DE69115279T2 - MICROWAVE PACKAGING WITH ELECTRICALLY CONDUCTIVE ELEMENTS THAT MODIFY THE MIRKOWAVE FIELD. - Google Patents

Abstract

A container such as, for example, a disposable carton or package, for use in microwave oven heating, cooking or baking of material (e.g., food) disposed in the container or in the package. Such a container may comprise, in combination, substantially microwave transparent wall portions, and at least one wall zone having a microwave field modifying coating thereon which is disposed to form an array of discrete, electrically conductive pattern elements. Preferably, the microwave field modifying coating material is pattern applied by printing; and, most preferably, by gravure printing. Different wall zones of the container may have the same or different microwave field modifying coating materials applied thereto in different patterns or arrays. Different patterns or arrays may be configured and coated to achieve different combinations of microwave reflectance, absorbance, and transmittance properties from zone to zone to achieve the desired degree of uniformity or asymmetry of the microwave field in a microwave oven to achieve desired heating, cooking, or baking for various contents of the container.

Description

TECHNICAL AREA

The invention relates to cartons, packages, cookware or the like for use in microwave heating, cooking and baking; and more particularly to such cartons, packages and cookware incorporating microwave field modifiers useful for such things as generating high surface heat on adjacent food material to effect browning and/or crisping; or balancing between surface heating and deep microwave heating of the underlying food product; or shielding underlying or adjacent food material from direct exposure to microwave energy to prevent overcooking and/or overheating of the food material; or simply providing a more uniform microwave energy field.

BACKGROUND OF THE INVENTION

Microwave ovens have the ability to heat, cook or bake objects, especially food, extremely quickly. Unfortunately, microwave heating also has its disadvantages. For example, microwave heating in today's microwave ovens often fails to achieve such desirable results as flatness, uniformity, browning, crisping and reproducibility. Contemporary attempts to achieve these and other desirable results with microwave ovens include the use of microwave field modifying devices such as microwave susceptors and/or microwave shields. Such devices have been incorporated into microwave packaging; the latter here includes packing materials, cartons, containers, cookware or the like.

Microwave susceptors and reflectors, like other materials and structures, have a degree of microwave reflectivity (R), absorbivity (A) and transmittance (T); collectively called RAT properties. RAT properties are measured as the percentage of microwave energy reflected (R), absorbed (A) and transmitted (T) by a material or structure. Thus, the total of R, A and T values equals 100%.

In general, a microwave shield is relatively opaque to microwave energy. Expressed in RAT values, the shield will have a relatively low T-value. An example of microwave shields are such highly electrically conductive materials as aluminum foil. Although shields are generally considered non-heating elements, a shield could also be a susceptor, i.e., heat considerably, and vice versa. Thus, a shield is a relatively low T-value element, regardless of its heat generating capacity.

Generally speaking, microwave susceptors are devices that, when placed in a microwave energy field, such as exists in a microwave oven, react to produce a significant amount of heat. The susceptor absorbs a portion of the microwave energy and converts it directly into heat energy, which is useful for, for example, crisping or browning foods. Thus, microwave susceptors generally have a relatively high microwave absorption value (A). In addition to the high absorption capacity, susceptors have a mechanism for converting the absorbed microwave energy into heat. For example, the heat may result from a microwave-induced intramolecular or intermolecular effect; from induced electrical currents leading to so-called 1²R losses in electrically conductive devices; or by dielectric heating of a dielectric material located between electrically conductive particles, elements or regions, which type of heating is also referred to as stray field heating or capacitive heating.

As mentioned, microwave susceptors and reflectors as well as other materials and constructions have an effect on the microwave energy distribution in the microwave oven. That is, due to their RAT properties, they interact with the microwave energy in the oven and cause a modification of the microwave energy field. Accordingly, devices and constructions which cause a modification of the microwave field or the microwave energy distribution in a Microwave oven, referred to here collectively as microwave field modifiers.

The patent literature is replete with a variety of teachings regarding the use of materials and constructions as microwave heaters (e.g., susceptors) and reflectors in microwave ovens. For example, U.S. Patent 4,230,924, issued October 28, 1980 to William A. Brastad et al., discloses a "Method and Material for Pre-Packaging Foods to Achieve Microwave Browning." This material may be a dielectric packaging sheet with a flexible metal coating, such as aluminum, in the form of a relatively thin film or a relatively thick foil, the coating being divided into a number of individual metallic islands or patches separated by crisscrossing non-metallic gaps formed by exposed dielectric strips on the packaging sheet. An orthogonal pattern of such square metallic islands is shown; and ranges for island size and spacing between islands are given.

In the thin film embodiments of Brastad et al., there is a balance between microwave heating in the interior of the thin metal coatings (i.e., current heating in thin vapor-deposited metallic coatings) and the degree of microwave transparency that allows direct microwave heating of, for example, a food product wrapped inside such a packaging sheet. In the relatively thick film embodiments, there is a balance between microwave-induced heating in the dielectric substrate strips located between the microwave-reflective, film-covered islands (i.e., fringe field heating) and the degree of microwave transparency of the sheet through the uncovered dielectric strips.

US Patent 4,883,936, issued November 28, 1989 to Maynard et al., discloses "Control of microwave interaction heating by patterned deactivation." This document deals with deactivating sections of thin film susceptors as a way to provide susceptors with patterned active areas. US-A-4 594 492 describes a microwaveable package with a microwave shield attached to the side wall and microwave interaction layers on the inside of the top wall.

U.S. Patent 4,864,089, issued September 5, 1989 to Tighe, discloses "Localized microwave radiation heating using a coating medium": This document states that the conversion efficiency can be controlled by the selection and amount of conductive and semi-conductive materials in the medium.

U.S. Patent 4,866,232, issued September 12, 1989 to James L. Stone, discloses a food package for use in a microwave oven which, as stated, may include deposits of metallized ink on those areas of a container where increased heat is desired and/or deposits of metallized ink on certain areas of a container to provide microwave protection there. Stone states (without supporting data) that such ink deposits may have equal or different thicknesses and densities to provide different degrees of heating or shielding.

U.S. Patent 4,904,836, issued February 27, 1990 to Turpin et al., discloses a "microwave heater and method of making the same" wherein heating is effected in a microwave lossy coating material having specified regions of inverse penetration depths; and wherein coatings are provided having regions of greater or lesser depth than other regions of the lossy material coating.

