CA1274126A - Composite material containing microwave susceptor materials - Google Patents

Abstract

TITLE

COMPOSITE MATERIAL CONTAINING MICROWAVE
SUSCEPTOR MATERIALS
ABSTRACT OF THE DISCLOSURE
A composite material useful for controlled generation of beat by absorption of microwave energy is disclosed. The material comprises a dielectric substrate, e.g., polyethylene terephthalate film, coated with a mixture of an electrically conductive metal or metal alloy in flake form in a thermoplastic dielectric matrix, e.g., a polyester copolymer. In a preferred embodiment, the coating of flake/thermoplastic is applied to as to yield an isotropic coating with good heating performance reproducibility. The use of circular flakes with flat surfaces and smooth edges contributes substantially to good heating performance reproducibility.

Description

TITLE
COMPOSITE MATÆRIAL CONTAINING MICROWAVE
SUSCEPTOR MATERIALS
BACKGROUND OF THE INVENTION
This invention relates to novel composites useful for controlled generation of heat by absorption of microwave energy.
Food preparation and cooking by means of microwave energy has, in recent years, become widely practiced as convenient and energy efficient. Along with the growth in the use of microwave cooking has been a growth in the sale and use of foods specially packaged for microwave cooking. Such special microwavable packages attempt to alleviate some of the problems inherent in microwave cooking, for example, lack of browning or crispening of the surface of a cooked food item or uneven cooking due to development of "hot spots" in the food item. Examples of packaging materials developed for use in microwave cooking are those disclosed in U.S. Patents 4,518,651, 4,267,420, 4,434,197, 4,190,757, 4,706,108, UK Patent Application No. 2,046,060A tpublished 1980 November 05) and European Patent Application Publication No.
63,108 (published 1982 October 20).
U.S. 4,518,651 to Wolfe discloses composite materials exhibiting controlled absorption of microwave energy based on the presence of electrically conductive particles such as particulate carbon in a polymeric matrix bouncl to a porous substrate. The resulting composite materials are to have a surface resistivity of 100 to 1000 ohms per square. Wolfe teaches that it is critical that at least some of the polymeric matrix be beneath the surface of the substrate, be substantially free of electrically conductive particles and be intermingled with the ~ fl~

substxateO This is achieved by a lamination process at certain temperaJ,ures and pressures.
U.S. 4,267,420 to Brastad discloses a packaging material which is ~ plastic film or other dielectric substrate having a thin semiconducting coating. The semiconducting coating generally has a surface resistance of 1 to 300 ohms per sguare, and the preferred coating is evaporated aluminum. Similar materials, i.e., films with a continuous layer of electrically conductive ~aterial deposited thereon, are also disclosed in UK Patent Application

2,046,060A.
U.S. 4,434,197 to Petriello et al~ discloses a multi-layer structure having five layers including outside layers of polytetrafluoroethylene, two intermediate layers of pigmented polytetrafluoroethylene and a central layer of polytetrafluoroethylene containing an energy absorber.
The energy absorber can be a material such as colloidal graphite, ferric oxide and carbon and should have a particle size such that it will uniformly disperse with particles of polytetrafluoroethylene to form a co-dispersion.
U.S. 4,190,757 to Turpin et al. discloses a microwaveable package composed of a non-lossy dielectric sheet material defining a container body and a lossy microwave absorptive heating body connected thereto, the heating body possibly comprising a multiplicity of particles of microwave absorptive material of different particle sizes and a binder bonding those particles together~ Absorptive materials include zinc oxide, germanium oxide, iron oxide, alloys such as one of manganese, aluminum and copper, oxides, carbon and graphite. The binders for these materials are ceramic type materials such as cement, plaster of paris or sodium silicate, and the resulting materials are therefore not flexible. The package also requires a shield, for example, a metal foil sheet adapted to reduce by a controlled amount the direct transmission of microwave energy into the food product. A ~omewhat similar disclosure is found in U.S. 4,706,108 to Anderson et al. This patent also discloses a microwave heatin~ device comprising a microwave reflective ~ember positioned adjacent to a magnetic microwave absorbing material.
European Patent Application Publication No.
63,108 disclose~ a packaging material such that at least a region of one side thereof is provided with a coating comprising heat reflecting particles in a predetermined pattern, in ~or instance flake or particle shape. The heat reflecting particles preferably consist of metal particles of aluminum or another food-stuff inert metal and are preferably included within a layer of polyester, polymethylp~ntene or another material having corresponding heat resistance characteristics. The , content of heat reflecting particles amounts to 0.01-1% by weight of the surface weight of the coating, and the heat resistant layer has a surface weight of 15 to 30 grams per square meter.
Despite the many developments tc date in the field of microwaveable packaging, certain needs gtill exist. Many existing materials function in one way or another to convert a portion of the microwave energy into heat, but the materials offer little control to the packager in terms of how much heat is generated and how quickly. For example, some of the materials tend to heat uncontrollably in a microwave oven, leading to charring or even arcing, ignition and burning of the packaging material. Other available . ,. 1~7~

