NL8900899A - A PATTERNED SUSCEPTOR MATERIAL FOR USE IN A MICROWAVE OVEN. - Google Patents

Description

Patterned susceptor material for use in a microwave oven.

The invention relates to a method for manufacturing a so-called susceptor material by applying a metal coating layer to a plastic film, the thickness of which has been chosen such that the coating layer serves as an absorbing layer for microwave radiation, after which the coating on this metallized film is adhered to a support layer.

The problem of getting a good browning and a crispy crust with dough products or products that are provided with a (crust) dough layer such as pasties, spring rolls and the like on the outside, when they are cooked in a microwave oven or reheated therein, has been solved at least in part by the use of so-called susceptor materials. These are materials that generally consist of a plastic film on which a thin metal coating layer has been applied, which combination is then laminated on a supporting paper or cardboard layer. Packagings in which a food product can be packaged can be manufactured from such a material. If the food product is now prepared in a microwave oven without removing the packaging, the layered material will absorb micro-wave radiation and thereby be heated quickly, so that a hot plate or hot layer is created around the food product, as it were. Particularly when heating dough products, such as pies and the like, not only will the outer surface become hot, but it will also be dried, crisp and brown.

A modification of such a general browning function is to achieve browning of the food surface only in certain areas according to a predetermined pattern. In such a case, the susceptor layer (the microwave absorbing layer, which becomes hot in use and provides the tanning by transfer of heat radiation) will only have to be present in certain areas according to the predetermined pattern. A special example is the generation of brown stripes on the food product, which simulates that the product in question has been prepared on a barbeque or grill.

There are several obvious ways to achieve such a local effect. For example, the paper or cardboard support layer can be provided with metallized film strips or strips with intermediate webs where there is no metallized film. It is also possible to remove part of the metalized layer from the film before it is laminated to the paper or cardboard support layer, or to use a mask during the application of the metal coating layer, which locally covers the film in such a way that there is no metallization in the covered places. It is also possible to remove parts of this layered structure after the metallized film has been laminated to the support layer.

This may, for example, be strips or other shaped parts which are cut from the layered material in a predetermined pattern, so that openings are created in the material.

Susceptor materials incorporating a metallized film function as a microwave absorbing material in accordance with known electromagnetic theory. If aluminum is used for the metallization, which is a good electrical conductor, this aluminum, if applied in thicker layers, will function as a good reflector for electromagnetic radiation, such as visible light with a relatively short wavelength. , as well as microwave radiation with a much longer wavelength.

Only when the thickness of the metal layer is very small and the surface layer (which is limited to the so-called skin depth) that interacts with incident electromagnetic radiation shows considerable electrical resistance, the reflectivity is less and the absorption is much stronger. The theory predicts that, with a correctly chosen layer thickness of the metal cladding layer, the absorption will reach a maximum of 50%. If the metal layer is made even thinner, the electrical resistance of the metal layer will further increase, the absorption of microwave energy decreases and the layer becomes increasingly transparent to electromagnetic radiation in the microwave region and also in the region of the visible wavelengths. Typical susceptor materials generally have metal coatings that exhibit high absorption of microwave energy. The thickness of such a coating layer can be expressed using the optical density of the layer, which for aluminum will be between 0.2 and 0.3.

Surprisingly, it has been found that thin metallized films on plastic substrates, which initially have good absorption properties, after being subjected to relatively weak elongation or after light scratching, may lose their susceptor properties and become locally transparent to microwave radiation , even if apparently their interaction with visible light, that is, radiation within the spectrum visible to man, has remained virtually unchanged both in terms of reflection and absorption. It is believed that the local stretching of the material or the application of fine scratches, so-called micro-scratches that are barely perceptible to the human eye, lose the absorption properties for microwaves with a relatively long wavelength, while, however, the absorption properties for visible light of a much shorter wavelength are retained.