European Patent Application 0 345 523, filed on 23 May 1989, discloses a microwave susceptor having a multiple regions, at least one region having a varied microwave response achieved by interruptions in the susceptor surface.

Although some of the problems associated with achieving the desired heating, cooking and baking results in microwave ovens are to some extent overcome by other have been solved, they have not been solved in the same manner or to the same extent as offered by the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a microwaveable package containing a first wall and a first microwave field modifier. The microwave field modifier comprises an electrically conductive microwave active coating material pattern coated on a predetermined zone of the wall to define an array of electrically conductive discrete elements.

According to another aspect of the present invention, there is provided a microwaveable package comprising a top wall, a bottom wall and a peripheral side wall connecting the top wall to the bottom wall to form an enclosure. The side wall has a microwave field modifier attached or integral thereto which forms a substantially continuous vertically disposed annular shield. The microwave field modifier comprises an electrically conductive microwave active coating material pattern disposed on a predetermined region of the wall to define an array of electrically conductive discrete elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Although the specification concludes with claims which particularly point out and distinctly claim the subject matter regarded as the present invention, it is believed that the invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which like features or elements are alike designated throughout the several views, and wherein:

Figure 1 is a plan view of a discrete microwave field modifier that may be incorporated into, attached to, or formed integrally with packaging embodiments of the present invention;

Fig. 2 is an enlarged fragmentary plan view of the microwave field modifier shown in Fig. 1;

Figure 3 is an enlarged fragmentary plan view of an alternative microwave field modifier that may be incorporated into embodiments of the present invention;

Figure 4 is a plan view of another alternative microwave field modifier that may be incorporated into embodiments of the present invention;

Figure 5 is a plan view of another alternative embodiment of a microwave field modifier;

Fig. 6 is an enlarged fragmentary plan view of the alternative microwave field modifier shown in Fig. 5;

Figure 7 is a perspective view of a package of the present invention with an integral cover and in the open position and with an array of edible bakeable items disposed therein;

Fig. 8 is a fragmentary cross-sectional view taken along section line 8-8 of Fig. 7;

Fig. 9 is a perspective view of the package of Fig. 6 without edible bakeable articles disposed therein and with a side panel removed to better illustrate the microwave field modifier disposed thereon;

Fig. 10 is a plan view of a blank which can be erected to form the package of Fig. 7;

Figure 11 is a perspective view similar to Figure 9 of a second preferred package of the present invention;

Fig. 12 is a plan view similar to that of Fig. 10, which can be erected to form the package of Fig. 11;

Figure 13 is a perspective view similar to Figure 9 of a third preferred package of the present invention ;

Fig. 14 is a perspective view similar to that of Fig. 6 of a fourth preferred package of the present invention with separate top and bottom portions; and

Fig. 15 is a plan view of a blank which can be erected to form either the top or bottom portion of the package of Fig. 14.

DESCRIPTION OF THE INVENTION

In short, the present invention provides microwaveable packages containing one or more microwave field modifiers The preferred microwave field modifiers (an example of which is generally indicated at 20 in Figure 1) generally comprise a substrate 22 and an array of a plurality of discrete electrically conductive elements 24 disposed thereon. The discrete electrically conductive elements 24 are preferably formed by applying a coating material to portions of the substrate 22 in a pattern; and even more preferably by printing. The elements 24 preferably have an elongated section and/or are laterally offset from one another.

In general (and therefore without specific reference numerals) and as used herein, "elongated" has its usual meaning: i.e., a shape that is noticeably long compared to its width. In addition, the elongated elements described here are preferably substantially straight, although it is not intended to thereby exclude serpentine, wavy and curved shapes. Also, the elongated elements preferably have rounded ends, as shown, to reduce the tendency toward electrical arcing.

"Staggered" relationship, as used here, includes (but is not limited to) shapes such as elongated, square and rectangular elements that are adjacent to each other but whose ends are offset from each other. The offset does not necessarily have to be uniform across the entire group.

The microwave field modifiers of the present invention may be a discrete unit covering an entire substrate or only a portion thereof. Similarly, multiple modifiers having different effects may be placed in different zones of the same substrate. A modifier may consist of layered groups. For example, a group may be placed on each surface (side) of a substrate. Alternatively, a modifier or multiple modifiers may be integrated into selected portions of packages, which here include packaging materials, cartons, containers, cookware or the like.

In addition, the modifier may form a layered or laminated structure with, for example, one or more additional Layers for such purposes as reinforcement, arc suppression, and interaction modification functions. For example, although not shown 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 mass of food product; and to protect the modifier from electrical arcing in the event that the modifier is placed in close proximity to electrically conductive objects. In addition, such layered constructions may be made by coating a thermoplastic film with electrically conductive coating material in a pattern and then laminating the pattern-coated film to paper or cardboard or other dielectric substrate.

The discrete electrically conductive elements are preferably formed by applying a coating material to the substrate in a pattern. Among the advantages of this structure are significant cost and equipment savings over current thin film susceptors. It is even more preferable if the coating material is applied to the substrate by printing, or most preferably by rotogravure printing. Printing offers advantages such as cost and efficiency savings over other coating methods, and rotogravure equipment is generally currently available to carton manufacturers.

The coating material of the present invention generally contains a binder system comprising a resin and a solvent, and electrically conductive particles. Additionally, the coating material may contain various other components.

The binder system is used to bond the electrically conductive particles together in contact with each other. The binder system also preferably serves to bond the coating material to the dielectric substrate. A binder system contains a resin and a solvent. Exemplary preferred resin materials include nitrocellulose, Ethyl cellulose, polyvinyl butyral, polyvinyl pyrrolidone, poly(methyl vinyl ether/maleic acid) copolymer resins and acrylic resins. A nitrocellulose can be obtained from General Printing, Ink Division, Sun Chemical Corporation, Cleveland, Ohio as a 40% solution of 18-25 CPS RS nitrocellulose. (This solution contains 17% by weight isopropyl alcohol, 23% by weight ethyl acetate and 20% by weight n-propyl acetate). An ethyl cellulose can be obtained from Hercules Inc., Wilmington Delaware as Ethyl Cellulose N-4. A poly(methyl vinyl ether/maleic acid) copolymer resin can be obtained from GAF Corporation of Wayne, New Jersey under the trade name Gantrez. Gantrez is a registered trademark of GAF Corporation. Gantrez currently exists in three basic forms: Gantrez AN, Gantrez ES, and Gantrez S. Gantrez ES-225 is preferred. A polyvinyl butyral can be obtained from Hoechst-Celanese of Somerville, NJ under the trade name Mowital. It is available in various molecular weights. The preferred types are coded by Hoechst-Celanese as B-20H, B-30H, and B-30T. Mowital is a registered trademark of Hoechst-Celanese. Polyvinylpyrrolidone can be obtained from GAF Corporation, Wayne, New Jersey, and from Sigma Chemicals of St. Louis, Missouri.

