WO1990012477A1 - Patterned susceptive material for use in a microwave oven - Google Patents

Abstract

Method and system for manufacturing susceptor materials by providing a metal coating (12) on a plastic film (10), the thickness of said coating being selected such that the coating will act as an absorber for microwave radiation and adhering said metallised film to a support layer. After metallizing the film, but before adhering the metallised film to the support layer the metal coating is in a predetermined pattern locally disrupted to such extend that the susceptor locally becomes transparent for microwave radiation. The disruption step is carried out for instance by locally stretching the metallised plastic film within the elastic deformation range of the plastic material, or by sliding a friction surface tool (18, 20, 22x) in accordance with said predetermined pattern across the metallised coating causing thereby at least micro-scratching of said coating.

Description

Patterned susceptive material for use in a microwave oven.

The invention relates to a method for manufacturing susceptor materials by providing a metal coating on a plastic film, the thickness of said coating being selected such that the coating will act as an absorber for microwave radiation and adhering said metal- lised film to a support layer.

The problem of achieving good browning and crisping of pastry products or products which have an outside (puff) pastry layer such as pies, spring rolls etc., in microwave cooking or rewar ing has been, at least partially, solved by the use of susceptor materials. These are typically a lightly metallised plastic film laminated to a supporting paper or paperboard, . From such material packages can be prepared for packing food products. When a food product is cooked in a microwave oven without removing the packing then the layered material will absorb microwave radiation becoming quickly heated thereby to provide a hot plate or hot surround which not only heats the outer surface, but also dries, crispens and browns the outer surface.

A modification off this general browning function is to generate the browning of the food product outer surface only in specified areas according to a predetermined pattern. In that case the susceptor layer (the microwave absorbing layer which will become heated in use and which will cause the browning by transferring heat radiation) only has to be present in predeter¬ mined areas depending on said predetermined pattern. A particular example is the generation of brown stripes on the food product to simulate that said product is cooked on a barbecue or grill.

There are several obvious ways of achieving this localized effect. One may for instance use a paper or paperboard layer comprising strips or bands of metallized film with intermediate paths without metallized film. One may also remove sections of the metallized film layer, before the layer is laminated onto the paper or cardboard carrier, or one can make use of a mask for locally covering the plastic film during the manufacturing of the metal coating, such that at the masked locations no metal coating is obtained. It is also possible to remove parts of the layered structure after the metal coating is laminated to the carrier layer. One may for instance remove stripes or otherwise formed sections which are cut from the layered material in a predetermined pattern such that openings in the material are created. Susceptor materials comprising a metallized film act as microwave absorbers in accordance with well-established electro¬ magnetic theory. Aluminium, which is a good electrical conductor, in thick layers acts as a good reflector of electromagnetic radiation, both of relatively short wavelength visible light and of the much longer wavelength microwave radiation.

Only when the metal layer is very thin and the surface layer (restricted to the so-called skin depth) which interacts with incident electromagnetic radiation shows significant electrical resistance, does the reflectivity diminish and absorption becomes significant. Theory predicts that with a suitably selected layer thickness of the metal coating absorption reaches a maximum of 50%. With further thinning and increase in electrical resistance of the metal layer, absorption of microwave energy decreases and the layer becomes increasingly transparant to electromagnetic radiation in the microwave range and also to radiation with visible wavelengths. Typical susceptor materials comprise in general a metal coating showing a high absorption of microwave energy. The thickness of such a metal coating can be expressed by means of the optical density of the layer which for aluminium will have a value between 0,2 and 0,3.

Surprisingly it has been found that thin metallised films on plastic substrates which normally show good absorption properties, when subjected to relatively minor stretching or scratching, can lose their susceptor properties and become transparant to microwave radiation, even though they remain apparently unchanged in their reaction with visible light, i.e. the visible light transmission, reflection and absorption remains essentially unchanged. It is believed that stretching or applying fine scratches, so-called micro-scratching which are not or hardly visible to the human eye, destroys the absorption properties of long wavelength microwaves but not the absorption properties of short wavelength visible light. In this invention use is made of this unexpected charac¬ teristic to generate local areas on an initially continuous susceptor surface, in which local areas the material not behaves anymore as a susceptor but is mainly transparant for microwaves. According to the invention this effect is obtained within the scope of a method as defined in the first paragraph in that after metallizing said film, but before adhering the metallised film to said support layer the metal coating is in a predetermined pattern locally disrupted to such extend that the susceptor locally becomes transparant for microwave radiation.

