CA2033353A1

CA2033353A1 - Method and apparatus for producing microwave susceptor sheet material

Info

Publication number: CA2033353A1
Application number: CA002033353A
Authority: CA
Inventors: Yigal Peleg
Original assignee: Societe des Produits Nestle SA; Stouffer Corp
Current assignee: Societe des Produits Nestle SA; Stouffer Corp
Priority date: 1990-01-16
Filing date: 1990-12-28
Publication date: 1991-07-17
Also published as: DE69028088T2; AU634329B2; JPH05220889A; GR3021103T3; AU6920491A; US5126519A; ES2090084T3; EP0437853A3; DK0437853T3; DE69028088D1; EP0437853A2; EP0437853B1; JP2931702B2; ATE141215T1

Abstract

METHOD AND APPARATUS FOR PRODUCING
MICROWAVE SUSCEPTOR SHEET MATERIAL

Abstract of Disclosure Microwave susceptor sheet of the paperboard type, and method of making such susceptor sheet for use in a disposable food appliance adapted to heat a quantity of food in the appliance by microwave energy which susceptor sheet and method comprises vacuum deposition of a layer of aluminum onto D thin plastic film and then bonding the film onto a support sheet of paperboard, wherein the thin plastic film has a thickness of less than about 1.0 mil and is formed from PCTA copolyester plastic having a melting point of greater than 500°F.

Description

METHOD AND APPARATUS FOR PRODUCING
~ICROWAVE SUSCEPTOR S}IEET nATERIAL
Di~closure The present invention relates to the art of disposable ` containers for micro~ave reconstitution of food items and more particularly to a novel lamina~ed susceptor sheet material and method of making this material for use in ~licrowave heating of food items.
INCORPORATION BY REFE~ENCE
Many pa~en~s have issued which relatc t~ the conce~t o depositing a thin metallized layer onto a plastic film which is, in ~urn, mounted on a ~aperboard for the purposes of forming a microwaveable susceptor sheet capable of converting microwave energy into heat as the microwaves pass through the shcet. These microwave susceptor sheets are lamina~ed stock and are disclosed in prior United States Patents, such as Seiferth 4,825,025, Seifer~h 4,641,005, Quick 4,713,510, Brastad 4,267,420> Beall 4,258,08~ and Brascad 4,230,924.