Electrically conductive particles that can be used to make coating materials include pure metal particles, some metal oxides, metal alloy particles, carbon particles, and graphite particles. Furthermore, the conductive particles preferably have irregular shapes; more preferably they are relatively flat; and most preferably they have different shapes and sizes; all of which promote electrical contact between the elements.

A preferred type of conductive particle which has proven successful is Nickel Flake HCA-1 which can be obtained from Novamet Company, Wyckoff, NJ. The Novamet Nickel Flake HCA-1 are dendritic particles formed from spheroids which have been interconnected and flattened. Thus, a preferred conductive particle has a flattened dendritic shape. A second preferred particle can be obtained from Cabot Corporation, Waitham, Mass. as Carbon Black Regal 99R. This particle is also flat and has a size of about 0.36 nm.

Some other components that may be used as components of coating materials include binder solvents, emulsifiers, acids and liquids that chemically combine with the other components of the coating material to cause solidification of the coating material after application in a fluidized state. In coating material applications that require a certain degree of flexibility, the coating materials may further contain plasticizers. In addition, suspending agents or other components may be included in the coating material formulations.

In addition, an undercoat applied to the substrate prior to printing increases conductivity. Similarly, an overcoat placed over the elements increases conductivity. Conductivity is further increased by using both an undercoat and an overcoat. If the binder of the undercoat and/or overcoat uses the same solvent as the binder of the coating material, conductivity is increased even further. Consequently, an undercoat or overcoat is used, more preferably an undercoat and an overcoat, even more preferably an undercoat or an overcoat using the same solvent as the coating material, and most preferably an undercoat and an overcoat using the same solvent as the binder of the coating material.

In addition, increasing the acidity of the binder appears 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, e.g. acid complexing additives. Without intending to be bound by this, it is believed that the surface chemistry effects of adsorption produce this benefit. Adsorption appears to allow closer contact of the metal particles with each other, which results in better conductivity. It is also possible that salts are formed with the oxides, which makes the metal more suitable for electrical conduction.

Fig. 1 shows an exemplary microwave field modifier, generally designated 20, embodying the present invention. The substrate 22 of Fig. 1 is 20-point paperboard, as is often made into such things as cartons for packaging microwavable food products: i.e., packages 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 materials for the substrate 22 include paperboard, coated paperboard, thermoplastic film, thermoplastic nonwovens, thermosetting plastics, or ceramics.

Disposed on the substrate 22 is an array formed of a plurality of discrete electrically conductive elements 24. In Fig. 1, the array extends over the entire top surface 28 of the substrate 22 except for a peripheral zone 29 which is free of coating material 26. The peripheral zone 29 serves to insulate the edges of the modifier 20 to substantially prevent arcing between the electrically conductive elements 24 and any metallic materials disposed adjacent to the modifier 20. Moreover, although the array of Fig. 1 extends over the entire surface of the substrate 22, the modifier may be confined to one or more zones of the substrate 22.

Referring to the enlarged fragmentary view of Fig. 2, the preferred modifier 20 of Fig. 1 comprises elements 24 that are the same size and shape, except at some row ends, and have lengths L and widths W, respectively. Thus, the array preferably comprises substantially identical elements 24. In addition, the elements 24 are preferably linearly aligned in straight, parallel rows. In the linear direction, the rows are spaced apart by a distance designated SL; and adjacent rows are spaced apart by a distance designated SW in the transverse direction. Furthermore, the rows are offset from one another so that side-by-side adjacent elements are offset in the linear direction by a distance denoted by OS. The degree of offset in percent of such a group is (OS/L) (100).

Referring to Fig. 3, an alternative embodiment of a microwave field modifier 120 of the present invention is shown in which the elongated elements 124 have a generally serpentine shape. The length L of each element 124 is measured along its centerline. Except for the shape of the elements 124 and the degree of offset, the modifier 120 is substantially similar in construction to the modifier 20 of Fig. 1. Accordingly, the substrate 122 may be the same dielectric material as the substrate 22 of Fig. 1, or may be made of a different dielectric material. The elements 124 may differ in size and/or shape; and the elements 124 may have the same or different coating materials (e.g., coating materials with a different surface resistivity). As shown, in this arrangement the elements 124 of adjacent rows are offset by approximately 50%.

The modifier 120 of Fig. 3 is more isotropic than the modifier 20 of Fig. 1. In other words, the effectiveness of the array as a shield is less dependent on the orientation of the array with respect to an incoming microwave. When the R values are plotted over a 360º angle of rotation using the RAT test described below, a sinusoidal curve results. For the modifier 20 of Fig. 1, the maximum R value would be relatively high and that portion of the curve would be relatively wide. In addition, the low R portion of the curve would be relatively narrow. For the modifier 120 of Fig. 3, the curve would have generally the same shape, but the maximum R value would be lower and the low R portion of the curve would be narrower, whereas the high R portion of the curve would be wider. Thus, a modifier 120 that is more isotropic than a second modifier 20 would provide better shielding in a microwave oven, even though its maximum R-value is lower, because in the microwave oven the waves arrive at all angles.

However, the modifier 20 of Fig. 1 can be made more isotropic by building up the array in layers (e.g., by printing both sides of a single substrate) such that the elements 24 in one layer are arranged substantially perpendicular to the elements 24 of the other layer (i.e., such that the high R region for one layer will shield waves arriving at a low R angle for the other layer). An example of this can be seen in Fig. 7, discussed later. This provides an isotropic modifier with even higher R than the modifier 120 of Fig. 3.

