PATTERN COATED MICROWAVE FIELD MODIFIER OF DISCRETE ELECTRICALLY CONDUCTIVE ELEMENTS

Abstract: A patterned, microwave field modifier (20) for use in microwave ovens which modifier may be discrete; or may be integrated into, for example, packages or packaging or wrapping type materials, or in durable or disposable cookware. The modifier may constitute a dielectric substrate (22) having a plurality of discrete surface areas which are coated with an electrically conductive coating material (24) to form discrete electrically conductive elements, and which discrete elements on a given surface are disposed in a predetermined array. Preferably the elements are elongate; and, preferably, the array constitutes a plurality of rows of linearly aligned elements which rows are in parallel relation, and in which the elements adjacent rows are in staggered relation. The electrically conductive elements are preferably formed by printing an electrically conductive ink-like coating material on the dielectric substrate. The relative values of microwave power reflectance, absorbance, and transmittance of such microwave field modifiers may be tailored to specific needs by varying the surface electrical conductance of the elements; the lengths, widths, spacing, and the degree of stagger between adjacent elements; the resistivity, shape, size, and aspect ratio of electrically conductive particles of the coating material; and the dielectric properties of the substrate and/or other constituents of the coating material.

Description: PATTERN COATED MICROWAVE FIELD MODIFIER OF DISCRETE ELECTRICALLY CONDUCTIVE ELEMENTS TECHNICAL FIELD The invention pertains to microwave field modifiers for use in microwave oven heating and cooking, and more specifically, to such microwave field modifiers which are useful for such things as generating high surface heat on adjacent food matter to effect browning and/or crispening; or for balancing surface heating and deep microwave heating of underlying food product; or for partially protecting underlying or adjacent food matter from direct exposure to microwave energy to obviate overcooking and/or overheating the food matter; or for simply effecting a more uniform microwave energy field. BACKGROUND OF THE INVENTION Microwave ovens possess the ability to heat, cook or bake items, particularly foodstuffs, extremely rapidly. Unfortunately, microwave heating also has its disadvantages. For example, microwave heating alone in today's microwave ovens often fails to achieve such desirable results as evenness, uniformity, browning, crispening, and reproducibility. Contemporary approaches to—achieving these and other desirable results with microwave ovens include the use of microwave field modifying devices such as microwave susceptors and/or microwave shields. Microwave susceptors and shields, like other materials and constructions have some degree of microwave reflectance (R), absorbance (A) and trans ittance (T); or collectively RAT properties. RAT properties are measured in terms of percentage of microwave energy reflected by (R), absorbed by (A), and transmitted through (T) a material or construction. Thus, the aggregate of the R, A and T values will total 100%. Generically, a microwave shield is relatively opaque to microwave energy. In terms of RAT, a shield will have a relatively low T value. Microwave shields are exemplified by such highly electrically conductive materials as aluminum foil. Although shields are generally thought of as non-heating elements, a shield could also be a susceptor, i.e., heat appreciably, and visa-versa. Thus, a shield is an element with relatively low T regardless of its tendency to generate heat. Generically, microwave susceptors are devices which, when disposed in a microwave energy field such as exists in a microwave oven, respond by generating a significant amount of heat. The susceptor absorbs a portion of the microwave energy and converts it directly to heat which is useful, for example, to crispen or brown foodstuffs. Thus, microwave susceptors generally have a relatively high microwave absorbance (A) value. In addition to high absorbance, susceptors include a mechanism to convert the absorbed microwave energy to heat. For example, heat may result from microwave induced, intramolecular or intermolecular action; or from induced electrical currents which result in so called I-squared-R losses in electrically conductive devices; or from dielectric heating of dielectric material disposed between electrically conductive particles, elements or areas which type of heating is hereinafter alternatively referred to as fringe field heating or capacitive heating. As noted, microwave susceptors and reflectors, and other materials and constructions, have an effect on the microwave power distribution within a microwave oven. That is, they interact with the microwave energy within the oven through their RAT properties and cause the microwave energy field to be modified. Accordingly, devices and constructions which act to modify the microwave field or microwave energy power distribution within a microwave oven are referred to herein collectively as microwave field modifiers. The patent literature is replete with a variety of teachings with respect to the use of materials and constructions for use in microwave ovens as microwave heaters (e.g., susceptors) and reflectors. For instance, United States Patent 4,230,924 which issued October 28, 1980 to William A. Brastad et al discloses a Method And Material For Prepackaging Food To Achieve Microwave Browning. Such material may be a dielectric wrapping sheet having a flexible metallic coating thereon such as aluminum, in the form of a relatively thin film or relatively thick foil, the coating being subdivided into a number of individual metallic islands or pads separated by criss-crossing non-metallic gaps provided by exposed dielectric strips on the wrapping sheet. An orthogonal pattern of square such metallic islands is shown; and ranges of island sizes and island spacing are stated. In the thin film embodiments of Brastad et al , there is a balancing of microwave heating within the thin metallic coatings (i.e., current heating in thin, vapor deposited metallic coatings), and the degree of microwave transparency which enables direct microwave heating of, for example, a food enclosed within such a wrapping sheet. In the relatively thick foil embodiments, there is a balancing of microwave induced heating in the dielectric substrate strips disposed between the microwave reflective, foil covered islands (i.e., fringe field heating), and the degree of microwave transparency of the sheet through the uncovered dielectric strips. United States Patent 4,883,936 which issued November 28, 1989 to Maynard et al discloses Control Of Microwave Interactive Heating By Patterned Deactivation. This deals with deactivating portions of thin film susceptors as a way of making susceptors having patterned active areas. United States Patent 4,864,089 which issued September 5, 1989 to Tighe discloses Localized Microwave Radiation Heating through the use of coating medium: it states that conversion efficiency can be controlled by the choice and amount of conductive and semi- conductive materials in the medium. United States Patent 4,866,232 which issued September 12, 1989 to James L. Stone discloses a Food Package For Use In A Microwave Oven which, it states, may comprise depositions of metalized ink on areas of a container where enhanced heat is desired, and/or depositions of metalized ink on areas of a container to provide microwave protection. Stone states, without supporting data, that such ink deposits may be of the same or different thicknesses and densities to effect different