SEALED CONTAINER FOR COOKING IN MICROWAVE RANGE

Abstract: A container body is sealed with a cover consisting of an insulating flexible sheet, and an antenna consisting of a con­ ductive band is laminated on the cover. Consequently, when the container body is exposed to microwaves in a microwave range, the energy of the microwaves is focused to a position in the vicinity of the tip portion of the antenna. This energy is converted into heat, with which the cover is opened. The cover is thus melt-opened reliably to a suitable size. ABSTRACT A container body is sealed by a lid made of a flexible sheet having an insulative property and an antenna made of an electrically conductive belt is laminated on the lid. According to this construction, when the container body is exposed to a microwave in a microwave oven, a microwave energy is concentrated at a portion near the front end of the antenna and the microwave energy is converted into a heat energy, whereby the lid is opened by the thus converted heat. The lid can thus be precisely fused and opened with suitable size.

Description: INDUSTRIAL FIELD This invention relates to a sealed container for accommodating food therein and, more particularly, to a sealed container to be cooked in a microwave oven in which an opening is automatically opened in a lid of the sealed container for discharging vapor generated at a time when the container filled with the food is heated in order to be cooked in the microwave oven. BACKGROUND ART In a case where it is required to heat and cook food contained in a sealed container made of a plastic or paper material, it is convenient to heat the container with the food contained in a wrapped condition. Namely, when the food is heated in an electronic oven, particularly a microwave oven, in a wrapped condition, the container is liable to be broken by an inner pressure resulting from the vapor generated during the heating process. In such a case, the food contained in the container may be scattered outwardly. On the other hand, when the food in the container is heated with the lid of the container removed, the food may be denatured due to the diffusion of the vapour generated during the heating process. In order to eliminate this problem, there is provided means for automatically opening the lid of the container when the food sealed in the container is heated in the microwave oven. According to the technique disclosed in the Japanese Patent Laid-open Publication No. 61-69576, a sticking layer made of non-metallic type microwave absorbing material is applied on the lid of the container. In addition, according to the technique disclosed in the U.S Patent No. 4,210,674, an electrically conductive material is applied on the lid of the container and the dimensions and shape of the conductive material are so designed as to absorb the microwaves. According to these methods, the lid is heated and fused to be opened by the provision of the microwave absorbing material. US 4 640 838 describes a sealed container for containing food to be cooked within a microwave oven, comprising: a container body having an opening defined within a upper portion thereof; a lid fabricated from a flexible sheet having insulative properties, for sealing the opening of the container body; and a microwave absorber laminated upon the lid. With the electronic oven or microwave oven, a standing wave is generated in the casing and, hence, the strength of the microwave is not uniformly distributed. With the methods of the conventional techniques described above, in which the microwave is absorbed and the lid is opened by the heat generating material or element, in the situation where the heat generating material is applied on a portion at which the strength of the microwave is weak, there may arise the case where the lid is not opened due to inadequate heating of that particular region. In the case where the heat generating material being of a large size is utilized to open the container lid in the low strength region of the microwave, if the heat generating material is positioned in the high strength region of the microwave, the widened opening is formed in the lid of the container, through which a large amount of the vapor is dispersed and the food will be denatured. Accordingly, this invention was conceived to solve the problems of the prior art described above, and an object of this invention is to provide a sealed container to be cooked in a microwave oven in which when a container sealed with a lid made of an insulative flexible sheet material is exposed to the microwaves in the electronic oven, the lid is rapidly heated in a short time to provide a fused opening having a precisely controlled size. Another object of this invention is to provide a sealed container to be cooked in a microwave oven in which a lid of the container is rapidly heated in a short time to provide a fused opening having a precisely controlled size even in the case where there exist regions in the interior of the oven at which the oscillation directions of the microwaves are different from each other. A further object of this invention is to provide a sealed container to be cooked in a microwave oven in which when a container sealed with a lid made of an insulative flexible sheet material or a container in form of a bag made of an insulative flexible material is exposed to the microwaves in the microwave oven, the lid of the container or the container bag or pouch can be surely opened after a predetermined time has elapsed, and also provide the same with reduced cost. DISCLOSURE OF THE INVENTION In order to achieve the objects described above, according to this invenion, there is provided a sealed container to be cooked in a microwave oven, characterized in that an antenna made of an electrically conductive material is laminated on a lid of a container body to be sealed by the lid made of an insulative flexible material, an energy caused by microwaves is concentrated near the opposing front portions of the antenna when the container body is exposed to the microwaves in the microwave oven, and the energy is converted into a heat energy by which the lid is opened. The heat generating operation is further increased by arranging the heat generating material to a region at which the microwave energy is concentrated. In such a case the microwave energy is converted into heat energy by arranging the heat generating material at