Referring to a second alternative embodiment, seen in Fig. 4, the discrete microwave modifier 220 includes a dielectric substrate 222 having a staggered array of square elements 224 disposed thereon and having an uncoated peripheral region 229. Except for the shape of the elements 224 and the degree of staggering, the modifier 220 is substantially similar in construction to the modifier 20 of Fig. 1. Accordingly, the substrate 222 may be the same dielectric material as the substrate 22 of Fig. 1, or may be of a different dielectric material, or may have a different size and/or shape; and the elements 224 may have the same or a different coating material (e.g., a coating material with a different surface resistivity). Although the elements 224 are shown with non-rounded corners, it is not intended to preclude square and rectangular elements from having rounded corners.

As shown, in this arrangement, the elements 224 of adjacent rows are offset by approximately 50%. The offset makes the modifier 220 with square elements 224 more effective as both a shield and a susceptor, i.e., heater, than squares that are orthogonally aligned.

In addition, the modifier 220 of Fig. 4 is more isotropic than the modifier 20 of Fig. 1 and the modifier 120 of Fig. 3. As a result, the modifier 220 of Fig. 4 also has a lower maximum R value than any of the previous embodiments.

Referring to Figures 5 and 6, another alternative microwave field modifier 320 is shown which includes an array of Y-shaped elements 324 on a dielectric substrate 322. Figure 4 is a partial enlarged view of the modifier 320. The Y-shaped elements 324 have three elongated sections and are arranged side by side in vertically extending rows. The length L of the elongated section in this embodiment is the maximum path along the centerline. In this embodiment, the three possible paths are all equal in length. In the rows of this embodiment, the stems of the Y-shaped elements 324 are partially received between the arms of adjacent Y-shaped elements 324. Also, the arms of the Y-shaped elements 324 in adjacent rows are partially received laterally between the arms of adjacent Y-shaped elements 324; and some of the arms of laterally interleaved Y-shaped elements 324 are parallel to each other. The stray field heating in such an arrangement can be varied by varying the spacing between portions of adjacent Y-shaped elements 324, as well as the size of the Y-shaped elements 324, the width of their elongated portions, and the dielectric loss properties of the substrate material 322. These modifiers 320 are more isotropic compared to modifier 20 similar to that of Figure 1; i.e., their RAT properties are less sensitive to position changes when such modifiers 320 are placed in microwave ovens and have lower maximum R values.

As previously mentioned, 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%. Because of 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., corners and edges, and generally slowing down the microwave cooking process.

Each modifier can be characterized by its microwave reflection, absorption and transmission values, i.e. its RAT values. However, because shields are characterized by their RAT values, and in particular their R and T values, the RAT values are particularly useful in determining shielding performance.

One method for 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 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 waveguide termination; and two S281A coaxial waveguide adapters. In addition, a digital millivoltmeter is used.

Connect the calibrated RF power output of the 8616A Signal Generator to the RF input of the 8743A Reflection Transmission Test Unit. The 8411A Harmonic Converter is connected to the 8743A Reflection Transmission Test Unit chassis and the 8410B Network Analyzer. Then, the 8410B Network Analyzer's test channel output, reference channel output, and test phase output are connected to the 8418A Auxiliary Display Holder's test amplitude input, reference input, and test phase input. The 8418A Auxiliary Display Holder has a chassis connection to the 8414A Polar Display Unit. The 8413A Phase Gain Indicator has a chassis connection to the 8410B Network Analyzer. The amplitude output and the phase output of the 8413A phase gain indicator are connected to the inputs of the digital millivoltmeter.

The settings of the 8616A signal generator are as follows: frequency is set to 2.450 GHz; the RF switch is ON; the ALC switch is ON to stabilize the signal; the DMB meter is zeroed using the ALC calibration output knob; and the attenuation is set for an operating range of 11 db. Set the frequency range of the 8410B network analyzer to 2.5, which should put the reference channel level meter in the "operating" range. The amplitude gain knob and the amplitude fine adjustment knob are set accordingly. to zero the voltmeter readings for the reflection and transmission measurements, respectively.

Microwave modifier samples are 3 1/2 inches in diameter.

For reflection, the 8743A reflection transmission unit is set to reflection mode. A coaxial waveguide adapter S281 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 waveguide adapter S281 and the low power waveguide termination S290A. The amplitude voltage is zeroed using the amplitude gain and fine tune knobs of the 8410B network analyzer. The shield is replaced by the susceptor probe. In other words, the probe is placed between the coaxial waveguide adapter S281A and the low power waveguide termination S920A and the attenuation voltage is measured. Typically, four measurements are taken per sample and averaged. Samples are rotated clockwise 90º per measurement. After the second measurement, the sample is flipped (top to bottom) for the final two measurements. For polarized, isotropic samples, care must be taken to orient the samples to obtain the maximum and minimum values. The R value is the maximum measurement. These samples can also be rotated in increments other than 90º.

For transmission, place the 8743A reflection transmission unit in transmission mode. A 10db attenuator is inserted into the transmission side of the line, between the "input" port of the 8743A reflection transmission unit and a second S281A coaxial waveguide adapter. The two S281A coaxial waveguide adapters are aligned and firmly connected. The amplitude signal voltage is set to zero using the 8410B network analyzer's amplitude gain and fine tune knobs. The modifier under test is placed between the two waveguide adapters and the attenuation voltage is measured. Four readings are taken for the reflection measurement as described above. The reflection and transmission values should be calculated in the same way; ie as mean 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 as they occur 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 equated to a circuit consisting of a generator connected to two parallel impedance loads. The generator represents the magnetron, while one impedance load represents the food load and the other impedance load represents the microwave modifier. The above network analyzer method does not include the food load resistance. Thus, when the food load is added to the circuit, its "impedance" relative to the resistance of the microwave field modifier is not known. If the "impedance" of the food load is significantly less than the "impedance" of the modifier, much of the microwave energy will pass through the food. Consequently, designing a modifier inevitably involves some trial and error, depending on the actual food being heated.

As previously described, a susceptor is a microwave field modifier that absorbs microwave energy and heats up appreciably when exposed to a microwave field. One method of determining the heating capability of a modifier (at least relative to another microwave field modifier) is the energy competition test described below. Using a carousel microwave oven having a power consumption of 30 kWh/min, measured at a load of 1000 g of water, an effective susceptor preferably has a ΔT at 2 min of about 90°F or more, more preferably about 150°F or more, and most preferably about 200°F or more.

