form an SO thin film on the PVA film. The thickness of the SO thin film is 100 to 5000 Å (10 to 500 nm). If the thickness of the SO film is less than 100 Å (10 nm) the moistureproofness is insufficient. On the other hand, if it exceeds 5000 Å (500 nm), curling is produced on the film, or the SO thin film itself is apt to cause cracking or separation. The second layer (B) is preferably bonded to the first layer (A) in such a manner that the transparent heat-resistant film is exposed on the outer surface. Owing to this structure, the SO thin film itself is protected from scratching or crush, so that the problem of lowering of the moistureproofness which may be caused by the breakage of the SO thin film is eliminated. (3) Third layer (C) The third layer (C) is bonded to the first layer (A) on the opposite side of the first layer (A) to the side to which the second layer (B) is bonded, preferably on the surface of the PVA film. The third layer (C) functions as a sealant and is composed of a heat-sealable resin layer. As the heat-sealable resin, general resins such as low-density polyethylene, ethylene-vinyl acetate copolymer, polypropylene and ethylene-vinyl acrylate copolymer (ionomer) are usable. When a heat-resistant moistureproof film is required to have a high moistureproofness, such as an EL device packaging film, an ethylene-acrylate copolymer (EAA) and an ethylene-ethyl acrylate copolymer (EEA) are preferred in order to prevent moisture permeation from the sealed surface. The laminate structure of these layers will now be described. In the case of bonding the second layer (B) and the third layer (C) to the first layer (A), a known method such as a dry-lamination method or an extrusion-lamination method using an adhesive such as a urethane adhesive, acrylic adhesive or polyester adhesive is adopted. The film of the third layer (C), may be either an unstretched film, or a uniaxially or biaxially streched film. The third layer (C) may be laminated either before or after the formation of the SO thin film of the first layer (A). The third layer (C) may be formed on the first layer (A), preferably on the PVA film without using an adhesive, by an extrusion coating method. The heat-resistant moistureproof film of the present invention may be used as a packaging film, especially as an EL device packaging film, but it is also preferably used as a packaging film for other things which are required to have a moistureproof property. Some contents to be packaged often require that the packaging film has an ultraviolet screening property as well as a moistureproofness property. It is possible to impart an ultraviolet screening property, if necessary, to the heat-resistant moistureproof film of the present invention. For example, it is possible to produce a moistureproof film having an ultraviolet screening property by adding a substance having an ultraviolet absorbing activity to the adhesive layers when the three layers are bonded to each other. As an ultraviolet absorber, at least one of the commercially available ultraviolet absorbers such as benzophenone and benzotriazole is used. The amount of ultraviolet absorber used is determined in accordance with the required ultraviolet absorbing activity. It goes without saying that if a film containing a substance having an ultraviolet screening activity is used as the film constituting each layer, ultraviolet screening activity is imparted to the transparent moistureproof film of the present invention. In the transparent moistureproof film of the present invention, the sum of thicknesses of the second layer (B) and the third layer (C) is preferably 50 to 1000 µm, and the total thickness of the moistureproof film of the present invention is preferably 100 to 1100 µm, more preferably 150 to 300 µm, from the viewpoint of mechanical strength, flexibility and economical efficiency. The ratio of the thicknesses of the three layers is preferably (A) : (B) : (C) : = 1 : 1 to 2 : 1 to 10. It is possible to incorporate a transparent plastic film between the first layer (A) and the second layer (B), or between the first layer (A) and the third layer (C), in order to adjust the total thickness of the moistureproof film. Although the plastic film is not specified so long as it is transparent, a film of the same kind as the film which constitutes the second layer (B) is preferably used. The thickness of the plastic film is selected with due consideration of the total thickness of the moistureproof film. An example of the structure of the moistureproof film of the present invention will be explained hereinunder. Figs. 1 to 5 are schematic sectional views of examples of the structure of a heat-resistant moistureproof film of the present invention. In the following explanation, "SO thin film" means a transparent SO thin film and "film" means a transparent heat-resistant film defined in the present invention unless specified otherwise. Fig. 1 shows the fundamental structure of a heat-resistant moistureproof film of the present invention. The