the corner portion of the antenna instead of arranging the same at the opposing front portions thereof, whereby the lid is opened by the heat energy. According to the structure described above, an alternating current passes the antenna constructed of a good electrically conductive material when the microwaves are generated, and high electric field and high magnetic field of high frequency are generated at a portion at which antennas are opposed or at a corner portion of the antenna. In the case where a resistance acting as a heat generating element is arranged at the portion where the high electric field and high magnetic field are created, a heat is generated due to the Joule heat caused by the current passing through the heat generating element, and where a dielectric element acting as a heat generating element is arranged at the portion of the high electric field region, a heat is generated by the dielectric heating of the dielectric element. Furthermore, in the case where a magnetic material acting as a heat generating element is arranged at a portion where the high magnetic field is created, a heat is generated by the hysterisis phenomenon of the magnetism. The antenna does not substantially generate the heat, so that the heat diffusion from the antenna is reduced even if a large-sized antenna is utilized, and since a large amount of the energy concentrated by the antenna having a large size is converted into the heat energy by the heat generating element having a small size, the lid of the container can be rapidly heated to a high temperature, and as a result the lid of the sealed container can be securely opened when the container is heated and cooked in the microwave oven. Furthermore, in order to achieve the objects of this invention, according to this invention, there is provided a sealed container to be cooked in a microwave oven, characterized in that an antenna made of an electrically conductive material is disposed on a lid of the container body sealed by the lid made of an insulative flexible sheet material, the antenna being provided with a portion inclined at an angle of more than 3° with respect to the horizontal plane so as to concentrate microwaves to a specified portion of the antenna and when the container is exposed to the microwaves in the electronic oven, energy concentrated by the antenna due to the microwaves is converted into heat energy, the lid is opened by the thus converted heat. Various kinds of antennas can be utilized. When a belt shape antenna is utilized, the microwave energy is concentrated at the front portion thereof, when a C-shaped antenna is utilized, the energy is concentrated at the opposing ends thereof, and when a plurality of antennas are utilized in combination, the energy is concentrated at a portion at which end portions of the antennas are arranged close to each other. The thus concentrated energy due to the microwaves is converted into heat energy by the heat generating element laminated on the lid, and the microwave energy may be also converted into the heat energy by the dielectric loss of the lid. According to the structure described above, standing waves are caused in the interior of the microwave oven and the microwaves in the microwave oven are reflected with the vertical and horizontal oscillation components on the vertical and horizontal walls thereof because of the rectangular configuration of the microwave oven. The vertical and horizontal oscillation components are sometime different from each other in their strengths. According to this invention the antenna for receiving the microwaves, since the antenna is provided with vertically and horizontally directed portions, the microwave energy can be effectively concentrated even if the vertical and horizontal oscillating components of the microwaves are different in their strength. The thus concentrated microwave energy is converted into heat energy by the heat generating element on the lid of the container to thereby fuse the opening of the lid. Accordingly in the case where the microwave oven includes the inner regions in which the oscillating directions of the microwaves are different, the lid of the sealed container can be rapidly heated and fused in a short time, thus surely forming an opening having a proper size. Furthermore, in order to achieve the objects, according to this invention, there is provided a sealed container to be cooked in a microwave oven, characterized in that an electrically conductive layer in shape of a linear belt for concentrating microwaves and a heat generating layer for converting the microwave energy into a heat energy are aranged at a distance of 0 to 3.5 mm, these layers being contained in a label and the label is sticked on a lid, made of an insulative flexible material, of a sealed container or on a bag or pouch made of a flexible material having an insulative property. On the label are laminated a base layer, an adhesive layer, a liner conductive layer and a heat generating layer, and the liner conductive layer has a length more than 20mm and a surface resistivity less than 1Ω /cm². According to the structure described above, since the conductive layer has an adequate length, the conductive layer can stably concentrate the microwave energy to the heat generating layer even in a location of the standing wave region of the microwaves. The heat generating element is preferrably made of a material suitable for converting the microwave energy into heat energy such as a conductive material, having a high resistivity, dielectric material or magnetic material, or a combination of these materials. The microwave energy is converted into heat energy by the resistance loss, dielectric loss, or magnetism loss and the lid of the container body of a part or a label of the bag is fused by the thus converted heat to thereby form an opening. The base layer is arranged so as to provide good rigidity to the label to easily release the label from a releasing layer or to stick the label to the lid, thus reducing the manufacturing cost. The adhesive layer serves to stick the label to the lid or the pouch. The conductive layer serving as the antenna has a long length of about 20mm, so that the microwave energy is effectively concentrated and the lid or bag can be opened regardless of the location of the container or bag in the microwave oven. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of a sealed container according to the first embodiment of this invention; Fig. 2 is a sectional view taken along the line II-II in Fig. 1; Fig. 3 is a plan view of a lid of a sealed container according to the second embodiment of this invention; Fig. 4 is a plan view of a lid of a sealed container according to the third embodiment of this invention; Fig. 5 is a plan view of a lid of a sealed container according to the fourth embodiment of this invention; Fig. 6 is a plan view of a lid of a sealed container according to the fifth embodiment of this invention; Fig. 7 is a plan view of a lid of a sealed container according to the sixth embodiment of this invention; Fig. 8 is a plan view of a lid of a sealed container according to the seventh embodiment of this invention; Fig. 9 is a plan view of a lid of a sealed container according to the eighth embodiment of this invention; Fig. 10 is a plan view of a lid of a sealed container according to the ninth embodiment of this invention; Fig. 11 is a side view of the sealed container shown in Fig. 10; Fig. 12 is a cross sectional view of the sealed container shown in Fig. 10; Fig. 13 is a side view of a sealed container according to the tenth embodiment of this invention; Fig. 14 is a side view of a sealed container according to the eleventh embodiment of this invention; Fig. 15 is a side view of a sealed container according to the twelfth embodiment of this invention; Fig. 16 is a plan view of a sealed container according to the thirteenth embodiment of this invention; Fig. 17 is a side view of the sealed container shown in Fig. 16; Fig. 18 is a plan view of sealed container according to fourteenth embodiment of this invention; Fig. 19 is a side view of the sealed container shown in Fig. 18; Fig. 20 is a perspective view of a sealed container according to the fifteenth embodiment of this invention; Fig. 21 is a perspective view of a sealed container according to the sixteenth embodiment of this invention; Fig. 22 is a perspective view of a sealed container according to the seventeenth embodiment of this invention; Fig. 23 is a perspective view of a sealed container according to the eighteenth embodiment of this invention; Fig. 24 is a perspective view of a sealed container according to the nineteenth embodiment of this invention; Fig. 25 is a perspective view of a sealed container according to the twentieth embodiment of this invention; Fig. 26 is a sectional view of a label according to the twenty-first embodiment of this invention; Fig. 27 is a plan view of the label shown in Fig. 26; Fig. 28 is a sectional view of a label according to the twenty-second embodiment of this invention; and Fig. 29 is a sectional view of a label according to the twenty-third embodiment of this invention. BEST MODES FOR EMBODYING THE INVENTION The embodiments according to this invention will be described hereunder with reference to the drawings. Figs. 1 and 2 represent the first embodiment according to this invention. Referring to the figures, reference numeral 1 designates a lid made of a plastic sheet having a thermoplasticity and adapted to heat seal a flanged portion 2a of a container body 2 which is made of a material having plasticity and into which a preserved food 3 is accommodated. A C-shaped antenna 4 made of an electrically conductive material and a heat generating element or material 5 made of a high resisting material are laminated on the surface of the lid 1. The heat generating element 5 is arranged at an upper portion of the heat seal portion of the lid 1 and the antenna 4 is positioned so that the heat generating element 5 is arranged between both the ends of the C-shaped antenna 4. The antenna 4 is prepared by the coating with a conductive coating agent containing conductive powders, a vacuum evaporation, a spattering method, a flame coating of a conductive material, a chemical placing method, or a bonding of a metallic foil, and the heat generating element 5 is formed by the coating with a binder including high resistance powders or the bonding of a conductive plastic sheet. These antenna 4 and the heat generating element 5 may be directly laminated on the lid 1 or once laminated on a plastic sheet to form a label, which is then bonded on the lid 1. In the case where the heat generating element 5 is formed by the coating with the binder including the high resistance powders, the binder is fused when the heat generating element 5 is heated to a temperature more than a predetermined value, and when the binder is fused, the heat generation of the heat generating element stops, so that the lid 1 cannot be abnormally heated thereby to prevent the lid from being subjected to the heat decomposition. A foamable material is disposed on the flange 2a of the container body 2 at a portion corresponding to the location of the heat generating element 5 arranged on the lid 1. When the enclosure thus constructed is exposed to the microwaves in the interior of the microwave oven, the current passing the antenna 4 creates a high electric field for the heat generating element 5 to pass the current through the heat generating element 5, whereby the heat generating element 5 generates heat and the sealed portion heated by the heat generating element 5 is fused and opened. After the cooking in the microwave oven, the lid 1 can be easily separated from the container body 2 by pulling the opened seal portion. The location of the foamable material at the seal portion can further facilitate the easy separation of the lid 1 from the container body 2. Fig. 3 is a plan view of a lid of a sealed container according to the second embodiment of this invention. Referring to the figure, reference numeral 4 designates a C-shaped antenna made of an electrically conductive material and the antenna 4 is laminated on the lid 1 as described with reference to the first embodiment. The lid 1 is heat sealed by substantially the same manner as that described in the first embodiment. According to this embodiment, a high electric field is created between the opposing ends of the C-shaped antenna 4 through which a current passes caused by the microwaves, whereby a portion 1' near the front ends of the antenna 4 is fused and opened by the dielectric heating. Fig. 4 is a plan view of the lid used for the sealed container of the third embodiment of this invention. Referring to the figure, reference numerals 4 designate antennas in form of linear belts made