degrees of heating and shielding. United States Patent 4,904,836 which issued February 27, 1990 to Turpin et al discloses a Microwave Heater And Method Of Manufacture wherein heating is effected in microwave lossy coating material having specified ranges of inverse penetration depths; and wherein coatings are provided which may comprise areas of lesser or greater depth than other area of the coating of lossy material. European Patent Application 0 345 523 which was filed May 23, 1989 discloses a microwave susceptor having a plurality of regions where at least one region has an altered microwave responsiveness which is achieved by disruptions in the susceptor surface. While some of the problems associated with achieving desired heating, cooking, and baking results in microwave ovens have been solved to some extent by others, they have not been solved in the same manner or to the same extent as is provided by the present invention. SUMMARY OF THE INVENTION In one aspect of the present invention a microwave field modifier is provided. The microwave field modifier includes a substrate which has a surface and a quantity of electrically conductive coating material. The coating material is disposed on the surface of the substrate to define a plurality of electrically conductive discrete elements. The discrete elements are elongate and disposed in a predetermined array. In a second aspect of the present invention a microwave field modifier is provided. This microwave field modifier includes a substrate which has a surface and a quantity of electrically conductive coating material. The coating material is disposed on the surface of the substrate to define a plurality of electrically conductive discrete elements. The discrete elements are substantially square and uniform in size and are disposed in a predetermined array having two or more substantially parallel rows in staggered relation. In a third aspect of the present invention a microwave field modifier is provided. The microwave field modifier includes a substrate which has a surface and a quantity of electrically conductive coating material. The coating material is disposed on the surface of the substrate to define a plurality of electrically conductive discrete elements. The discrete elements are configured to include an elongate portion and disposed in an array so that elongate portions of adjacent elements are in side by side relation. BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims which particularly point out and distinctly claim the subject matter regarded as forming the present invention, it is believed the invention will be better understood from the following description taken in conjunction with the accompanying drawings in which identical features or elements are identically designated in the several views, and in which: Figure 1 is a plan view of a discrete microwave field modifier which is an exemplary embodiment of the invention; Figure 2 is an enlarged fragmentary plan view of the microwave field modifier shown in Figure 1; Figure 3 is an enlarged fragmentary plan view of an alternate microwave field modifier which is of the present invention; Figure 4 is a plan view of an alternate embodiment microwave field modifier which is an embodiment of the invention; Figure 5 is a plan view of another alternate embodiment of a microwave field modifier of the present invention; Figure 6 is an enlarged scale, fragmentary portion of the alternate microwave field modifier embodiment shown in Figure 5; Figure 7 is a perspective view of a preferred package having an integral cover and in the open position and having an array of comestible bakeable articles disposed therein; -and Figure 8 is a fragmentary sectional view taken along section line 8-8 of Figure 7. DESCRIPTION OF THE INVENTION An exemplary microwave field modifier of the present invention, indicated generally as 20 in Figure 1 basically includes a substrate 22 and an array formed from a plurality of discrete electrically conductive elements 24 disposed thereon. The discrete electrically conductive elements 24 are preferably formed from pattern coating a coating material to areas of the substrate 22; and even more preferably by printing. The elements 24 preferably have an elongate portion and/or preferably are staggered relative to each other side to side. As used herein, elongate has its ordinary meaning: i.e., having a form notably long in comparison to its width. Additionally, the elongate elements 24 are preferably substantially straight; albeit it is not intended to thereby exclude serpentine, wavy, and curved shapes. Also, the elongate elements 24 preferably have radiused ends as shown to lessen the propensity for electrical arcing. Staggered relation, as used herein, is intended to include but not be limited to shapes such as elongate, square, and rectangular elements 24 which are in side by side relation but which have their ends offset from one another. The offset need not be necessarily uniform throughout the array. Generally, (and therefore without specific reference numbers) the microwave field modifier of the present invention may be a discrete unit covering an entire substrate or only a portion thereof. Likewise, several modifiers having varying effects can be located in different zones on the same substrate. A modifier may consist of layered arrays. For example, an array can be disposed on both surfaces (sides) of a substrate. Alternatively, a modifier, or modifiers, may be integrated into selected portions of packages which herein includes wrapping materials, cartons, containers, cookware and the like. Additionally, the modifier can be a layered or laminated structure comprising, for instance, one or more additional layers for such purposes as strength, arc suppression, and interactive modifier functions. For example, albeit not depicted in the figures, a thermoplastic or thermosetting coating or film may be applied to the modifier structure to cover the electrically conductive coating material to preclude direct contact between the electrically conductive coating material and an adjacent load such as a quantity of food product; and to protect the modifier from electrical arcing in the event the modifier is placed in close proximity to electrically conductive articles. Furthermore, such a layered construction can be made by pattern coating a thermoplastic film with electrically conductive coating material, and then laminating the pattern coated film to paper or cartonboard or some other dielectric substrate. The discrete electrically conductive elements are preferably formed by pattern coating a coating material onto the substrate. Among the advantages of this structure are significant cost and equipment savings relative to current thin film susceptors. It is even more preferable if the coating material is applied to the substrate by printing and most preferably by rotogravure printing. Printing offers advantages such as cost and efficiency savings over other coating processes and rotogravure printing equipment is generally currently available to carton manufacturers. The coating material of the present invention generally includes a binder material system, which comprehends a resin and a solvent, and electrically conductive particles. In addition the coating material may include various other components. The binder system is used to bind the electrically conductive particles together in contacting relation. The binder system also preferably functions to bind the coating material to the dielectric substrate. Included in a binder system is a resin and a solvent. Exemplary preferred resin materials include nitrocellulose, ethyl cellulose, polyvinylbutyral, polyvinylpyrrolidone, poly (methyl