materials are simply not capable of generating enough heat quickly enough to be of use in certain applications (e.g., providing fast heat-up and high bag temperatures to provide ef~icient popping of popcorn in a microwave oven). And many of the available materials are simply not ~uitable for the mass disposable-packaging market because they are ~imply too expensive to produce.
SUMl~ARY OF THE IN~IENTION
New packaging materials for microwave u~e have now been found which ~olve some of the problems inherent in prior art materials. Specifically, this invention relates to composite materials for controlled generation of heat by absorption of 15 microwave energy comprising ~a) a dielectric substrate substantially transparent to microwave radiation and (b) at least one coating on at least one surface of the substrate, the coating comprising (i) about 5 to 80% by weight of a susceptor material in flake form capable of converting microwave energy to heat, and (ii) about 95 to 20% by weight of a thermoplastic dielectric matrix, wherein the surface weight of said coating on the substrate is în the range of about 2.5 to 100 y/m2. The D.C. surface resistance of the resulting composite material is generally at least 1 x 106 ohms per square. These new materials offer the advantages of being economical to produce and of being easily adaptable so as to ma~ch the degree of heat generated to the requirements of the food which is packaged in it. The materials can be adapted to heat to very high temperatures within a very short time and thus find utility as packaging materials for food items for which bxowning is desired but which are cooked for relatively short periods of time (e.g., breadstuffs or pizza) and also for food items for which high temperatures and rapid heat-up are needed to insure efficient microwave cooking (e.g., popcorn).
Despite the high degree of heat which these materials axe capable ~f ge~erating, the amount of i 5 susceptor material and thermoplastic matrix can be adapted to avoid charxing, arcing or burning of the packaging materials ~s often results from use of prior art materials.
DETAILED DESCRIPTION OF THE INVENTION
The substrate material used in this invention is a carrier web or film which has sufficient thermal and dimensional stability to be useful as a packaging material at the high temperatures which may be desired for browning or rapidly heating foods in a microwave oven (generally, as high as 150 degrees C and above, preferably 220 ~ degrees C and above.) Polymeric films, including i polyester films ~uch as polyethylene terephthalate films and polymethylpentene films, and films of other thermally stable polymers such as polyarylates, polyamides, polycarbonates, polyetherimides, polyimides and the like can be used. Porous structures such as paper or non-woven materials can also be used as ~ubstrates 60 long as the required thermal and dimensional ~tability is satisfied. For flexible packaging, the ~ubstrate is preferably about 8 to 50 micrometers thick. Thicker, non-flexible materials~ ~uch as found in trays, lidding, bowls and the like, could also be used. The preferred substrate is biaxially oriented polyethylene terephthalate which is preferably about 12 micrometers thick.
As previously indicated, the substrate must have ~ufficient dimensional stability at the elevated temperature~ involved in microwave cooking to prevent distortion of the substrate which may result in , - 6 -non~uniform cooking from loss of intimate contact of the packaging material with the food to be cooked.
Substrates lacking such high temperature dimensional ~tability can be used if they are laminated with yet another substrate layer meeting the thermal ~tability requirements of the original ~ubstrate. The lamination can be accompli~hed either by taking advantage of the adhesive properties of the thermoplastic ~atrix coating on the original substrate or by using any number of conventional adhesives to aid in forming a ~table laminate. For example, a composite of this invention ~uch as a polyester copolymer coated polyethylene terephthalate film can be thermally ~ealed to another polyester film or to paper or heavier ovenable paperboard. Alternatively, another adhesive can be applied from ~olution prior to lamination to increase the strength of the laminate.
These supplemental adhesives can be selected from a number of commercially available candidates with required thermal stability. These include copolyesters, copolyester-polyurethanes and cyanoacrylates.
The thermoplastic dielectric matrix used in the composite of this invention can be made from a variety of polymeric materials with 6uffi~ient thermal stability to allow for dimensional integrity of the final packaging material at the elevated temperatures associated with microwave cooking of food. The dielectrical properties at 915 megahertz and 2450 megahertz of the matrix is ~180 an important variable in terms of the heat generated in unit time at 2450 MHz. The dielectric matrix has a relative dielectric constant o~ about 2.0 to 10 with a preferred value of 2.1 to 5.0, and a relative dielectric loss index of about 0.001 to 2.5, preferably 0.01 to 0.6. The ~L~7~ 6 matrix ~150 preferably displays adhesive characteristics to the substrate in the composite and any additional cubstrate to which the composite may be laminated to increase dimensional tability. For best results, the peel strength of the matrix to substrate(s) ~eal should be at least 400 to 600 g/in.
A variety of polymeric materials known in the art meet these reguirements. Examples include but are not limited to: polyester~, polyester copolymers, curable resins such as copolyester-polyurethanes and epoxy resins, polycarbonates, polyethersulfones, polyarylsulfones, polyamide-imides t polyimides polyetheretherketones, poly 4'4-isopropylidene diphenylene carbonate, imidazoles, oxazoles, and thiazoles. These materials may be crystalline or amorphous. The preferred matrix is a polyester copolymer. These are reaction products of a glycol and a dibasic acid. Suitable glycols include ethylene glycol, neopentyol, mixtures of 1,4-butane diol, diethylene glycol, glycerin, trimethylethanediol and trim~thylpropanediol. Suitable dibasic acids include azealic, ~ebacic, adipic, iso-, tere- and ortho-phthalic, and dodecanoic acids. The preferred polyester copolymer is a copolymer or mixture of copolymers, of ethylene glycol with terephthalic and azealic acid or with terephthalic and isophthalic acid~
The susceptor materials used in this invention are metals and metal alloys which are capable of absorbing the electric or magnetic portion of the microwave field energy to convert that energy to heat. Suitable such materials include nickel, antimony, copper, molybdenum, bronze, iron, chromium, tin, zinc, silver~ gold, and the preferred material, aluminum. Other conductive materials such as graphite ~7~

