Microwave energy susceptible conformable laminate packaging materials

A laminated wrap for packaging articles of food requiring browning and crispening and a degree of shielding during microwave cooking is disclosed. The wrap comprises a layer of flexible, heat resistant, microwave transparent plastic film, a layer of flexible, heat resistant, heat stable, microwave transparent film, and a layer of substantially continuous microwave susceptor material located on an interior surface of a film of the laminate.

This invention relates to packaging materials and structures used in 
microwave cooking, and specifically to microwaveable packaging of food 
items which require surface browning and/or crispening during cooking. 
There has been much interest recently in packaging materials which aid in 
browning and crispening of food items in a microwave oven. U.S. Pat. No. 
4,267,420, Brastad, discloses a food item wrapped with plastic film having 
a very thin coating thereon. An additional sheet or film of plastic is 
optionally laminated to the coating for abrasion protection. Other 
exterior support by more rigid dielectric materials such as paperboard and 
the like is also disclosed. The coating converts some of the microwave 
energy into heat which is transmitted directly to the surface portion of 
the food so that a browning and/or crispening is achieved. 
U.S. Pat. No. 4,641,005, Seiferth, discloses a disposable food receptacle 
for use in microwave cooking, which includes a provision to brown the 
exterior of the food in the receptacle. A thin layer of an electrically 
conductive material is incorporated into the receptacle on the food 
contacting surfaces thereof, so that the conductive layer will become 
heated by the microwave radiation and will, in turn, brown the exterior of 
the food in the receptacle. The receptacle includes a smooth surfaced 
plastic film, as a protective layer, and a support means formed of paper 
stock material. 
U.S. Pat. No. 4,713,510, Quick et al., discloses a microwave ovenable 
package including a layer of material that will convert a portion of the 
microwave energy to heat and a layer of paperboard interposed between the 
energy-converting layer and the food. The energy-converting layer may be 
carried on a plastic film, and an additional layer of paperboard may be 
used to sandwich the energy-converting layer and the plastic film between 
layers of paperboard. For the purpose of providing a more intense heating 
effect, two energy-converting layers, each on a dielectric substrate, 
sandwiched together between layers of paperboard, are disclosed. 
Laminates of plastic films with thick layers of vacuum deposited metal are 
also known as packaging materials. For Example, U.S. Pat. No. 4,559,266, 
Misasa et al., discloses a laminated material comprising (A) a layer 
composed mainly of polyolefin, (B) a layer composed mainly of, e.g., 
polyester resin, (C), a metal-vacuum deposited layer, and (D) a layer 
composed mainly of a transparent thermoplastic resin. This laminated 
material is used for its superior gas barrier properties and light 
shielding properties, etc. Such laminates, in order to provide significant 
gas barrier properties for packaging applications, require deposition of 
metal (typically aluminum) in sufficient amounts to impart optical 
densities of greater than 1.0, typically at least 4.0. Such materials are 
substantially opaque and have light shielding properties, but are not 
suited for use for microwave heating applications, for which much lower 
optical densities are required. 
In order to properly brown or crispen foods which are irregular in shape or 
which have nonplanar surfaces, it is desirable to have a packaging 
material which is readily conformable to the food. It is also desirable 
that the material supply enough heat energy to the surface of the food, 
and provide some degree of microwave shielding for the interior of the 
food so that the surface can be properly browned or crispened in a short 
time without the interior becoming overcooked. The present invention 
provides a laminar film which conforms closely to the shape of a food item 
during cooking, provides a high degree of heat to the surface of the food, 
and provides shielding to the interior portion. 
SUMMARY OF THE INVENTION 
The present invention provides a conformable multilayer laminated structure 
useful for packaging food for microwave cooking, comprising: 
(a) at least one layer of flexible, microwave transparent plastic film able 
to withstand a temperature of about 220.degree. C. without melting or 
degrading; 
(b) at least one layer of flexible microwave transparent plastic film able 
to withstand a temperature of about 220.degree. C. without melting or 
degrading, which film further exhibits at most about 2% shrinkage when 
exposed for 30 minutes to a temperature of 150.degree. C.; and 
(c) at least one layer of substantially continuous microwave susceptor 
material located on a surface of a film of the laminate, said layer being 
located between film (a) on one side and film (b) on the other side, and 
being present in sufficient thickness to cause the multilayer structure to 
heat under microwave cooking conditions to a temperature suitable for 
browning or crispening of food placed adjacent thereto. 
The present invention also provides packages suitable for containing food 
and browning or crispening of the surface of food in a microwave oven 
prepared from such film.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention consists of a multilayered structure having at least 
two layers of heat resistant microwave transparent plastic film, and at 
least one substantially continuous layer of microwave susceptive material. 
This microwave susceptive material is coated on at least one of the 
interior surfaces or interfaces formed between the plastic films of the 
laminate. 
