Microwave susceptor sheet stock with heat control

Abstract: In a blankable, foldable microwave reactive heat susceptor sheet stock (S) of the type having two generally parallel surfaces (12,14) and comprising a thin paperboard sheet with a laminated, continuous, microwave reactive layer including a microwave permeable support film with a microwave reactive layer including a microwave permeable support film with a microwave reactive stratum, there is provided an improvement comprising a graybody layer (40,GB) with an absorptivity over about 0.50 and microwave permeable with the graybody layer (40) being applied coextensively with at least a portion of the parallel surfaces (12,14) so that heat created by the interactive stratum will be absorbed and available from the graybody layer.

Description: This invention relates to the art of microwave reactive heat susceptor sheet stock and more particularly to an improved susceptor sheet stock with heat control means allowing selective manipulation of the available heat from the sheet stock. The present invention relates to an improvement in the microwave reactive heat susceptor sheet stock of the type having a paperboard base and described in many prior patents, such as Seiferth 4,641,005, Jaeger 4,891,482 and EP-A-0 317 203. A susceptor sheet is also known from EP-A-0 312 333 which describes an absorbing composite having absorbing and transmitting zones. By using the improvement according to the invention, there can be a substantial reduction in the needed amount of microwave reactive heat susceptor sheet stock needed for the heating utensil or appliance that is employed to reconstitute frozen food items, such as frozen French Bread pizza. In addition, this improvement allows for a crisping, browning effect on the lower bread crust, without the necessity of an additional sheet stock layer or other modifications of the sheet stock. By using the present invention, it has been found that a heating utensil or appliance employing the present invention can be used for a number of reconstitution cycles so that a single heating utensil can be supplied with a package containing two or more slices of French Bread pizza slices. In the past, it has been conventional wisdom to employ separate and distinct heating utensils for each slice of pizza provided in a single package. Thus, if two slices are provided, two separate and distinct heating utensils or appliances were supplied. By using the present invention, an appliance can be provided for each slice or a single heating unit can be supplied to the customer. A single unit results in a further reduction in the cost of the product. No matter how many heating units are provided to the customer, each unit uses a lesser amount of sheet susceptor stock required for each heating utensil. In accordance with the present invention there is provided an improvement in a blankable, foldable microwave reactive heat susceptor sheet stock of the type having two generally parallel surfaces and comprising a thin paperboard sheet with a laminated, coextensive, microwave reactive layer including a microwave permeable support film with a microwave reactive stratum. The improvement comprises a graybody layer with a high absorptivity and microwave permeable. This layer, thus, allows free passage of microwave energy. The graybody layer has an absorptivity value greater than said laminated microwave reactive layer. The graybody layer is applied coterminously with at least a portion of the parallel surfaces in the sheet stock. The invention is in the use of a layer of high heat absorbing material applied by coating, laminating, deposition, i.e. printing or photographic, etc. The heat from the microwave reactive stratum is absorbed by the absorbing material. Normally the material is black and approaches the ideal blackbody with an absorptivity of 1.0. Thus, the preferred embodiment uses a black layer with an absorptivity over 0.90. This is more technically defined as a graybody; however, a graybody has a wide variety of absorptivity. To illustrate that the invention uses high absorptivity, the graybody layer of the present invention is defined as having a high absorptivity, this can be defined as the general range of 0.50-0.97. This resistivity approaches a blackbody (1.0) at the high range of absorptivity. A "graybody" is generally known as a high absorptivity substance which can approach the unobtainable black body status. Such layer can be provided by printing, coating, or laminating. Application of the graybody layer can be on the surface of the various constituent portions of the susceptor sheet stock. For instance, it can be on the outside, exposed surface of the paperboard. It can be on the inside, hidden surface of the paperboard. Further, it can be applied to the protective plastic film or over the aluminum microwave reactive stratum. No matter how the graybody layer is applied to the sheet stock or where it is located, it still has the same heat absorbing, thermodynamic characteristic. Heat from the parallel, closely spaced microwave reactive material layer is absorbed by the graybody layer in the area of the sheet stock containing the absorbing layer. Consequently, the graybody layer provides a heat sink to absorb and hold the