Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
MICROWAVE PACKAGE WITH EASY OPEN SEAL
BACKGROUND OF THE INVENTION
This invention relates to packaging
materials suitable for microWave cooking, particularly
packages which have seams which are easy to open after
microwave cooking.
There has been much intere~t rece~tly in
packaging materials which aid in browning and crisping
of food items in a microwave oven. Such packages can
take the form of a sheet which is wrapped about a food
item and sealed in its wrapped conformation. One
persistent pxoblem with such packages, however, has
been the difficulty with which such seals or seams are
opened after cooking in order to remo~e the food
contents of the package. In many such packages the
sheet material itself is liable to melt or fuse
together on heating creating an unattractive or
impenetrable seam. In o~her packages the adhesive
used is such that the pac~age, after heating, remains
as firmly sealed as before. The present invention
provides a package with a seam or seal which is easy
to open after microwave cooking, thus avoiding the
pro~lems o~ the prior art packages.
U.S. Patent 4,gll,938 discloses a package
useful for cooking food in a microwave oven~
comprising a thermally stable film wrapped about the
food, a layer of heat releasable thermoplastic
materis.l locat~d on the. ~urface o~ the thermally
stable ~ilm and forming a seal between at least two
sur~ace areas thereof, and a microwave susceptor
material in close proximity to at least a part of said
seal. The seal is selectively releasable upon
exposure to microwave energy and resultant heating of
AD-5891 35 the m~crowave sUsceptor material under microwave
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cooking conditions. The release of the seal permits
venting of pressure or accommodativn o~ yrowth of the
food. Suitabl.e thermally stable films include
polyesters, polymethylpentene, polyarylates,
polyamides, polyimides, polycarbonates, or cellophane.
U.S. Patent 4,640,838 discloses a vapor
ti~ht package which has a deposit comprising
nonmetallic, microwave absorbing particles such as
graphite dispersed in a polymeric binder. When heated
in a microwa~e oven, heat built up in the particles
may soften and weaken the underlying packaging
material, thus venting the package. The package can
be made with a heat-resistant plastic film such as
cellophane. The deposit can be positioned over an
opening, slit, or score in the package, and the heat
can soften and weaken the deposit to vent the package.
The deposit may be placed at a position to enhance the
opening of the package tD remove its contents.
European application 0 340 037 discloses a
conformable multilayer laminated structure use~ul for
packaging food for microwave cooking, comprising a
layer of flexi~le, heat-resistant microwave
transparent plastic film, a layer o~ ~lexible, heat
resistant, heat stable, microwave transparent plastic
~ilm, and a layer o~ substanti~lly continuous
microwave susceptor material located on a surface of a
~ilm o~ the laminate. The layers ~f plastic film can
be prepared ~rom a number of ~aterials, including as
one option cellophane. The laminated structure can
also include a layer of heat sealable thermoplastic
resin, and applications are disclosed in which food is
wrapped and sealed in the structure.
U.S. Patent 4,735,513 discloses a flexible,
sheet structure comprising a base sheet of e.g.
polyester, having a microwave coupling layer. The
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s~eet may be laminated to a backing sheet of
dimensionally stable flexible material transparent to
microwaves. Examples of such backing sheets include a
synthet~c sheet formed from synthetic plastic fibers
of a non-thermoplastic and dimensionally stable
composition, or, preferably, paper~ The ~icrowave
coupling layer can extend to the edge of the paper but
is preferably present as an island covering selected
areas of the sheet. The sheet may be formed into e.g.
lo a bag.
U. S. Patent 4,734,288 discloses a package
~or an expandable food product including an outer
container and an expandable plastic food pouch within
the container. The expandable food pouch may contain
a designed fault along its peripheral edge to
facilitate the controlled release of pressure. One
sidewall of the package (paperhoard) may include a
flap provided with a heat sensitive adhesive which :;s
released when exposed to microwave energy.
U.S. Patent 4,267,420 discloses a plastic
film or other dielectric substrate having a very thin
coating thereon which controls the microwave
conduc~ivity when a paekage wrapped with such film is
placed within a microwave oven.
U. S. Pa~ent 4,571,337 discloses a bag and a
mixture of edible popcorn ingredients ~uitable ~or use
in microwave ovens. Portions of the panels of the bag
contain a coating that is sensitive to pressure and
heat, ~orming a seal along the top edge o~ the panels.
The seal has sufficient strength to withstand the
internal steam pressure generated by the moisture
content of the kernel~ for at least one-half of the
popping process~ Pre~erably, the bag will vent at the
top ~eam b2fore khe process is comple~ed to allow
steam to escape.