BACKGROI)ND OF INVI~NTlON
During the last few years, a subs~antial effort has been devoted to production of thin paperboard laminated sheet material incorporatin~ a food engagin~ plastic film havin~ a vacuum deposition layer of metal, such as elemental aluminum, which metallized layer forms a microwave interactive layer. This sheet stock is referred to as a ST-8~72 2~33~3 6usceptor and is produced in large qu3ntities for the food industry. When the susceptor sheet material is wrapped around a food item, such as a pizza, or is formed into a container for encircling a fvod item, microwave energy can pcnetrate the metallized layer of the susceptor sheet material causing eddy current to flow in the metallized layer to increase the temperature of the metallized layer and, thus, the lamination sheet material itself.
Consequently, the la~ination sheet material is a heat source for cooking food items adjacent ~he susceptor material or hin a container forme(l by the susceptor material. The susceptor mat~rial h~s the characteristics of a ~hin paperboard and can be die cut, formed and glued into var;ous shapes. This sheet material is widely employed in the food industry and is primarily applicable to heating of crust type food items, such as the crust of a ~izza or the crust of a pot pie.
By allowing the susceptor sheet material to be an independent heat source activated by microwave energy, the susceptor sheet material can obtain a relatively high temperature such as over 400F which will heat the adjacent ood item either by radiation or contact conduction. In this latter instance, wi~h ~he temperature of the susceptor shee~ increasing to a temperature substantially higher than the temperature possible by micro~ave heating of the food item directly, the susceptor sheet can cause crispin~ and browning of the crust of the food i~em. Consequently, one of the pri~lary objectives of a microwave susceptor sheet s~ock is the ability to reach a rela~ively high temper~ture, which is a function of the amount of vacuum deposited aluminum forming the microwave interactive layer and the applied microwave energy. As the metal applied to the ~ ~ ~ 3 3 ~ 3 plastic film o the metallized surface incrcases, the obtainable temperature of the susceptor sheet will first rise and then drastically decrease. This decrease is caused by a complete covering of the plastic ilm. If the metallized aluminum layer is a continuous lay~r of aluminum, microwave energy will be reflected fronl the layer and will create a minor amount of eddy curren~ heating in the metallized layer. Thus, the metallized aluminum must hsve a relatively thin thickness allowing passage of microwave encrgy to create heating of the interactive metal layer.
The thickness of the metalliæed layer con~rols the ultimate temperature reached during exposure of the susceptor sheet to microwave energy. Since the aluminum is vacuum d~posited upon a thin film of plastic material, the temperature to which the interactive aluminum layer can increase is limited by the thermal characteristics of the thin plastic film onto which the interactive netallized layer is vacuum deposited. To obtain the necessary temperature for rapid and effective crisping snd browning of crust material, polyethylene film has generally been replaced by polyester film. In addition, the thickn~s~ of the plastic film is generally greater than 1.0 mils to wi~hstand the te~lperature to which the interactive layer is elevated during the cooking process. It has been found that even wi~h a relatively thick layer (5 mils) of PET
(polyethylene terepllthalate) the temperature to which the interactive layer could bc elevated during the cooking proces~ was seriously limited. When using the more standard PET material, which is readily available in thin film, a sufficient amount of deposited aluminum on the PET film ~o create the desirPd cookin~ temperature caused crazing of ~he film during ~he cooking process. The ~emperature of the 2~3~3~3 sheet would exceed 400F and the PET would craze at nbout 400~F. To prevent such crazing, the temperature of the interactive material was reduced by limiting the amount of me~alliza~ion on the surface o~ the film. This, in turn, reduces the effectiveness of the susceptor sheet material for heating the food adjacent the susceptor sheet.
In view of this situation, ~here has been a substantisl deman~ for a thin film material which would not craze at temperatures desired for effective cooking of food by the lamination microwave susceptor sheet material. It was sugges~ed that laminated high ~emperature plas~ic material could be used for supporting the metallized microwave in~eractive layer. Higher temperature plastic materials were not available in ~hin film. Less than 5 mils is an acceptable film thickness for the purposes of use in a microwave susceptor sheet of the type including a thin plastic film which is metallized and supported on a paperboard. Further, even at the high range of acceptable thickness, high temperature plastic was too expensive for susceptor sheet.
Efforts to increase the thickness of the plastic film for the purposes of withstanding higher temperatures has proven ineffective. The increased thickness resulted in an insulation layer bet~/een the interactive n)etallized layer and the food bein~ cool~ed. In ~uick 4,713,510 the crazing problem was solved by placing the food against ~he paperboard instead of against the plastic film. Thus, crazing of ~he film which supported the met~llized microwaveable interactive material was irrelevant. This solution was not effective because this paperboard created even a greater insulation barrier betweerl the interactivP