To perform the energy competition test, a 150 ml Pyrex beaker containing 100 g of distilled water is placed on a carousel in a microwave oven with a 3 3/4" diameter Pyrex Petri dish containing 30 g of Crisco oil. These items are placed side by side in competition with their centers approximately 9" apart. An initial temperature measurement of the oil is taken. These items are then subjected to the full power of the microwave field for a total of 2 minutes; at 30 second intervals the microwave oven is opened and the oil is stirred with a thermocouple, measured and the temperature recorded. This measurement should be taken as quickly as possible to minimize cooling of the oil. This procedure provides a control measurement.

The above procedure is repeated with a 3 1/2 inch diameter sample of microwave field modifier immersed in the oil. Start with the oil at approximately the same initial temperature as the control measurement. It may be necessary to place an inert weight, such as a glass rod, on top of the modifier to keep it immersed in the oil. The data can be normalized by setting the initial temperature to a standard of 70ºF and subtracting or adding the initial temperature deviation of 70ºF from each of the recorded temperatures.

Once the test has been conducted, one method that can be used to compare the performance of the various microwave field modifiers is to compare the temperature change over the 2 min time interval. Thus, the 2 min ΔT is calculated by subtracting the 2 min AT of the oil alone from the 2 min ΔT of the oil and the susceptor. Additionally, the 2 min AT of the susceptor is normalized by adding or subtracting any initial temperature deviation of the oil from 70ºF.

As with measuring RAT values by using a network analyzer, the energy competition test cannot accurately predict how well a modifier will heat in the microwave oven in conjunction with a food load. However, the use of water is intended to simulate the modifier competing with a load. The larger the The more the microwave properties of the actual food load deviate from the properties of a water load, the less accurate this test will be. Consequently, it may be desirable to use a different amount of water or competing load in a particular application comparing potential microwave field modifiers to reduce the amount of trial and error required to achieve the desired results with the actual food load. In any event, however, some trial and error will inevitably be necessary.

Despite the above, a microwave field modifier of the present invention can be designed with a minimal amount of trial and error to have the desired RAT values and heating properties by selectively changing certain variables. Thus, a modifier with a wide range of shielding and heating properties can be developed. Some of these variables relate to the array arrangement, i.e., element shape, size, orientation and arrangement relative to other elements; some to the coating material; and finally, some to the substrate or possible overcoatings and undercoatings. Generally speaking, the directional influence of the various variables with respect to tailoring the RAT properties is described below with reference to Figure 2. However, the directional influence applies to all embodiments of the present invention.

Among the array variables are the length, width, spacing and degree of offset between adjacent elements 24. One variable is the length L of element 24. In general, the maximum length L of 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 1/2 cm or more, and most preferably 1 cm or more. In any event, the length L should be less than the value that would cause electrical arcing. In this general range, increasing the length L increases the microwave reflectivity and decreases both the microwave absorptivity and transmittance of the modifier 20. In addition, increasing the length L of the elements 24 increases the magnitude of the alternating voltage and current induced in the elements 24 and therefore also increases the tendency for arcing across the gaps between the ends of adjacent elements 24 and for overheating inside the elements 24.

With respect to width W, a width of from about 0.001 inch to about 1.0 inch is preferred; with a width W of from about 0.010 inch to about 0.10 inch being more preferred. In this general range, increasing width W decreases the microwave reflectivity and increases the bandwidth of the wavelength response of the modifier 20, making the modifier 20 less sensitive to wavelength changes that can be caused by such things as contact with a food load. In addition, the elongated 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.

The end-to-end distance SL is preferably about 0.010 inches to about 0.100 inches, and in this range, increasing end-to-end distance SL or increasing side-to-side distance SW decreases the microwave reflectivity, increases both the microwave absorptivity and transmittance of the modifier 20, and reduces the stray field heating of the intervening portions of the dielectric substrate 22.

The side-to-side distance SW is preferably between about 0.010 inches and about 0.10 inches. Increasing side distance SW in this range reduces both stray field heating and shielding.

The degree of dislocation is preferably about 30% or 35% for maximum shielding and heating in highly conductive coating materials.

Among the coating material variables are the resistivity, shape and size of the conductive particles; and the dielectric properties (i.e., dielectric constants, loss factors and breakdown strengths) and the electrical surface conductivity of the dried coating material. The electrical surface conductivity of the conductive elements 24 is relatively high, ge measure as surface resistance: preferably 100 ohms per square or less; more preferably 10 ohms per square or less; even more preferably 3 ohms per square or less; even more preferably 1 ohm per square or less; and most preferably 0.1 ohm per square or less. Increasing the surface electrical conductivity of the elements 24 in this region directly increases the microwave reflectivity and decreases the microwave transmittance of the modifier 20.

Achieving a dried coating with a conductivity in the desired ranges listed above is aided by certain measures. For example, depositing a sufficient amount of coating material is essential. The more coating material, i.e. the thicker the coating material, the higher the conductivity. The coating material thickness is preferably between 0.0001 inches and about 0.003 inches, and even more preferably between about 0.0005 inches and about 0.002 inches.

Also, the viscosity of the coating material may be important, depending on 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 Zahn No. 3 cup. In any event, the viscosity should be such that the coating material is suitable for the coating process chosen; be it painting, spraying, printing, screen 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 generally done in printing processes.

In addition, an undercoat applied to the substrate prior to printing increases conductivity. Similarly, an overcoat applied over the elements increases conductivity. Conductivity is further increased when both an undercoat and an overcoat are used. If the binder of the undercoat and/or overcoat uses the same solvent as the binder of the coating material, conductivity is increased even further. Consequently, an undercoat or an overcoat is used, more preferably an undercoat and an overcoat, even more preferably an undercoat or an overcoat using the same solvent as the coating material, and most preferably an undercoat and an overcoat using the same solvent as the binder of the coating material. In addition, an overcoat of Gantrez AN or Gantrez S, for example, may be useful to create an FDA-approved protective layer.

In addition, increasing the acidity of the binder appears to have a beneficial effect on conductivity. The more acidic the binder, the greater the conductivity. Thus, it may be advantageous to add acidic binder additives, such as acid complexing additives, to the coating material. Although not intended to be bound by this, it is believed that the surface chemistry effects of adsorption are the cause of this benefit. Adsorption allows the metal particles to be in closer contact with one another, resulting in better conductivity. It is also possible that salts are formed with the oxides, making the metal more available for electrical conduction.