In a first embodiment of the method according to the invention the local disruption of the film is obtained by locally stretching the metallised plastic film within the elastic deformation range of the plastic material. Another preferred embodiment of the invention is charac¬ terized in that the local disruption is obtained by sliding a friction surface tool in accordance with said predetermined pattern across the metallised coating causing thereby at least micro- scratching of said coating. The invention will be described in more detail with reference to the figures in which schematically examples are illustrated of apparatusses destined to carry out the method in practice.

Figure 1 illustrates a cross-section through a susceptor material which is known as such. Figure 2 illustrates in which way small scratches can be made in the semiproduct consisting of the metallized plastic film in such a manner, that because of these scratches the susceptor material becomes locally transparant for microwave radiation.

Figure 3 illustrates a more extended embodiment of figure 2. Figure 4 shows schematically the use of rollers to obtain disturbances of the metal coating in a predetermined pattern.

Figure 5 illustrates in a strong magnification the influence of the ribs on the surface of one of the rollers in figure 4.

Figure 6 shows another embodiment of an apparatus in which use is made of rollers, in this case destined to obtain another pattern of disturbances in the metal coating.

Figure 1 illustrates a partial cross-section through a susceptor material comprising a plastic film 10 onto which a metal coating 12 is applied. This metal coating 12 can be obtained for instance by sputtering, evaporating, etc. This metallized plastic film 10, 12 is adhered to a carrier layer 14, which in known embodiments may consist of paper or cardboard.

The plastic film is for instance a polyethylene thereph- thalate film with a thickness of 12 micron. The thickness of the metallization 12 is selected such that, when this material is brought in a microwave oven, this layer 12 will absorb the major part of the incident microwave radiation, for instance will absorb at least 15% of the incident power. The thickness of the metal coating is thereby so small that the thickness will be expressed in terms of optical density. If aluminium is used as material for the layer 12 than the optical density of this layer will have a value in the rane between 0,15 and 0,8 and preferably between 0,2 and 0,3.

Before the metallized plastic film 10, 12 is adhered to the carrier layer 1 the metallization 12 is locally disturbed in such a manner that the integrety and continuity of the metal coating will be lost locally. The effect thereof is that the metal coating layer Will locally not function as absorption layer anymore but will become transparant to microwave radiation. That implies that when applying this material as enclosure for food products which have to be cooked in a microwave oven, the material will locally absorb ener^r, becomes heated thereby and will start radiating heat, whereas at other places the microwave energy will be transmitted without any generation of heat radiation. The result thereof is that the food product will be locally browned under the influence of the heat radiation whereas at other places where the microwave radiation is transmitted. No or at least less browning will result.

There are various apparatusses conceivable to obtain the desired local disturbances. A very simple embodiment is illustrated in figure 2 and comprises at least one pin or pencil 16 which in the illustrated embodiment has a somewhat rounded or bended outer end. This outer end of the pin 16 is pressed against the metal coating 12, whereby simultaneously the metallized film 10, 12 is moved underneath the pin in the arrow direction thereby supported for instance on a somewhat spherical surface 13 which may form part of a roller or can be a fixed surface. Because the pin 16 will be pressed with some force against the metal coating layer 12 this layer 12 will become scratched so that the integrity of the layer 12 would be lost locally. The area around the scratch will become therefore transparent to microwave radiation. The areas not touched by the pin 16 will stay unchanged as far as their microwave absorbing character is concerned. It is remarked that means for moving the metallized film 10, 12 in the arrow direction are not shown in the figure.

Figure 3 illustrates a further developped embodiment of such an apparatus in which use made of a number of pins 16a 16d which if they are pressed against the metallized film 10, 12, while the film is moving in the direction of the arrow 15 in figure 3, will cause mutually parallel scratches in the metal layer as indicated by the -.-lines in figure 3. the resulting metallized film 10, 12 will show parallel strips which still have their absorbing properties, which strips are separated from each other by paths in which the metal coating 12 is transparant for microwave radiation.