of an electrically conductive material, which are to be laminated on the lid 1 by substantially the same manner as that described with reference to the first embodiment. A heat generating element 5 is also laminated on the lid 1 so as to connect the opposing ends of the antennas 4 and 4. The heat generating element 5 may be formed from the same material as that of the antenna 4 or from a different material different from that of the antenna, but it is necessary for the heat generating element 5 to have a resistance per unit length larger than that of the antenna 4. In the case where the heat generating element 5 is made of the same material as that of the antenna 4, the heat generating element 5 should be designed to have a width or thickness smaller than that of the antenna 4. In the illustrated embodiment, the heat generating element 5 is connected to the antennas 4, but the heat generating element 5 may also be disposed close to the end portions of the antennas 4 and 4. When the lid 1 of the construction described above is utilized in the manner described with reference to the first embodiment, the high electric field created between the antennas 4 enables the passage of the current through the heat generating element 5, thereby heat is generated by the heat generating element 5 generates heat. Fig. 5 is a plan view of the lid used for a sealed container according to the fourth embodiment of this invention. Referring to the figure, reference numeral 4 designates an antenna made of an electrically conductive material and formed substantially in shape of a belt, which is to be laminated on the lid 1 as described with reference to the first embodiment. A heat generating element 5 made of a high resistance material is laminated on the lid 1 in the manner such that the heat generating element 5 contacts the front end of the antenna 4. In the illustrated embodiment, the heat generating element 5 contacts the front end of the antenna 4, but the heat generating element 5 may also be disposed close to the antenna 4. When the lid 1 of the construction described above is utilized in the manner described with reference to the first embodiment, the high electric field created by the antenna 4 is applied to the heat generating element 5 so as to heat the heat generating element 5, thereby the lid 1 is fused and opened. Fig. 6 is a plan view of a lid used for a sealed container according to the fifth embodiment of this invention. Referring to the figure, reference numerals 4 designate antennas made of an electrically conductive material and laminated on the lid 1 as described with respect to the first embodiment in the manner such that the front end portions of the antennas 4 are close and opposite each other. When the lid 1 of the construction described above is utilized in the manner described with reference to the first embodiment, the high electric field is created between the antennas 4, and the portion 1′ of the lid 1 which is close to the antennas 4 is fused and opened by the dielectric heating. Fig. 7 is a plan view of a lid used for a sealed container according to the sixth embodiment of this invention. Referring to the figure, reference numeral 4 designate antennas made of an electrically conductive material and formed in arcuate shapes, and an arcuate heat generating element 5 made of a ferrite SF-547 manufactured by TODA KOGYO and an epoxy resin and laminated on the lid 1 in close to and along the arcuate longitudinal sides of the antennas 4 as described with reference to the first embodiment. When the lid 1 having the construction described above is utilized as in the first embodiment, the heat generating element made of the ferrite and the epoxy resin generates the heat, whereby the lid 1 is opened in a linear shape. Fig. 8 is a plan view of a lid used for a sealed container according to the seventh embodiment of this invention. Referring to the figure, reference numeral 4 designate antennas made of an electrically conductive material and the antennas 4 are laminated on the lid 1 in the state that a heat generating element 5 made of a metallic aluminum evaporation film is laminated on the lid 1 as described with reference to the first embodiment in close to and between the opposing ends of the antennas 4. When the lid 1 having the construction described above is utilized as in the first embodiment, the heat generating element made of the evaporation film is heated by the electric field caused between the antennas 4 and the lid 1 is thereby opened. Fig. 9 is a plan view of a lid of a sealed container according to the eigthth embodiment of this invention. Referring to the figure, reference numeral 4 designates a C-shaped antenna made of an electrically conductive material, and reference numeral 5 designates a heat generating element made of a barium titanate and an epoxy resin and laminated on the lid 1 as described with reference to the first embodiment close to the antenna 4. When the lid 1 is utilized as described in the first embodiment, the heat generating element 5 made of the barium titanate-epoxy resin is dielectrically heated by the high electric field caused between the end portions of the antenna 4, and as a result the lid 1 is opened. Figs. 10 to 12 represent the ninth embodiment according to this invention. Referring to the figures, reference numeral 1 designates a lid made of a flexible sheet having an insulative property, and the lid 1 is heat sealed to a flanged portion of a container body 2 made of a plastic material into which a preserved food 3 is accommodated. A lamination layer composed of an antenna 4 made of an electrically conductive material and formed in shape of a linear belt and a flexible sheet 6 having an insulative property is bonded to the lid 1. A heat generating element 5 is bonded to the lid 1 close to the antenna 4. The flexible sheet 6 is composed of two sheet elements which have portions bonded to each other and the remaining portions, not bonded to each other, are bonded to the lid 1 so as to extend in opposite directions. The portions laminated due to the elasticities thereof extend vertically upwardly. The flexible sheet 6 is bent substantially horizontally by temporarily applying a lid 8 as shown in Fig. 12 when the sealed container is prepared, but when the sealed container is entered into the electronic oven, the lid 8 is removed and the flexible sheet 6 then returns the standing position