With respect to the conductive particles of the coating material of element 24, the electrically conductive particles are preferably less than 25 µm in size (i.e., in their maximum dimensions); and more preferably less than 10 µm. Finer particles result in greater coating conductivity for a given weight percentage of particles in a given coating material. Also, a mixture of different particle sizes causes an increase in surface electrical conductivity for a given weight percentage of particles in a given coating material. The particle aspect ratio - i.e., the ratio of the longest dimension to the shortest dimension of a particle - is also essential. A high aspect ratio (i.e., greater than 10:1) is preferred because it increases electrical contact between particles in consolidated coating materials. and such conductive particles are more amenable to microwave heating than particles with smaller aspect ratios. In addition, particles with higher resistivity cause a decrease in the surface conductivity of the coating material; and particles with indented shapes and edges tend to promote electrical contact between the conductive particles in a coating material and therefore increase the surface conductivity of the elements 24.

With respect to the dielectric properties of the dried binder and other fluids, a high dielectric loss factor increases dielectric heating (i.e., capacitive heating inside a conductive element 24 between conductive particles); a high dielectric strength reduces the tendency to arc; and a high dielectric constant increases microwave reflectivity and power withstand capability.

Among the remaining variables are the dielectric properties (i.e., dielectric constants, loss factors, and breakdown strengths) of the substrate 22 and any optional overcoating materials. With respect to the substrate 22, a high dielectric constant causes an increase in microwave reflectivity and power withstand capability; a high dielectric loss factor causes an increase in dielectric heating; and a high breakdown strength prevents breakdown at higher induced voltages. Thus, a high breakdown strength reduces the tendency for arcing and therefore tends to prevent arc blackening of the substrate 22. This protects the modifier 20 from breakdown insofar as carbon particles resulting from arcing would tend to short the elements across the gaps therebetween. The same applies to any optimal overcoating or undercoating that can be applied to prevent the modifier from directly contacting the food.

The directional information given above regarding the various variables is generally true. However, there is a complex relationship between the various variables. Due to this complex relationship, a change in a specific variable, while keeping the other variables constant, may have a minimal effect in a specific situation; i.e. a specific coating material and coating pattern. In contrast, a change in the same variable may have a truly significant effect in another situation.

Without wishing to limit the invention, one possible method of designing a microwave field modifier of the present invention with the desired RAT properties is as follows. Take a coating material having the desired conductivity range when dried (see above) and a suitable substrate. Determine the maximum line length that can be used without arcing occurring. This is done by applying a series of lines of varying lengths up to about 4 cm and preferably about 3 mm wide to the chosen substrate. Place these lines in the microwave field together with 100 g of water in a beaker (to simulate a food load).

Choose a line length approximately 20% smaller than the minimum line length that will form an arc. Create a pattern coating with this line length L, a width W of 1 mm, gaps of 1 mm 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 arrangement. The modifier can be tested in a network analyzer to determine the RAT values. Also perform an energy competition test to determine heating capacity. Alternatively, this arrangement 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 reduce the size of the gaps. The transition to a more conductive coating material or the Depositing a thicker coating (which increases conductivity) also increases heating and shielding. If excessive heating occurs and a reduction in shielding effectiveness is tolerable, make an opposite adjustment.

If this arrangement simply produces too much heating for the required shielding, then alter the dielectric properties in the coating material, the substrate, and any overcoating or undercoating to reduce the loss materials, and perhaps move to a more conductive coating material. Another possible way to increase shielding is to increase the degree of dislocation. The above procedure and list of adjustable variables is not intended to be an exhaustive list, but merely illustrates how adjustment to the desired RAT values can be made.

As indicated above, the relative RAT values of the microwave field modifier of the present invention can be specifically tailored to the specific needs of different foods by varying such parameters as the surface electrical conductivity or surface electrical resistance of the elements; the lengths, widths, spacing and degree of displacement between adjacent elements; the resistivity, shape, size and aspect ratio of the conductive particles; and the dielectric properties (i.e., dielectric constants, loss factors and breakdown strengths) of the substrate and/or dielectric binder.

To illustrate the versatility of the microwave field modifiers of the present invention, three exemplary embodiments are 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 characteristics can be described with reference to Figs. 1 and 2. The assembly is screen printed on 20 point cardstock using a 109 mesh screen made of 0.0032 inch diameter monofilament polyester. The coating material consists of about 60 weight percent silver particles and has a dry surface resistivity of less than 0.5 ohms per square. Such a coating material can be obtained from Acheson Colloids Company, a division of Acheson Industries, Inc., Port Huron, Michigan, and is sold under the designation Electrodag 477SS.

Still referring to Figures 1 and 2, the elongated elements 24 are 2.5 cm long, 0.040 inches wide, have an end-to-end spacing of 0.160 inches and a side-to-side spacing of 0.160 inches, and an offset (pattern offset) OS of about 25%. The pattern can be printed with the elongated elements 24 horizontally on the front side of the substrate 22 and with the elongated elements 24 vertically on the back side of the substrate 22.

Example 2

An exemplary microwave field modifier of the present invention having high shielding properties and substantially high heating properties can be described with reference to Figures 1 and 2. In this example, the heat is generated primarily as a result of stray field heating or capacitive heating between the elements 24 due to the relatively high conductivity and relatively high dielectric properties of the coating material components. However, some heat is also generated by the I²R heating of the elements 24 themselves. The surface resistance of the dried coating material is less than about 2 ohms per square.

The assembly is screen printed on a 20 point paperboard substrate 22 using a 109 mesh 0.0032 inch diameter monofilament polyester screen. The coating material used is a 47% copper and 53% acrylic binder system coating material available from Acheson Colloids Company, Port Huron, Michigan as Acheson Copper Electrodag #437.

The assembly has an appearance similar to that shown in Figures 1 and 2. The elongated elements 24 are 2 cm long and 0.032 inches wide. The assembly has an end pitch of 0.045 inches, a side pitch of 0.027 inches and an offset of 30%.