In figure 4 a different apparatus is illustrated destined to realize a predetermined pattern of disturbances in the metal coating 12. This apparatus comprises two rollers 18 and 20 in between which the metallized film 10, 12 is conveyed. The various moving directions 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 instance made of a relative 1 soft material such as rubber or comprises another surface of a relatively soft material. The upper roller 20 is made of a relatively hard material such as steel or has an outer surface of a relatively hard material and comprises furthermore projecting ribs 22a 22e. In the embodiment of figure 4 these ribs are running concentrically around the roller 20. The influence of these ribs on the metal coating 12 of the metallized film 10, 12 is illustrated in more detail in figure 5. Figure 5 shows in a large magnification the influence of a rib 22x, present on the upper roller 20, on the metal coating 12. The metallized film 10, 12 is in figure 5 clamped in between the rollers 18 en 22. Only the rib 22x presses deeper into the metal coating 12 and will locally disturb this coating such that the homogenity or continuaty of the layer 12 will disappear. Also the plastic film 10 will become stretched somewhat by the impressive action of the rib 22x. The dimensions of the rib 22x and the pressure prevailing between both rollers 20 en 18 during the transfer of the metallized film 10, 12 are within the scope of the invention selected such that the deformation of the plastic film 10 will stay within the elastic range or area, so that the plastic film 10 after leaving the split between both rollers 18 and 20 will recapture its original shape. However, the metal coating 12 will stay locally disturbed.

In figure 4 the result of this roller operation is shown. The metal layer 12 comprises a number of line-shaped disturbances these line-shaped disturbances are representing sections of the metal layer 12 which are now transparant to microwave radiation. The not disturbed sections in between will stay microwave radiation absorben . In general the disturbances will not be visible to the human eye or hardly visible so that, in case such a susceptor material is used for cooking or preparing a food product in a microwave oven, a surprising element will be introduced.

The apparatusses which are shown in the figures 2 until 5 result in line-shaped disturbances of the metal coating 12, but the invention is certainly not restricted thereto. In figure 6 another embodiment,of an apparatus is illustrated, much similar to the embodiment of figure 4, however, now comprising another pattern of ribs onto the roller 20. In this case, the ribs, which are not referenced by reference characters separately, are running in a mutual perpendicular diamond shaped pattern. THe result obtained therewith will be a diamond shaped pattern of disturbances pressed in the metal coating 12 of the metallized film 10, 12 as is schematically indicated in figure 6 with help of the -.-lines. It will be clear that any arbitrary pattern of ribs can be made on the roller 20, including linear and curved lines and including eventually special patterns in case this is desirable for instance for advertising purposes. Perhaps a bit superfluous figure 7 illustrates a pattern of ribs on the roller 20 to create circular shaped disturbances of the continuaty of the metal coating 12. Finally it is remarked that applications of the material according to the invention are not restricted to pastry products. In principle the material can be used in combination with all possible food products which during the preparing or cooking thereof in a microwave oven will have to be browned locally.

Claims

1. Method for manufacturing susceptor materials by providing a metal coating on a plastic film, the thickness of said coating being selected such that the coating will act as an absorber for microwave radiation and adhering said metallised film to a support layer, characterized in that after metallizing said film, but before adhering the metallised film to said support layer the metal coating is in a predetermined pattern locally disrupted to such extend that the susceptor locally becomes transparent for microwave radiation.

2. Method for manufacturing susceptor materials according to claim 1, characterized in that the local disruption of the film is obtained by locally stretching the metallised plastic film within the elastic deformation range of the plastic material.

3. Method for manufactoring susceptor materials according to claim 1, characterized in that the local disruption is obtained by sliding a friction surface tool in accordance with said predeter¬ mined pattern across the metallised coating causing thereby at least micro-scratching of said coating.

4. System for carrying out the method according to one of the preceding claims, characterized by at least a pencil-shaped svibject comprising a rounded outer end, a supporing surface for locally supporting said metallized film and means for conveying the metallized film across said supporting surface, whereby during operation said pencil-shaped object is pressed against that part of the metallized film which is supported by said supporting surface.

5. System for carrying out the method according to one of the claims 1 until 3, characterized by two rollers between which during operation the metallized film is conveyed, whereby that roller, which is pressed against the metal coating comprises standing out parts arranged in a predetermined pattern, the dimensions of said parts being such that the stretch, caused thereby in the plastic film remains within the elastic deformation range of the plastic material.