vinyl ether/maleic acid) co-polymer resins and acrylics. A nitrocellulose can be purchased from General Printing, Ink Division, Sun Chemical Corporation, Cleveland, Ohio as a 40% solution of 18-25 CPS RS nitrocellulose, (This solution includes 17% isopropyl alcohol, 23% ethyl acetate and 20% n-propyl acetate by weight.) An ethyl cellulose can be purchased from Hercules Inc., Wilmington, Delaware under the trade name Ethyl Cellulose N-4. A poly (methyl vinyl ether/maleic acid) co-polymer resin can be purchased from the GAF Corporation of Wayne, New Jersey under the trade name Gantrez ® . Gantrez ® is a registered trademark of the GAF Corporation. Gantrez ® currently comes in three basic forms; Gantrez® AN, Gantrez ® ES and Gantrez ® S. Gantrez ® ES-225 is preferred. A polyvinylbutyral can be purchased from Hoechst-Celanese of Somerville, NJ under the trade name Mowital®. It is available in several molecular weights. The preferred types are coded B-20H, B-30H and B-30T by Hoechst-Celanese. Mowatol ® is a registered trademark of Hoechst-Celanese. Polyvinylpyrrolidone may be obtained from GAF Corporation, Wayne, New Jersey and Sigma Chemicals of St. Louis, Missouri. Electrically conductive particles which may be used to make coating materials include pure metallic particles, some metallic oxides, metal alloy particles, carbon particles and graphite particles. Furthermore, the conductive particles preferably have irregular shapes; even more preferably are also relatively flat; and most preferably are also of differing shapes and sizes; all of which promote electrical contact between the elements. A preferred conductive particle which has been found successful is Nickel Flake HCA-1 which may be purchased from the Novamet Company, Wyckoff, N.J. The Novamet Nickel Flake HCA-1 is a dendritic particle formed of spheroids which have been connected together and flattened. Thus, a preferred conductive particle has a flattened dendritic shape. A second preferred particle may be purchased from Cabot Corporation, Waltham, Mass as Carbon Black Regal ® 99R. This particle is also relatively flat and has a size of about .36 nanometers. Some other components which may be used as constituents of coating materials include binder solvents, emulsifying agents, acids and liquid materials which will chemically unite with the other constituents of the coating material to cause the coating material to solidify after being applied in a fluidized state. In applications of coating materials which require some flexibility, the coating materials may further comprise plasticizer material. Additionally, anti-settling agents or other constituents may be included in coating material formul tions. Additionally, an undercoating placed on the substrate prior to printing increases conductivity. Likewise, an overcoating placed over the elements increases conductivity. Conductivity is further increased by using both an undercoating and an overcoating. If binder of the undercoating and/or overcoating uses the same solvent as the binder of the coating material conductivity is increased even further. Consequently, an undercoating or overcoating is used, more preferably an undercoating and an overcoating is used, even more preferably an undercoating or overcoating uses the same solvent as the coating material and most preferably an undercoating and overcoating uses the same solvent as the binder of the coating material . Furthermore, increasing the acidity of the binder seems to have a beneficial effect on conductivity. The more acidic the binder the greater the conductivity. Thus, it may be beneficial to add acidic binder additives to the coating material, such as acid complex forming additives. While not intending to be bound it is believed the surface chemistry effects of adsorption may be providing this benefit. The adsorption may allow for closer contact of the metal particles with each other giving rise to better conductivity. Also, it is possible salts are being formed with the oxide making the metal more free for electrical conduction. Figure 1 illustrates an exemplary microwave field modifier, indicated generally as 20, which embodies the present invention. The substrate 22 of Figure 1 is twenty point cartonboard such as is commonly converted into such things as cartons for packaging microwaveable food products: i.e., packages which are suitable for being placed in a microwave oven to heat, cook or bake the contents of the package without removing the contents from the carton. Other exemplary substrate 22 materials include cartonboard, coated cartonboard, thermoplastic film, thermoplastic nonwovens, ther oset plastics, or ceramic. Disposed on the substrate 22 is an array formed from a plurality of discrete electrically conductive elements 24. In Figure 1, the array extends over the entire top surface 28- of the substrate 22 except for a perimetric zone 29 which is devoid of coating material 26. The perimetric zone 29 acts to insulate the edges of modifier 20 so as to substantially obviate arcing between the electrically conductive elements 24 and any metallic material disposed adjacent the modifier 20. Furthermore, although the array of Figure 1 extends over the entire surface of the substrate 22, the modifier may be limited to one or more zones of the substrate 22. Referring now to the enlarged fragmentary view of Figure 2, the preferred modifier 20 of Figure 1 includes elements 24 which are uniform in size and shape except at some row ends, and have lengths L and widths W, respectively. Thus, the array preferably substantially comprises elements 24 which are uniformly configured. Additionally, the elements 24 are preferably linearly aligned in straight rows parallel to each other. The rows are linearly spaced apart by a distance designated SL; and side by side rows are spaced widthwise a distance designated SW. Further, the rows are in staggered relation so that side by side adjacent elements are linearly offset by a distance designated OS. The degree of stagger, in percent, of such an array is (OS/L)(100). Referring to Figure 3, an alternative embodiment of a microwave field modifier 120 of the present invention is illustrated wherein the elongate elements 124 have a generally serpentine shape. In this embodiment the length L of an element 124 is taken along its center line. But for the shape of the elements 124, and the degree of stagger, modifier 120 is substantially similar in construction to modifier 20 of Figure 1. Accordingly, substrate 122 can be the same dielectric material as substrate 22 of Figure 1, or can be of different dielectric material. The elements 124 may be different in size and/or shape; and the elements 124 may comprise the same or different coating material (e.g., coating material having a different surface resistivity). As shown, in this array, the elements 124 of adjacent rows are staggered about 50% percent. The modifier 120 of Figure 3 is more isotropic than the modifier 20 of Figure 1. In other words, the effectiveness of the array as a shield is less dependant upon the orientation of the array relative to an incoming microwave. If the R values are plotted against the degree of rotation throughout 360° using the RAT test described hereinafter a sinusidal-type curve will result. For the modifier 20 of Figure 1, the maximum R would be relatively high and this portion of the curve would be relatively broad. In addition, the low R portion of the curve would be relatively narrow. For the modifier 120 of Figure 3, the curve would have generally the same shape, however, the maximum R would be less and the low R portion of the curve would be narrower, while the high R portion of the curve would be wider. Thus, a modifier 120 which is more