Example 3

An exemplary embodiment with low shielding capability and significant heating capability can be described with reference to Figure 4. In this example, the heat is generated substantially within the elements 224 themselves, due to the relatively low conductivity of the elements 224 and the dielectric properties of the coating material components.

The assembly is patterned onto a 20 point paperboard substrate 222 using a 109 mesh 0.0032 inch diameter polyester monofilament screen or a similar 18F multifilament screen. The coating material consists of 60% nickel and 40% nitrocellulose by weight. The nickel can be obtained from Novament Company of Wyckoff, New Jersey and is sold under the designation Nickel HCA-1 Flake. The nitrocellulose can be obtained from General Printing Ink Division, Sun Chemical Corp., Cleveland, Ohio and is sold as Nitrocellulose Solution #266-133.

The arrangement may have an appearance similar to that shown in Figure 4. The elements 224 are 7 mm squares spaced 0.6 mm apart on all sides. Additionally, the elements 224 have an offset of 50%.

Additional exemplary embodiments using different binder systems are listed below:

Example 4

This is an exemplary embodiment using a 10% solution of Mowital B30H polyvinyl butyral. A 10% solution can be obtained by dissolving 5 g of B30H powder in 45 g of methanol (methyl alcohol). To this is added 75 g of Novamet nickel; resulting in a solution of 60% nickel and 40% resin solution (10% solid resin). Thus, the final solution consists of 75 g of nickel and 50 g of 10% resin solution.

This coating material can be screen printed in a pattern similar to that of Fig. 1. The dimensions of this pattern can be as follows: end gap SL 0.045 inches, side gap SW 0.275 inches, length L 0.787 inches, width W 0.035 and overlap 31%.

Furthermore, when this coating material is screen printed onto a substrate such as cardboard which has been pre-coated with a layer of polyvinyl butyral solution (i.e. 10% solution in methanol without nickel) using a doctor blade or Mayer bar, the conductivity and reflectivity (R) can be increased.

If the first sample is coated with the same 10% solution to create an overcoat of the susceptor coating, the change in RAT properties can be similar to that of the second sample which has an undercoat.

If the sample with the polyvinyl butyral undercoat is now overcoated to produce a nickel printed sample that has both an undercoat and an overcoat, the conductivity and reflectivity (R) can be further increased.

Example 5

This example uses a 20% solution of Gantrez ES-225. A 20% solution can be obtained by starting with 20 g of the material as supplied (50% resin and 50% ethanol solvent). This gives 10 g resin and 10 g solvent. Add 30 g ethanol solvent to the solution; creating a solution of 20% resin and 80% solvent. This gives 10 g resin and 40 g solvent, or 50 g total solution. To this add 75 g of Novament Nickel; creating a plating material of 60% nickel and 40% resin solution (20% resin). Thus, the final solution is 75 g nickel and 50 g 20% resin solution.

This solution is then screen printed or gravure printed (with only minor viscosity adjustments) in a pattern similar to that of Fig. 1. The dimensions of this pattern can be as follows: end gap SL 0.045 in.; side gap SW 0.0275 in.; length L 0.787 in.; width W 0.035 in.; and offset 31%.

Example 6

This example uses a 10% solution of polyvinylpyrrolidone. A 10% solution can be obtained by dissolving 5 g of polyvinylpyrrolidone powder in 45 g of methanol (methyl alcohol). To this add 75 g of Novament Nickel; giving a solution of 60% nickel and 40% resin solution (10% resin solids). Thus, the final solution contains 75 g of nickel and 50 g of 10% resin solution.

This solution can then be screen printed in a pattern similar to that of Fig. 1. The dimensions of this pattern can be as follows: end gap SL 0.045 inches, side gap SW 0.275 inches, length L 0.787 inches, width W 0.035 inches and overlap 31%.

Example 7

This example uses a 5.4% solution of ethyl cellulose. A 5.4% solution can be obtained by starting with 2.2 g of ethyl cellulose resin. To this add 0.5 g of a suspending agent such as Bentone SD2, available from National Lead Chemicals, Hightstown, New Jersey. Add 4.4 g of a modifier such as Uni-Rez 7055 (fumaric acid modified rosin acid ester binder) available from Union Camp Corp., Wayne, New Jersey; and 1.8 g of a plasticizer such as Herculon D (hydrogenated rosin acid methyl ester) available from Hercules Chemical Corp., Wilmington, Delaware. Also add 32.1 g of n-propyl acetate as a solvent, yielding a 5.4% ethyl cellulose solution. To this add 59 g of Novament Nickel HCA-1 flakes; resulting in a coating material of 59% nickel and 41% (5.4% resin) ethyl cellulose resin solution. Thus, the final solution consists of 59 g of nickel and 41 g of 5.4% resin solution.

This solution is then screen printed or gravure printed (with only minor viscosity adjustments) in a pattern similar to that of Fig. 1. The dimensions of this pattern can be as follows: end gap SL 0.045 in.; side gap SW 0.0275 in.; length L 0.787 in.; width W 0.035 in.; and offset 31%.

From the previous examples, which show different combinations of a relatively high or a relatively low shielding capacity with a relatively high or relatively low heating capacity, it can be seen that modifiers with virtually any properties can be designed. In the above examples, high shielding capacity is defined as a transmittance of about 10% or less, and high heating capacity as a ΔT at 2 min of about 125°F or more.

The microwave field modifiers of the present invention described above can be used, for example, in packages for heating, baking, cooking, etc., various food products. Referring to Figures 7 and 8, a package 30 with printed microwave field modifiers for baking a pound cake batter product is shown. The carton 30 includes a box-like container portion 31 and an integral lid 32. The box-like container portion 31 of the carton 30 includes a bottom wall 35, side walls 41, 42, 43 and 44, and four adhesive tabs, designated 45. The integral lid 32 of the carton 30 includes a top wall 36, side walls 46, 47 and 48, and two adhesive tabs 49.

The carton 30 has microwave field modifiers disposed thereon and is designed to bake a pound cake batter product 52. The carton 30 contains eight commercially available susceptor trays 54, i.e., paper trays lined with a thin vapor-deposited metallic coating, available from IVEX Corporation, Newton, MA, into which the batter 52 is dispensed. The trays 54 may alternatively contain a microwave field modifier as described above, designed to heat. These trays 54 are filled with batter 52 and arranged in a circular array around a centrally located thermoplastic measuring tray 55. Such a configuration has been found to be very effective in achieving uniform soaking, rising and browning of pound cakes.