as shown in Fig. 11. In the state shown in Fig. 11, the antenna 4 extends in the vertical and horizontal directions substantially in an L shape, so that the antenna 4 can effectively direct the microwave energy to the heat generating element 5 even in the case where either one of vertical or horizontal component of the microwaves is strong, whereby the heat generating element 5 generates heat adequate to fuse and open the lid 1. Fig. 13 is a side view of a sealed container according to the tenth embodiment of this invention. Referring to the figure, reference numeral 4 designates an antenna made of an electrically conductive material which is laminated with flexible sheets 6 and 6' having an insulative property in a sandwitched manner. The coefficient of thermal contraction of the flexible sheet 6' in the longitudinal direction thereof is larger than that of the flexible sheet 6. A part of the thus formed lamination layer is bonded to the lid 1 made of an insulative flexible sheet through a layer 7 of a bonding agent. A heat generating element 5 is bonded to the lid 1 at a portion close to one end of the antenna 4. When the sealed container thus formed is heated in the electronic oven, the antenna 4 is bent so as to have a vertical component as shown by two dots and dash lines in Fig. 13 by the thermal contraction of the flexible sheets 6 and 6'. Fig. 14 is a side view of a sealed container according to the eleventh embodiment of this invention. A lid 1 made of a flexible sheet having an insulative property is heat sealed to a flanged portion of a container body 2 so as to have an inclination α with respect to the bottom surface of the container body 2. It is desired that the inclination α is a value more than 3° . The lamination layer composed of the antenna 4 and the flexible sheet 6 is bonded to the lid 1 so that the lamination layer inclines at the largest magnitude in the longitudinal direction of the lid 1. The heat generating element 5 is bonded to the lid 1 at a portion close to the end of the antenna 4. Accordingly, in this embodiment, the antenna 4 has the vertical and horizontal components in the longitudinal direction thereof. Fig. 15 is a side view of a sealed container according to the twelfth embodiment of this invention. A lid 1 made of a flexible sheet having an insulative property is heat sealed to a flanged portion of a container body 2. A lamination layer composed of the conductive antenna 4 and the insulative flexible sheet is bonded to the lid and to the side of the container body 2. A heat generating element 5 is bonded to the lid 1 at a portion close to the end of the antenna 4. In this embodiment, the antenna 4 also has the vertical and horizontal components in the longitudinal directions thereof. Figs. 16 and 17 represent the thirteenth embodiment according to this invention, in which a lamination layer 14a and 14b each composed of a belt-shape antenna and an insulative flexible sheet are utilized. The lamination layer 14b is entirely bonded to the lid 1, but the lamination layer 14a is bonded to the lid 1 at one portion and the remaining portion, not bonded to the lid, extends vertically by the elastic property itself. The microwave energy is concentrated between the lamination layers 14a and 14b where the energy is converted into the heat energy due to the dielectric loss of the lid 1 thereby to fuse and open the lid 1. Figs. 18 and 19 represent the fourteenth embodiment according to this invention, in which a lamination layer composed of an antenna 14a and an insulative flexible sheet 14b are disposed on the lid 1 in form of a belt. The lamination 14b is entirely bonded to the lid 1 and the lamination 14a is bonded to the lid 1 and the side of the container body 2. The microwave energy is concentrated to a portion between the lamination layers 14a and 14b, where the energy is then converted into the heat energy by the dielectric loss of the lid 1 thereby to fuse and open the lid 1. In the fifteenth embodiment according to this invention shown in Fig. 20, a C-shaped lamination layer 14a composed of an antenna and an insulative flexible sheet is utilized, and a part of the lamination layer 14a is bonded to the lid 1 and the remaining part extends substantially vertically as viewed by the self-elasticity of the lamination layer 14a. The microwave energy is concentrated to a portion between the opposing ends of the C-shaped lamination layer 14a and this energy is converted into heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. In the sixteenth embodiment according to this invention shown in Fig. 21, a C-shaped lamination layer 14a of an antenna and an insulative flexible sheet is utilized, and the lamination layer 14a is bonded both to the lid 1 and to the side of the container body 2. The microwave energy is concentrated to a portion between the opposing ends of the C-shaped lamination layer 14a and this energy is converted into the heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. In the seventeenth embodiment according to this invention shown in Fig. 22, two types of lamination layers 14a and 14b constituting the antennas and the insulative flexible sheets are utilized. The lamination layers 14b and 14b are entirely bonded to the lid 1, but the lamination layer 14a is partially bonded thereto. The remaining portion of the lamination layer 14a extends substantially vertically as viewed by the self-elasticity of the lamination layer 14a. The microwave energy is concentrated to a portion between the lamination layers 14b and the lamination layer 14a, where the microwave energy is converted into the heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. In the eighteenth embodiment according to this invention shown in Fig. 23, two types of lamination layers 14a and 14b constituting the antennas and the insulative flexible sheets are utilized. The lamination layers 14b and 14b are entirely bonded on the lid 1, but the lamination layer 14a is bonded both to the lid 1 and to the side of the container body 2. The microwave energy is concentrated to a portion between the lamination layers 14b and the lamination layer 14a, where the microwave energy is converted into the heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. In the nineteenth embodiment according to this invention shown in Fig. 24, a C-shaped lamination layer 14b composed of an antenna and an insulative flexible sheet and a belt-shaped