layer and the food item being cooked by the microwave energy.
In accordance with the present invention, PCTA
copolyester plas~ic film sold by Eastman Chemical Products a subsidiary of Eastman Kodak Company under the No. 6761 has proven extremely advantageous. This material has a melting point of 545F with ~n inherent viscosity 0.96. The crystalline peak melting point is 545F. The temperature of crystallization on cooling is 375F. Glass transition temperature is 208F. This material is sold under the trademark T}I~R~X and is not available in thin film. In accordance with the present invention, this plastic ma~erial is to be extrllded into a sheet having a thickness of less than l.0 mils. Directly onto this mat~rial vacuum deposited aluminum is to bc added with a metallization thickness sufficient to raise the temperature above over 400F, and preferably sver 45VF. PCTA copolyester is a polymer of cyclohexanedimethanol and terephthalic scid with another acid substituted for a portion of the terephthalic ~cid.
In accordance with the present invention, there is provided a laminated susceptor sheet for use in a disposable container adapted to heat A quantity of food in the container when exposed to micrownve energy. The lamination comprises a microwave interactive layer of electrically conductive me~al having a thickness which is sufficiently small to cause the microwave interactive layer, when subjected to mic1owave energy, to heat up to ~ temperature of over 400F, which temperature is sufficient to heat th~
surface of the food in heat transfer relationship with the sus~eptor material. Of course, between the interactive layer and the food there is provided a protective thin plastic fil~l having sufficient stability at high tempera~ure 2~333~3 that it will n~t degrade when the lamination is subjected to microwave energy to heat the surface of ~he quantity of food. In accordance with the present invention, the protective plastic film onto which the interactive layer of electrically connective netal is deposited, is PCTA
copolyester film having a thickness of less than about l.0 mil with a melting point of over 500F. The protective plastic film wi~h the inLeraCtiVe layer of electrically conductive metal is supported onto a paper stock matcrial having a s~lfficient s~ructural stability at high temperature necessary Çor hea~in~ the sur~ac~ of the quantity oE ~ood to maintain i~s physical shape nt such high temperatures. The interactive metallized layer of conductive metal is formed onto the plastic film in accordance with standard practice of vacuum depositing the microwaveable interactive layer onto the plastic ilm ater which the film is bonded directly to the paper stock material in a manner to cause the film and interactive layer to be held in bonded relationship with the paper stock.
In accordance with another aspect of the invention, there is provided a method of making a laminated susceptor sheet for use in a disposable container adapted to hea~ a quantity of food. This method includes ex~ruding a thin plastic film of less than about 1.0 mils from PCTA
copolyester plastic, vacuum depositing 8 layer o elemental aluminum onto the plastic film with aluminum having a thickn~ss which is sufficiently small to cause the aluminum layer, when subjected to microwave energy, to heat to the desired t~mperature of over 400F and bonding the plastic film directly onto one side of 8 paperboard stock material.
The prim~ry object of the present invention is the provision of a unique microwave susceptor sheet formed by 21~333~3 laminating a thin plastic film, onto which there is deposited a metallized layer, onto a paperboard sheet, which susceptor s1~eet does not craze at high temperatures and can be heated to a temperature exceeding about 450F without crazing or degrading of ~he plastic film against which the food being cooked is placed.
Another object of the present invention is the provision o~ a heat susceptor sheet, as defined above, which sheet is relatively inexpensive to manufacture, can be employed for browning and crisping crust material at relatively hi~h temperatures and can employ normal microwave susceptor technology.
These and other objects and advantages will become apparent from the description of the preferred embodiment.
PREF~RRED EMBODIMENT
A film having a thickness of less than about l.0 mils and preferably between 0.5 and 0.l mils is extruded from PCTA copolyQ6ter 676l material sold by ~astman Chemical Products and is stored on a supply roll. This roll of thin film is ed through a vacuum deposition chamber where elemen~al alu~inum is deposited along one surface of the thin metal film to a metallized thickness needed to create the desired heating effect which is substantially ~reater than the thickness normally applied ~o P~T plastic ~i.e.
sur~ace resistivi~y of 0.40 - 8 ohms per inch) so that the ~emperature a~ which the interac~ive layer will be heated by a given microwave energy will be substantially greater than the temperature that would cause cr~zing of PET which i6 approximately 400F. After the metallized surface has been applied onto the t11in (i.e. Q.l - l.0 mils) PCTA film, the thin film is bonded to a paperboard wi~h ~he metallized surace encapsulated between the plastic protec~ive film and ~ 7 ~