heat created by the parallel microwave interactive stratum. The stratum itself has a low absorptivity and high reflectivity. Thus, the stratum can not form an efficient heat sink. Further, the reactive layer can not form a heat sink because it is so thin, being in the neighborhood of substantially less than about 1 micron in thickness. In accordance with the invention, there is an interaction between the heat generated in the reactive stratum and the parallel graybody layer added to the standard sheet stock. There is a direct and immediate transfer of heat to the graybody. This absorption rate is greater for layers with high absorptivity. For instance, if the graybody has a black color such as carbon black or soot, the absorptivity may approach 0.97. As the color changes to shades of gray and possibly blue, a lesser amount of heat is absorbed due to a lowering of the absorptivity. The lower limit of absorptivity is probably in the general range of 0.50 found in two coats of linseed oil. The actual value of the absorptivity can be varied as desired, as long as the inventive concept of the invention is employed. The invention concept is the interaction of a closely spaced microwave reactive layer and a parallel graybody layer. The interactive layer transfers heat immediately to the graybody layer which holds the heat for the purpose of creating both a high temperature in the neighborhood of 149-177°C at the interface between the thin sheet stock forming the heat susceptor and the food engaged by the susceptor sheet stock and high temperature heat energy to the heated food product. In the past, the sheet stock appliances were usually modified to give a high browning temperature. The present invention accomplishes that objective without advancing the temperature much above 149°C. In addition, more heat energy is provided to, and concentrated in, the bread area. In summary, the present invention relates to the application of a graybody layer onto a standard heat susceptor sheet stock. This layer can be selectively applied adjacent the portion of the sheet stock adjacent the bottom of the pizza or other bread item for the purposes of causing efficient browning and crisping of the bread surface without high surface temperatures. The graybody layer can have a preselected pattern, or can be located in only certain areas of the sheet stock, for controlling the desired heating effect in an appliance made from a sheet stock using the invention. In accordance with another aspect of the present invention, there is provided a heating utensil or appliance employing the present invention wherein the heating utensil is essentially a lower platform formed from a microwave susceptor sheet stock wherein a graybody layer is applied on the lower surface of the platform. This graybody then absorbs the heat from the reactive layer which is parallel and just above it for the purposes of absorbing the heat and providing the heat directly to the lower surface of the food item on the platform. The graybody layer is permeable to microwave energy so that microwave freely passes through the layer; consequently, the layer absorbs and holds the heat, but does not in any way interfere with the microwave energy passing through the reactive stratum. In this fashion, the stratum can have the desired surface resistivity to cause the desired heating while the parallel, closely spaced graybody layer can absorb, retain and transfer high levels of heat energy to the food item. Brief Description of the Drawings The following drawings are included: FIGURE 1 is a pictorial view of a heating utensil employing the present invention; FIGURE 2 is an enlarged cross-sectional view taken generally along line 2-2 of FIGURE 1; FIGURE 3 is an enlarged partial cross-sectional view taken from the encircled area of FIGURE 2 and containing certain relationships relating to absorptivity of a graybody; FIGURE 4 is an enlarged cross-sectional view similar to FIGURE 3 illustrating a modification of the embodiment of the present invention shown in FIGURE 3; FIGURE 5 is a further view similar to FIGURE 3 showing still a further modification of the invention; FIGURES 6-9 are enlarged, partial cross-sectioned views of the susceptor sheet showing different positions of the graybody layer; FIGURES 10-13 are enlarged schematic views showing several pattern concepts that can be employed in accordance with the present invention; FIGURE 14 is a cross-sectional view of a pot pie heating utensil employing the present invention; FIGURE 15 is an enlarged cross-sectional view illustrating a further modification of the present invention wherein an aluminum reflective layer is employed within the sheet stock; FIGURE 16 is an enlarged cross-sectional view showing a further modification of the present invention employing both a combination of a graybody and an aluminum reflective layer; and, FIGURE 17 is a flow chart showing a method of using the preferred embodiment of the present invention as illustrated in FIGURES 1 and 2 for reconstituting a number of frozen pizza slices. Preferred Embodiments Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiment of the invention only, and not for the purpose of limiting same, FIGURES 1-3 show a heating utensil or appliance 10 for