lamination 14a composed of an antenna and an insulative flexible sheet are utilized. The lamination layer 14b is entirely bonded to the lid 1, but the lamination layer 14a is partially bonded to the lid 1 and the remaining part extends substantially apart from the lid 1 as viewed by the self-elasticity of the lamination layer 14a. Microwave energy is concentrated to a portion between the lamination layer 14a and the opposing ends of the C-shaped lamination layer 14b, where this energy is converted into the heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. In the twentieth embodiment according to this invention shown in Fig. 25, a C-shaped lamination layer 14b composed of an antenna and an insulative flexible sheet and a belt-shaped lamination layer 14a composed of an antenna and an insulative flexible sheet are utilized. The lamination layer 14b is entirely bonded to the lid 1, but the lamination layer 14a is bonded both to the lid 1 and to the side of the container body 2. The microwave energy is concentrated to a portion between the lamination layer 14a and the opposing ends of the C-shaped lamination layer 14b, where this energy is converted into the heat energy by the dielectric loss of the lid thereby to fuse and open the lid 1. Sealed containers according to the ninth to twentieth embodiments of this invention and sealed containers having structures substantially the same as those of the described embodiment except that the longitudinal sides of the antennas extend horizontally were prepared (in which the lids 1 were made of lamination of polyester/nylon/polypropylene layers). A rice boiled together with red beans (SEKIHAN) having a volume of 200g was filled and sealed in the respective containers, which were then set in the central portion of the microwave oven (National Electronic Oven NE-M200, Output: 500W, Lower Stirrer-type) and heated for two minutes. The containers prepared according to the described embodiments were all opened in 10 to 30 seconds, but some of the containers provided with the horizontally extending antennas were not completely opened in this heating time. This invention is not limited to the described embodiments and many changes and modifications may be made. For example, the antennas of the thirteenth to twentieth embodiments may be utilized in the combined manner and such antennas may be entirely bonded to the lid of the eleventh embodiment and the heat generating element may be positioned at a portion between the antennas between which the microwave energy can be concentrated. In the foregoing embodiments, the heat generating elements utilizes not only the high resistance substance but also the other substances which generate heat by absorbing the microwave energy, that is, the substances which generate heat due to the magentic loss, dielectric loss or discharge in substitution for the resistance loss. As an element or substance which generates heat due to the magnetic loss, there will be listed up a ferromagnetic alloy made of, for example, ferrite, iron, cobalt, nickel, which are represented by MOFe₂O₃ (where M is a divalent metallic ion) and the combination of some of these substances. A high molecular compound such as epoxy resin, neoprene, bakelite or nylon will be utilized for the heat generating element which generates heat due to the dielectric loss, and a substance having a large dielectric loss such as a barium titanate will be also utilized for the heat generating element. As the heat generating element which generates heat due to the discharge is utilized a vacuum evaporation film of metallic material such as aluminum, iron or nickel, vacuum evaporation film of semiconductor material such as silicon, or fibrous substance made of these substances. Embodiments of the container sealed with lids or containers in form of pouches or bags to which labels are bonded according to this invention will be described hereunder. A label of the twenty-first embodiment according to to this invention is shown in Fig. 26, in which an electrically conductive layer 12 and a heat generating layer 13 is laminated on a base layer 11 and a protection layer 14 is further laminated so as to enclose the conductive layer 12 and the heat generating layer 13. A layer 15 of a bonding agent or adhesives is also laminated below the base layer 11 and a releasing layer 16 is temporarily bonded to the adhesive layer 15. The base layer 11 is composed of single lamination layer of a thermoplastic sheet, thermosetting plastic sheet or paper material having a thickness suitable for providing the proper rigidity to the label. The conductive layer 12 is prepared by a substance which may form a thin film having a surface resistivity below 1 Ω /cm² such as metallic foil, flame coating film or conductive coating film. If the conductive layer 12 is composed of a substance having a surface resistivity more than 1 Ω /cm², the substance itself absorbs the microwave energy and generates the heat and, hence, it would not proper to use the layer (12) as an antenna for concentrating the microwave energy. It is necessary for the conductive layer 12 to have a length more than 20mm, since if the length is below 20mm, the conductive layer is affected by the electric field distribution in the electronic oven and the heat is not generated stably. The end shape of the conductive layer may be made linear, curved or polygonal, or combination of these shapes. The heat generating layer 13 is prepared from a substance which absorbs the microwave energy and generates heat. As an example, a ferrite or a ferromagentic alloy having a large magnetic loss, barium titanate having a large dielectric loss, or conductive coating film, conductive plastic or vacuum evaporation film having a large resistance loss may be utilized as a substance for the heat generating layer 13. When the substance having a large resistance loss is utilized, it is desired for the substance to have a surface resistivity more than 11Ω /cm². If the substance has a surface resistivity less than 11Ω /cm², the substance not only absorbs the microwave energy but also reflects the same, thus reducing the heat generating function. The heat generating layer 13 may be formed in a circular or polygonal shape, or a combination of these shapes. It is necessary that the distance C between the conductive layer 12 and the heat generating layer 13 is 0 to 3.5 mm. If the distance is greater