isotropic than a second modifier 20 may provide better shielding in a microwave oven even though its maximum R value is less since within the microwave oven the waves come in at all angles. The modifier 20 of Figure 1, however, can be made more isotropic by layering the array (as by printing both sides of a single substrate) such that the elements 24 in one layer are substantially perpendicular (i.e., so that the high R range for one will shield waves coming in at an angle of low R for the other) to the elements 24 of the other layer. An example of this can be seen in Figure 7 which will be discussed hereinafter. This provides an isotropic modifier of even higher R than the modifier 120 of Figure 3. Referring to a second alternate embodiment seen in Figure 4, discrete microwave field modifier 220 includes a dielectric substrate 222 on which a staggered array of square elements 224 are disposed, and which has an uncoated perimetric zone 229. But for the shape of the elements 224, and the degree of stagger, modifier 220 is substantially similar in construction to modifier 20 of Figure 1. Accordingly, substrate 222 can be the same dielectric material as substrate 22 of Figure 1, or can be of different dielectric material, or different in size and/or shape; and the elements 224 may comprise the same or different coating material (e.g., coating material having a different surface resistivity). Albeit elements 224 are shown to have unradiused corners, it is not intended to thereby preclude square and rectangular elements from having radiused corners. As shown, in this array, the elements 224 of adjacent rows are staggered about fifty (50) percent. Being staggered, the modifier 220 of square elements 224 are both more effective as shields and as susceptors, i.e., heaters, than squares which are orthogonally al igned. In addition, the modifier 220 of Figure 4 is more isotropic than the modifier 20 of Figure 1 and the modifier 120 of Figure 3. Consequently, the modifier 220 of Figure 4 will also have a lower maximum R than either of the previous embodiments. Turning now to Figure 5 and Figure 6, another alternate microwave field modifier 320 is shown to comprise an array of Y-shape elements 324 on a dielectric substrate 322. Figure 4 is a fragmentary, enlarged scale view of modifier 320. The Y-shape elements 324 have three elongate portions, and are disposed in side by side, vertically extending rows. In this embodiment the length L of the elongate portion is the maximum path along a center line. In this embodiment the three possible paths are identical in length. In the rows of this embodiment, the stems of the Y-shape elements 324 are partially nested between the arms of adjacent Y-shape elements 324. Also, the arms of the Y-shape elements 324 in adjacent rows are partially sideways nested between the arms of adjacent Y-shape elements 324; and some of the arms of sideways nested Y-shape elements 324 are in parallel relation. Fringe field heating can be varied in such an array by varying the spacing between portions of adjacent Y-shape elements 324, as well as the the size of the Y-shape elements 324, the width of their elongate portions, and the dielectric loss property of the substrate 322 material. This modifier 320, as compared to modifiers 20 similar to Figure 1, are more isotopic; i.e., their RAT properties are less sensitive to positional variances when such modifiers 320 are placed in microwave ovens, and have lower maximum R values. As previously described a shield is relatively opaque to microwaves. In other words, a shield has a relatively low T value. Thus, a shield preferably has a T value of less than about 40%, more preferably less than about 10% and most preferably less than about 5%. Due to their low T values, shields also generally have high R" values. Shields are particularly useful for such purposes as preventing overheating in certain areas, i.e., at corners and edges, and generally slowing down microwave cooking. Any modifier can be characterized by its microwave reflectance, absorbance, and trans ittance values: i.e., its RAT values. However, since shields are characterized in terms of RAT, and particularly R and T, RAT values are particularly helpful with regard to determining shielding ability. One method of measuring RAT values uses the following Hewlett Packard equipment: a Model 8616A Signal generator; a Model 8743A Reflection-Transmission Test Unit; a Model 8411A Harmonic Frequency Converter; a Model HP-8410B Network Analyzer; a Model 8418A Auxiliary Display Holder; a Model 8414A Polar Display Unit; a Model 8413A Phase Gain Indicator; a Model S920 Low Power Wave Guide Termination; and two S281A Coaxial Waveguide Adapters. In addition a digital millivolt meter is used. Connect the RF calibrated power output of the 8616A Signal Generator to the RF input of the 8743A Reflection-Transmission Test Unit. The 8411A Harmonic Frequency Converter plugs into the 8743A Reflection-Transmission Test Unit's cabinet and the 8410B Network Analyzer. Connect the test channel out, reference channel out, and test phase outputs of the 8410B Network Analyzer the test amplitude, reference and test phase inputs, respectively, of the 8418A Auxiliary Display Holder. The 8418A Auxiliary Display Holder has a cabinet connection to the 8414A Polar Display Unit. The 8413A Phase Gain Indicator has a cabinet connection to the 8410B Network Analyzer. The amplitude output and phase output of the 8413 Phase Gain Indicator is connected to the digital millivolt meter's inputs. The settings of the 8616A Signal Generator are as follows: Frequency is set at 2.450GHz; the RF switch is on; the ALC switch is on to stabilize the signal; Zero the DBM meter using the ALC calibration output knob; and set the attenuation for an operating range of lldb. Set the frequency range of the 8410B Network Analyzer to 2.5 which should put the reference channel level meter in the "operate" range. Set the amplitude gain knob and amplitude vernier knob as appropriate to zero the voltage meter readings for reflection and transmission measurements respectively. Microwave field modifier samples are three and one-half inches in diameter. For Reflection place the 8743A reflection-Transmission Unit in the reflection mode. A S281 Coaxial Waveguide-Adaptor is connected to the "Unknown" port of the 8743A Reflection-Transmission Test Unit. A perfect shield (aluminum foil) is placed flat between the reflection side of the S281 wave guide adaptor and the S290A Low Power Guide Termination. The amplitude voltage is set to zero using the amplitude gain and vernier knobs of the 8410B Network Analyzer. The shield is replaced by the sample of the microwave field modifier. In other words the sample is placed between the S281A Coaxial Waveguide Adaptor and the S920A Low Power Waveguide Termination and the attenuation voltage is measured. Normally, four readings are taken per sample and averaged. The samples are rotated clockwise ninety degrees per measurement. After the second measurement the sample is turned over (top to bottom) for the final two measurements. For polarized, isotropic samples care must be taken to orient the samples such that the maximum and minimum readings are obtained. The R value is the maximum reading. These samples may also be rotated in increments other than ninety degrees. For Transmission place the 8743A Reflection-Transmission Unit in the transmission mode. A lOdb attenuator is placed in the transmission side of the line, between the "In" port of the 8743 Reflection-Transmission Unit and a second S281A Coaxial-Waveguide Adaptor. The two S281A Coaxial-Waveguide Adaptors are aligned and held together securely. The amplitude signal voltage is zeroed using the amplitude