first layer (A) is a single-layer of a transparent laminated film produced by forming an SO thin film 2 on one side of a PVA film 1. The second layer (B) composed of a single-layer of a transparent laminated film produced by forming an SO thin film 4 on one side of a film 3 is bonded to the SO thin film 2 of the first layer (A) through an adhesive layer (a), so that the SO thin film 4 faces inside. The third layer (C) composed of a heat-sealable resin is bonded to the PVA film 1 of the first layer (A) through an adhesive layer (a). Fig. 2 shows another structure example of a heat-resistant moistureproof film according to the present invention. This example shows a film in which a laminate of two films is used as the second layer (B) in the structure example in Fig. 1. In the second layer (B) shown in Fig. 2, a laminated film composed of a film 5 and an SO thin film 6 formed on one side thereof is bonded to a transparent laminated film which is produced by forming the SO thin film 4 on one side of the film 3, through an adhesive layer (a) so that the SO thin film 6 faces inside. The second layer (B) composed of this laminate is bonded to the SO thin film 2 of the first layer (A) through the adhesive layer (a), so that the SO thin film 4 faces inside. In the structure shown in Fig. 2, the film 3 may be a transparent plastic film other than the transparent heat-resistant film defined in the present invention. Fig. 3 shows still another structure example of a heat-resistant moistureproof film according to the present invention. In this example, a laminate of two films is used as the second layer (B) in the same way as in the structure shown in Fig. 2. This example is different from that shown in Fig. 2 in that only one layer of the SO thin film is formed in the second layer (B). In the second layer (B) shown in Fig. 3, a film 5 without forming any SO thin film is bonded through the adhesive layer (a) to the SO thin film 4 of the transparent laminated film produced by forming the SO thin film 4 on one side of the film 3. The second layer (B) constituted by this laminate is bonded to the SO thin film 2 of the first layer (A) through the adhesive layer (a), so that the film 3 faces inside. In the structure shown in Fig. 3, the film 3 may also be a transparent plastic film other than the transparent heat-resistant film defined in the present invention. Fig. 4 shows a further structure example of a heat-resistant moistureproof film according to the present invention. In this example, a laminate is used as the second layer (B) in the same way as in the structures shown in Figs. 2 and 3. This example is different from those shown in Figs. 2 and 3 in that an ordinary transparent plastic film which does not have a high heat-resistance, is incorporated, thereby obtaining the laminate composed of the three transparent plastic films. In the second layer (B) shown in Fig. 4, an ordinary transparent plastic film 5 not having a high heat resistance with the SO thin film 6 formed on one side thereof is bonded to the SO thin film 4 of the transparent laminated film produced by forming the SO thin film 4 on one side thereof, through an adhesive layer (a), so that the film 5 faces inside, and a film 7 without forming a SO thin film is bonded to the SO thin film 6 through the adhesive layer (a). The second layer (B) constituted by this laminate is bonded to the SO thin film 2 of the first layer (A) through the adhesive layer (a), so that the film 3 faces inside. In the structure shown in Fig. 4, the film 3 may also be a transparent plastic film other than the transparent heat-resistant film defined in the present invention. Fig. 5 shows a still further structure example of a heat-resistant moistureproof film according to the present invention. This example shows a film in which a laminate is used as the first layer (A) in the structure example shown in Fig.4. Two transparent laminated films each produced by forming the SO thin film 2 on one side of the PVA film 1 are bonded to each other through an adhesive layer (a), thereby obtaining a laminate as the first layer (A) . The second layer (B) composed of the three layers as shown in Fig. 4 is bonded to the SO thin film 2 of the first layer (A) through the adhesive layer (a), so that the film 3 faces inside. In the heat-resistant moistureproof film according to the present invention, the moistureproofness as the fundamental function is realized by the protection of the SO thin film formed on the surface of the PVA film of the first layer (A) by means of the second layer (B). The excellent moistureproofness is guessed to be realized on the basis of the following action. The bonding energy of silicon of the SO thin film formed on a PVA film generally exhibits a specific value in the direction of thickness of this thin film. In the vicinity of the PVA film, silicon has a larger bonding energy than in the surface layer portion or in the intermidiate portion of the SO thin film. The PVA film with such an SO thin film formed thereon has an