According to the invention, use is now made of this unexpected effect by creating local areas in an initially continuous susceptor surface according to a predetermined pattern, in which areas the material no longer behaves as a susceptor, but is essentially transparent to microwaves. This is achieved according to the invention in a method of the type mentioned in the preamble, in that after the metalization of the film, but before the attachment of the metallized film to the support layer, the continuity of the metal coating layer is locally disturbed in a predetermined pattern. to such an extent that the composite film becomes locally transparent to microwave radiation.

In a first preferred embodiment of the method according to the invention, the local disturbance of the film is obtained by locally stretching the metallized plastic film within the elastic deformation range of the plastic material.

Another preferred method according to the invention is characterized in that the local disturbance is obtained by frictionally sliding a tool locally over the metal cladding layer in a predetermined pattern, whereby at least micro-scratches occur in said cladding layer.

The invention will be discussed in detail below with reference to the figures, which schematically show examples of devices with which the method can be carried out in practice.

Figure 1 shows a cross section through a known susceptor material.

Figure 2 shows how in the semiproduct consisting of the metallized plastic film small scratches are made in such a way that the susceptor material at these scratches becomes transparent to microwave radiation.

Figure 3 shows a more extensive embodiment of figure 2.

Figure 4 schematically shows the use of rollers to apply a predetermined pattern of disturbances in the metal cladding layer.

Figure 5 shows in high magnification the effect of the ribs, which are arranged on one of the rollers in Figure 4.

Figure 6 shows another embodiment of an apparatus using rollers, now intended to realize a different pattern of disturbances in the metal cladding layer.

Figure 1 shows a partial section through a susceptor material consisting of a plastic film 10 on which a metal coating layer 12 has been applied. This metal cladding layer 12 can be applied, for example, by sputtering, evaporation, etc. This metallized plastic film 10, 12 is then adhered to a carrier layer 14, which in known embodiments can consist of paper or cardboard.

The plastic film is, for example, a polyethylene terephthalate film with a thickness of 12 microns. The thickness of the metallization 12 is chosen such that, when using this material in a microwave oven, this layer 12 will absorb a significant portion of the incident microwave radiation, for example, absorb at least 15% of the power. The thickness of the metal cladding layer is so small that the thickness is expressed in terms of optical density. If aluminum is used to realize the layer 12, the optical density of this layer will be in the range between 0.15 and 0.8 and preferably 0.2 and 0.3.

Before the metallized plastic film 10, 12 is adhered to the support layer 143, distortions are made in the metallization 12 in such a way that the integrity or continuity of the coating layer is locally lost. The effect of this is that the metal coating layer no longer functions locally as an absorption layer, but becomes transparent to microwave radiation. This means that when this material is used as an envelope for food products to be prepared in a microwave oven, the material will locally absorb energy, thereby be heated and emit radiant heat, while in other places the microwave energy will be transmitted and no radiant heat is produced. The result of this is that the food product will be browned locally under the influence of the radiant heat, while in other places where the microwave radiation is transmitted there will be no, or at least much less browning.

Various devices are conceivable with which the desired local disturbances can be realized. A possible very simple embodiment is illustrated in figure 2 and consists of at least one pin 16, which in the shown preferred embodiment has a slightly rounded or bent end. This end of the pin 16 is pressed against the metal layer 12, while the metallized film 10, 12 is moved under the pin in the direction of the arrow, supported by, for example, a slightly convex surface 13, which can be part of a roller or a solid surface. Because the pin 16 will be pressed against the metal clad layer 12 with some force, scratches will occur in this layer 12, as a result of which the integrity of the layer 12 will be lost on site. The area around the scratch therefore becomes transparent to microwave radiation. The areas not touched by a pin 16 remain unchanged in their microwave absorbing character. It is noted that the means for moving the metallized film 10, 12 in the direction of the arrow are not shown in the figure.

Figure 3 shows a more extensive embodiment of such a device, in which use is made of a number of pins 16a ..... 16d, which when pressed against a metallized foil 10, 12 while the foil is in the direction of the arrow 15 in figure 3 moves will cause parallel scratches in the metal layer, as indicated by the lines in Figure 3. The resulting metallized film 10, 12 will therefore have parallel webs, which still have absorbent properties, which webs are separated from each other by strips, in which the metal layer 12 is transparent to microwave radiation.