and semiconductive ~aterials such as silicon carbides and magnetic ~aterial such as metal oxides, if available in *lake form, may also be operable ~usceptor materials and are deemed equivalent to the susceptor materials claimed herein.
The ~usceptor ~aterial must be in flake form. For the purpose o~ this invention, a particle is in flake form if its aspect ratio, defined as the ratio of the largest dimension of its face to its ~ thickness is at least about 10. Generally speaking, the conductive materials use~ul as ~usceptors in this invention will have an aspect ratio in the range cf 10 to 300. The preferred aluminum ~aterials will generally have an aspect ratio in the range of 20 to 200. Those preferred aluminum materials also generally have a largest dimension of 1 to 48 micrometers and a thickness of 0.1 to 0.5 micrometersO
As variables, the amount and the physical size, shape and surface characteristics of the susceptor flakes used in the coating and the amount of that coating applied to the substrate depend on the type and portion size of the food to be cooked. It is by altering these variables that one may control the generation of heat exhibited by the material when it is used in a microwave oven. An advantage of the composites of this invention is that they can be tailored to heat to high temperatures in relatively short periods of time in conventional microwave ovens, e.g., to temperatures of about 150C or above, preferably 190C or above, in 120 ~econds when subjected to microwave energy of 550 watts at 2450 megahertz.
The susceptor level in the thermoplastic matrix will generally range from about 5 to 80~ by 35 weight of the combined susceptor/matrix. The optimum ~ ~7~
.
_ g _ level will vary according to the particular susceptor material selected, its size and shape. It has been found that *or aluminum flakes, the pre~erred amount is 20 to 70 weight % of the ~usceptor/matrix. The amount of ~usceptor/ma~rix applied to the substrate will generally range from about 2.5 to as high as 100 gfm2. This will lead to a dry coating thickness in the range of as low as 1 to as high as 75 micrometers.
The amount of susceptor/ matrix coating used will, of 10 course, vary with the end use of the packaging material. For applications where browning and crispening o~ a ~ood product is desired, e.g., cooking pizza, the amount of coating might be 50 to 75 g/m2.
For other applications where high temperatures and 15 rapid heat-up are desired, e.g., cooking popcorn, the amount might be 2.5 to 15 g/m2.
The composite of this invention can be made by a number of methods. In one method, the dielectric matrix is dissolved in any number of common organic 20 solvents such as tetrahydrofuran, methylene chloride, ethyl acetate, methyl ethyl ketone or similar solvents, and then the susceptor is dispersed in this ~olution. The solution is then applied to the carrier film or web by ~ny number of coating processes 6uch as 25 metered doctor roll coating, gravure coating, reverse roll coating or ~lot die coating. The solvent is driven off after application of the coating by conventional oven drying techniques. A second technique is useful for melt stable matrices. The 30 matrix material is melted in conventional eguipment and the susceptor particles blended with the melt.
This mixtuxe is then extrusion or melt coated on the ~ilm or web substrate. In either case, the application of the susceptor/matrix is a well 35 controlled process that can be readily altered to vary ~7~
, the temperature range of the composite material when used in a microwave ~ven. This contrul is ~uperior to that used in prior art vacuum metallizing processes and the coating process can operate at much higher 5 speeds since no vacuum is reguired. Conceptually, the susceptor/matrix can be applied in patterns that would allow a variety o~ temperature properties in a single sheet of composite material.
Ideally, packaging materials of the type disclosed herein should have reproducible heating performance. A consumer should be able to rely on a ~pecific mat~rial heating to a specific temperature range within a specific time frame whenevex exposed to microwaYe radiation in his microwave oven. In the absence of such reproducible heating performance, a packaging material would lac~ wide commercial utility.
To achieve heating performance reproducibility, it has been found that the susceptor coating should be uniform and isotropic. The term isotropic as used herein means that the composite with the susceptor/matrix coating will exhibit substantially the same properties (i.e., heat to substantially the same temperature) when exposed to the electric field component of microwave radiation in any direction. Tests indicate that an oblong flake of susceptor material capable of coupling with the electric field, for example, will couple better when the incident electric field is parallel to the flake's largest dimension. Therefore, the heat generated from an oblong flake will vary from a maximum when the incident electric field is parallel to the largest dimension to a minimum when the incident electric field is perpendicular to the largest dimension. If the susceptor/matrix coating is isotropic, then, regardless of the fact that the susceptor material is an oblong flake, the degree nf coupling of the susceptor material with the incident electric field, and, thus, the heat generated from the susceptor coating, will not vary ~ubstantially with the direction of the incldent electric field.
(For simplicity, this discussion is limited to ~usceptors which couple with the electric portion of the microwave field energy. Susceptors which couple with the magnetic portion of the microwave field energy are deemed to be equivalent, and the principles disclosed herein apply equally to the incident magnetic field in such cases.) A substantially isotropic coating can be achieved using oblong flakes of susceptor materials if at least two coating layers are provided, the direction of alignment of the flakes (i.e., the direction Df the longest surface dimension of the flakes) in one layer being oxiented at about ninety degrees to the direction of alignment of flakes in the second layer. To illustrate, when a coating of oblong flake susceptor/matrix is applied to the substrate, the flakes tend to be aligned lengthwise in one direction, e.g., the direction in which the coating was stroked onto the 6ubstrate. To achieve an isotropic coating, a second layer of coating is 6troked on in a direction perpendicular to the direction in which the first layer was applied.
Multiple successive cross-passes of coating may be applied in this manner. One possible way in which the multiple layers of coating may be applied to achieve isotropy i6 by 45 degree opposing gravure printing.
The preferred way to achieve a substantially isotropic coating i6 to use circular flakes of ~usceptor material. These flakes tend to be flatter and have smoother edges than other commercially ~ 12 -available flak~s and are substantially round; it is believed that their ellipticity (ratio of largest to smallest ~urface dimensions) is in the range of about 1:1 to 1:2, preferably about 1:1 to 1:1. 5 . This is in contrast to other commercially available aluminum flakes which are oblong, and generally have e~lipticities greater than 1:2, ~ometimes as high as 1:4. Circular aluminum flakes are available commercially from Kansai Paint Company, Hiratsuka, Japan, under the designations "Aluminum Y" and "Aluminum X". Cir~ular flakes will provide an isotropic coating 60 long as they are applied so as to be parallel to the ~ilm surface and in a manner which avoids fragmentation of the flakes which can lead not only to irregularly shaped ~lakes but also to their random agglomeration.
To achieve best results, the manner in which the susceptor/matrix is applied to the substrate has ~een found to be important. First, it has been found that the susceptor/matrix should be applied to the substrate in such a way that the plane of the large dimension of the flake is ~ubstantially parallel to the surface of the substrate. Second, the ~lakes ~hould be dispersed in the thermoplastic matrix so that they are ~ubstantially insulated from each other.
A number of factors can be controlled to achieve these goals. The selection of the flake susceptor material can greatly affect the ability to achie~e a uniform and isotropic coating with properly aligned flakes. Our work indicates that the smoother and flatter the flakes are, the easier they will be to disperse in the thermoplastic matrix, thus reducing agglomeration. The smaller the aspect ratio (largest dimension to thickness) of the flakes, the less mechanical damage the ~lakes will encounter during the ~ ~74~

coating process and, thus, the less fragmented debris, capable of agglomerating, will result. The circular flakes described above have ~any of these desired features, e.g., smooth ~dges, flat surfaces and low aspect ratioO
Apart from the selection of th2 flake susceptor itself, the manner in which the susceptor coating is ~pplied to th substrate plays a ~ajor role in achieving the flake orientation that will lead to heating performance reproducibility. While the susceptor thermoplastic matrix coating can be applied in a single coating layer, it has been found that the desired flake orientation can more easily be achieved by application of a plurality of thin, dilute coats Df the material. Each coating layer is applied from a dilute (e.g., about 15-35% total solids) dispersion of susceptor and matrix in solvent. The ideal amount of susceptor in the coating layers varies according to the susceptor material selected. Generally, it has been found that good results are achieved when coatings are used in which circular aluminum flakes comprise about 40-70% o~ the total solids (susceptor and th~rmoplastic matrix), or in which oblong aluminum flaXes comprise about 20-60% of the total solids, or in which non-aluminum flakes comprise about 10-40% of the total solids.
The susceptor/matrix coating can be applied in a single coating layer if coating methods which insure laminar flow are utilized, e.q., slot coating with a small gap and a long land length. When only a single coating layer is to be applied, a high solids dispersion of the susceptor/matrix should be used, and the amount o~ ~usceptor in the solids should also be high.

4~

As previ~usly mentioned, a uniform and isotrspic usceptor/matrix coating is desired because the heating performance of a composite so ooated will have superior reproducibility. For the purpose of measuring and quantifying heating performance reproduciblity, the following test can be used.