, ST-8072 ~3~3~3 the paperboard. This susceptor sheet is then employed for a container or for a heating surface within a microwave heating utensil or container. The term "container" is intended to be generic for both a surface heating application of the present invention and the concept of surrounding the food being heated.
In ~he past, the protective film was often greater than 1.0 mil. Indeed, increasing the film thickness was employed for preventing the crazing which has been eliminated by implèmentation o the present invention.
Although PCTA ilm has been available, it was used in thickness greater than 1.0 mils. This film has been used only for laminated hi~h temperature heating utensils and then only occasionally. Basically PCTA has hsd limited applications. It is novel to extrude PCTA copolyester into the thin film of less than about 1.0 mils and then use this thin film for the food engaging protective ~eans in a lamination to be used as a microwave susceptor sheet. In the past, thP protective sheet means for use in susceptor sheets have been selected based upon a compromise between cos~ and heat resistibility. The present invention relates to the concept of employing a high temperature plastic material in thin film, which has not heretofore been available in thin film and which surprisingly reduces the tendency of crazing at temperat~res of over 450F that has plagued the paperboard type heat susceptor art for many years.

Claims

1. A paperboard type microwave susceptor sheet material for use in a disposable food appliance adapted to heat a quantity of food associated with said appliance when the appliance is exposed to microwave energy, said susceptor sheet material comprising:
(a) a microwave interactive layer of electrically conductive metal having a thickness which causes said microwave interactive layer, when subjected to microwave energy, to heat up to a temperature which is sufficient to heat the surface of food in heat transfer relationship therewith;

(b) protective means for said microwave interactive layer, said protective means including a smooth surfaced, thin plastic film with an extruded thickness of less than about 1.0 mil having sufficient stability at high temperature that it will not degrade when the susceptor sheet is subjected to sufficient microwave energy to heat the surface of the quantity of food, said plastic being PCTA
copolyester with a melting point of over 500°F; and, (c) support means for providing structural support for said interactive layer and said plastic film, said support means being formed of thin paper stock material having sufficient structural stability at the high temperature .
necessary for heating the surface of the quantity of food to maintain its physical shape; wherein said susceptor sheet is formed by the process including the successive steps of vacuum depositing said microwave interactive layer onto a smooth surface of said thin plastic film followed by the step of bonding directly said thin plastic film upon which said microwave interactive layer has been vacuum deposited to one side of said paper stock material in a manner to cause said thin plastic film and said microwave interactive layer to be held in bonded relationship with said support means to cause the surface of a quantity of food, when in heat transfer relationship therewith, to be heated when the susceptor sheet is subjected to microwave energy.

2. A susceptor sheet material according to claim 1, wherein the step of bonding said thin plastic film to said paper stock material includes the step of bonding directly to said paper stock material the side of said plastic film upon which said interactive layer has been vacuum deposited.

3. A susceptor sheet material according to claim 1, wherein said thin plastic film is adapted to directly contact the surface of the quantity of food.

4. A susceptor sheet material according to claim 1, wherein said step of vacuum depositing includes the step of vacuum vapor depositing said metal onto said thin plastic film.

5. A food package including a susceptor sheet material according to claim 1, wherein said food package includes means for at least partially encompassing food to be surface heated by said susceptor sheet when exposed to microwave energy.

6. A susceptor sheet material according to claim 1 wherein said extruded thickness of said thin plastic sheet is in the general range of 1.0-0.1 mil.

7. A susceptor sheet material according to claim 1 wherein said extruded thickness is in the general range of 0.5-0.1 mil.

8. A susceptor sheet material as defined in claim 1 wherein said thickness of microwave interactive material can cause said interactive layer to exceed about 400°F when said interactive layer is subjected to microwave energy.

9. A susceptor sheet material as defined in claim 1 wherein said interactive layer is vacuum deposited elemental aluminum .

10. A method of making a paperboard type microwave susceptor sheet for use in a disposable food appliance adapted to heat a quantity of food in said appliance, said method comprising the steps of:
(a) extruding a thin plastic film of less than about 1.0 mil from PCTA copolyester plastic;
(b) vacuum depositing a layer of elemental aluminum onto said plastic film, with said aluminum having a thickness which causes said aluminum layer, when subjected to microwave energy, to heat to a desired temperature;
(c) bonding said film directly onto one side of a paper stock material.

11. A method as defined in claim 10 wherein the thickening of said aluminum layer can cause said aluminum layer to exceed about 400°F when said aluminum layer is subjected to microwave energy.