JPH0464254B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a food container used when cooking food by microwave heating, and more particularly to a food container for uniformly heating the food or heating the outer surface particularly strongly. [Prior Art] When heating food using microwaves, the food is heated from the inside, and the outside surface of the food is cooled by the surrounding air and the container, making it difficult to heat the food. Over time, the outer surface of the food may not be fully cooked, and even if the food has already been cooked, the outer surface of the food may not be as pleasant to eat. Also, since traditional cooking methods used fire, the outside of the food was burnt, and people have come to prefer this type of food. Additionally, intense heating of the outer surface of the food produces a pleasant aroma. For this reason, conventionally, when cooking food using microwaves, a method was used in which the food was heated in a container. A porcelain container that absorbs microwaves is known as such a container. However, such containers are expensive and heavy, so they are not suitable for disposable retort food containers, and they have drawbacks such as the container being heated and difficult to cool, making it inconvenient to remove it from the microwave oven after cooking. A technique for making such a container using a laminate made of plastic sheets and conductive materials has also been demonstrated (Special Publication Publication in 1983).
−15548). A layer of such a conductive material is formed by vapor depositing a metal such as aluminum on a plastic sheet, but since metal has a low electrical resistance, the surface resistivity of the layer formed by it is low, and it is necessary to Heating is difficult. Reducing the thickness of the metal layer increases the surface resistivity and increases the heating rate, but in order to do so, the thickness of the metal layer must be, for example, approximately 40 Å, and it is difficult to stably manufacture this by vacuum evaporation. There was a problem in that it was difficult to form a uniform thickness over the entire surface of the plastic sheet. [Problems to be Solved by the Invention] The present invention has been made to solve the problems of the microwave cooking container according to the prior art.
It is suitable for disposable containers such as retort food, and can be manufactured stably at low manufacturing costs.
Moreover, it is an object of the present invention to provide a microwave cooking container capable of sufficiently heating the outer surface of food. [Means for solving the problem] A conductive material such as a metal element is coated with fine stripes in the thickness distribution on a relatively thick base material of an insulator such as paper or plastic sheet, or on a protective sheet of an insulator such as a thin plastic sheet. The laminates are laminated to form a pattern, and the laminate thus formed is formed into a container shape to produce a microwave food container. A first specific example of the fine striped pattern of the conductive material is formed as a checkered pattern as shown in FIGS. 2 and 3. For example, such a checkered pattern is created by masking an insulating sheet with a metal mesh and depositing a conductive material to form a thick layer with a large number of squares arranged in a row.Then, the masking is removed and the entire surface is made conductive. By vapor depositing the material, a lattice-like thin layer is formed to create a layer of conductive material whose thickness distribution forms a fine striped pattern. In order to achieve the object of the present invention, the thickness of the thin layer portion must be 20 Å or more and less than 80 Å, and each side of the square of the thick layer portion must be 5 mm or less. A second example of the fine striped pattern of the conductive material is obtained by depositing the conductive material on the surface of an insulating sheet having a surface roughness of R _Z =2 to 40 μm. When a material is deposited on an uneven surface, the layer of the deposited material takes on a striped pattern with alternating thick and thin parts, as shown in FIG. In order to achieve the purpose of the present invention, the surface resistivity of the vapor-deposited layer of the conductive material is 12Ω squared to 1500Ω/square, and the apparent resistivity calculated from the average thickness of the conductive material is the true value of the conductive material. It is necessary that the resistivity is 23 to 500 times that of . [Operation] When a conductive substance is exposed to microwaves, a current flows and generates heat due to Joule heat. When a conductive material is laminated with a uniform thickness on the surface of an insulator, the amount of heat generated per unit area of the surface is a function of the resistivity of the conductive material and the laminated thickness. In the case of materials, a large amount of heat generation cannot be obtained unless the laminated layer thickness is extremely thin, around 40 Å.
However, when the thickness distribution of the laminated layer is formed into a fine striped pattern that appears alternately between thick and thin layers, the thickness of the thin part becomes 20 Å to 80 Å.
It has been found that it is possible to perform vapor deposition in the range of .ANG., and that even if the average thickness of the laminated layer is increased, a very large amount of heat generation can be obtained per unit area of the surface. Although it is difficult to mathematically calculate the amount of heat generated by a conductive material layer whose thickness changes two-dimensionally, the results obtained through experiments will be clarified in the following examples. [Examples and Comparative Examples] Examples and comparative examples of the present invention will be described below with reference to the drawings. As shown in FIG. 1, a conductive material 3 is laminated on the surface of a base material sheet 2 of plastic or paper, in which metallic aluminum is vapor-deposited so as to form a fine striped pattern in the thickness distribution.
A plastic protective sheet 4 is adhered thereon to form a laminate 5. The laminate 5 is molded to make the container 1. When food to be cooked is placed in the container 1 and heated in a microwave oven, the conductive material layer 3 generates heat and the portion of the food that comes into contact with the container is strongly heated. Therefore, the part of the food that comes into contact with the container becomes burnt and emits an odor. The conductive material layer may generate spark discharge depending on its shape, and the amount of heat generated varies depending on the shape, so the results of experiments on various shapes of the conductive material are shown below. It should be noted that the examples shown are those that meet the conditions of the embodiment of the present invention, and those shown as comparative examples are those that do not meet the conditions. Experiment 1 As shown in Figures 2 and 3, a 50μ thick biaxially stretched polyethylene terephthalate film (PET) 2 was covered with mesh 80 (American mesh).
Metal aluminum 3 was vacuum-deposited to a thickness of about 700 Å by masking with a wire mesh having a diameter of 0.10 mm. Next, metal aluminum was deposited on the entire surface, excluding the mesh, by an additional thickness of t, to produce the laminate shown in the figure. Therefore, the thickness T of the square-shaped thick laminated portion in the figure is 700 Å+t. The thickness of aluminum vapor deposition was measured as follows. A part of the smooth glass plate was masked and placed in a place where the deposition conditions were the same as the sample during the above deposition, and after the deposition was completed, the masking was removed and the step at the end of the deposition was repeatedly measured using reflection interferometry. The value was taken as the thickness of the deposited aluminum layer. Subsequent deposition thicknesses were measured in the same manner. A biaxially stretched polyethylene terephthalate film (PET) with a thickness of 12μ is adhered to the laminate shown in the figure using an adhesive.
A laminate was made in this order and used as a heating element for experiments. Commercially available microwave oven [Manufactured by Matsushita Electric Industrial Co., Ltd., National Microwave Oven NE-M200, operating frequency 2450MHz,
A Teflon table with a height of 3 cm was placed in a chamber (with an output of 500 W), and the heating elements described above (of various thicknesses, i.e., those with the thickness of the thin part of the conductive material) and a commercially available freezing gyoza were placed on top of the table and heated by microwave irradiation for 70 seconds. Changes in the heating element and burnt and hardened state (crispyness) of the pickled vegetables were observed. The results are shown in Table 1.
Shown below.