Test for Heatinq Performance Re~roducibility Six l-cm by 2-cm pieces taken from a sample composite are heated in a 2450 MHz microwave electric field of 243 V/cm. (This simulates the hot spot electric field in a typical 700 watt microwave oven.3 The samples are divided into two groups.
Samples in Group 1 are oriented so that the electric field is parallel to the longitudinal or machine direction (MD) of the sample, and samples in Group 2 are oriented so that the electric field is parallel to the cross or transverse direction (TD) of the sample. The temperature of each composite ~ample is measured after exposure to the microwave electric field ~or four minutes. The mean temperatures ~or ~ach group of samples as well as for all six samples taken as a whole are determined. With this test, the sample composite is deemed to possess heating performance reproducibility if:

(1) MD and ~D are each within Temp + 5%, (2~ Each MD temperature is within MD + 10%, 7~

and (3~ Each TD temperature is within TD + 10%;

S where ~D is the mean temperature for the samples of Group 1, TD is the mean temperature for thP
samples of ~roup 2, Temp is the mean temperature for all six ~amples, MD temperature is the temperature for any ~ample in Group 1, and TD temperature is the temperature for any sample in Group 2, all temperatures being in degrees Centigrade.

A non-resonant 2450 MHz wavequide system, ~uch as described below, can be used to obtain the 25 data required for the Reating Performance Reproducibility Test. The system comprifies a microwave generator feeding 254 watts through a microwave circulator into a 6ection of WR284 rectangular waveguide terminated with a shorting 30 plate. (WR284 is a rectangular waveguide with an interior cross-section of 7.2 cm. by 3.4 cm.). The reflected wave from the ~hort circuit establishes a 243 V/cm pure electric ~ield at the standing wave maxima in the waveguide section as long as the sample 35 perturbation is small and the reflected energy is - 15 ~

7~

dissipated by the matched termination connected to the third port of the microwave circulator before it can make a third pass through the sample assembly. Th~
microwave heating of the l-cm by 2-cm 6ample is measurPd by recording the temperature reading of a Luxtron Fluoroptic temperature probe which was s~ndwiched between the l-cm by 2~cm ilm sample and a 5 milimeter diamater Teflon tR) p~lytetrafluoroethylene (E.I. du Pont de Nemours and Co., Wilmington, Delaware) rod. The probe-film assembly is secured to the rod by a Teflon (R) polytetra1uoroethylene tape. The whole tape-film-probe-rod assembly is inserted through an aperture into the ~ample holder position in the waveguide, located at a distance of (n/4)(23.1 cm), where n is an odd integer, from the end of the end shorting plate.
(23.1 cm is the full wavelength.) A waveguide phase shifter and an electric field probe is used to shift the electric field maximum to the ~ample position.
The temperature versus time heating profile was recorded for each sample piece over a period of at least four minutes.
The composite materials of this invention are further illustrated by the following examples. In ~5 each of these examples, the surface D.C. resistances of the exemplified composite materials are greater than 1 x 10~ ohms per square. D.C. 6urface resistances can be measured by methods known in the art (e.g., ASTM D257-78) using conventional, commercially available instruments. All temperatures are in degrees Centigrade.
The samples prepared in Examples 1-8 and Comparative Example A were tested in a commercial microwave appliance rated at 550 watts at a fre~lency of 2450 megahertz. Tests in the microwave oven of the ~4~6 invention wer~ run both in the pr~sence and ~bsence of ~ood. Two types of temperature monitor~ were used.
One was a ~inglQ optioal pyrometer probe used wi~h a Vanzetti Optical Pyrometer. Thi~ non-cont~ct probe which i5 dependent on the ~mmissivity of the ~rticle who~e temperature ~ being ~easured. The ~econd temperature ~onitsr used was a Luxtron Fluoroptic four channel device with contact thermo probes. Temperature measurement6 made in the ~bsence of food were carried ou~ by ~uspending ~ ~wo-inch ~quare of test material (either the coated film or the coated film laminated to paper or paperboard) in the microwave oven in generally the geometri~ center of the cavity. The test ite~ i~ attached to the Luxtron thermo probe ~nd to a 6tring which enters the cavity ~rom ~ ~ole drilled throu~h the exterior cabinet and into the interior cavity. ~he ~tring $t~elf $s the ~uspending ~gent with the test item att~ched t~ it with a piece of non-lossy ~dhesive tape. Temperature is recorded at ~ifteen 6econd intervals over the course of 3 minutes and 15 ~econds. The oven is cooled to room temperature between tests.
~eE~
E~L
This example ~hows the heat gener~ting capabilities of the ~ombined ~etal flAke/dielectric ~atrix with 6upp~rt film compared to the ~upport film itself or the support film coated with the dielectric matrix but in the absence of the ~etal flake.
The matrix coating was prepared in the following manner. The matrix polymer, in this case 15.8 weight p~rt~ of the copolymer condensation product of 1.0 ~ol of ethylene glycol with 0.53 ~ol of terephthalic ~cid and 0.47 mol of azelaic ~cid was combined with 0.5 weight part6 of eruc~mide ~nd 58 7~

weight parts of tetrahydrofur~n in ~ heated glass reactor vessel equipped with paddle ~tirrer. After dissolution of th~ ~olid~ ~t 5S-C, 0.5 weight parts of ~agnesium ~ilicate and 25 weight parts of toluene were blended in. Finally 35 weight parts of dxy aluminum flaXe ~Alcoa Alumirlite*flake, grade 16633 W2S blended in. These flakes have a diame~er distribution of 1 to 48 micrometers (88% in the 4 to 24 micrometer range), a thickness in the 0.1 to 0.5 ~icrometer r~nqe, and a 6urface ~rea in the range of 1 to 15 ~2/gram.
A second ~atrix coating was prepared in the same fashion ~s that above except that no aluminum flake was added. ~ach of these coating dispersions were cast, in ~eparate experiments, on 12 ~icrometer thickness, biaxially oriented polyethylene terephthalate film to a wet coating thickness of 230 ~icrometers. The wet coated films were allowed to dry. The dry coating weight of the dispersion containing ~luminum Plake was 54 grams per square meter with the aluminum comprising 67% by weight of the dried coating. In the ~econd coatin~ dispersion without aluminum flake added, the dried coating weight was 19 grams per 6quare ~eter. Coating weiyht ~s determined by ~tripping the film of the dried coating and gravimetricnlly deter~ning unit weight of coated nnd stripped film. In these two cases the ~m~unt of copolymer matrix i~ approximately equal.
Samples of each coated film and an uncoated piece of the carrier film detailed above were cut to two-inch ~guare6. ~emperature measurements were carried out in the microwave oven as described earlier~ Results of the heating test are ~et out below in Table I.
*denotes trade mark .. .

-- 19 ..

Temp. ~-r) ~fter microwave exposure _ _or -Sample 30 sec. 60 sec. 90 ~ec. 195 sec.

Vncoated carrier film 56 65 68 77 Coated film without Aluminum Flake 58 65 70 78 Coated film with Aluminum Flake 190 213 87* --*Film has melted.
TABLE I (Continued) Total Coating Wet Coating Weight Weight Thickness Thermoplastic Sam~le a/m2_ ~micrometers~ ~atrix (q~m2 Uncoated carrier film -~
Coated film without Aluminum Flake 19 230 19 Coated film with Aluminum Flake 54 230 18**
2S **Thermoplastic matrix comprises 33% by weight of dry coating.