The susceptor material is a substantially continuous electrically 
conductive material which is present in sufficient thickness to cause the 
multilayer structure to heat under microwave cooking conditions to a 
temperature suitable for browning or crispening of food placed adjacent 
thereto, but not so thick as to completely prevent penetration of 
microwave energy to the interior of the food. The term "substantially 
continuous" is meant to refer to materials which exhibit measurable 
conductivity, typically having a surface resistance in a range of 200-2000 
ohms/square. This is in contrast to materials in which the susceptor is 
particulate matter in the form of e.g. flakes, which may be dispersed in 
an insulating matrix. Such materials typically exhibit a surface 
resistance of at least 10,000 ohms/square and often in excess of 10.sup.5, 
10.sup.6, or even 10.sup.8 ohms/square. A preferred susceptor material is 
vacuum metallized aluminum, which will preferably be present in sufficient 
amounts to impart an optical density of about 0.10 to about 0.35, 
preferably 0.16 to about 0.22, to the film. Stainless steel having a 
surface resistivity of about 200 to about 2000 ohms/square is also 
preferred. Other metals, of course, may be used, including gold, silver, 
mu-metal, nickel, antimony, copper, molybdenum, bronze, iron, tin, and 
zinc. Although the optical density may vary with the metal used, the test 
to determine the correct amount of coating is whether the coating will 
heat to the proper temperature and provide sufficient heat flux for 
browning or crispening of food items. The required temperature may depend 
on the particular food item used, but for many applications, it is at 
least about 180.degree. C. The thin layer of susceptor material is 
preferably imparted to the film surface before lamination by vacuum 
metallizing. Other methods may also be used if they provide a 
substantially continuous layer of the desired thickness. 
The layer of susceptor material is coated onto at least one surface of heat 
resistant, microwave transparent plastic film. This film may be made from 
any suitable plastic film which has the desired properties of heat 
resistance and microwave transparency. The term "heat resistant" refers to 
the ability of the film to withstand the temperatures generated in a 700 
watt microwave oven during cooking without melting or degrading. When the 
film is made into the laminate of the present invention, as will be shown, 
temperatures of up to about 220.degree. C. or more may be encountered 
under microwave cooking conditions, so the film should maintain its 
integrity at such temperatures. Thus a heat-stable film may be defined as 
one which does not melt or degrade when exposed to temperatures of about 
200.degree. C.; preferred films will similarly not melt or degrade when 
exposed to temperatures of 230.degree. C., 260.degree. C., or even higher. 
Certain polyesters, such as polyethylene terephthalate (PET), having a 
melting point of about 260.degree. C., are particularly suitable for this 
purpose. Other suitable materials may include certain types of polyesters, 
polyamides, cellophane, cellulose triacetate, ethylene 
chlorotrifluoroethylene copolymers, fluorinated polyethylene, polyimides, 
polysulfones, polyvinyl alcohol polymers, polyetheretherketones, 
polytetrafluoroethylene, and others. 
The layer of film on which the thin layer of microwave susceptor is located 
should have at least a moderate degree of resistance to deformation or 
shrinkage upon heating. Ordinary PET film (as opposed to shrink film) is 
suitable for use as this layer because it shrinks only about 1.5 to 3% at 
150.degree. C. over a period of thirty minutes. However, ordinary PET by 
itself does tend to shrink more than this amount under the intense heat 
encountered in a microwave oven when the film is coated with susceptor 
material. Excessive shrinkage observed under such conditions, of up to 
60%, is not desirable because many foods, such as breads, can be crushed 
by the action of a such a film. Furthermore, excessive shrinkage tends to 
disrupt continuity of the layer of susceptor material, thereby degrading 
the heating properties. 
In order to provide a controlled amount of shrinkage, this invention 
provides that at least one layer of film in the laminate should be a heat 
stable film. The term "heat stable" refers to film which exhibits only a 
minimal amount of shrinkage at elevated temperatures. For example, heat 
stabilized PET typically shrinks less than 0.6% in thirty minutes at 
150.degree. C. This is in contrast to ordinary PET film which, as 
indicated above, shrinks about 1.5 to about 3% under such conditions. Thus 
heat stabilized PET may be defined as PET which is treated so as to shrink 
less than about 2% when heated to 150.degree. C. for thirty minutes. 
Preferably it will shrink less than about 1.5% or 1% when so heated, and 
most preferably about 0.6% or less. Heat stabilized PET is made from a 
regular grade of PET film by a stabilization process involving a series of 
heat treatment and relaxation steps, and is well known to those skilled in 
the art. A heat stabilization process for PET is more fully described in 
Bulletin E-50542, "Thermal Stabilization of Mylar.RTM.," from E. I. Du 
Pont de Nemours and Company. Heat stable films, of course, may include 
films other than heat stabilized PET, including those listed above, 
provided such films have the desirable properties of minimal shrinkage 
under microwave cooking conditions. 