than 3.5 mm, it is impossible to apply an adequate heating amount to a portion to be opened. The protective layer 14 serves to protect the conductive layer 12 and the heat generating layer 13. It is prepared from an insulative plastic sheet or a coat film which is bonded, fused or coated to the conductive layer 12, the heat generating layer 13 and the base layer 11. The adhesive layer 15 is prepared from an acrylic bonding agent or another bonding agent utilized for the bonding of a usual label. The releasing layer 16 serves to protect the adhesive layer 15 and is prepared from a film essentially consisting of a plastic material having a low adhesive capacity, a releasing layer essentially consisting of a surface treated film or paper, or a substance prepared by coating a paper with a plastics material. The label having the structure described above is provided with a cut line 17 in the base layer 11 around the conductive layer 12 and the heat generating layer 13 as shown in Fig. 27, and the upper portion of the releasing layer 16 is peeled off along the cut line 17. The label is thus bonded to the lid of the sealed container or the bag through the adhesive layer 15. It is desired to display an object of the usage of this label for a user, and for this purpose, a printing may be directly made to the conductive layer 12 or protective layer 14. In the twenty-second embodiment according to this invention shown in Fig. 28, the base layer 11 function as the protective layer, and the conductive layer 12 and the heat generating layer 13 are laminated between the base layer 11 and the adhesive layer 15. The other structures and functions of the embodiment shown in Fig. 28 are substantially the same as those described with reference to Fig. 27. In the twenty-third embodiment (Figure 29) according to this invention, the adhesive layer 15 is composed of a hot metal type bonding agent. The adhesive layer 15 is disposed on the surface of the lid or bag of the container and heat pressed by a heating matter from the base layer 11, whereby the adhesive layer 15 is softened and bonded to the lid of the container or the bag with the releasing layer 16, the conductive layer 12 and the heat generating layer 13 removed from the base layer 11. The twenty-first embodiment according to this invention will be described more concretely hereunder. (Example - 1). A polyethylene terephthalate film, having a thickness of 25 µ m, prepared by a biaxial orientation method was utilized as the base layer 11, and an electrically conductive silver paste, having a length of 45mm and a thickness of 5 µ m, prepared as the conductive layer 12 was coated on the base layer 11. The surface resistivity of the thus prepared conductive layer was of 0.2 Ω /cm². An electrically conductive thin film of carbon paste (thickness: 3µ m; surface resistivity: 400 Ω /cm²) was applied on the base 11 as the heat generating layer 13 by the screen printing method. A polyethylene terephthlate film, having a thickness of 25 µ m, prepared by the biaxial orientation method, was prepared as the protection layer 14, and the protection layer 14 was bonded to the base layer 11 by a urethane bonding agent. An acrylic solvent type substance was utilized as the adhesive layer 15 and a polyethylene laminate glassing paper was utilized as the releasing layer 16. Curry was accommodated in a cup made of polypropylene suitable to contain 200g, the cup was then sealed by a lid made of a lamination of polyester/nylon/polypropylene layers. A label was bonded on the lid. The thus prepared sealed container was inserted into a microwave oven (output power: 500 W; National Electronic Oven NE-M 200), in which the container was exposed to the microwaves to examine the lid opening test. The results of the several containers indicated that the lids were opened constantly in about 20 seconds and steam was jetted through the opening irrespective of the locations of the containers in the electronic oven. (Example - 2) A lamination of polyester/nylon/polypropylene layers was utilized for prepareing a container in the form of a pouch or bag, with the polypropylene layer inwardly, having outer dimension of 170 x 130mm, and the 200 gr of rice was filled in the container, which was then sealed. A label having substantially the same structure as that of the Example 1 was bonded to the container. The thus prepared sealed container was inserted into a microwave oven (output power: 500 W; National Electronic Oven NE-M 200), in which the container was exposed to the microwaves to examine the lid opening test. The results of the several containers indicated that the lids were opened in about 10 to 30 seconds and steam was jetted through the opening without breaking the bag container irrespective of the locations of the containers in the microwave oven. It is to be noted that this invention is not limited to the described embodiments and various changes and modifications may be made. For example, the heat generating layer (13) and the conductive layer (12) may be laminated separately on the front and rear surfaces of the base layer (11) without having to be laminated on the same side of the base layer. Further, the flexible sheets having an insulating property in the respective embodiments described involve the sheets prepared by single or combined material of thermoplastic sheet, thermosetting plastic sheet, and paper sheet. As a thermoplastic resin, there will be provided linear polyester such as polyethylene terephthalate, polyolefine resin such as ionomer, polyethylene, polypropylene, ethylene-propylene copolymer, acrylic resin, polystyrene resin, AS resin, ABS resin, polyamide resin, polyimide resin, polyimine resin, polyvinyl chloride resin, polyvinylidene chloride resin, vinyl chloride-vinyl acetate copolymer, vinylidene copolymer, acetal resin, diallyl phthalate resin, fluoride resin, or resin prepared by the combination of some of these resins. As she thermosetting plastic resin, there will be provided a urea resin, phenol resin, epoxy resin, melamine resin, urethane resin, xylene-formaldehyde resin, or a resin prepared by the combination of some of these resins. INDUSTRIAL USAGE As described hereinbefore, the sealed container according to this invention is suitable to be heated in an electronic, particularly microwave, oven or a microwave generating box to cook the food accommodated in the container in the sealed condition.