~4~

ExamPle 2 This example shows the effect of the amount of aluminum flake on a weight basis in the dried coating on the te~perature reached ~y the c~mposite of the aluminum/matrix coating on carrier film. Thi~
example also shows the effect of the total aluminum/
matrix unit weight on the carrier film on the temperature generated.
Dispersions of aluminum flake in the matrix binder dispersion were made in the same ~ashion from the same materials as given in Example 1. In three ~eparate experiments the coating dispersion without aluminum flak~ was prepared as in Example 1. To one dispersion 1.1 weight parts of aluminum flake was added. Likewise 5.6 weight parts of aluminum flake was added to the second dispersion and 11~2 weight parts to the third dispersion. Each of these dispersions were used to prepare coated film on 12 mil biaxially oriented polyethylene terephthalate as described in Example 1. Wet coating thicknesses at 100, lS0 and 200 ~icrometers were cast with coating knives from each of the aluminum ~lake dispersions above. Coated samples were allowed to dry. Each of the dispersions will give, on a dry ~olids basis, 10, 25 and 40 weight percent ~luminum flake, respectively.
Temperature measurements were carried out in the microwave oven as described in earlier. Results of these heating tests are ~et out below in Table II.

~AB~LE II
Weight % Wet Coatiny Temp. (-C) after microwave Al/CoatingThickness, exposure for -Weiqht a/m2 mm *0 sec.120 sec. 195 sec.
**1~/6100 75 78 81 **10/12 15~ 7~ 75 80 ~*10/17 200 79 85 89 - 10 25/16 lS0 89 91 94 40/9100 11~ 126 133 40/20 lS0 166 180 194a 40/25 200 181 191 201a a-Film shrin~s *Weight % Al, dry basis based on total Al/
thermoplastic matrix **Comparative examples ~ 2~
. ~
. - 22 -Example 3 This example shows that aluminum flake as a high ~olids paste in mineral ~pirits or high flash naptha, in the presence or absence of leafing agents, can be substituted for the dry aluminum ~lake used in Example 1 ~nd 2.
Copolymer matrix dispersions were prepared as described in Example 1. Successive aluminum flake coating dispersions were ~ade substantially as described in Example 1. In Test A 52.5 weight parts aluminum paste (Alcoa leafing paste grade 6205, 65 weight ~ non-volatiles in Rule 66 mineral ~pirits) was used. In Test B 52.1 weight parts of aluminum paste (Alcoa leafing paste grade HF905, 65.5 weight %
non-volatiles in high ~lash naptha) was used. In Test C 52.1 weight parts aluminum paste (Alcoa non-leafing paste grade HF925, 65.5 weight %~non-volatiles in high flash naptha) was used.
The above aluminum dispersions were used in coating of 12 micrometer thick biaxially oriented polyethylene terephthalate using a coating knife to give a wet coating thickness of 230 micrometers as described in Example 1.
Heating tests on the dried films were carried out in the microwave oven as described earlier with the results set forth below in Table III.

TAE~LE I I I
~Temperature ~-CL aft_ Coating Wt. _ exposure for - _ Test Jm215 sec.30 6ec.45 sec.~20 sec.
A 31 151 149 146 156a B 54 163 --b ~: 47 161 196 221 52 10 a-Film ignited b-Film arced 2Ind melted at 19 seconds c-Film melted ~7~
i~ .

~xample 4 This example illustrates that aluminum flake with different surface area, as expressed in covering range in sguare centimeters per gram of flake, can be substituted for that given in the first example.
Dispersions o~ the matrix copolymer were prepared as described in Example 1. Successive dispersions were then prepared as described in Example 1 using aluminum flake with differing cov~ring power.
Test A employed the very same dispersion as described in Example 1 using 35 weight parts of the Alcoa dedusted Aluminite flake grade 1663 with a covering range of 20,000 squarP centimeters per gram. Test B
employed 34.1 weight parts of aluminum flake (Alcoa 15 dedusted Aluminite flake grade 1651 with a covering range of 12,000 square centimeters per gram3. Test C
employed 47.7 weight parts of aluminum paste ~Alcoa leafing paste grade 6678, 71.5 weight % non-volatiles in Rule 66 mineral spirits and a covering range of 28,000 to 30,000 square centimeters per gram) was used.
These aluminum flake dispersions were cast on 12 micrometer khick biaxially oriented polyethylene terephthalate with a coating knife to give a 230 25 micrometer wet coating thickness as described in Example 1.
The dried films were tested in the microwave oven as described earlier and the results are set forth below in Table IV.

~ ~7~;26 ~ABLE IV
DryTemp. (-C) aEter microwave Coating Wt._exposure for ~
~ q/m~ _ 30 sec. 45 sec. 195 sec.
A 54 213 87a B 61 - b C 84 153 172 67a a-Film melted b-Ignited in 7 seconds ` ~2~L2~

Example 5 This example will illustrate the substitution o~ a higher softening point matrix copolymer for the copolymer described in Example 1.
A dispersion of the ~ame copolymer was prepared as described in Example 1 with the addition of 1.8 weight parts o~ a copolymer made by reacting 1.0 mol of ethylene glycol with 0.~5 ~ol of terephthalic acid and 0.45 ~ol of isophthalic acid.
lD To this mixed copolymer dispersion is added 5.6 weight parts of aluminum flake (Alcoa dedusted Aluminite flake grade 1663~ as described in Example 1.
This coating dispersion is cast on 12 micrometer biaxially oriented ethylene terephthalate film using a coating knife to ~chieve a 200 micrometer wet coating thickness as described in Example 1.
Testing of a dried example of this coated film is carried out in a microwave oven as described earlier. For comparison, a coated film sample with nearly the ~ame aluminum content, on a dry basis, as prepared in Example 2 was tested. The results are presented in Table V.