One other such heat stable film which is particularly useful in the present 
invention is cellophane. Cellophane is a non-fibrous, material derived 
from cellulose. It does not melt even at temperatures well in excess of 
260.degree. C., although at such high temperatures some yellowing may 
occur. The shrinkage of cellophane at high temperatures is also minimal 
and is believed to be due to evolution of absorbed water. Thus it is known 
that plain, uncoated cellophane in a dry state (7% moisture), upon further 
heating, may shrink about 0.4-0.7% in the machine direction and about 
0.1-0.3% in the transverse direction. Commercial grades of cellophane, 
however, often contain glycerine or propylene glycol as a softener, and 
such materials may result in additional retention of water, particularly 
when the film has been equilibrated in a humid environment. In such cases 
the shrinkage of a film of cellophane upon heating is somewhat greater. 
When the moisture level in a cellophane film is reduced from that 
characteristic of 35% relative humidity to that characteristic of 5% 
relative humidity, the resulting shrinkage is about 1-1.25% in the machine 
direction and about 0.7 to 1.0% in the transverse direction. Starting from 
a higher relative humidity would result in a somewhat larger shrinkage, 
e.g, from 60% R. H. the shrinkages would be about double the above 
numbers. Thus the characteristic shrinkage of cellophane upon heating, for 
purposes of the present invention, is defined as that measured on a film 
in its dry state, i.e., containing about 7% moisture or less. 
In its simplest form, the invention contemplates use of a single layer of 
heat stable film in combination with a single layer of ordinary heat 
resistant film. These layers provide not only a support or substrate for 
the susceptor but also serve to protect the layer of susceptor and to 
provide a heat sink to prevent overheating of the film. The laminate 
results in improved dimensional stability and improved heating 
characteristics of the laminate, as described below. The thickness of the 
layer or layers of the heat stable film is not precisely limited. However, 
the thickness of the heat stable film should be sufficient to impart a 
degree of dimensional stability to the laminate. It is desired for the 
reasons discussed above that the shrinkage of the laminate, in any 
direction, be limited and controlled, preferably no greater than about 20 
percent upon exposure to the energy of a microwave oven. It is more 
preferred that the shrinkage obtained will be no greater than about 15 
percent, and a shrinkage of about 10 percent is most desirable. It has 
been found that for laminates comprising a single layer of PET, 0.013 mm 
thick, a single layer of heat stabilized PET about 0.013 mm thick is 
adequate to provide this limited degree of shrinkage. It is preferred, 
however, that at least two layers of heat stable film be used, one layer 
on either side of the thin layer of microwave susceptor material. 
Laminates prepared using heat stabilized PET provide superior results 
compared with known laminates in which the outer layers are paper or 
paperboard. Laminates with paperboard do not generally exhibit the 
improved heating properties found in the present invention, and are not 
conformable, transparent, or particularly flexible. 
Other layers may be present in the laminate for particular purposes. For 
example, the susceptor layer can be deposited on a layer of ordinary PET 
film, which in turn is laminated between two layers of heat stabilized 
PET, one on either side of the layer of ordinary (heat resistant) PET. In 
addition, one or more adhesive layers may be used to bond the layers of 
the laminate together. The adhesive may be any known to those skilled in 
the art to be suitable for such purposes. For some applications the 
adhesive layer is preferably a thin layer of an ethylene/carboxylic acid 
copolymer, which may, if desired, be crosslinked after formation of the 
multiple layer structure, by ionizing radiation. This feature is described 
in more detail in copending U.S. application 07/388,923, filed Aug. 3, 
1989, the disclosure of which is incorporated herein by reference. In 
addition, one or more layers of a heat sealable thermoplastic resin may be 
used as surface layers, overlying the heat stable plastic layers. Such 
layers may be useful for sealing the film to itself or to other items, for 
packaging applications. Suitable heat sealable thermoplastic resins are 
known in the art; particularly suitable resins are polyesters selected 
from the group consisting of copolymers of ethylene glycol, terephthalic 
acid, and azelaic acid; copolymers of ethylene glycol, terephthalic acid, 
and isophthalic acid; and mixtures of these copolymers. 
FIG. 1 shows a simple embodiment of a laminate of this invention. There are 
two layers of flexible, heat resistant, microwave transparent film, 10 and 
12, at least one of which is heat stable film. A layer of susceptor 
material, 14, such as vacuum metallized aluminum, is located on an 
interior surface of film 12. There is also an adhesive layer, 16, used to 
join the layers of film together in the laminate. 