Claims: A sealed container for containing food (3) to be cooked within a microwave oven, comprising: a container body (2) having an opening defined within a upper portion thereof; a lid (1), fabricated from a flexible sheet having insulative properties, for sealing the opening of the container body; and a laminate (4), fabricated upon the lid (1) ; characterised in that the laminate (4) comprises an antenna (4) fabricated from an electrically conductive material and in that the antenna (4) has opposed ends for generating an electrical field therebetween in response to the antenna (4) interacting with microwaves generated within the microwave oven whereby a portion of the lid, defined between the opposed ends of the antenna (4), is heated and partially melted by means of dielectric heating so as to develop a vent opening within the lid (1). A sealed container for containing food to be cooked in a microwave oven according to claim 1, wherein the antenna (4) has a C-shaped configuration. A sealed container for containing food to be cooked in a microwave oven according to claim 1 or claim 2, wherein a heat generating substance (5) is disposed adjacent to one of the ends of the antenna (4). A sealed container for containing food (3) to be cooked within a microwave oven, comprising: a container body (2) having an opening defined within a upper portion thereof; a lid (1), fabricated from a flexible sheet having insulative properties, for sealing the opening of the container body; and a laminate (4) fabricated upon the lid (1) characterised in that the laminate (4) comprises an antenna (4) fabricated from an electrically conductive material, and further characterised by a heat generating substance (5), disposed adjacent to an end of the antenna (4) interacting with microwaves generated within the microwave oven whereby a portion of the lid, defined where said heat generating substance (5) is disposed, is heated and partially melted by means of dielectric heating so as to develop a vent opening within the lid (1). A sealed container for containing food to be cooked in a microwave oven according to any one of the preceding claims wherein the portion of the lid (1) to be heated and partially melted is a peripheral portion of the lid (1) to be sealed with the container body (2). A sealed container as claimed in any one of claims 2 to 4, wherein the antenna (4) comprises at least two antennae laminated upon the lid (1). A sealed container for containing food to be cooked within a microwave oven, as claimed in any one of claims 1 to 4 wherein the antenna (4) has a portion inclined an an angle greater than 3° with respect to the plane of the base of the container body (2), for concentrating microwave energy toward a specific portion of the lid (1) when the sealed container is exposed to microwaves. A sealed container for containing food to be cooked in a microwave oven as claimed in claim 4, wherein the antenna (4) has a belt shape and the heat generating substance (5) is laminated on the lid (1) adjacent to the antenna (4). A sealed container for containing food to be cooked in a microwave oven as claimed in claim 7, wherein the microwave energy concentrated by the antenna is converted into heat energy due to the dielectric loss of the lid. A sealed container for containing food to be cooked in a microwave oven as claimed in any one of claims 7, 8 or 9, wherein the lid (1) is horizontally disposed and the antenna (4) is laminated on a flexible sheet (6) having an insulative property, the flexible sheet (6) having a first portion bonded to the lid (1) while a second remaining portion extends in a direction away from the lid as a result of the self-elasticity of the flexible sheet. A sealed container for containing food to be cooked in a microwave oven as claimed in claim 8 or claim 9, wherein the lid (1) is horizontally disposed, at least two kinds of flexible sheets (14a, 14b) having insulative properties and having different coefficients of thermal contraction are laminated upon opposite sides of the antenna (4), a portion of a first lamination layer is bonded to the lid (1), and the remaining portion of the first lamination layer not bonded to the lid is displaced so as to extend away from the lid due to the thermal deformation of the remaining portion of the first lamination layer. A sealed container for containing food to be cooked in a microwave oven as claimed in any one of claims 9 to 11, wherein when the container body (2) is disposed so that the bottom surface thereof is disposed within a horizontal plane, at least a portion of the lid (1) is inclined at an angle greater than 3° with respect to the horizontal plane and the antenna (4) is laminated on a portion of the lid (1) in the inclined direction. A sealed container for containing food to be cooked in a microwave oven as claimed in any one of claims 9 to 12, wherein the antenna (4) is bonded to the lid and a side surface of the container body (2). A sealed container for containing food to be cooked within a microwave oven, as claimed in claim 4 having an electrically conductive layer (12) in the form of a linear belt having a longitudinal length of at least 20 mm and a surface resistivity of less than 1Ω per square centimetre, for concentrating the microwave energy toward the heat generating layer (13), the electrically conductive layer (12) being disposed adjacent to the heat generating layer (13) with a distance of 0-3.5 mm defined therebetweeen. A sealed container for containing food to be cooked within a microwave oven, as claimed in claim 14 wherein the container is in the form of a sealed pouch, the lid comprising a label applied thereto. A sealed container for containing food to be cooked in a microwave oven as claimed in claim 14, wherein the label further comprises a base layer (11), the conductive layer (12) and the heat generating layer (13) both being laminated on the base layer (11), a protection layer (14) arranged so as to enclose the conductive layer (12) and the heat generating layer (13), and a releasing layer (16) releasably applied to the adhesive layer (15) which is laminated below the base layer (11). A sealed container for containing food to be cooked in a microwave oven as claimed in claim 14, wherein the label includes a base layer (11), the conductive layer (12) and the heat generating layer (13) both laminated below the base layer (11), the adhesive layer (15) arranged so as to enclose the conductive layer (12) and the heat generating layer (13), and a releasing layer (16) releasably applied to the adhesive layer (15). A sealed container for containing food to be cooked in a microwave oven as claimed in claim 14, wherein the label includes a base layer (11), a releasing layer (16) releasably applied to the lower side of the base layer (11), the conductive layer (12) and the heat generating layer (13), both laminated below the releasing layer (16), and a hot melt adhesive layer (15) laminated so as to enclose the conductive layer (12) and the heat generating layer (13) while being bonded to the releasing layer (16).

Inventors: FUJIWARA, TAMIO, ISHIBASHI, KAZUHISA, NAMIKI, TSUNEHISA, SHIBASAKI, KYUICHI, SUGIYAMA,IKUO, TANIKAWA, ISAO, YAMADA, MUNEKI
Applicants: TOYO SEIKAN KAISHA, LTD.
Application year: 1988
Main classification: A47J27
IPC codes: B65D81, B29C65, B65D77