~7~

Weight % Temp . ( D C) after microwave Al/Dry exposure for -Copol,vmer Coati~a Wt. ~O ~ec. 120 sec~ 195 sec.
Single (Example 2) 25/21 g/m2 109 119 127 Mixed (Example 5) ?3/27 g/m2 118 131 139 xample 6 This example illustr~tes the use of a ~econdary ~uppoxt web to promot~ dimensional ~tability of the primary ~truc~ure of ~he invention ~ described in Example 1.
Samples of film ~oated with the aluminum flake/polyester cop~lymer dispersiGn ~s described in Example 1 or 2 ~s treated with an ~dhesive colution on t~e uncoate~ 6ide of ~aid 6tructure. The ~dhesive used was a 601ution of ~ ~isture curable, isocyanate ended copolyester (~orton Chemicals Adcote*76FS93, 3 weight parts of the adhesive diluted with ~ weight parts o~ ethyl acetate ~s recommended by the manufacturer) ~nd was applied by ~ typical laboratory aerosol ~pray device. The adhesive as applied was dried briefly with ~id of ~ ~ot air gun and then a 6uitably ~ized piece of bleached white paper ~160 micrometer thic~ness) applied with the aid of a rubber roller. The laminate was ~tored under a weighted glass plate ~r a minimum of 1~ hours prior to use.
The laminates as described above were tested in a microwave oven ~s described earlier. In these tests the ~uspension ~tring was attached to the paper 6ide of the laminate nnd the fiber optic probe to the coated side of the film. The results of these tests are presented below in Table VI.

* denotes trade mark

3~

d q' ~
~9 --TABLE VI
Weight %
Al/D~yWet Temp (-C) after microwave CoatingCoating . exposure for -5 Test Sample Wt. hickness* 60 sec. 120 sec. 195 sec.
Unlaminated (see40/25 Example 2) ~/m2 200 ~81191 201a L~minated 40/25 g/m2 200 167 178180 a-Film ~hrinks * micrometers ~;~7~

Example 7 This example and the following Example 8 will illustrate the utility of this invention in the preparation of foods in a microwave oven. These examples illustrate the range Df heat generating capability of the artirles of this invention in preparation of foods requiring additional heat to improve cooking food performance or to improve visual appearance or textural consiskency of the cooked food.
~ In this example, it will be shown that popping performance of commercial microwave popcorn packages can be improved by incorporation of the article of invention as part of the microwave popcorn packagev A laminate of the primary ~tructure as described in Example 1 and a paper 6econdary ~upport web as described in Example 6 are used. ~he primary 6tructure before lamination consisted of 40 weight %
of aluminum flake dispersed in the polyester c~polymer matrix (dry solids basis) and applied to 12 micrometer thick biaxially oriented polyethylene terephthalate at a wet cast thickness such as to achieve a dry coating weight of 11 grams per square meter of which 3.6 grams per square meter was aluminum flake. The dry coating weight was determined by gravimetric techniques wherein a convenient ~ized piece of the coated film is soaked in tetrahydrofuran until the coating is stripped. After rinsing with additional tetrahydrofuran, the stripped support film is oven dried and weighed. The aluminum flake composition o~
the coating is readily determined on the coated film either directly by x-ray fluorescence techniques or by pre-digestion of a ~ample in strong mineral acid ~ollowed by determination of aluminum using standard atomic absorption techniques.

~ commerci~l ~icrowave popcorn bag paper made ~rom a copolye~ter-coated polyethylene terephthalate laminate was altered for use in this test. A three by five inc~ reot~ngle o~ a laminate as described above was a~fixed to the bottom of the bag using a cy~noacrylate adhesive, The 6aid piece was affixed with the coated ~ide on the in~ide bottom of the bay and the paper ~ide upward. A 100 gram plug of the combined popcorn and oil from a purchased bag of microwave popcorn was tr~nsferred to the bag with heater pad ~ffixed. The 100 gram plug of popcorn and oil was found to contain 554 kernel~ of popcorn. The test bag was then 6ealed at its top opening using a bar sealer (at 125-C ~nd 35 kilopascal~ for ~ne ~econd).
The test bag and ~ control bag (commercial bag as described above) were then tested in t~e 550 watt microwave oven as described in Example 1. A
fiber optic probe for the Luxtron Fluoroptic*
thermometer described in Example 1 was inserted in the exterior bottom flap of the package so that the sensor end was located below the approximate geometric center of the test pad ~nd separated from ~t by ~ust ~ne layer of the bng. In thefie tests the bag (test or 25 control) was x~ised ~rom the met~l floor of the interior cavity of the ~icrowave ~ven with the ~se of ~n inverted paperbo~rd tray 15 centimeters ~quare by 3 centimeters in height (the tray is fabricated from unbleached, pressed, ovenable paperboard of 50 30 ~icrometer ~h$ckness).
The time for popping ~3 minutes, 15 6econds) was within the range recommended on the commercial package. Once each bag had been popped, the bags were cooled and opened. The bag content~ were poured into 35 a graduated 2~00 cubic centimeter beaker nnd its * denotes trade mark \

volume measured. ~he pGpp~d 2nd unpopped kernels are then separated ~nd a count ~ade of the unpopped kernels from which the percentage of the unpopped kernels out of the total content was calculated.
5 These results ~re set forth in Table VII.
TABLE VII
Pop Volume Count of ~ax. Temp. C (Cubic Unpopped %
Baa at ba~ bottom (Centimeters) kernels Unpopped 10 control 236 1875 15~ 29 Test 257 2000 143 26

4~%1E;

Example 8 In this example the utility of the invention in providing sufficient heat in a microwave oven to effect brownin~ and crispening of microwave pizza is illustrated.
An article of this invention as described in Example 1 is used to prepare a tray for cooking of commercially ~vailable microwaveable pizza. In this example the primaxy structure consisted of a 12 micrometer thickness film of biaxially oriented polyethylene terephthalate to which was applied, according to the description to Example 1, a dispersion of aluminum flake (Alcoa dedusted Aluminite flake grade 1651) in the polyester copolymer binder eolution as described in Examples 1 and 4, applied to a dry coating weight o~ 61 g~m2. The aluminum flake content of the liquid matrix dispersion is 67 weight %
on a dry solids basis and the wet coating thickness used was 230 micrometers. The coated side of the dried film was affixed to the top cide of an inverted paperboard tray using a cyanoacrylate adhesive. The 20 centimeter square by 3 centimeter height tray was constructed of pressed ovenable paperboard with a thickness of 50 micrometers.
A commercial microwaveable pizza (255 gram cheese pizza) was removed from its freezer package and centered on the tray described above. The tray with pizza was then placed on the floor of the 550 watt microwave oven described in Example 1 and cooked for two minutes. The top of the pizza was bubbling hot with aesthetically pleasing appearance judged from cheese melted but retaining its ~hredded appearance.
The bottom of the pizza crust immediately after removal from the microwave oven was dxy to the touch and had no visible moisture. The bottom crust was ~7~6 ~ 34 - _ browned with a few small areas beginning to show signs of charring which is the expected appearance of pizza crust. The crust was noticeably crisp when a knife was scraped across it and was definitely crisp when cut with the knife. A control pizza was cooked using the tray incorporated in a commercial package, a tray lined with lightly metallized polyethylene terephthalate film. It too gave atisfactory appearance ~f the top and crust but this was achieved only after the recommended cooking time of 3 minutes and 30 seconds.
Com~arative ExamPle This example illustrates the importance of the flake structure for optimum performance in terms ~f temperatures generated.
A copolymer dispersion is prepared as described in Example 1 using 11.2 weight parts of powdered aluminum (less than 75 micrometer particle size). This dispersion is cast on 12 micrometer thick biaxially oriented polyethylene terephthalate film with a coating knife to achieve a wet coating thickness of 200 micrometers as described in Example 1. .
The dried coated film was tested in A
microwave oven as described earlier. The test results, and the results for a comparable film in which aluminum flake was used as the susceptor material ~from Example 2) are set forth in Table A.