FIG. 2 shows an alternative embodiment of this invention. The top layer, 
18, is a layer of heat sealable resin, which overlies a layer of flexible, 
heat resistant, microwave transparent plastic film, 20. Layers 22 and 24 
are adhesive resins. Under the first layer of plastic film 20 is a central 
layer of film, 26, with a coating of susceptor material, 28. Laminated to 
the other surface of the central film is another outer layer of plastic 
film 30. Either film layer 20 or 30, or preferably both, are heat stable. 
The central layer, 26, may also be of heat stable film, if desired, 
although this is not required. 
FIG. 3 shows another embodiment, in which there are two internal susceptor 
layers, 32 and 34, located on two adjacent layers of plastic film, 36 and 
38. The layers of plastic film which carry the susceptor material are 
laminated front to back, so that the susceptor layers are separated by the 
thickness of one layer of plastic film. The upper layer of susceptor 
material is protected by an outer layer of plastic film 40 which overlies 
it. The layers of film are held together by adhesive layers 42 and 44. The 
upper layer of film, furthermore, is overlain by a layer of heat sealable 
resin, 46. At least one layer 36, 38, or 40 is heat stable film, and 
preferably at least the two outer layers, 38 and 40 are heat stable film. 
The embodiment illustrated by this figure provides additional heating 
ability, and also additional microwave shielding, compared with that of 
FIG. 2, because of the additional layer of susceptor material. The 
presence of multiple layers of susceptor material results in an increased 
optical density, when the susceptor material is, for example, aluminum. 
The heating ability of such multiple layer heaters is improved somewhat 
when the layers of susceptor material are separated from each other as 
shown, i.e., when the films on which they are carried are not laminated 
together face to face. It is also permissible, therefore, when multiple 
layers of susceptor material are used, that one layer be located on each 
of the two surfaces of the central layer of plastic film. Alternatively, 
the susceptor material may be on the interior surfaces of the outer layers 
of film. In such arrangements both layers of susceptor material are 
covered by an outer layer of film and remain separated by the thickness of 
the central layer of film. It is apparent that many similar arrangements 
of film and susceptor layers are permissible within the scope of this 
invention. 
The multilayer laminates of this invention have several advantages over 
monolayer metallized films. The first advantage is that these laminates 
provide a significantly higher heat flux than do the single layer, 
susceptor coated films. This is important because heat flux is a measure 
of the ability of the laminate to rapidly heat the surface of the food 
with which it is in contact. The reasons for this improvement are not 
clearly understood. It is believed, however, that the additional layers of 
film provide additional support and protection during heating for the 
susceptor layer. This support, then, prevents "mud cracking," or breaking 
up of the susceptor layer into small islands upon exposure to microwave 
radiation. Such cracking of the susceptor layer is believed to reduce the 
efficiency of the heating. This improvement in heating efficiency is 
illustrated in the examples which follow. 
Another advantage is observed when films of the present invention are used 
to heat food items. It has been found that the interior of food items 
heats somewhat more slowly when the food is wrapped in films of the 
present invention, compared with single layer films. Thus the films 
provide an added measure of shielding to the interior of the food, further 
aiding the cooking by preventing overcooking of the food interior while 
the surface is being browned and crispened. 
Yet another advantage of having the susceptor layer buried between other 
film layers is that the cover layer protects the film from oxidation or 
corrosion by the air. When a thin layer of aluminum on PET film was 
exposed to the air for seven months, the optical density was observed to 
decreases from an initial value of 0.23 to 0.18. Another sample, held at 
38.degree. C. and 90% relative humidity for 14 days decreased in optical 
density from 0.17 to 0.15, and developed streaks, while a comparable film 
laminated to a film of PET remained unchanged. 
One of the most significant advantages of the present invention is that the 
presence of at least one layer of a heat stable film such as heat 
stabilized PET, provides a laminate which exhibits controlled shrinkage. 
Although the PET used is not a shrink film, single layers of microwave 
susceptor films under microwave conditions shrink and degrade or form 
holes. Even multiple layer structures may show excessive shrinkage, of up 
to about 60%, under microwave conditions, if at least one heat stable film 
layer is not used. When heat stabilized PET is used as at least one layer, 
and preferably two outer layers, however, the shrinkage is controlled to 
about 10%. This level of shrinkage is necessary for many applications, in 
order to permit the film to retain a snug configuration about the item of 
food to be cooked. Thus packages made of the laminates of the present 
invention will shrink just enough to achieve or maintain conformity about 
the food item, without crushing it. Such behavior is particularly 
desirable for packaging of such fragile items as bread. The conformity 
with the surface of the food item permits efficient transfer of heat 
energy from the film to the food surface, for efficient browning or 
crispening of the food. 
The controlled shrinkage, furthermore, results in structures which do not 
exhibit large scale nonuniform deterioration, such as holes, developing 
upon heating in a microwave oven. Such deteriorations or other distortions 
are observed when single layer, susceptor coated films that are not heat 
stable are used for wrapping foods for microwave cooking. These 
deteriorations prevent the film from conforming to the shape of the food, 
and thus prevent uniform cooking. 