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SUMMARY OF THE INVENTION
The present invention provides an
easy-to-open package useful for cooking of food in a
microwave oven, comprising a flexible multiple layer
structure which comprises:
(a) a layer of non-melting cellulosic
material;
(b~ a support layer of polyethylene
terephthalate film;
(c) a layer of microwave susceptor material
comprising vacuum dep~sited metal loc~ated on said
support layer in an amsunt suitable to supply heat to
an adjacent food item upon exposure to microwave
energy and extending over at leas~ about 50% of the
surface area of said support layer; a~d
(d) a layer of hea~ releasable thermopla~tic
material;
said multiple layer structure being wrapped
about said food and being sealed in its wrapped
2~ conformation by means of the heat releasable
thermoplastic material, the seal formed thereby being
in close proximity to said layer of ~icrowave
susceptor material, whereby the strength of said sea}
is reduced after exposure to microwave energy,
rendering said ~eal more easily openable a~ter removal
of said package from said microwave oven.
BRIEF~DESCRIPTION OF THE DRAWINGS
Figure 1 is a viaw o~ a package o~ the
present invention.
Figure 2 is a partial cross-sectional view
of the package of Figure 1.
Figure 3 is an alternative package
configuration of the present invention,
DETAILED DESCRIPTION OF THE INVENTION
The packages of the present invention are
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prepared from multilayer film structures which are
wrapped around or otherwise encompass a food item~
The multilayer film structures comprise, first, at
least one layer of non-melting callulosic material.
As used herein this term includes paper, paper board,
and related produc~s as well as csllophane
(regenerated cellulose). Such materials are
non-melting, thus providing resistance to gross
melting or structural deformation of the package upon
haating in a microwave oven. It is preferred that
this layer is sufficiently thin that the final
structure will have suitable flexibility to be wrapped
around a foodstuff. Thus a layer of cellophane should
preferably be about 13 to about 50 micrometers (1/2 to
~ mils) thick and most preferahly about 20 to abou~ 40
micrometers (3/4 to 1-1/2 mils) thick. Paper or paper
products should preferably be about 25 to 250
micrometers ~1 to 10 mils~ thick, most prefexably
about 50 to about 150 micrometers (2 to 6 mils) thick.
Certain materials, such as cellophane, are initially
quite flexible in thin sheets, but a~ter heating to
e.g. 200~C and subsequent cooling they may exhibit an
increase in stiffness. This behavior may actually ~e
desirable since a package exhibiting an increase in
stiffness after cooking may be easier to open.
Adjoined to the layer of non-melting
cellulosic material is a layer o~ microwave suscept~r
material on a support layer of polyethylene
terephthalate (PET) ~ilm. PET film is a staple item
of commerce which can be readily treated with a
microwave ~usceptor material. It is a reasonably
temperatura-stable ma~erial, having a melting point of
about 2~0-C, and so i~ ~uitable ~or use in many
microwave cooking applications. However, when used
alone with an active, heat generating microwave
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susceptor material, as described below, it can be
overheated a~d subject to melting; hence the
requirement that a layer of non-melting cellulosic
material be present. The thickness of the layer of
PET is not particularly important, provided that it is
thick enough to survive ordinary handling and thin
enough not to unduly limit the flexibility of the
final product. Common thicknesses of suitable PET
films range ~rom about 5 to about 130 micrometers,
especially about lo to about 50 micrometers.
There is a layer of microwave susceptor
material deposited on the PET film. Susceptor
materials are well known and are used to convert
microwave energy into heat which can be used e.g. to
~5 brown and crisp the surface of a food item. In the
present invention the base PET film is coated with a
thin layer of susceptor metal by well-known vacuum
deposition techniques. The susceptor material is a
substantially continuous electrically conductive
material which is present in suPficient thickness to
cause the mul~ilayer structure to heat under microwave
cooking condition~ to a ~emperature 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 susceptor will cover a substantial part of
the base sheet, preferably at least about 50% and most
preferably nearly 100%, in order to 3ssure utility ~or
supplying heat to the food item.
A preferred susceptor material is vacuum
metallized aluminum or stainless steel, which will
preferably be present in sufficient amounts to impart
an optical density, in the case of aluminum, o~ about
0.10 to about 0.35, preferably 0.16 to about 0.25, to
the film. (Such film, as an additional benefit, will
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be substantially transparan~ to vlsible l~ght îf the
cellulosic materlal ls cellophane.) Other metals, of
course, may be used, including gold, silver, mu-metal,
st~inl~ss steel, nickel, antimony, copper, molybdenum,
bronzel iron, tin, and zinc. Suitabl~ films will
generally have a surface resistivity of about 60-1000
ohms~square, preferably about 100-600 ohms~square.