gain and vernier knobs of the 8410B Network Analyzer. The modifier to be tested is placed between the two waveguide adaptors and the attenuation voltage is measured. Four readings are taken as described above for the reflection measurement. Reflection and transmission values should be calculated in the same manner; i.e. average or maximum. Absorption is calculated by subtracting the transmission measurement and the reflection measurement from 1.00. It should be noted that RAT values as measured in the network analyzer may be different from actual RAT values when a microwave field modifier is placed in competition with a food load. The food and the microwave field modifier compete for the available microwave energy. The competition can be analogized to a circuit consisting of a generator connected to two impedance loads in parallel. The generator represents the magnetron while one impedance load represents the food load and the other impedance load represents the microwave field modifier. The network analyzer procedure above does not include the food load resistor. Thus, when the food load is added to the circuit its "impedance" relative to the microwave field modifier's resistance is not known. If the food load's "impedance" is significantly less than the modifier's "impedance" most of the microwave energy will flow through the food. Consequently, design of modifier's inevitably involves some trial and error based upon the actual food to be heated. As previously described, a susceptor is a microwave field modifier which absorbs microwave energy and heats appreciably when exposed to a microwave field. One method for determining the ability of a modifier to heat (at least relative to other microwave field modifiers) is the Energy Competition Test described below. Using a carousel microwave oven which has a power rating of 30 BTU/ in as measured with a 1000 gram water load an effective susceptor preferably has a ΔT at two minutes of about 90°F or more, more preferably about 150°F or more, and most preferably about 200 β F or more. To conduct the Energy Competition Test, place a 150 ml pyrex beaker containing 100 grams of distilled water in a microwave oven on a carousel along with a three and three quarter inch diameter pyrex petri dish containing 30 grams of Crisco ® Oil. These items are placed side by side about nine inches on center apart. Take an initial temperature reading of the oil. Subject these items to the full power of the microwave field for a total of two (2) minutes; at 30 second intervals open the microwave oven and stir the oil with a thermocouple measuring and recording the temperature. This measurement should be taken as quickly as possible to minimize cooling of the oil. This procedure provides a control. Repeat the above procedure with a three and one half inch diameter sample of a microwave field modifier submerged in the oil. Begin with the oil at about the same initial temperature as with the control. It may be necessary to place an inert weight, such as a glass rod, on top of the modifier to keep it submerged in the oil. The data can be normalized by adjusting the initial temperature to a standard 70 β F by subtracting or adding the initial temperature deviation from 70 * F to each of the temperatures recorded. Once the test has been run and, one method which can be used for comparison of various microwave field modifiers is to compare the change in temperature over the two minute time interim. Thus, the two minute ΔT is calculated by subtracting the two minute ΔT of the oil alone from the two minute ΔT of the oil and susceptor. Additionally, the two minute ΔT of the susceptor is normalized by adding or subtracting any initial temperature variance of the oil from 70°F. As with measuring RAT through the use of a network analyzer, the Energy Competition Test may not predict exactly how well a modifier will heat in the microwave in conjunction with a food load. However, the use of water is intended to simulate the modifier in competition with a load. The greater the variance in microwave properties of the actual food load from the properties of the water load, the less accurate this test will be. Consequently, it may be desirable to use another amount of water or competing load in a particular application comparing possible microwave field modifiers to reduce the amount of trial and error necessary to achieve the desired results with the actual food load. In any event some trial and error will inevitably be necessary. Despite the above, a microwave field modifier of the present invention may be tailored with minimal trial and error to provide desired RAT values and heating characteristics by selectively altering certain variables. Thus, a modifier can be designed having a wide range of shielding and heating properties. Some of these variables relate to the array, i.e., the element shape, size, orientation and arrangement relative other elements; some to the coating material; and lastly, some to the substrate or possible overcoatings or undercoatings. Generally speaking, the directionality of the various variables, with respect to tailoring its RAT properties will be discussed below with reference to Figure 2. The directionality, however, applies to all embodiments of the present invention. Included among the array variables are the lengths, widths, spacing, and the degree of stagger between adjacent elements 24. One variable is the length L of the element 24. In general, the maximum length L of the element 24 is preferably less than about 4 cm and even more preferably less than about 3 cm. In addition the length L is preferably one-half (1/2) cm or greater and most preferably one (1) cm or greater. In any case, L should be less than the amount that would induce electrical arcing. In this general range increasing length L increases microwave reflectance, and decreases both microwave absorbance and transmittance of the modifier 20. Additionally, increasing the length L of the elements 24 increases the magnitude of the alternating voltage and current which is induced in the elements 24, and thus will tend to increase the tendency for arcing to occur across the end to end gaps between adjacent elements 24 and overheating within the elements 24. With regard to width W, a width of from about 0.001 inches to about 1.0 inches is preferred; with a width W from about 0.010 inches to about 0.10 being more preferred. In this general range increasing width W decreases microwave reflectance, and increases the wavelength bandwidth response of the modifier 20 which makes the modifier 20 less sensitive to wavelength changes which can be induced by such things as contact with a food load. Additionally, the elongate elements 24 preferably have a L/W ratio of from about 2:1 to about 200:1; and even more preferably from about 10:1 to about 40:1. End to end spacing SL is preferably from about 0.010 inches to about 0.100 inches and in this range increasing end to end spacing spacing SL or side to side spacing SW decreases microwave reflectance, increases both microwave absorbance and transmittance of the modifier 20, and decreases the fringe field heating of the intervening portions of dielectric substrate 22. Side to side spacing SW is preferably from about 0.010 inches to about 0.10 inches. Increasing side spacing SW in this range decreases both fringe field heating and shielding. The degree of stagger is preferably about 30% or 35% for maximum shielding and heating for highly conductive coating materials. Included among the coating material variables are the resistivity, shape and size of the conductive particles; and the dielectric properties (i.e., the dielectric constants, loss factors, and dielectric strengths) and surface