It is also possible to take advantage of the limited shrinkage of the 
laminates prepared with heat stabilized PET in a different manner. FIG. 4 
shows a sheet of such film 48 containing slits 50 piercing through the 
thickness of the film. These slits may be covered by a label affixed by 
pressure sensitive adhesive (not shown) during storage, which is removed 
before cooking. Upon microwave treatment, the slits open to form the 
openings 52 shown in FIG. 5. A laminate containing one or more such 
openings may be useful for venting of vapor from the food item during 
cooking. 
COMATIVE EXAMPLES C1-C3 
Comparative Examples C1-C3, reported in Table I, show some of the 
properties of single layers of PET, not heat stabilized, metallized with 
aluminum. Films B, C, and D are films of polyethylene terephthalate, 0.013 
mm thick (0.5 mils, 50 gauge) which have been vacuum metallized with 
aluminum to the optical density indicated. 
The shrinkage of these film upon exposure to a 700 W microwave field for 10 
seconds is also reported in Table I. For such measurements, A strip of 
film, 127 mm.times.25 mm, is clamped in a holder assembly made of 
polytetrafluoroethylene. One end of the film is attached to a clamp 
rigidly mounted onto the base of the holder assembly. The other end of the 
film is attached to a clamp which mounted on the base. As the film heats, 
shrinkage of the film moves the slidably mounted clamp along a scale so 
that the relative amount of shrinkage can be measured by comparing the 
position of the clamp before and after heating. Tests are reported of 
samples in which the long dimension is in the machine direction as well as 
in the transverse direction. During such tests, these single layer films 
fail mechanically by formation of holes. This mechanical failure results 
in the anomalously low measurements of shrinkage for these single layer 
films, as reported in Table I. (This hole formation is suggestive that the 
films, without the presence of additional layers of film or contact with 
the surface of a food item to act as a heat sink, may heat in this test to 
above the melting point of the PET, which is about 250.degree.-260.degree. 
C.) 
The heating parameters for the films are shown in Table I. The maximum 
temperature and heat flux are determined by measuring the temperature rise 
of a sample of oil. The oil, 5 g of microwave transparent oil (Dow-Corning 
210H heat transfer silicon oil), is placed in a Pyrex.TM. borosilicate 
glass tube, 125 mm long, 15 mm outside diameter. A sample of film to be 
tested, 46.times.20 mm, is wrapped around the tube, with the long 
dimension of the film along the length of the tube and the top edge of the 
film located at the level of the surface of the oil. The film sample is 
secured by use of microwave transparent tape prepared from 
polytetrafluoroethylene, about 6 mm larger than the film sample, and the 
tube assembly is supported in a holder also of polytetrafluoroethylene. 
The temperature rise of the oil upon heating the assembly in a microwave 
oven is measured at 15 second intervals using a "Luxtron" temperature 
probe placed in the oil sample and connected to suitable recording 
instrumentation. Maximum heat flux is taken from the plot of oil 
temperature versus time, and is reported as the slope of a straight line 
originating at time=0 minutes and terminating at the time which gives the 
greatest slope, normally about 1 minute. The values so obtained are often 
approximately 2/3 of the maximum instantaneous increase in oil temperature 
as a function of time. 
EXAMPLES 1-5 AND COMATIVE EXAMPLES C4-C13 
Examples 1-5 and Comparative Examples C4-C13, in Table I, show the 
properties of various multiple layer laminated film structures. The 
structure of these films is shown schematically. One layer (B, C, or D) is 
a layer of polyethylene terephthalate metallized with aluminum, as in 
Comparative Examples C1-C3. The metallized film is laminated to at least 
one other layer of film, using a layer of adhesive, "Adcote" 506-40 
(crosslinkable copolyester, from Morton Thiokol). After lamination the 
laminate is stored, rolled on a paper core for at least three days, and 
normally 7 to 14 days at room temperature in order to ensure complete 
curing of the adhesive. The substrate is either paper (parchment, 0.13 mm, 
5 mils) or polyethylene terephthalate film. The PET film is indicated in 
the Table by the letter "H" for heat stabilized PET film, or by the letter 
"P" for PET film which is not heat stabilized. The thickness of the PET 
film substrate is about 0.012 mm or about 0.023 mm, indicated by the 
numbers "48" (48 gauge) or "92" (92 gauge), respectively. In some cases 
the PET films are also coated with a layer of heat sealable polyester 
resin, located on the outer surface. This heat sealable layer is the 
condensation product of 1.0 mol ethylene glycol with 0.53 mol terephthalic 
acid and 0.47 mol azelaic acid, also containing small amounts of erucamide 
and magnesium silicate. This layer is indicated in the Table by the plus 
(+) symbol. It is seen from Comparative Examples C4, C5, C9, C11, and C12 
that the laminates in which the films are not heat stable tend to exhibit 
excessive shrinkage upon heating in a microwave oven, evidenced by 
excessive MD or TD shrinkage, and exhibiting significant variability among 
samples. When at least one film of the laminate is heat stabilized PET, 
however, the shrinkage is much less and much more uniform and predictable. 