Referring to Fig. 9, the side walls 41, 42, 43 and 44 of the cardboard base 31 have a microwave field modifier 20 similar to Example 1. The inner surface of the side walls 41, 42, 43 and 44 is provided with an array of elongated elements 24 which run horizontally and the outer surface of the side walls 41, 42, 43 and 44 has an identical array of elongated elements 24 arranged vertically. A portion of the side wall 43 is peeled away (and an adhesive tab removed) to show this arrangement. Such two-layer arrangements of pattern elements 24 (ie, on both surfaces of the side walls) provide a better isotopic response to positional differences when such structures are placed in microwave oven fields. The modifier of the side walls 41, 42, 43 and 44 provides shielding around the sides to slow baking at the edges and also to make the baking process more uniform.

The top wall 36 of the carton 30 includes a second modifier 220 similar to Figure 3, except that the elements 224 are orthogonally oriented. The interior surface of the top wall 36 of the carton 30 is coated with an array of generally square elements 224. This modifier 220 may be similar to that of Example 3. The modifier 220 generates a significant amount of heat which browns the surface of the cupcakes and helps to give them the traditional domed appearance.

The carton 30 can be made from the blank shown in Fig. 10. The blank shows the modifier 220 applied to the top wall 36. The blank also shows the side wall modifiers 41, 42, 43 and 44 applied to the panels 41, 42, 43 and 44, although the elongated elements 24 applied to the back of these panels 41 to 44 are not shown. The printed patterns of coating material are spaced from the distal and end edges of the walls 41 to 44, and also from the edges of the top wall 36. This spacing serves to ensure that adjacent portions of the coated walls do not touch each other in the erected carton 30; and this again ensures that no arcing occurs in these areas of the box 30.

Fig. 11 is a perspective view of an alternative microwaveable carton 130 which is substantially similar to carton 30, Fig. 9, except that the carton 130 does not have electrically conductive pattern elements 24 on the outer surfaces of its side walls 141, 142, 143 and 144; and that the arrangement of square pattern elements 242 on the top wall 136 is in the staggered relationship of Fig. 4 rather than in the orthogonal arrangement of the carton 30, Fig. 9. Fig. 12 illustrates a blank which can be erected to form the carton 30 of Fig. 11.

Fig. 13 is a perspective view of an alternative microwaveable carton 230 which is substantially similar to carton 30 except that it does not have an array of electrically conductive pattern elements 224 on the inwardly facing surface of top wall 236. When such carton 230 is filled with cupcakes similar to those shown in Fig. 7, the cupcakes become dome-shaped, gold-colored, moist, and fairly uniform in appearance, but not quite to the same extent as package 30, Fig. 11. An advantage of this embodiment, however, is that only one side of the blank needs to be coated with coating material.

Fig. 14 is a top view of a cut, scored and pattern coated carton blank 3308 which can be erected to form either the container section 331 or the lid section 332 of the carton 330 of Fig. 15. This view shows that the printed patterns of coating material are spaced from the distal and end edges of the walls 341 through 344, and also spaced from the edges of the top wall 336. This spacing serves to ensure that adjacent portions of the coated walls do not touch in the erected carton; and this in turn ensures that arcing does not occur in these areas of the carton 330.

The lid 332 is sized relative to the base 331 such that the lid 332 fits over the base 331. The inner sides of the side walls 341, 342, 343 and 344 of the base 331 and the inner sides of the side walls 346, 347, 348 and 350 of the lid 332 are provided with arrays of electrically conductive pattern elements 24 like those of the carton 30 of Fig. 9. Thus, when the lid 332 covers the base 331, two layers of pattern elements 24 are provided over the double side walls. of the carton 330. Functionally, the side walls of this construction are believed to have approximately the same microwave field modifying effect as the side wall portions of the carton 30, Fig. 9. Although this embodiment is shown without the top wall modifier, as in the embodiment of Fig. 13, a top wall modifier could be added as in the embodiment of Fig. 11. Again, an advantage of this embodiment is the need to print the coating material on only one side of the blanks.

Claims (8)

1. A microwaveable package comprising a first wall and a first microwave field modifier, characterized in that the microwave field modifier comprises an electrically conductive microwave active coating material pattern coated on a predetermined zone of the wall to define an array of electrically conductive discrete elements. 2. Microwaveable packaging according to claim 1, further characterized by a second wall and a second microwave field modifier, the second microwave field modifier comprising an electrically conductive microwave active coating material pattern coated on a predetermined zone of the second wall to define an array of electrically conductive discrete elements. 3. Microwaveable packaging according to claim 1, comprising a top wall, a bottom wall and a peripheral side wall which connects the top wall to the bottom wall to form an envelope, characterized in that the side wall has an attached or integral microwave field modifier which forms a substantially continuous vertically arranged annular shield, the microwave field modifier comprising an electrically conductive microwave active coating material pattern which is arranged on a predetermined zone of the wall to define a group of electrically conductive discrete elements. 4. A microwaveable package according to claim 3, further characterized by a second microwave field modifier located on the inside of the top wall to generate a substantial amount of heat, the top wall being separated from the contents of the package by a headspace, and wherein the microwave field modifier comprises an electrically conductive microwave active coating material pattern disposed on a predetermined zone of the wall to define an array of electrically conductive discrete elements. 5. Microwaveable packaging according to claims 1 to 4, in which the discrete elements have uniform length and are arranged parallel to one another in several rows, the rows being arranged in such a way that a predetermined offset results between the discrete elements in adjacent rows. 6. Microwaveable packaging according to one of claims 1 to 5, wherein the microwave field modifier comprises two layers of discrete electrically conductive elements; the first layer of elements being applied as a pattern on the inside of the wall and the second layer of elements being applied as a pattern on the outside of the wall. 7. A microwaveable package according to claim 1, wherein the first wall has multiple layers and the microwave field modifier comprises two layers of discrete electrically conductive elements; the first layer of elements being applied as a pattern on a surface of a first layer of the first wall and the second layer of elements being applied as a pattern on a surface of a second layer of the first wall. 8. Microwaveable packaging according to one of claims 6 or 7, in which the first layer of discrete electrically conductive elements is substantially perpendicular to the second layer of discrete electrically conductive elements.