electrical conductivity of the dried coating material. The surface electrical conductivity of the conductive elements 24 is relatively high as measured in terms of surface resistivity: preferably one-hundred (100) ohms per square or less; more preferably ten (10) ohms per square or less; Still more preferably three (3) ohms per square or less; even more preferably one (1) ohm per square or less; and most preferably one-tenth (0.1) ohm per square or less. Increasing surface electrical conductivity of the elements 24 in this range directly increases microwave reflectance, and decreases microwave transmittance of the modifier 20. Achieving a dried coating having conductivity in the desired ranges identified above is aided by certain features. For example, putting down a sufficient quantity of coating material is important. The more coating material, i.e., the thicker the coating material, the more conductive. Coating material thickness is preferably from about 0.0001 inches to about 0.003 inches and even more preferably from about 0.0005 inches to about 0.002 inches. Also, viscosity of the coating material can be important depending upon the coating process used. The viscosity of the coating material is preferably from about 50 cps to about 7000 cps. For rotogravure printing the viscosity is preferably from about 100 cps to about 175 cps as measured with a #3 zahn cup. In any event the viscosity should be such that the coating material is suitable for the chosen coating process used; be it painting, spraying, printing, silkscreen printing or rotogravure printing. Achieving the desired viscosity may require the addition of resin, solvents or other additives after the initial mixing of the coating material as is commonly done in printing processes. Additionally, an undercoating placed on the substrate prior to printing increases conductivity. Likewise, an overcoating placed over the elements increases conductivity. Conductivity is further increased by using both an undercoating and an overcoating. If binder of the undercoating and/or overcoating uses the same solvent as the binder of the coating material conductivity is increased even further. Consequently, an undercoating or overcoating is used, more preferably an undercoating and an overcoating is used, even more preferably an undercoating or overcoating uses the same solvent as the coating material and most preferably an undercoating and overcoating uses the same solvent as the binder of the coating material. Additionally, an overcoating of Gantrez® AN or Gantrez ® S for example, may be useful to provide an FDA approved barrier. Furthermore, increasing the acidity of the binder seems to have a beneficial effect on conductivity. The more acidic the binder the greater the conductivity. Thus, it may be beneficial to add acidic binder additives to the coating material, such as acid complex forming additives. While not intending to be bound it is believed the surface chemistry effects of adsorption may be providing this benefit. The adsorption may allow for closer contact of the metal particles with each other giving rise to better conductivity. Also, it is possible salts are being formed with the oxide making the metal more free for electrical conduction. With respect to the conductive particles of the coating material of elements 24: the electrically conductive particles are preferably less than twenty-five microns in size (i.e., their maximum dimensions); and more preferably less than ten microns in size. Finer particles result in greater coating conductivity for a given weight percent of the particles in a given coating material. Also, a mix of different particle sizes acts to increase surface electrical conductivity for a given weight percent of the particles in a given coating material. Particle aspect ratio --i.e., the ratio of the longest dimension to the shortest dimension of a particle-- is also important. A high aspect ratio (i.e., greater than ten to one) is preferred because it promotes electrical contact between the particles in solidified coating materials, and such conductive particles are more susceptible to microwave heating than particles of smaller aspect ratios. Additionally, particles of higher resistivity will act to decrease surface conductivity of coating materials; and particles having jagged shapes and edges tend to promote electrical contact between conductive particles in a coating material, and will thus tend to increase the surface conductance of the elements 24. With regard to the dielectric properties of the dried binder and other fluids a high dielectric loss factor will act to increase dielectric heating (i.e., capacitive heating within the conductive element 24 between conductive particles); a high dielectric strength reduces the tendency to arc; and a high dielectric constant will increase microwave reflectivity and power handling ability. Included among the remaining variables are the dielectric properties (i.e., the dielectric constants, loss factors, and dielectric strengths) of the substrate 22 and any optional overcoating material. With respect to the substrate 22, a high dielectric constant will function to increase microwave reflectance and power handling capacity; a high dielectric loss factor will act to increase dielectric heating; and high dielectric strength prevents breakdown at higher induced voltages. Thus, high dielectric strength reduces the tendency to arc, and thus tends to obviate arc charring of the substrate 22. This protects the modifier 20 from breakdown inasmuch as carbon particles which result from arcing would tend to short circuit the elements across the intervening gaps. The same is true with respect to any optional overcoating or undercoating which may be applied to separate the modifier from direct contact with the food. The above directionality information with respect to the various variables is generally true. However, there is a complex relationship among the various variables. Due to this complex relationship, manipulation of a particular variable while keeping the remaining variables constant can have a minimal effect in a particular situation; i.e., with a particular coating material and pattern. In contrast, manipulation of the same variable may have a truly significant impact in a second situation. While not wishing to limit the invention, one possible method of designing a microwave field modifier of the present invention having desired RAT properties follows. Pick a coating material which will have the desired conductivity range when dried (see above) and a suitable substrate. Determine the maximum line length that can be used without arcing. This is done by coating a series of lines having diffe r ing lengths up to about 4 cm and preferably about 1 mm wide onto the chosen substrate. Place these lines in the microwave field along with lOOg s of water in a beaker (to simulate a food load). Choose a line length approximately 20% less than the minimum line length that arcs. Coat an array pattern with this line length L, 1mm wide W, gaps of 1mm between the ends SL and the sides SW of the lines and a 25% offset between adjacent rows of lines. Determine the performance characteristics of the array. The modifier may be tested in a network analyzer to determine the RAT. Also perform an energy competition test to determine the heating ability. Alternatively, this array can be tested in conjunction with an actual food load to determine its performance characteristics. To increase heating and shielding, increase the line length and decrease the size of the gaps. Changing to a more conductive coating material or depositing a thicker coating (which increases conductivity) will also increase heating and shielding. If