NL9002540A - LOW TEMPERATURE SUSCEPTOR. - Google Patents

Description

Low temperature susceptor.

The invention relates to a layered susceptor material comprising an active microwave absorbing substructure consisting of: an inert plastic film substrate layer and an active metal layer deposited on said substrate layer, the thickness of this metal layer being chosen such that the metal layer is microwave absorbency of between 10% and 50%, and comprising a base material layer that supports said active microwave absorbing substructure and is adhered to this substructure by a layer of an adhesive or adhesive.

For example, a similar susceptor, but described as having much lower electrical surface resistance of the metal layer, is the subject of U.S. Patent No. 4,641,005. In the context of this description, a susceptor can be defined as a material that absorbs microwave energy to such an extent that the material is heated quickly and provides a hot surface to aid in food preparation processes.

Generally, in such susceptors, a polyester film, for example a polyethylene terephthalate film, is used as the substrate layer, which film after being metallized, is adhered or glued to a paper or cardboard layer that functions as said base material layer. Susceptors of this type have been widely accepted as browning aids for a wide range of microwavable food products. These susceptors are used, for example, as an active packaging material to create a crispy surface and to brown all kinds of dough products, which would otherwise not be successfully reheated in a microwave oven. Another important field of application is self-inflating popcorn bags, as described, for example, in U.S. Patent 4,571,337, wherein a susceptor increases the effectiveness of the pop process and reduces the preparation time.

At present, there are two interrelated problems in the use of susceptors that deserve attention. If the susceptor surface is in close contact with a food product creating an effective heat dissipation capability for the heat generated in the susceptor, the susceptor can heat itself to temperatures well above 200 ° C. These high temperatures, for which the packaging materials and components are generally not intended, can lead to the release of volatiles from the polyester film surface and degradation products from the polyester film surface very close to the food. The paper or cardboard substrate can also become brown and charred, releasing unwanted (potentially harmful) vapors and gases.

A second problem also arising from the high temperatures generated by the susceptor is crackle formation or cracking in the polyester film. This is often invisible to the naked eye, although in some products it becomes immediately apparent because the paper or cardboard layer is exposed as the metallized polyester film shrinks. Surface examination of spent susceptor materials using a scanning electron microscope has shown that the laminating adhesive or adhesive layer and the underlying paper or cardboard layer can be released in this cracking process.

The food product, which was originally insulated from direct exposure to the adhesive or adhesive laminating layer and to the paper base layer, is now exposed through the cracks to the hot glue layer and to the paper and any hot (> 200 ° C) ) surfaces released volatile components.

It is believed that the temperature limit of conventional susceptors is determined by the melting point of the polyester film, namely 265 ° C. Above this temperature, this film no longer provides a solid base for the microwave absorbing metal layer. The thermal stresses cause the metal layer to become discontinuous and to function as a much weaker microwave absorption layer (without this self-automatic limiting property, temperatures would increase even further to the point where the susceptor would ignite).

The objects of the invention are twofold: to construct a susceptor with heating capabilities similar to existing products, but with a maximum temperature peak lower than more than 200 ° C of current products; and constructing a susceptor that provides a continuous shielding surface in contact with the food during the heating process and its use.

In accordance with this objective, the susceptor material described in the introductory paragraph of the invention is characterized in that said inert plastic film substrate layer is made of a plastic material having a melting point of less than 200 ° C and further that the substrate layer material is is in an oriented state.

Preferably, the layered susceptor material comprises a further layer of a material having a melting point above the melting point of said substrate layer, which further material layer is adhered to said active microwave absorbing substructure on that side facing away from the side to which the base material layer is adhered . This further layer functions as a coating layer which, due to the higher melting point temperature, will remain stable under all conditions and will isolate the food product from the layers of the said substructure during use of the susceptor.

The invention will now be described in more detail with reference to the drawings.

Figure 1 illustrates a cross-sectional view through a layered susceptor material according to the invention.

Figure 2 illustrates a cross-sectional view through a preferred embodiment of a layered susceptor material according to the invention.

Figure 3 illustrates another embodiment of the layered susceptor material of the invention.