Figure 4 shows another device with which a predetermined pattern of disturbances in the metal layer 12 can be realized. This device is provided with two rollers 18 and 20, between which the metallized film 10, 12 is moved. The various directions of movement of the rollers 18 and 20 and of the metallized film 10, 12 are indicated by arrows in Figure 4. The lower roller 18 is, for example, made of a relatively soft material such as rubber or has an outer surface of a relatively soft material. The top roller 20 is made of a relatively hard material, such as steel, or has a coating of a relatively hard material and is further provided with protruding ribs 22a ..... 22e. In the embodiment of Figure 4, these ribs extend concentrically around the roller 20. The influence of these ribs on the metal cladding layer 12 of the metallized film 10, 12 is illustrated in more detail in Figure 5. Figure 5 shows the influence in high magnification which has a rib 22x, which is located on the top roller 20, on the metal cladding layer 12. The metallized foil 10, 12 is clamped in figure 5 between the rollers 18 and 20. Only the rib 22x presses deeper into the metallization layer 12 and will locally disturb this layer such that the homogeneity or continuity of the layer 12 is lost. The plastic film 10 will also be stretched somewhat by the pressing action of the rib 22x. Within the scope of the invention, however, the dimensions of the rib 22x and the pressure prevailing between the two rollers 20 and 18 during the passage of the metallized film 10, 12 are such that the deformation occurring in the plastic film 10 within the elastic area, so that after leaving the nip between the two rollers 18 and 20, the plastic film 10 returns to its original condition. However, the metal layer 12 will remain locally disturbed.

Figure 4 shows the result of this rolling operation. A number of line-shaped disturbances have been induced in the metal layer 12.

The lineal perturbations represent parts of the metal layer 12 that are now transparent to microwave radiation. The intermediate undisturbed parts remain absorbent to microwave radiation.

The disturbances will generally be barely perceptible to the human eye, so that if such a susceptor material is used in the preparation of a food product in a microwave oven, an element of surprise is introduced.

Although the devices shown in Figures 2 to 5 result in linear disturbances of the metallization layer 12, the invention is certainly not limited thereto. Figure 6 shows another embodiment of a device, very similar to the embodiment of figure 4, but now provided with a different pattern of ribs on the roller 20. In this case, the ribs, which are not individually marked, mutually perpendicular to each other in a diamond-shaped pattern. The result will therefore be that a diamond-shaped pattern of perturbations is pressed into the metal layer 12 of the metallized foil 10, 12, as is shown schematically in Figure 6 by means of lines.

It will be appreciated that any roll pattern, including straight and curved lines and including any special patterns, may be applied to the roll 20, if desired, for example, for advertising purposes and the like. Perhaps unnecessarily, figure 7 shows a rib pattern on the roller 20 with which circular disturbances of the continuity of the metal coating 12 can be realized.

Finally, it is noted that the use of the material according to the invention is not limited to dough products. In principle, the material can be used with all food products which must preferably be browned locally in a microwave oven during preparation.

Claims (4)

A method of manufacturing susceptor materials according to claim 1, characterized in that the local disturbance of the film obtained by locally stretching the metallized plastic film within the elastic deformation range of the plastic material. A method of manufacturing susceptor materials according to claim 1, characterized in that the local disturbance is obtained by frictionally sliding a tool locally over the metal cladding layer in a predetermined pattern, causing at least micro-scratches in said cladding layer originate. Apparatus for carrying out the method according to any one of the preceding claims, characterized by at least one pin-shaped object having a rounded end, a support surface for locally supporting the metallized film and means for advancing the metallized film over said support surface, wherein during operation said pin-shaped object is pressed against that part of the metallized film supported by the support surface. Apparatus for carrying out the method according to any one of claims 1 to 3, characterized by two rollers between which the metallized film is transported, said roller, which presses against the metallization, is provided with projections arranged in a predetermined pattern parts whose dimensions are such that the elongation thereby induced in the plastic film remains within the elastic deformation range of the plastic material.