reconstituting slices of pizza. This utensil is formed from a single blank of foldable microwave reactive heat susceptor sheet stock S and includes parallel folded sides 12, 14 on the opposite edges of a lower support portion or platform 20 onto which the bread or crust portion of the pizza slice is positioned for microwave reconstitution in an appropriate domestic microwave oven. When blank S is folded to form sides 12, 14, legs 22, 24 and 26 protrude from side 14 and legs 32, 34 and 36 protrude from side 12. The upper portion of the pizza slice which contains the non-bread constituents, faces away from utensil 10. Utensil 10 is placed in the frozen food package in a flat, unfolded form to consume a relatively minor amount of head room. Blank S is formed from a standard material available from such companies as James River Corporation of Richmond, Virginia and is standard well known sheet stock used in many packaging applications for microwave reconstitution of food products. By positioning a pizza slice on platform 20, microwave passes through the lower platform into the crust area of the pizza slice. At the same time microwave energy is directed through the top portion of the pizza slice. In use of the utensil so far described, the pizza would become fully cooked and reconstituted without appropriate browning, crisping and other reconstitution of the exposed bread surface at the bottom of the pizza. To accomplish this crisping and browning, in accordance with the invention, a layer 40 formed from a graybody is applied to the lower surface of platform 20. This layer can be coated, laminated, deposited, printed, photographically applied or otherwise applied on the under surface. The layer is a graybody with a absorptivity in a general range of 0.50-0.97. At the higher range, this approaches an ideal blackbody, which is not obtainable. In accordance with a preferred embodiment of the invention, graybody layer 40 is formed from 3-5 mills of polycarbonate with a black pigment. This produces an absorptivity in the general range of about 0.90 which is on the high range and is essentially considered to be a blackbody. The term "graybody" means that the absorptivity is not 1.0 which is the technical definition of a blackbody and the value against which the resistivity of a graybody is measured. In the preferred embodiment, a graybody approaching a blackbody and having an absorptivity of 0.90 is employed for the layer 40. This is formed from a layer of plastic material having a black pigment which is glued to the underside of platform 20. As shown in FIGURE 3, the remainder of the platform 20 is a standard susceptor sheet stock including thin paperboard sheet 100 in a thickness of 7-8 mm. A thin plastic film 102 has vacuum deposited thereon elemental aluminum forming a microwave reactive stratum 104 coated onto the under surface film 102 and adhered by adhesive to the top surface of paperboard 100. This is a standard paperboard sheet stock S onto which is applied a high absorptivity graybody layer 40 which constitutes improvement of the present invention. Referring now in more detail to FIGURE 3, microwave energy represented by arrows MW passes downwardly through the food item on platform 20 and through sheet stock S. As the microwaves pass through stratum 104, the stratum converts microwave energy into heat causing heat energy schematically illustrated as wavy arrows H to move upwardly into the under surface of the food product being heated. At the same time, heat energy is conducted through paperboard 100 to the graybody layer 40, also referred to as layer GB. Since the absorptivity of layer 40 is extremely high, there is a rapid transfer of heat from stratum 104 to layer 40. This thermodynamic action absorbs the heat and layer 40 becomes a heat sink. This heat sink directs heat through the paperboard to the under surface of the food product. Thus, a rapid and immediate heating of the under surface is accomplished. Heat is not lost from the lower portion of sheet S. The inner face at the upper surface of platform 20 does not exceed about 149°C. However, the substantial amount of heat is transferred across this interface to cause browning and a decrease in entrapped moisture. By using a high absorptivity layer on the under surface of platform 20, a high concentrating of heat transfer is accomplished. There is no loss of heat through paperboard 100. This has proven to create a high browning effect without a high interface temperature. Also, there is no entrapped moisture since the high immediate transfer of energy to the under surface of the food item rapidly drives moisture from between the food item and the platform 20. This can not be accomplished merely by elevated temperature at the interface. All of these phenomenons work in some fashion to maintain relatively low interface temperature while affecting rapid and highly desirable browning and crisping of the under surface of the bread crust. In the past, such browning was accomplished only by increasing the temperature at the interface which is not necessary by using the present invention. Consequently, as will be more fully explained later, the utensil 10 does not have any tendency to degrade the integrity of film 102. Consequently, the utensil can be used