Figure 1 illustrates a cross-sectional view through a layered susceptor material comprising a substrate layer 10 and a metal layer 12 applied to the substrate layer 10. The substrate layer consists of a thin inert plastic film, which forms the material that is vacuum-coated with the metal layer 12. These two layers together form a substructure which can be produced as a separate product. A base material layer 16 is adhered or glued to said substructure by a layer of an adhesive or adhesive 14.

In the illustrated embodiment, the base layer is adhered to the metal layer 12.

According to the invention, the substrate layer 10 is made of a plastic material with a melting point of less than 200 ° C and is preferably made of biaxially oriented polypropylene film of a quality suitable for use in food packaging applications and suitable for metallization. Within the context of this description, the term "melting point" not only represents the temperature at which a polymer changes from the solid phase to the liquid phase, but also refers to the softening point or temperature range at which non-crystalline plastic material transitions from the solid to the viscous state.

Preferably, the substrate layer is a biaxially oriented film made of polypropylene. The thickness of the film 10 is preferably between 5 and 50 / h.

The metal layer 12 can be any metal or alloy that is stable as a thin film and corrosion resistant, but preferably consists of aluminum or stainless steel, which are metals which are generally accepted in the art of food packaging and food preparation.

The metal layer is applied to the substrate layer by a suitable vacuum deposition process, such as a thermal evaporation process or a sputtering process. In such processes, the substrate film is conveyed at an adjustable speed across the evaporation source such that the vapor or substrate can condense to achieve the desired film thickness within fractions of 1 micron.

The final metal coating preferably has a surface resistance in the range of 50 to 500 ohms /. The optical density for an aluminum metallized film should preferably be in the range of 0.18 to 0.28 so that the metal layer has a micro wave absorbency between 10% and 50%. The correct thickness, as is known to those skilled in the art, can be achieved by controlling the metal evaporation or sputtering rate and by controlling the speed of the film above the metal evaporation source.

Preferably, the base material layer 16 consists of paper or cardboard. The metallized film is laminated to this paper or cardboard using an adhesive or adhesive suitable for use in food packaging, the metallized surface of the substrate film being in contact with the adhesive layer 14.

A cross-sectional view through a further developed embodiment of a layered susceptor material according to the invention is illustrated in Figure 2. Those layers, which are also present in the embodiment illustrated in Figure 1, are denoted by the same reference numerals. The difference between Figure 1 and Figure 2 lies in the fact that a further plastic film 18 is adhered or glued by means of an adhesive layer 20 on the top of the metallized substrate film 10. The further plastic layer has a melting point temperature above the melting point temperature of the ( polypropylene) substrate film and preferably consists of polyester. The purpose of this further layer is to provide a protective separating layer between the actual food product and the microwave absorbing substructure. Since the substructure, comprising the layers 10, 12, has a self-limiting property at a temperature of about 170 ° C, determined by the melting point of the polypropylene film, there is no danger that the polyester film 18 will start to release volatiles or degradation products caused by overheating of this layer. On the other hand, any volatiles or degradation products released by the polypropylene film 10 will be effectively blocked from the food product by the further polyester layer.

Figure 3 illustrates a cross-sectional view through another embodiment of the susceptor material of the invention, which differs only from the embodiment illustrated in Figure 2 by the orientation of the substructure consisting of the polypropylene film 10 and the metalized layer 12. It will be Fig. 3 it is clear that the paper or cardboard base material layer 16 in this embodiment is not glued or adhered to the metal layer 12, but is adhered to the polypropylene layer 10. Thereafter, the further plastic layer 18, consisting of polyester, is provided by means of the adhesive layer 20 glued to the metal layer 12. This embodiment also has the self-limiting property at a fairly low temperature determined by the melting point of the polypropylene film (170 ° C).

The following three examples demonstrate the invention. More specifically, the browning properties, the necessity to apply an oriented base film and the necessity to apply a temperature resistant layer in contact with the food will be demonstrated.