excellent moistureproofness at normal temperatures. In a high-temperature and high-humidity atmosphere, however, the PVA film moistens and the dimensions thereof change, so that the SO thin film formed on the surface of the PVA film is broken and the inherent moistureproofness is impaired. This problem is solved by protecting the PVA film by another film, but the moistureproofness of an ordinary film, polyvinylidene chloride (PVDC) film or a PVDC-coated film greatly deteriorates at a high temperature, and such a film is hardly expected to have a preventing effect to the moisture absorption of the PVA film at a temperature of not lower than 50°C. In contrast, in a transparent laminated film having an SO thin film used as the second layer (B) in the present invention, the moistureproofness thereof is lowered only slightly at a high temperature, so that it can effectively protect a transparent film of the first layer (A) and maintain the excellent moistureproofness of a PVA film with an SO thin film formed thereon even in a high temperature range. The heat-resistant moistureproof film according to the present invention includes a transparent heat-resistant film having the sum of the absolute values of the percentages of heat-shrinkage in the machine direction and transverse direction when heated at 150°C for 30 minutes, of not more than 1%. For this reason, it is considered that during the EL device sealing process, for example, not only is the generation of cracking on the SO thin film of the second layer (B) prevented, but also the shrinkage of the base film of the first layer (A) which is bonded to the SO thin film is suppressed, so that the generation of cracking on the SO thin film on the surface of the base film is effectively prevented. Accordingly, the heat-resistant moistureproof film according to the present invention is usable as a packaging film not only for back light of a liquid crystal display but also for an EL device which is used in an automobile or an illumination. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 5 are sectional views of heat-resistant moistureproof films according to the present invention. In the Figures, (A) represents a first layer, (B) represents a second layer, (C) represents a third layer, (a) represents an adhesive layer, (1) represents a polyvinyl alcohol film, (2), (4) and (6) represent silicon oxide thin films, and (3), (5) and (7) represent transparent films. The present invention will now be explained in more detail in the following Examples and Comparative Examples. In the following Examples, the physical properties were measured by the following methods. (1) Thickness of SO thin film The thickness of an SO thin film was measured by a quartz film thickness tester. (2) Heat-shrinkage Percentage (%) A film was heated in a hot-air oven at 150°C for 30 minutes, and the heat-shrinkage percentage of the film was obtained from the change in the dimensions of the film in the machine and transverse directions after heating. (3) Moisture permeability (g/m 2 ·24H) A three-side 70 × 70 mm sealed bag (sealing width: 5 mm) was made of a moistureproof film. Thick paper having a high moisture-absorbing property and in the absolutely dry state was inserted into the bag instead of an EL device. The remaining side was sealed by passing the bag through two heating rubber rollers to prepare a sample with thick paper enclosed therein. The sealing conditions were a heating temperature of 150°C and a rolling pressure of 10 kg/cm. 10 samples with thick paper enclosed therein were left to stand in an atmosphere at 50°C and 90% RH for about 500 hours, and the moisture permeability of each moistureproof film was obtained from a change in the total weight. (4) Transparency (%) The visible light transmittance of the film was measured by a spectrophotometer produced by Hitachi Ltd.. The transparency was expressed by the light transmittance at 550 nm. Example 1 The following films were used in this Example. 〈Laminated film (A)〉 A biaxially stretched film (stretch ratio: 3 × 3 times, thickness: 12 µm) composed of PVA having a saponification degree of 99.9 mol% was used as a base film, and a transparent SO thin film of 1000 Å (100 nm) thickness was formed on one side thereof by evaporating silicon oxide (SiO) having a purity of 99.9% under heating by an electron beam under a vacuum of 5 × 10 -5 torr (6.7 × 10 -3 Pa). 〈Laminated film (B)〉 A combination of the following laminated films (B - 1) and (B - 2) was used. (1) Laminated film (B - 1) Two biaxially stretched polyethylene terephthalate (hereinunder referred to as "PET") films each having an SO thin film of 1000 Å (100 nm) thickness formed on one side thereof in the same way as in the laminated film (A), a stretch ratio of 3 × 3 times and a thickness of 12 µm were prepared. The SO thin film of one of the PET films was bonded to the non-SO thin film side of the other PET film through a urethane adhesive. (2) Heat-resistant film (B - 2) An unstretched polyether sulfone film of 50 µm thickness was prepared. The percentage of shrinkage of the film after being heated at 150°C for 30 minutes was 0.10% in the machine direction (MD) and 0.09% in the transverse direction (TD), and the light transmittance was 88%. 