``\

TABLE A
Weight % Wet Coating Temp ( ^ C~ after microwave Al/Dry thk., _ exPosure for - _ ~ Coatinq Wt Micrc~meters 60 sec.120. sec. 195 sec.
Powdered 40/28 g/m2 200 78 84 90 Flake 40/25 g~m2 200 181 191 ~Ola a Film ~hrunk ~0 ~5 ~.274~
.

Exam~les 9-27 Numerous film samples were prepared to investigate the factors important for providing reproducible heating performance. Each of the samples listed in Table VIII was prepared by hand-coating polyethylQne terephthalate film with ~ doctor-knife type draw bar with a coating of aluminum flake in a polyester copolymer matrix as used in Example 1. The types of ~luminum flake used were as follows:

C-l: circular flake, average diameter of 10 microns, ~Aluminum X", available from Kansai Paint Company, Hiratsuka, Japan C-2: circular flake, average diameter of 20 microns, ~Aluminum Y~, available from Xansai Paint ICompany, Hiratsuka, Japan E-l: oblong flake, average diameter of 35 microns, '~OBP-8410", available from Obron Corporation, Painesville, Ohio E-2 : oblong flakef average diameter of 2-5 microns, ~'L-875-AR", available from Silberline Manufacturing Company, Lansford, Pennsylvania Circular flakes C-l and C-2 were flatter and had smoother edges than oblong flakes E-1 and E-2.

Six l-cm by 2-cm pieces taken from each coated film ~ample were heated in a microwave electric field of 243 V/cm, using the procedure described previously, three with the electric field parallel to the machine direction of the film, and the other three with the electric field parallel to the transverse direction of the film. (Films were hand coated in the machine direction of the film.) The temperature of the film was ~easured over a period of about ~ive ~inutes. Mean temperature data are presented in Table VIII which also indicates whether the samples passed the Heating Performance Reproducib.ility Test ~et forth previously~

., -~ABLE VI I I
Wet # of ~6 Al of Elake . ThicknessCoatirJg Dry Ex. TYPerMIL) * Passes Coat g C-2 2 3 20 11 C-l 2 3 60 14 ~ 1 20 C-l 6 ~ 60 1016 ~-2 2 3 33 18 C-l 6 1 33 19 C-l 2 3 2 0 24 ~-2 2 3 33 E-l 2 3 33 2~ C-l 2 3 33 ~Per layer of coating -- 3~ --: ~7~2~
-- 3~ --TABLE VIII [Continued~
Passes Heating Perfor-Ex. 4'_MD 4' TD ~ ance Reproducibility ~est?
943.541.7 42.6 Yes 1043.343.6 43.4 Yes ll233,6226.6230.1 Ye~
12215.9207.12~1.5 Yes 1353.657.6 55.6 Yes 1445.044.4 44.7 No 15213.0170.8l91.9 No 16184.6168.8176.7 No 10 17205.4194~2199.8 No 1859.468. t) 63.7 No 1951.446.4 48.9 No 20*190.0184.1187.1 No 2181.394.4 87.8 No 22129.2119.0124.1 No 23141.6130.2135.9 No 2473. 264.9 69.1 No 15 25219.3175.3197.4 No 26105.9125.7115.8 No 2798.6133.1 115.9 No 4'MD - Mean temperature of MD samples at 4 ~inutes 4'TD - Mean temperature of TD samples at 4 minutes 20 4~ Temp - Mean temperature of all samples at 4 minutes *3 minute MD, TD, Temp values used for this experiment.

. -- 39 --~ 2~4~

These data show that, in general, the coatings of the two circular flakes, C-l and C-2, produce substantially less variation in temperature when exposed to external E-field of a widely varying polari2ation angle than coatings of tha two oblong flakes. As a result, the films coated with the circular flakes have ~uperior temperature reproducibility.
To compare data for films attaining temperatures above 190 degrees C after four minutes, one may review Examples 11, 12 and 25. Figures 1 and 2 graphically present the temperature data obtained for the films in respective Examples 11 and 12, bot~
films coated with circular flakes which pass the Heating Performance Reproducibility Test. In contrast, Figure 3 presents the temperature data for the film in Example 25, one coated with oblong flakes which failed the Heating Performance Reproducibility Test. Temperature vs. time data for each of the six pieces of film in each example are presented in the figures. nE//MD~ indicates that the piece was heated in the microwave electric field with the electric fîeld parallel to the machine direction of the film;
~E//TDn indicates that the piece was oriented with the electric ~ield parallel to the transverse direction of the ~ilm. rrhe figures ~how that for the film of Example 25, in which an oblong aluminum flake material was used as ~usceptor material, the temperature of the six pieces after four minutes exposure to a microwave electric field of 243 V/cm varied by as much as 90 degrees C. By comparison, Figures 2 and 3 show that for the films of Examples 11 ~nd 12, in which a circular ~luminum flake material was used as susceptor ~ 40 -.. ~ 2~

material, the temperature o~ the ~ix pieces after four minutes varied by no more than about 25 degrees C.

1~

. - 41 -~'~7~6 ExamPles 28- 32 This set of examples ~h~w the improvement which can be obtained in the temperature repr~ducibility o~ a film coated with oblong ~lake ~usceptor ~aterial when the material is applied in a ~anner to produce a ~ubstantially isotropic coating.
The susceptor ~aterial utilized in this example is a ~oncircular aluminum flake, designated ~Reynolds LSB-548, available from Reynolds Aluminum Company, Louisville, Kentucky. The matrix was prepared as in Claim 1. Samples of PET ~ilm were hand-coated with the ~usceptor/matrix coating, the first coating being applied in the machine direction, the ~econd coating lS being appliced in the tr~nsverse direction, and subsequent coatings being applied alternately in the MD and the TD. Six pieces of each film sample were exposed to a microwave electric field of 24~ V/cm for four minutes, three with the electric field parallel to MD, and the other three with the electric field parallel to TD. The averaye temperatures for each ~ample, MD and TD, are presented in Table IX.