[Table] Good results were obtained in Examples 1, 2, and 3.
In Comparative Example 1, the outer surface of the pickpocket was not heated, and in Comparative Example 2, spark discharge occurred at the heating element. Implementation 2 The thickness of the thick part of metal aluminum is 750Å,
A heating test was conducted under the same conditions as in Experiment 1, with the thickness of the thin part being 50 Å, and the length of one side of the square of the thick part (interstitial distance l) being varied.
The results are shown in Table 2.

[Table] In Comparative Examples 3 and 4, spark discharge occurred in the heating element, and the charcoal in the vicinity of the spark discharge was carbonized. Other parts were not heated well. On the other hand, in Examples 4 to 8, no spark discharge occurred and the heating conditions were good. Experiment 3 Surface resistivity and evaluation were performed on laminates formed by vacuum-depositing metallic aluminum to various thicknesses on insulating sheets with two types of surface roughness (R _Z = 5.4 μ and R _Z = 0.1 μ or less). The ratio of the resistivity of the bulk (ρ'Ω·m) to the resistivity of the bulk (ρΩ·m) was measured, and a heating test was conducted using the microwave oven used in Experiments 1 and 2. In this experiment, the surface roughness R _Z of the insulating sheet was measured by the 10-point average roughness measurement method of JISBO601. Further, the surface resistivity ρ ⁰ (Ω/hole) was measured according to the metal film resistance testing method of JISC2316. The apparent resistivity ρ′ (Ω・m) of the deposited film is expressed by the formula ρ′=ρ ⁰
×
Calculated at t. Here, t is the average deposition thickness of the deposited film. Note that the resistivity (bulk) of metal aluminum is 2.75×10 ⁻⁸ Ω·m. The heating test is the fifth
The experiment was carried out using the apparatus shown in the figure. In the figure, 11 is a microwave oven, 8 is a Teflon stand, 5 is the above-mentioned laminate, and 6 is a piece of leather. Giyoza skin 6
The interface temperature between the temperature sensing element 7 (LUXTRON
Fluoroptic Thermometer Temperature
Probe (LSA)], a thermometer 9, and a recorder 10. In this way, the microwave irradiation time required to raise the temperature of the outer surface of the Giyoza skin from room temperature to 200°C was measured, and the heating performance of the laminate was determined. The results are shown in Tables 3 and 4.