Example 1

A roll of cast, i.e., non-oriented, polypropylene film (Karl Dickel "Safran" PP 350 MET) was metallized by vacuum evaporation of aluminum to produce a coating or coating with an optical density of 0.2. The metallized film was laminated using a water-based adhesive (Duraflex 56) applied with a dry coating weight of 3 g / m2 on the metallized side, on a 240 g / m2 cardboard.

Pieces of this laminated board cut to a size of 11x18 cm (about 200 cm2) were tested as a susceptor for browning pieces of fresh pizza dough weighing 190 g.

Two pieces of dough were also placed on a 200 cm 2 susceptor layer in a 600 W microwave oven and prepared at full power, i.e. with continuous power, for 5 minutes. As intended and also expected, rapid heating of the layered structure occurred, but surprisingly, in an uncontrolled manner, the heating went well beyond the melting point of polypropylene. Burning occurred in "extra hot spots" (hot spots) of the material. The oven was filled with smoke from the burning base material, polypropylene and dough.

This demonstrated the strong microwave absorption properties of the susceptor, but also demonstrated the absence of a necessary limitation in the absorbing properties at the melting point of the plastic film.

Example 2

A roll of biaxially oriented polypropylene film (ICI "Propafilm" MVG) as in Example 1 metallized to an optical density of 0.2 and then laminated to the same board base and with the same adhesive as in Example 1.

The susceptor was tested in a pizza dough microwave oven as in Example 1.

After a 5 minute cooking time, the pizza dough was observed to be lightly browned on the bottom which was in contact with the susceptor. An examination of the surface of the susceptor used showed that the surface had become dull. Examination under a scanning electron microscope revealed that the surface had changed from the glossy, smooth surface of the original film to a textured surface of a melted and solidified polymer. The surface was continuous, apart from small "pinhole holes" of a few microns in diameter and some slightly larger holes up to 100 µm (0.1 mm) in diameter.

This example demonstrated the browning function of the susceptor, and the temperature limiting property that prevented burning or overheating of the susceptor and the food product and also showed the melting of the susceptor surface, also in contact with the food.

Example 3

To the laminated susceptor material of Example 2 (metallized biaxially oriented polypropylene on a cardboard substrate), a 12 µm polyester film was added by means of a further lamination process on the polypropylene surface (ICI "Melinex S", a biaxially oriented polyethylene terephthalate film for food packaging Objectives This is a film of the same type which is used as a metallization base to produce conventionally constructed susceptor products.

Squares of this material (200 cm2) were retested in the 600 W microwave oven with 190 g pieces of pizza dough. After a 5 minute cooking time, the pizza dough had browned on its underside, the susceptor itself had not burned, and the polyester film in contact with the food remained continuous, unbroken and showed no evidence of melting.

Claims (7)

A layered susceptor material comprising an active microwave absorbing substructure consisting of: an inert plastic film substrate layer and an active metal layer deposited on said substrate layer, the thickness of this metal layer being selected such that the metal layer has a microwave absorbency of between 10% and 50%, and comprising a base material layer that supports said active microwave absorbing substructure and is adhered to said substructure by means of an adhesive or gluing agent layer, characterized in that said inert plastic film substrate layer is manufactured is of a plastic material having a melting point of less than 200 ° C and further the substrate layer material is in an oriented state. Layered susceptor material according to claim 1, characterized in that the layered susceptor material consists of a further layer of a material having a melting point above the melting point of said substrate layer, which further material layer is adhered to said active micro wave absorbing substructure. on that side facing away from the side to which the base material layer is adhered. Layered susceptor material according to any one of the preceding claims, characterized in that the oriented substrate layer is made of a biaxially oriented material. Layered susceptor material according to any one of the preceding claims, characterized in that the substrate layer consists of a film made of polypropylene. Layered susceptor material according to any one of the preceding claims, characterized in that the metal layer consists of aluminum with a thickness corresponding to an optical density in the range of 0.18 to 0.3. Layered susceptor material according to any one of the preceding claims, characterized in that the metal layer has a surface resistance in the range of 40 to 40 ohm /. Layered susceptor material according to any one of the preceding claims 2-6, characterized in that the further layer of a material with a melting point above the melting point of the substrate layer is made of polyethylene terephthalate, polyamide or polycarbonate.