The amount of susceptor material applied to
the film may be varied within certain limits which
lo will be apparent to one skilled in the art. The test
to determine the correct amount of material is whether
the coatiny will heat to the proper temperature and
provide sufficient heat flux for browning or
crispening of food items. The re~uired temp~ratur~
may dep~nd on the particular food item used but for
many applications is at least about lso~c.
Normally a layer of an adhesive material
will be used to join ~he layer o~ cellulosic material
to the vacuum metallized PET sheet. Any of a number
of materials are suitable for this purpose, including
crosslinking adhesives such as "Adcote" crosslinkabl~
copolyester from ~orton Chemical, ethylene acid
copolymers and adhesives based thereon, acrylic
adhesives, polymeric vinyl acetate or vinyl
alcohol-based adhesives, and certain radiation
crosslinked materials. Certain suitable adhesive
materials are described in more detail in copending
application U.S.S.N. 07/388,923, ~iled August 3, 19~9,
the disclosure of which i~ incorporated herein by
re~erence. The adhesive layer can be applied by
conventional laminating techniques in general, such as
extrusion coating, lamination, or printing processes.
The joining together of the cellulo~ic layer and the
metallized PET layer can be by conventional lamination
followed by curing, as necessary.
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on at least a part of one outer layer of the
structure is applied a layer of heat releasable
thermoplastic material. This is an adhesive type
mater al which loses much of its strength upon
heating, e.g. by heat generated by adjacent microwave
susceptor material in a microwave oven. Suitable heat
releasable thermoplastic materials include polyester
copolymers, particularly copolymers of ethylene
glycol, terephthalic acid and azelaic acid; copolymers
of ethylene glycol, terephthalic acid, and isophthalic
acid; and mixtures of these copolymers. An especially
effective copolymer is that prepared by the
condensation of ethylene glycol with terephthalic acid
and azelaic acid in the mole ratio of about 50:50 to
about 55:45. This resin composition can be applied as
a solution of e.g. 15.B parts solid in a mixture of
toluene (25 parts) and tetrahydrofuran (58 parts).
Small amounts of other materials such as erucamide and
magnesium silicate can also be pre~ent in the mixture.
After solvent removal J the heat releasable
thermoplastic material can be heat ~ealed to itself vr
to another film surface by application of heat with
e.g. an iron.
Fi~ures 1 and 2 ~how a food item wrapped in
such a multiple layer film. Figure 1 shows the food
item 11 contained between two sheets or within a
single folded sheet of the present invention. The
she~t may be folded alony a crease 17 providing
contacting edges 15; or if two separate sheets are
used all edges, 15 and 17 can be non-creased edges.
In either arrangement the ~ood item can be said to be
"wrapped" in th~ multiple layer structure in the sense
that it is enclosed therein. The structure is sealed
in at least one seal 13 about the food item. Although
35 Figure ~ illustrates a rectangular package, other
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geometrical arrangemants are equally a part of the
present invention. For instance, the seal need not
completely surround the ~ood item; vent areas can be
remain unsealed or can be created by cutting off
corners of the package if desired. However, th~ seal
will generally extend along at least a considerable
length of the food item.
Figure 2 shows the seal or seam in greater
detail. (In this figure the thicknesses of the
various film layers are exaggerated for illustration.)
Upper and lower sheets or leaves 21 and 21' enclose
the ~ood item 11 and form a seal in region 13~ Outer
layers 23 and 23' are the non-melting cellulosic
material. Layers ~5 and 25' are the PET substrate
layers on which vacuum deposited metal layers 27 and
27' reside. Layers 31 and 31' are adhesive layers.
Layers 29 and 29' are the layers of heat releasable
thermoplastic material~ Although Figure 2 shows the
heat releasable thermoplastic material 29 adjacent to
the PET sheet, it is understood that this need not be
so; it could be adjacent to the cellulosic sheet. In
addition, the microwave susceptor material 27 need not
be located between the support layer 25 and the
cellulosiG material 23 as shown, although ~hat
arrangement may be de~irable to protect the metal
layer from the environment. Furthermore, the seal need
not be a symmetrical face-to-face seal as shown. For
instance, it is permissible to use only a single layer
of the heat releasable thermoplastic material to make
the seal. It is important, however, that the portion
of the heat releasable thermoplastic material which
provide~ the seal is adjacenk to a heat generating
layer of microwave susceptor material 27.