more than once and only a single utensil need be supplied in a package having more than one pizza slice. The radiation blackbody formula is provided in FIGURE 3 illustrate that the amount of radiation from the graybody layer 40 is a function of the fourth power of the absolute temperature T. This is the Stefan-Boltzmann law wherein the Stefan-Boltzmann constant (5.670X10 -12 ) is the multiplier of the temperature factor T4. Radiation and absorption are related concepts. A blackbody is an ideal radiator and an ideal heat absorber. As is known, the blackbody radiation relationship is reduced by the absorptivity of a graybody. This is indicated in the lower portion of FIGURE 3 where the absorptivity is less than 1.0. The graybody is defined generally as having an absorptivity in the general range of 0.50-0.97 at 149°C. The present invention relates to the use of layer 40 having an absorptivity which is relatively high with respect to the other constituents of sheet stock S. This use of a highly absorptivity layer or coating on the sheet stock controls the temperature at the upper surface adjacent layer 102. It has been found that use of a high absorptivity layer 40 produces a highly satisfactory reconstituted slice of pizza with only the platform type utensil shown in FIGURES 1 and 2. The modification in FIGURE 4 employs graybody layer 40a on the upper surface of sheet stock S adjacent film 102. In this instance, heat generated by stratum 104 is directly transferred through layer 40a to intensify the heating effect for the food product without increasing the temperature of film 102. In FIGURE 5, the modification in FIGURE 4 is further changed to include the previously described layer 40 at the lower portion of paperboard 100. In this embodiment, both layers 40, 40a absorb energy from microwave interactive stratum 104 to concentrate heat in the under surface of the product being reconstituted on platform 20. Referring now to FIGURES 6 and 7, the graybody layer GB is applied onto film 102 before it is laminated onto the paperboard 100. In FIGURE 6, graybody layer 40b is applied by deposition, printing, and/or lamination, onto the aluminum reactive stratum 104. In FIGURE 7, the graybody layer 40c is applied to the upper surface of film 102. In either instance, this film is then laminated by adhesive to the paperboard 100 for forming a microwave reactive heat susceptor sheet stock. These modifications, especially the modification shown in FIGURE 6, illustrates that the graybody layer can be inside sheet S between the two external surfaces of the sheet stock without departing from the invention. FIGURES 8 and 9 show modifications of the invention wherein graybody layer 40d is applied to the upper surface of paperboard 100. In FIGURE 8, layer 40d is captured between the stratum 104 and paperboard 100 as a replacement for lower layer 40. In FIGURE 9, both layers 40, 40d are applied onto paperboard 100. The application of layer 40 can be either before or after lamination of the microwave reactive layer 104 onto paperboard 100. A further modification obtainable by employing the present invention is illustrated in FIGURES 10 and 11 wherein the graybody layer 40e is applied to the under surface of paperboard 100. In this instance, layer 40e has a preselected area A 1 which is distinguished from an area A 2 of sheet stock S which does not have a coterminous graybody layer. Thus, one portion of the sheet stock (A 1 ) is heated by using the present invention. The other portion of the sheet stock is not modified. By adjusting the areas A 1 and A 2 , a variety of heating relationships can be accomplished by using the present invention. This concept is also shown in FIGURE 11 wherein a further partial covering of paperboard 100 is accomplished by upper graybody layer 40f. This divides the upper surface of paperboard 100 into area A 3 , A 4 , the first of which is subjected to the heat intensifying aspects of the present invention. For illustrative purposes, areas A 1 and A 3 are illustrated as being overlapping in area A 5 . Thus, area A 5 has the advantages associated with two separate layers of graybody material whereas the other portions A 2 , A 4 subjected only to a single graybody layer. Figures 10, 11 are provided to illustrate that several modifications can be made in implementation of the present invention. The areas A 1 -A 5 are the total areas of the heat susceptor sheet even though they are illustrated as applicable only to an association with paperboard 100. To illustrate a further possible implementation of the present invention, FIGURES 12 and 13 show areas 120, 120a which are not provided with a graybody layer. The other areas of the heat susceptor sheet stock are provided with a graybody layer 110 or blocks 110a of graybody material, respectively. In this manner, the amount of heating caused by the graybody layer can be modulated by a pattern and/or the arrangement of the graybody layer to the non-graybody areas of susceptor sheets. The blocks shown in FIGURE 13 could be photographically printed in black ink on paperboard 100 or on the upper surface of film 102. Another cooking utensil or heating utensil applying the present invention is illustrated in FIGURE 14 wherein a pot pie dish 200 is provided