there is excessive heating and a reduction in shielding is tolerable adjust oppositely. If this array simply produces too much heating for the shielding required then change the dielectrics in the coating material, substrate and any overcoatings or undercoatings to lower loss materials and possibly change to a more conductive coating material. Another possible way to increase shielding is to increase the amount of the offset. The above method and list of adjustable variables is not intended to be all-inclusive but illustrates how adjustment to desired RAT values might be made. As stated above, the relative RAT values of microwave field modifiers of present invention may be tailored to meet specific needs of various foodstuffs by selectively varying such parameters as the surface electrical conductance or resistance of the elements; the lengths, widths, spacing, and the degree of stagger between adjacent elements; the resistivity, shape, size, and aspect ratio of the conductive particles; and the dielectric properties (i.e., the dielectric constants, loss factors, and dielectric strengths) of the substrate and/or the dielectric binder. To illustrate the versatility of the microwave field modifiers of the present invention three exemplary embodiments will be presented: one with high shielding properties and low heating properties; one with high shielding properties and high heating properties; and one with low shielding properties and high heating properties. Example 1 An exemplary microwave field modifier of the present invention having relatively high shielding properties and relatively low heating properties can be described with reference to Figures 1 and 2. The array is silk screen printed onto twenty point cartonboard using a 109 mesh, 0.0032 inch diameter monofilament polyester silkscreen. The coating material is comprised of about sixty percent by weight of silver particles, and manifest a dried surface resistivity of less than one-half (0.5) ohm per square. Such a coating material can be purchased from Acheson Colloids Company, A Division Of Acheson Industries, Inc., Port Huron, Michigan, and is identified as Electrodag 477SS. Still referring to Figures 1 and 2, the elongate elements 24 are 2 1/2 cm. long, 0.040 inches wide, spaced 0.160 inches end to end, spaced 0.160 inches side to side, and have stagger (pattern offset) OS of about 25%. The pattern may be printed with the elongate elements 24 horizontally on the front side of the substrate 22 and with the elongate elements 24 oriented vertically on the back side of the substrate 22. Example 2 An exemplary microwave modifier of the present invention having high shielding properties and substantial heating ability can be described with respect to Figures 1 and 2. In this example the heat is primarily generated as a result of fringe field or capacitive heating between the elements 24 due to the relatively high conductivity and the dielectric properties of the coating material components. Some heat, however, is generated by I-squared-R heating of the elements 24 themselves. The surface resistivity of the dried coating material is less than about two ohms per square. The array is silk screen printed onto twenty point cartonboard substrate 22 using a 109 mesh, 0.0032 inch diameter monofilament polyester silkscreen. The coating material used is forty-seven percent (47%) copper and fifty-three percent (53%) acrylic binder system coating material which may be purchased from Acheson Colloids Company, Port Huron, Michigan as Acheson copper Electrodag #437. The array might have an appearance similar to that illustrated in Figures 1 and 2. The elongate elements 24 are two (2.0) centimeters long and 0.032 inches wide. The array has an end gap of 0.045 inches, a side gap of 0.027 inches and and offset of 30%. Example 3 An exemplary embodiment having low shielding and substantial heating ability can be described with reference to Figure 4. In this example the heat is primarily generated in the elements 224 themselves due to the relatively low conductivity of the elements 224 and the dielectric properties of the coating material components. The array is pattern coated onto a twenty point cartonboard substrate 222 using a 109 mesh, 0.0032 inch diameter polyester monofilament or a similar 18-F ultifilament silkscreen. The coating material is comprised of 60% nickel and 40% nitrocellulose by weight. The nickel may be purchased from the Novamet Company of Wyckoff, New Jersey and is identified as Nickel HCA-1 Flakes. The nitrocellulose may be purchased from the General Printing Ink Division, Sun Chemical Corp., Cleveland, Ohio, and is identified as nitro cellulose solution #266-133. The array might have an appearance similar to that illustrated in Figure 4. The elements 224 are seven (7) millimeter squares spaced 0.6 mm apart on all sides. In addition, the elements 224 are offset 50%. Additional exemplary embodiments using various binder systems are provided below: Example 4 This is an exemplary embodiment using a ten percent solution of Mowital ® B30H polyvinylbutyral. A ten percent solution can be obtained by dissolving 5 grams of B30H powder in 45 grams of methanol (methyl alcohol). To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (10% solid resin) resin solution. Thus, the final solution consists of 75 grams nickel and 50 grams of 10% resin solution. This coating material may then be screen printed in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .045 inches, side gap SW 0.275 inches, length L of .787 inches, width W of .035, and overlap of 31% Furthermore, if this coating material is screen printed onto a substrate such as cartonboard which has been pre-coated with a layer of the polyvinylbutyral solution, (i.e., 10% solution in methanol without nickel) by using a doctor blade or a mayer rod the conductivity and the reflectance (R) may be increased. If the first sample is coated with the same 10% solution such as to produce an overcoating of the susceptor coating, the change in RAT properties may be similar to those of the second sample which has an undercoating. If the sample with the undercoating of polyvinylbutyral is now overcoated to produce a printed sample with nickel which has both an undercoating and an overcoating, the conductivity and the reflectivity (R) may be further increased. Example 5 This Example uses a twenty percent solution of Gantrez ES-225. A twenty percent solution can be obtained by starting with 20 grams of material as supplied (50% resin and 50% ethanol solvent) This provides 10 grams resin and 10 grams solvent. Add 30 grams of ethanol solvent to the solution; creating a 20% resin and 80% solvent solution. This provides 10 grams resin and 40 grams solvent or 50 grams of total solution. To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (20% resin) resin solution coating material. Thus the final solution consists of 75 grams nickel and 50 grams of 20% resin solution. This solution is then screen printed or rotogravure printed (with only minor adjustments to viscosity) in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.045 inches; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%. Example 6 This example uses a ten percent solution of polyvinyl- pyrrolidone. A ten percent solution can be obtained by dissolving 5 grams of polyvinylpyrrolidone powder in 45 grams of methanol (methyl alcohol). To this add 75 grams of Novamet nickel; creating a 60% nickel and 40% (10% solid resin) resin solution. Thus, the final solution consists of 75 grams nickel and 50 grams of 10% resin solution. This solution may then be screen printed in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of .045 inches, side gap SW 0.275 inches, length L of .787 