〈Sealant film (C)〉 An unstretched film of an ethylene-ethyl acrylate copolymer (EEA) having a thickness of 50 µm was prepared. The SO thin film side of the laminated film (A) was first bonded to the non-SO thin film side of the laminated film (B - 1) through a urethane adhesive (hereinunder referred to simply as "adhesive"). The SO thin film side of the bonded laminated film was then bonded to the heat-resistant film (B - 1) through an adhesive, thereby producing a four-layered laminated film. The sealant film (C) was bonded to the laminated film (A) of the four-layered laminated film through an adhesive, thereby producing a transparent five-layered film, as shown in Fig. 4. Examples 2 to 7 and Comparative Examples 1 to 5 Various laminated films were obtained in the same way as in Example 1 except that the structures of the films were varied as shown in Table 1. The moisture permeability and the transparency of the laminated films obtained were measured by the above-described methods. The results are shown in Table 1. In Table 1, the heat-resistant films (B - 2) are represented by the following. PES: a film of a polyether sulfone having a repeating unit of formula (1) [TALPA-1000 (trade name) produced by Mitsui Toatsu Chemical Inc. ] (Tg: 225°C). PSF: a film of a polysulfone having a repeating unit of formula (2) [SUMILITE FS-1300 (trade name) produced by Sumitomo Bakelite Co., Ltd.] (Tg: 223°C), PAr: a film of a polyarylate having a repeating unit of formula (11) [Espex-R (trade name) produced by Sumitomo Chemical Co., Ltd.] (Tg: 193°C). PI: a film of a polyimide having a repeating unit of formula (17). PET: a polyethylene terephthalate film. The sealant films in Examples 2 to 7 and Comparative Examples 1 to 5 have the same structure as that in Example 1. As is seen from Table 1, a heat-resistant moistureproof film having improved moisture permeability and transparency is produced when the second layer (B) includes a predetermined transparent heat-resistant film. The heat-resistant moistureproof film of the present invention has excellent transparency and excellent moistureproofness. It has also flexibility, high mechanical strength and economical efficiency. Even if the film is used under severe conditions for a long time, the moistureproofness does not deteriorate. Accordingly, the heat-resistant moistureproof film of the present invention is suitable as a packaging film for an EL device for a back light of a liquid crystal display, which is used under severe conditions for a long time and hence required to have a high moistureproofness. Thus, the present invention has a high industrial value.

Claims: A heat-resistant moistureproof film comprising a first layer (A), a second layer (B) bonded to one side of said first layer, and a third layer (C) bonded to the other side of said first layer; said first layer (A), second layer (B) and third layer (C) being defined as follows: (a) the first layer (A) is a single transparent laminated film or a laminate thereof, the transparent laminated film comprising a base film composed of a polyvinyl alcohol having a saponification degree of not less than 99 mol% and a transparent silicon oxide thin film having a thickness of 100 to 5000 Å (10 to 500 nm) formed on at least one side thereof; (b) the second layer (B) is a single transparent plastic film other than a polyvinyl alcohol film, or a laminate thereof, at least one transparent silicon oxide thin film having a thickness of 100 to 5000 Å (10 to 500 nm) being formed on at least one side of at least one of said plastic films, and at least one of said plastic films being a transparent heat-resistant film having a sum of absolute values of the percentages of heat-shrinkage in the machine direction and transverse direction when heated at 150°C for 30 minutes of not more than 1% and a light transmittance of not less than 85% (ASTM D-1003); and (c) the third layer (C) is a heat-sealable resin layer. A film according to Claim 1, wherein said transparent heat-resistant film has a thickness of 10 to 100 µm. A film according to Claim 1 or 2, wherein said transparent heat-resistant film is composed of a polyether sulfone having a repeating unit of formula: A film according to any one of Claims 1 to 3, wherein said silicon oxide thin film is formed by a vacuum deposition method, sputtering method or ion plating method. A film according to any one of Claims 1 to 4, wherein said base film of said first layer (A) has a thickness of 5 to 400 µm, and the total thickness of said second layer (B) and said third layer (C) is 50 to 1000 µm.

Inventors: OHASHI SHINICHI, SAWADA TSUTOMU, YOSHIDA SHIGENOBU, OHASHI, SHINICHI, SAWADA, TSUTOMU, YOSHIDA, SHIGENOBU
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1991
Main classification: B32B9
IPC codes: B65D65, B32B27, H05B33, G02F1