The laminates of the present invention, furthermore, show improved 
temperature and heat flux characteristics compared with the single layer 
films, when the susceptor layer is located in the interior of the 
laminate. (See Example 1 and Comparative Example C2.) When the susceptor 
layer is located on an outer surface of the laminate, however, there is no 
improvement in heat flux. (Comparative Examples C6, C7, and C8.) When 
paper, rather than film, is used to prepare the laminate (Comparative 
Examples C10 and C13) there is no significant improvement in heating 
properties compared with single films. It should be noted that laminates 
which exhibit larger heat flux would, because of the higher temperatures 
generated, tend to shrink more than would structures exhibiting lower heat 
flux, independently of the composition of the film layers. This factor 
should be taken into account when comparing shrinkage values. 
EXAMPLE 6 AND COMATIVE EXAMPLES C14-C20 
These examples are prepared by laminating two layers of aluminized film, as 
indicated. In Comparative Examples C14 and C16 the layers of metal are 
adjacent, separated only by a layer of adhesive. In Example 6 and 
Comparative Examples C15 and C17 the layers of metal are separated by the 
thickness of the PET layer. In the examples when two layers of susceptor 
coating are separated by a layer of film, further improvement in heating 
efficiency is seen. This result permits a film structure such as that of 
Example 6 to be used for the microwave cooking of certain food products, 
such as pizza or bread, which require a high surface heat, but where the 
interior of the product requires a degree of shielding to prevent 
overcooking. 
TABLE I 
______________________________________ 
Max 
Flux 
% Shrink 
Max T (kcal/ 
Ex Composition OD MD TD (.degree.C.) 
m.sup.2 -min) 
______________________________________ 
C1 B* 0.13 8 15.sup.c 
C2 C* 0.16 7 8.sup.c 
181 133 
" 140 68 
" 0.19 183 108 
" 167 93 
" 180 116 
" 0.18 7 8.sup.c 
C3 D* 0.28 4 5.sup.c 
" 0.23 87 28 
" 73 21 
" 90 33 
C4 B*/48P+ 0.14 5 38 
C5 C*/48P+ 0.17 29 48 202 175 
C6 *C/48P+ 0.18 168 116 
C7 *C/48H 0.19 14 3 128 59 
" 174 105 
1 C*/48H+ 0.14 10 9 205 186 
" 0.16 a 7 188 160 
" a a 184 145 
C8 *D/48H 0.21 5 2 178 106 
" 171 101 
C9 D*/48P+ 0.25 10 4 
2 D*/48H+ 0.19 8.sup.b 
4.sup.b 
193 160 
" 192 170 
C10 C*/Paper 175 143 
C11 48P/C*/48P+ 0.15 55 60 212 200 
C12 92P/C*/48P+ 0.16 30 25 206 195 
3 48H/C*/48H+ 10 8 206 222 
" 206 209 
4 48H/C*/48H+ 0.16 8 8 212 189 
" 0.17 18 6 201 148 
" 200 150 
5 48H/D*/48H+ 0.19 5 7 200 171 
" 191 140 
C13 Paper/C*/Paper 161 115 
C14 C*/*D 0.34 a a 195 199 
" 199 190 
C15 C*/D* 0.38 a 7 215 230 
" 213 221 
C16 D*/*C/48P+ 0.35 197 195 
C17 D*/C*/48P+ 0.35 213 214 
6 48H/C*/D*/48H+ 
0.34 16 14 213 218 
" 213 217 
C18 *D/92P+/*C 0.37 20 22 210 187 
" 216 194 
C19 *D/48P+/*C 0.40 20 22 201 199 
" 204 199 
C20 Paper/C*/C*/Paper 200 197 
______________________________________ 
*indicates location of susceptor layer. 
+: indicates location of heat sealable polyester layer. 
a: sample broke. 
.sup.b a duplicate measurement, believed to be in error, showed 45 and 50 
for MD and TD, respectively. 
.sup.c samples developed holes during test. 
EXAMPLE 7 
Loaves of Italian bread (454 g) were purchased from a local supermarket 
packaged in white paper bags. They were stored for two days, some in their 
original bags, and some sealed in pouches formed from composite film of 
similar to that of Example 4. The pouch was reasonably transparent, 
permitting a good view of the bread. After two days storage, the unwrapped 
samples were hard and stale. (See Table II.) Both the unwrapped and the 
wrapped samples were heated for two minutes in a 700 watt microwave oven. 