Figure 3 is an alternative package of the
present invention which is sealed with two end seams
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41 and ~3 and a fin seam 45. The other edges 47 and
49 may be creased or uncreased (folded over without
creasing). Any one or more of the seams can contain
the heal releasable thermoplastic resin in close
contact with the susceptor material and so be easily
openable. Preferably all three seams will be
releasable in this manner.
In practice the package such as shown in
Figure 1 is placed into a micxowave oven and cooked
lo for a suitable time to cook and brown or crispen the
~ood item contained therein. During this time the
heat generated from the interaction of the microwave
energy with the susceptor material not only serves to
brown and crisp the food item but also softens the
heat-releasable adhesive and tends to cause it to
release. Even upon subsequent cooling the seal is
weakened and is easy to open by manually pulling it
apart. In some cases the seal releases completely
upon heating and does not reform upon cooling, for
example, when the leaves of the seal physically
separate. Such behavior results in a package which is
particularly easy to open after cooking. Certain
materials such as cellophane, when used in the
multiple layer structures of the present invention,
tend to become somewhat more rigid after heating and
may distort slightly during the heating process; such
packages are more likely to automatically open in this
fashion. The complete or partial release of the seal
need not depend on any internal pressure generated by
the cooking of the food.
Examples
For the Examples a piece of microwave active
sheet, described below for each example, is wrapped
around a commercially available frozen burrito and
heat sealed in its wrapped conformation using a seal
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formed bet~een adjacent layers of heat releasable
thermoplastic material ~except as noted). Each
wrapped burrito is cooked in a 700 watt microwave oven
on full power for 3.5 minutes; the package removed
~rom the oven and the seal evaluated for ease of
opening.
Example 1
A multilayer ~ilm is prepared using a first
layer of cellophane about 25 micrometers thick (about
l mil, 175 gauge) and a substrate layer of PET film,
12 micrometers thick (0.48 mil, 48 gauge), vacuum
metallized with aluminum to an optical density of
about 0.26. These layers are joined together
~aluminized surface of the PET facing inward; using an
lS adhesive layer about 12 micrometers thick of ethylene
methacrylic acid copolymer containing g wsight percent
methacrylic acid and having a melt index of 10 dg/min.
The layers are joined by extrusion laminationJ
followed by irra~iation crosslinking of tha adhesive
layer with 10 megarads of electron beam radiation.
The outwardly fac.ing surface of the PET is coated with
a 3 g/m~ layer of heat releasable thermoplastic
material comprising the condensation product of 1.0
mol ethylene glycol with 0.53 mol terephthalic acid
and 0.47 mol azelaic acid.
After cooking, the film has shrunk slightly
and wrinkled, and the package is very easy to open.
Some o~ the seal area has already spontaneously
opened. The film is noticeably stiffer a~ter cooking
and has taken the shape of the contents, that is, it
develops a memory o~ its configuration in the oven.
Example 2.
A similar structure similar to that of
Example 1 is prepared replacing the acid copolymer
adhesive with an acrylic adhesive (Hycar~ 26373 from
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B. F. Goodrich), applied at a thickness of about 2 . 3
g/m2 (1.5 pounds per ream (1.~ lb/~s2~ooo in2)). The
results are the same as in Example 1.
A multilayer structure is prepared using as
a core layer a film of PET 12 micrometers thick (48
gauge~, vacuum metallized on one side with 304
stainless steel to a surface resistance of 515 ohms
per s~uare. A layer of PET 12 micrometers thicX ~48
gauge~ is applied to each side of the metallized film
layex using, for each such layer, a 3 g/m2 adhesive
layer of the heat releasable thermoplastic material
described in Example 1. To one outer surface o~ the
resulting structure is applied a 3 g/m2 layer of the
same heat releasable thermoplastic material.
After cooking, this material has shrunk
slightly but shows less wrinkling than observed in
Example 1. The film is somewhat stiffer than before
cooking, but not markedly so. The seals of this
package have fused and cannot be separated. (It
appears that this film does not generate enough heat
to adequately crisp the burrito; in addition, there is
no venting of steam ~rom the package through the
seams.)
Com~arative_Example C2
A layer of aluminized PET films as in
Example 1 is adhered to a layer of paper, aluminized
side facing inward, using ~dcote~ adhesive. No
s~parate layer of heat releasable resin is used as
external adhesive. The package is sealed by heating
to fuse adjacent PET layers to form a seam. After
cooking the paper shows little change in appearance
and is not noticeably stiffer.
The seal in such a package/ formed directly
between two layers of high-melting PET, is more
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di~ficult to Eorm than seals involving heat releasable
thermoplastic resins. Such a seal is therefore more
likely to be defective and to come open during
handling, before cooking. To the extent that such a
5 seal is well formed, it will not release readily
during or after cooking.
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