【table】

[Table] In Comparative Examples 6 and 5, spark discharge occurred and stable temperature measurement was impossible. Vapor deposition from Examples 9 to 13
It can be seen that the heating performance is highest when the Al layer thickness is 290 Å. The evaporation thickness of 290 Å is a thickness at which vacuum evaporation can be easily and stably performed. Experiment 4 A test similar to Experiment 3 was conducted using various insulating sheets. The thickness of metal aluminum vapor deposition is
It is 290Å. The results are shown in Table 5.

[Table] It can be seen that the heating performance of the example is superior to that of the comparative example. Experiment 5 Metallic aluminum was deposited on paper materials with various surface roughnesses, and a biaxially stretched polyethylene terephthalate film with a thickness of 12 μm was adhered thereon using an adhesive to create a laminate, and a tray was made from this laminate.
The size of the tray is 5 x 10 cm and 2 cm deep, and metal aluminum is vapor-deposited only on the bottom of the tray. Place commercially available frozen gyoza in this tray,
It was heated for 70 seconds using the microwave oven used in Experiment 1.
The results are shown in Table 6.

[Table] In Comparative Example 13, the paper used in Example 19 was coated with a thermosetting resin to give a surface roughness of 0.1μ. The examples show good heating performance. Experiment 6 A container was made by gluing a laminate of various sheet materials with metal aluminum vapor-deposited onto the entire outer surface of a flanged polypropylene cup (inner volume: about 70 g) with a diameter of 6 cm and a depth of 3 cm (approximately 70 g). The metal aluminum evaporated side of the laminate was adhered to the outer surface of the cup with an adhesive. Into this container was placed 60 g of a paste solution made by mixing a 30% aqueous potato starch solution with an iodine dyeing solution and heated for 30 seconds in the microwave oven used in Experiment 1. Since the iodine staining solution fades when the temperature exceeds about 60°C, the temperature distribution of the contents can be easily determined. The results of the heating test are shown in Table 7.

〔Effect of the invention〕

By laminating a conductive material on an insulator in a fine striped pattern that appears alternately in thick and thin layers, a laminate with good microwave heating performance can be obtained. The food container for microwave cooking of the present invention made from the above-mentioned laminate is easy to manufacture because the conductive material is relatively thick, and also has high heating performance. It can also be applied to plastic disposable containers.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a container which is an embodiment of the present invention;
FIG. 2 is a plan view of a laminate constituting an embodiment of the present invention, FIG. 3 is a sectional view taken along line A-A in FIG. 2, and FIG.
The figure is a sectional view of a laminate constituting another embodiment of the present invention, and FIG. 5 is a schematic diagram of an apparatus for carrying out a heating test on a laminate constituting an embodiment of the present invention. 1... Container, 2... Base material sheet, 3... Conductive substance,
4... Protective sheet, 5... Laminated body, 6... Giyoza skin, 7... Temperature measuring element, 8... Teflon stand, 9... Thermometer,
10...Recorder, 11...Microwave oven.

Claims (1)

[Claims] 1. A food container made of a laminate in which a conductive substance is laminated on an insulator base material or an insulator protective sheet, in which the thickness distribution of the conductive substance has a fine striped pattern. A food container for cooking in a microwave oven, characterized in that: 2. The electronic device according to claim 1, wherein the fine striped pattern of the conductive material is a lattice stripe, the thickness of the thin part of the conductive material is 20 Å or more and less than 80 Å, and the distance between the lattice stripes is 5 mm or less. Food containers for microwave cooking. 3. Directly on the surface of the insulator base material or the insulator protective sheet whose surface roughness R _Z is 2μ to 40μ, the apparent resistivity calculated from the average thickness of the conductive material is 23% of the bulk resistivity. 2. The food container for microwave cooking according to claim 1, which is laminated to have a surface resistivity of 12 Ω/square to 1500 Ω/square.