Only the loaves sealed in the susceptor pouches were properly restored. 
(Properly restored loaves have a crisp, dry crust, but an interior which 
is not dry, but slightly moist, and not tough.) During the microwave 
heating process, the pouches shrank somewhat by "microwrinkling" around 
the bread to give a snug fit. (Since the loaves were too long to use a 
turntable, browning and crisping was not completely uniform.) Internal 
temperatures of the loaves was measured using "Luxtron" probes, and 
recorded as a function of cooking time, shown in Table III. 
TABLE II 
______________________________________ 
PERCENT WEIGHT LOSS 
Unwrapped 
In Pouches 
______________________________________ 
1 day 6 negligible 
2 days 11 0.2 
after microwave 15 1.7 
heating 
______________________________________ 
TABLE III 
______________________________________ 
Time Temperature, .degree.C. 
(sec) Unwrapped Pouch 
______________________________________ 
0 20 21.5 
15 32 26 
30 45 30.5 
45 66 35 
60 88 40.5 
75 100 47 
90 102 52.5 
105 103 62 
120 103 66 
______________________________________ 
Average of two loaves heated. Deviation from the average temperature di 
not exceed .+-. 7 C.degree.. 
It is believed that the fact that the internal temperature in the wrapped 
sample did not quickly rise to the boiling point is important to 
successful reconstitution of the bread. This difference is believed to 
result from the shielding effect of the low microwave transmission of the 
film used. 
EXAMPLE 8 
Three frozen egg rolls (La Choy.TM., "Almond Chicken" filling) weighing 
about 120 g, about 127 mm long and about 38 mm diameter were cooked in a 
microwave oven as in Example 7, for 4 minutes. Sample A was cooked without 
any susceptor wrapping. Sample B was wrapped in a single layer metallized 
PET having an optical density of 0.16 (film C in Comparative Example C2). 
The wrapping was sealed with a polyimide based tape. Sample C was wrapped 
and sealed with a film similar to that of Example 4. The results are shown 
in Table IV and FIG. 6. 
TABLE IV 
______________________________________ 
Temperature, .degree.C. 
Time A B C 
(min) unwrapped monolayer laminate 
______________________________________ 
0.00 0 0 0 
1.00 5 0 0 
1.25 98 0 5 
1.50 100 5 10 
2.00 100 35 28 
3.00 100 90 68 
3.50 100 100 85 
4.00 100 100 100 
______________________________________ 
Desirable serving temperature for most hot foods is 70.degree. C. The 
desired cooking period for frozen egg rolls is about 3 minutes, in order 
to obtain the best browning. Only the package of this invention (C) meets 
these requirements. Both of the other two egg rolls were overdone, the 
unwrapped one (A) especially so. The unwrapped egg roll, in addition, was 
soft and soggy on the outside. It is believed that the favorable results 
from package C derive in large part from reduced transmission of 
microwaves to the bulk of the food item and increased shielding provided 
by the laminate, compared with the monolayer film. 
EXAMPLE 9 
Example 8 was repeated, using instead of the egg rolls, fresh "steak" 
rolls. The results are shown in Table V and FIG. 7. As in Example 7, the 
unwrapped roll did not attain surface crispness and was internally 
overcooked. The results also show that the laminate of this invention (C) 
provide greater shielding fo the interior of the food item than does a 
monolayer structure (B). 
TABLE V 
______________________________________ 
Temperature, .degree.C. 
Time A B C 
(sec) unwrapped monolayer laminate 
______________________________________ 
0 24 22 20 
5 28 28 23 
10 47 47 33 
15 79 67 47 
20 102 86 60 
25 103 92 74 
30 103 102 83 
35 103 103 88 
______________________________________ 
EXAMPLE 10 
Two frozen dinner rolls, 150 mm long.times.64 mm diameter were cooked 
separately in a 700 watt microwave oven. The first was cooked unwrapped, 
as removed from the store package, as a control. The second was sealed in 
a pouch of film as in Example 7. After 60 seconds of heating, the 
unwrapped sample was soft on the outside and steaming on the inside (about 
93.degree. C.). The wrapped sample, after 65 seconds of heating, was 
crispy and brown on the outside and properly hot (about 70.degree. C.) on 
the inside. 