with an outer layer of surrounding aluminum foil 202. Aluminum foil 202 is reflective and is not permeable to microwave energy. Thus, no microwave heating occurs around the edge of a pot pie in utensil 200. The lower portion of utensil 200 includes an opening in the aluminum foil 202 which is filled with a graybody layer 204. Microwave energy MW passes through this layer into the standard microwave susceptor heat stock S. Thus, a higher heating occurs adjacent the lower surface of the pot pie. This can be employed for the purposes of selective heating and/or browning of a food item within receptacle 200. This receptacle has a round, generally frustoconical shape, which is only representative in nature and is employed for the purposes of illustrating a use of the present invention. A still further way of using the present invention is illustrated schematically in FIGURES 15 and 16. In these illustrations, an aluminum foil is employed for the purposes of creating a reflective layer in combination with the graybody layer. In FIGURE 15, the aluminum foil layer 220 is laminated over film 102. Above the aluminum foil is provided graybody layer 40g. Thus, as heat energy is created by the double exposure of stratum 104 to the microwave energy MW, a substantial heat energy is created. This heat energy is directed to foil 220 which is a good heat transmitter. Heat is transmitted through the aluminum foil and is absorbed by layer 40g . This layer immediately heats the food product. In FIGURE 16, aluminum foil 220a and graybody layer 40h are on different areas of lower surface of sheet stock S. This produces a hot area adjacent the graybody and a cooler area adjacent the aluminum foil 220a. The invention involves a high absorptive layer. It is defined as being microwave permeable. However, this coating or layer, could conceivably, be placed upon a reflective substrate as an aluminum foil as shown in FIGURE 15. As previously described, it has been found that for various reasons, the use of layer 40 on the under surface of platform 20 allows heating of the pizza slices while providing brown crisp under surface for the bread in contact with the platform 20 without the crazing of film 102. The flow chart shown in FIGURE 17 indicates that this advantage of the present invention allows the use of heating utensil 10 more than once for slices P1, P2 and Pn. In this manner, a single utensil or appliance 10 can be supplied in a package having several pizza slices. The same utensil can be used more than once. This is a substantial reduction in the cost of the utensil necessary for reconstituting frozen pizza slices. In addition, by using the invention, the upper portion of utensil 10 is open to allow direct exposure of the pizza with the microwave energy. This is different from a sleeve; however, the invention could be employed with the sleeve of sheet stock S if that is desired. The present invention has been described with its primary use in reconstituting a frozen food item. There is no intent to limit the invention to that particular application; therefore, the invention can be used in various domestic and industrial heating applications where it is desired to concentrate the heat created by the standard susceptor sheet stock S.

Claims: A foldable microwave reactive heat susceptor sheet stock (S) of the type having two generally parallel surfaces and comprising a thin paperboard sheet (100) with a laminated, coextensive, microwave reactive layer including a microwave permeable support film with a microwave reactive stratum (104), characterised by a graybody layer (40, 68) with an absorptivity over about 0.50 and microwave permeable, said graybody layer (40) having an absorptivity value greater than said laminated microwave reactive layer and being applied coextensively with at least a portion of said parallel surfaces. The sheet stock defined in claim 1 wherein said graybody layer (40) is applied onto one of said parallel surfaces. The sheet stock defined in claim 1 wherein said graybody layer (40) is applied between said parallel surfaces. The sheet stock defined in any preceding claim including a second graybody layer (40) with an absorptivity of over about 0.50 applied coextensively with at least a portion of said parallel surfaces. The sheet stock defined in any preceding claim wherein said absorptivity is in the range of 0.50-0.97. The sheet stock defined in any preceding claim wherein said graybody layer (40) comprises a thin sheet (102) laminated with said sheet stock. The sheet stock defined in claim 6 wherein said thin sheet includes a surface area and said parallel surfaces have a combined surface area, said surface area of said thin sheet (102) being less than the surface area of said parallel surfaces. The sheet stock defined in claim 7 wherein said thin sheet has a preselected surface pattern (110a, 120a). The sheet stock defined in claim 6 or claim 7 wherein said thin sheet includes a single integral sheet. The use of the sheet stock (S) defined in any preceding claim as a blank to be formed into a microwave heating utensil (200).

Inventors: PELEG YIGAL, PELEG, YIGAL
Applicants: NESTLE SA, SOCIETE DES PRODUITS NESTLE S.A.
Application year: 1991
Main classification: B65D81
IPC codes: B65D81, H05B6