- ~3 -~ABLE_IX
# Coating Dry Coating Al in Dry Passes . Thickness Coating _ _ Ex ~ ~D mils %~2 4'MD 4'TD
28 4 0 1.3-1.5 2~10.079.255.7 29 5 0 1.6-1.7 2011.8104.271.7 30 6 0 1.7-1.9 2012.999.795.3 31 8 0 2.4-2.S 2017.51~3.5121.6 32 2 2 1.4-1.6 2010.7~8.9~0.0 33 3 2 2.3-3.1 2019.3147.4154.0 34 3 3 2.5-~.8 2019.~157.8159.7 10 35 4 4 3.3_3.4 2024.0162.3160.~
36 1 0 0.2 403.346.7 56.3 37 1 1 0.6-~.7 4010.7128.7131.3 38 2 2 1.4-1.7 4025.5162.0167.7 39 4 4 2.4-2.7 4042.0157.01~4.7 .

~74~

- ~4 -These data ~h~w that by increasing the isotropy of the coating (by applying layer(6) in which the alignment of ~lakes is oriented about ninety degrees to the alignment of flakes in another layer(s), ~s in Examples 32-35 and 37-39~, t~e temperature reproducibility ~f the coated film was i~proved.

Claims (28)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A composite material for controlled generation of heat by absorption of microwave energy during microwave cooking comprising (a) a dielectric substrate substantially transparent to microwave radiation, and (b) a coating on at least one surface of the substrate comprising (i) about 5 to 80% by weight of metal or metal alloy susceptor in flake form, and (ii) about 95 to 20% by weight of a thermoplastic dielectric matrix, wherein the surface weight of said coating on the substrate is in the range of about 2.5 to 100 g/m2.

2. A composite of Claim 1 where the coating contains about 25 to 80% by weight of metal or metal alloy susceptor and about 75 to 20% by weight of a thermoplastic dielectric matrix, and wherein the D.C.
surface resistance of the resulting composite material is at least 10 x 106 ohms per square.

3. A composite of Claim 1 or claim 2 where the thermoplastic dielectric matrix is selected from the group consisting of copolymers of ethylene glycol, terephthalic acid and azelaic acid, copolymers of ethylene glycol, terephthalic acid and isophthalic acid, or mixtures of said copolymers.

4. A composite of Claim 1 or Claims 2 where the susceptor is aluminum.

5. A composite of Claim 1 where the dielectric substrate is polyethylene terephthalate film, and the coating on at least one surface thereof comprises 30 to 60% by weight aluminum flake and 70 to 40% by weight of a copolymer of ethylene glycol with terephthalic acid and either isophthalic acid or azelaic acid or mixture of such copolymers.

6. A packaging material comprising a composite of Claim 1 laminated to a second dielectric substrate substantially transparent to microwave radiation.

7. A packaging material of Claim 6 where the second dielectric substrate is a polyester film or paper.

8. A packaging material comprising a composite of Claim 2 laminated to a second dielectric substrate substantially transparent to microwave radiation.

9. A packaging material of Claim 8 where the second dielectric substrate is a polyester film or paper.

10. A composite of Claim 1 or 2 capable of heating to a temperature of about 150°C or higher when subjected to microwave energy of 550 watts at 2450 megahertz for a period of 120 seconds.

11. A composite of Claim 1 or 2 capable of heating to a temperature of about 190°C or higher when subjected to microwave energy of 550 watts at 2450 megahertz for a period of 120 seconds.

12. A composite of Claim 1 where the susceptor comprises a circular flake having an ellipticity in the range of about 1:1 to 1:2.

13. A composite of Claim 12 where the susceptor comprises an aluminum flake.

14. A composite of Claim 13 where the susceptor comprises about 40 to 70% by weight of the coating.

15. A composite of Claim 1 where the susceptor comprises an oblong flake having an ellipticity greater than 1:2.

16. A composite of Claim 15 where the susceptor comprises an aluminum flake.

17. A composite of Claim 16 where the susceptor comprises about 20 to 60% by weight of the coating.

18. A composite of Claim 1 where the coating comprises at least two layers and the direction of alignment of susceptor flakes in at least one of said layers is oriented at about ninety degrees to the direction of alignment of susceptor flakes in at least one other of said layers.

19. A composite of Claim 18 where the susceptor is an oblong flake having an ellipticity greater than 1:2.

20. A composite of Claim 1, samples of which when exposed to a microwave electric field of 243 V/cm for four minutes, said electric field parallel to the longitudinal direction of the composite in half of said samples and said electric field parallel to the cross direction of the composite in half of said samples, meet the following requirements:

(1) MD and TD are each within Temp ? 5%;
(2) Each MD Temperature is within MD ? 10%, and (3) Each TD Temperature is within TD ? 10%, where MD Temperature is the temperature for any sample exposed with said electric field direction parallel to the longitudinal direction of the composite and MD is the mean temperature of all of such samples; TD
Temperature is the temperature for any sample exposed with said electric field direction parallel to the cross direction of the composite and TD is the mean temperature of all of such samples; and Temp is the means of all MD Temperatures and TD Temperatures, all temperatures being in Centigrade and measured after four minutes exposure to the microwave electric field.

21. A method for making a composite of Claim 1 comprising applying a plurality of thin, dilute coats of a dispersion of susceptor and thermoplastic matrix in a suitable solvent to the dielectric substrate.

22. The method of Claim 21 in which said thin, dilute coats are applied in a manner so that the direction of alignment of susceptor flakes in at least one said coat is oriented at about ninety degrees so the direction of alignment of susceptor flakes in at least one other of said coats.

23. A composite material for controlled generation of heat by absorption of microwave energy comprising (a) a dielectric substrate substantially transparent to microwave radiation, and (b) a coating on at least one surface of the substrate comprising (i) about 5 to 80% by weight of metal or metal alloy susceptor in flake form, and (ii) about 95 to 20% by weight of a thermoplastic dielectric matrix, wherein the surface weight of said coating on the substrate is in the range of about 2.5 to 100 g/m2, said composite being made by the method of Claim 22.

24. A composite of Claim 12 or Claim 15 capable of heating to a temperature of about 150 degrees C or higher when subjected to microwave energy of 550 watts at 2450 Mhz for a period of 120 seconds.

25. A composite of Claim 18 or Claim 20 capable of heating to a temperature of about 150 degrees C or higher when subjected to microwave energy of 550 watts at 2450 Mhz for a period of 120 seconds.

26. A composite of Claim 12 or Claim 15 capable of heating to a temperature of about 190 degrees C or higher when subjected to microwave energy of 550 watts at 2450 Mhz for a period of 120 seconds.

27. A composite of Claim 18 or Claim 20 capable of heating to a temperature of about 190 degrees C or higher when subjected to microwave energy of 550 watts at 2450 Mhz for a period of 120 seconds.

28. A composite of Claim 1 or Claim 2 in which the flakes are dispersed in the thermoplastic matrix so that said flakes are substantially insulated from each other.