inches, width W of .035 inches and overlap of 31%. Example 7 This Example uses a 5.4 percent solution of ethyl cellulose. A 5.4 percent solution can be obtained by starting with 2.2 grams of ethyl cellulose resin. To this add 0.5 grams of an anti-settling agent such as Bentone SD2 which is available from National Lead Chemicals, Hightstown, New Jersey. Add 4.4 grams of a modifier such as Uni-Rez 7055 (fumaric-acid modified rosin ester binder), available from Union Camp Corp., Wayne, New Jersey; and 1.8 grams of a plasticizer such as Herculon D (hydrogenated methyl ester of rosin), available from Hercules Chemical Corp, Wilmington, Delaware. Also add 32.1 grams of n-propyl acetate as a solvent providing a 5.4 percent ethyl cellulose solution. To this add 59 grams of Novamet nickel HCA-1 flakes; creating a 59% nickel and 41% (5.4% resin) ethyl cellulose resin solution coating material. Thus the final solution consists of 59 grams nickel and 41 grams of 5.4% resin solution. This solution is then screen printed or rotogravure printed (with only minor adjustments to viscosity) in a pattern similar to that of Figure 1. The dimensions of this pattern may be as follows: end gap SL of 0.045 inches; side gap SW of 0.0275 inches; length L of 0.787 inches; width W of 0.035 inches; and a stagger of 31%. It can be seen from the foregoing examples which include various combinations of relatively high and relatively low shielding with relatively high and relatively low heating ability, that modifiers having virtually any properties can be designed. In the above examples high shielding is defined as having a transmittance of about 10% or less and high heating is defined as having a ΔT at two minutes of about 125 β F or greater. The above described microwave field modifiers of the present invention can be used for example, in packages for heating, baking, cooking, etc., various food items. Referring to Figures 7 and 8, a package 130 having microwave field modifiers printed thereon for baking a cupcake batter product is illustrated. The carton 130 includes eight commercially available metalized- thin film susceptor cups 54 which may be obtained from IVEX Corporation, Newton, MA, into which the batter is divided. The cups 54 may alternatively be comprised of a microwave field modifier of the present invention which is designed to heat. These cups 54 are filled with batter 52 and placed in an annular orientation around a centrally located spacer 55 which also is used as a measuring cup. The side walls 41, 42, 43 and 44 of the carton 30 base are printed with a microwave field modifier 420 which included elements 24 which may be similar to the elements 24 of the modifier 20 of Example 1. The inner surface of the side wall 41, 42, 43 and 44 has an array of elongate elements 24 running horizontally and the outer surface of the side wall 41, 42, 43 and 44 has an identical array of elongate elements 24 oriented vertically. This modifier 420 provides a shield around the sides to slow down baking at the edges and to even out the baking. A second modifier 220 is printed on the inner surface of the lid 35 of the package 30. This may be a modifier 220 such as Example 3 and Figure 4 but for the elements 224 being orthoganally aligned. This modifier 220 generates a significant amount of heat which browns the surface of the cupcakes and helps give them the traditional domed top appearance. While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention. In particular, inasmuch as each of the arrays of elements described above comprise uniformly configured and sized elements, it is not intended to thereby preclude arrays comprising non-uniformly configured and sized elements.

Claims: CLAIM?; 1. A microwave field modifier comprising a dielectric substrate having a surface, and a quantity of electrically conductive coating material, characterized in that the coating material is disposed on the surface of the substrate to define a plurality of electricall conductive discrete elements, the discrete elements being elongate, and disposed in a predetermined array, preferably disposed in a predetermined array of a plurality of straight rows in end to end relation within the array, more preferably wherein the straight rows of discrete electrically conductive elements are disposed in side by side parallel relation, even more preferably wherein the discrete elements in adjacent the straight rows are disposed in staggered relation, even more preferably wherein the discrete elements are of uniform length. 2. A microwave field modifier comprising a dielectric substrate having a surface, and an electrically conductive coating material, characterized in that the coating material is disposed on the surface of the substrate to define a plurality of electrically conductive, discrete elements, the discrete elements being substantially square and uniform in size, and being disposed in a predetermined array comprising two or more substantially parallel rows in staggered relation. 3. A microwave field modifier comprising a dielectric substrate having a surface, and a quantity of electrically conductive coating material, characterized in that the coating material is disposed on the surface of the substrate to define a plurality of electrically conductive, discrete elements, the discrete elements being configured to each comprise an elongate portion, and so disposed in a array wherein elongate portions of adjacent the discrete elements are in side by side relation. 4. The microwave field modifier of Claim 1 wherein the length L of the discrete elements is sufficiently small to obviate arcing, preferably less than about 3 centimeters. 5. The microwave field modifier of any one of Claims 1-4 wherein the width W of the discrete elements is from about 0.001 inches to about 1.0 inches. 6. The microwave field modifier of any one of Claims 1-5 wherein the discrete elements have a thickness from about 0.0001 inches to about 0.003 inches, preferably from about 0.0005 inches to about 0.002 inches. 7. The microwave field modifier of any one of Claims 1-6 wherein the discrete elements have a resistivity of about 100 ohms per square or less. 8. The microwave field modifier of any one of Claims 1-7 wherein the surface of the substrate is coated with an undercoating or wherein the discrete elements are overcoated with an overcoating, or both. 9. The microwave field modifier of Claim 1 wherein the micro¬ wave field modifier further characterized by a second substrate surface having a plurality of discrete elements disposed thereon, the second substrate surface being disposed in layered relation to the first, preferably the discrete elongate elements on the sub-strate surface are perpendicular to the elongate elements on the second substrate surface. 10. The microwave field modifier of any one of Claims 1-9 wherein the two minute ΔT is about 125 * F or greater. 11. The microwave field modifier of Claim 10 wherein the discrete elements have a sufficiently low surface electrical resistance that heating within the elements is substantially obviated and a sufficiently small spacing between the elements to cause fringe field heating between the elements. 12. The microwave field modifier of Claim 10 wherein the coating material has a sufficiently high dielectric loss that substantially all the heating is caused by capacitive heating between electrically conducive particles within the coating material.

Inventors: MILENKEVICH JOSEPH ANTHONY, MUNK BENEDIKT AAGE, PROSISE ROBERT LAWRENCE, MILENKEVICH, JOSEPH, ANTHONY, MUNK, BENEDIKT, AAGE, PROSISE, ROBERT, LAWRENCE
Applicants: PROCTER & GAMBLE, THE PROCTER & GAMBLE COMPANY
Application year: 1991
Main classification: B65D81
IPC codes: B65D65, B65D81, F24C7, H05B6