EXAMPLE 11 
This example shows a packaging configuration other than a pouch. A round 
deep-dish pizza was packaged and cooked in a package made from the 
laminate of this invention. A 185 mm square piece of susceptor film as in 
Example 4 was conformed to the bottom and sides of the crust of the pizza 
by use of a mold assembly shown in FIG. 8. The mold assembly 54 contained 
a round cavity, 56 about 134 mm in diameter and 33 mm deep, a suitable 
size for containing the pizza. The film was placed over the cavity and 
pressed into the cavity by ram 58, so that the film conformed to the 
cavity, and wrinkles were pressed into place. The ram was removed and the 
pizza, frozen, weighing 184 g, 127 mm diameter, 25 mm height, was placed 
into the formed film. The excess film was trimmed even with the edges of 
the mold assembly. A cover sheet of 9 micrometer (35 gauge) aluminum foil 
laminated to 0.13 mm (5 mil) parchment paper was heat sealed to the film 
(by means of the heat sealable layer) to act as a shield over the top of 
the pizza. A diagram of the packaged pizza is shown in FIG. 9. The pizza 
with crust, 60, and filling, 62, is held within the laminated film, 64. 
The cover sheet, 66 is sealed to the lower, laminated film, 64, to form a 
circumferential rim, 68. The packaged pizza was removed from the mold 
cavity and cooked in a 700 watt oven with a turntable for 3 minutes and 45 
seconds. The bottom and sides of the crust were crisp and brown, and the 
filling was hot but not overcooked. The crisp, stiff crust provided good 
shape and integrity for the pizza. 
COMATIVE EXAMPLE C21 
An identical pizza was cooked in its original open ended plastic bag under 
the same conditions as Example 11. No browning or crispening of the crust 
was apparent. The crust, rather, was wet and soggy and the topping messy. 
The pizza itself was floppy and had poor shape integrity. 
EXAMPLES 12 and 13 
Laminated samples of films having multiple layers of heat stabilized PET 
are prepared by the procedure described for Example 1. The heating and 
shrinkage results from tests on these films are presented in Table VI, 
wherein the designation of the layers is the same as that used in Table I. 
The presence of additional layers of heat stabilized PET provides 
additional structural stability and avoids the problem of breakage 
occasionally encountered when a single layer of such film is used (see 
Example 1). 
TABLE VI 
______________________________________ 
Max 
Flux 
% Shrink 
Max T (kcal/ 
Ex Composition OD.sup.a 
MD TD (.degree.C.) 
m.sup.2 -min) 
______________________________________ 
12 C*/48H+/48H 0.16 20 16 190 246 
" 199 197 
13 C*/48H+/48H/48H 
0.16 25 12 203 176 
" 200 178 
______________________________________ 
.sup.a Optical density taken from the measurement of the film of Example 
1, from which the present films are prepared. 
COMATIVE EXAMPLE C22 
A sample is prepared in which a layer of the film designated above as "C*" 
is laminated to a layer of polyethylene. The results of tests, performed 
as described above, are indicated in Table VII, in which 200Peth indicates 
a layer of polyethylene which is (in this case) 2 mils thick (200 gauge). 
TABLE VII 
______________________________________ 
Max Flux 
Max T (kcal/ % Shrink 
Ex Composition (.degree.C.) 
m.sup.2 -min) 
MD TD 
______________________________________ 
C22 C*/200Peth 192 167 45 6 (holes/ 
broke) 
______________________________________ 
COMATIVE EXAMPLES C23-C26 
Samples are prepared in which a layer of the film designated above as "C*" 
is laminated with zero, one, or two layers of polyethylene (ordinary low 
density polyethylene unless otherwise indicated). The heating properties 
of such films are measured as described above, except that the size of the 
film samples is approximately 48 mm.times.11 mm and the glass tube 
containing silicon oil is supported in a small dewar flask rather than a 
holder of polytetrafluoroethylene. The maximum heat flux is determined by 
measuring the maximum temperature rise between adjacent measurement times. 
This provides a maximum instantaneous value, rather than the maximum value 
from time=0 as described above for Comparative Example 1. The results from 
the heating experiments are shown in Table VII. 
TABLE VII 
______________________________________ 
Max Flux 
Max T (kcal/ 
Ex Composition (.degree.C.).sup.a 
m.sup.2 -min) 
______________________________________ 
C23 C* 124 130 
" 92 85 
" 134 164 
C24 C*/200Peth 143 175 
" 151 192 
C25 200Peth/C*/200Peth 
142 141 
" 145 158 
C26 200HDP/C*/200HDP.sup.b 
171 237 
" 183 219 
______________________________________ 
.sup.a Recorded as temperature rise, delta T; converted to Max. T by 
assuming a starting temperature of 30.degree. C., believed to be accurate 
to within about 2.degree.. 
.sup.b HDP = linear, high density polyethylene, (2 mils thick) 
The results from these Comparative Examples show that addition to the 
metallized film of one or two layers of ordinary low density 
polyethylene--a material which is neither heat stable nor heat 
resistant--results in an increase in the maximum heat flux from about 126 
to about 166, or a increase of about 32%. (When high density polyethylene 
is used, the increase is greater.) In contrast, when one or two layers of 
heat stabilized PET is added to the same material ("C*") (See Examples 1, 
3, and 4 and Comparative Example 2), the increase in heat flux is greater, 
about 69%.