Note: Descriptions are shown in the official language in which they were submitted.
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WO90/16138 1 PCT/US90/02774
MICROWAVABLE DOUBLE-BAG FOOD CONTAINER
This invention relates to a package for foods in which
the contents may be heated by microwave radiation. In
one of its more specific aspects, this invention
relates to a food package for raw, cooked, o~ partially
cooked foods and shortening, e.g. butter or oil,
hydrogenated oil, normally solid vegetable oil, animal
fat, and the like. In another of its more specific
aspects, this invention relates to a package ~or corn
kernels and oil or shortening which includes a
microwave responsive heating element. A preferred
embodiment comprises a composite package or bag
comprising an inner liner or and a paper outer
container or bag having a microwave responsive heating
element on an outer surface of the inner liner or on
the outer or inner surface of the outer container.
Numerous containers, including paper or plastic boxes
and bags, have been devised for use as pàckages for
foods which are suitable also as utensils for heating
20 or cooking the food in a microwave oven. Many such -
containers are referred to in European Patent
Application Publication Number 256,791, incorporated
herein by reference. Some of the prior art containers
comprise a heating element whic~ is highly responsive
to microwave radiation to produce a more highly
elevated temperature in one or more selected areas of
the container than that in the rest of the container.
One type of heating element comprises metal particles,
usually aluminum, vacuum deposited on a polyester film
the metallized film is then laminated onto paper or
paperboard. The thus metallized paper or paperboard ~-
can then be used in a microwave oven to heat, brown or
sear food items placed on the metallized paperboard or
in packaqes comprising metallized paper~oard. While
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W090/16138 PCT/US90/02774
such structures can be very effective as heaters, the
vacuum deposition process does not readily lend itself
to coating selected areas of the film or the
application of varying amounts of metal particles ~r
metallized coating to selected areas of the finished
carton or structure.
Other methods of forming microwave responsive heating
elements involve dispersing particles which are
microwave energy absorbers, e.g. carbon, metals, and
metal alloys, in a binder and forming a sheet or
laminate comprising the dispersed particles. Turpin,
U.S. Patent No. 4,190,757 discloses heaters which
comprise a layer of particulate lossy materials, e.g.
lS carbon, iron and various metal oxides, or alloys, in an
inorganic binder, e.g. sodium silicate. U.S. Patent ` "
No. 4,264,668 to Balla discloses a heating element made
up of a layer of carbon black in an acrylate binder
laminated between a carrier layer, e.g. paper,
cardboard or plastic material, and an outer layer of
thermoplastic material. European Patent Application
Publication No. 242,952 discloses a heating element
made up of a metal or metal alloy in flake form in a
thermoplastic dielectric matrix on a dielectric
substrate.
The present invention provides an improved food package
comprising a microwavè interactive composition coated
or printed on papèr, polyester or paperboard forming a
part of the package.
.
One preferred embodiment of the present invention
includes a multi-layered bàg comprising an inner bag or
liner of greaseproof paper or polyester`film and an
3~ outer bag of paper with an integral microwàve heating
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WO90/16138 3 PCT/US90/02~74
element on the outer surface of the inner bag or on the
outer or inner surface of the outer bag. The microwave
heating element is made up of a microwave reactive
coating applied directly,suitably by a conventional
coating or printing process, on a selected surface of
the bag or on a suitable substrate. In a preferred
embodiment of the invention the microwave reactive
coating is applied to the inner surface of the outer
bag or container. Alternatively, the microwave
reactive coating may be applied directly to the outer
surface of the inner or outer bag or container. A
preferred microwave interactive coating composition is
that disclosed in the commonly assigned copending
United States patent application of Kenneth A. Pollart
15 et al, Serial No. 07/239,544, incorporated herein by
reference. A preferred coating composition comprises
carbon black, at least one finely divided metal or
metal oxide, clay, and a dielectric solid organic
binder in a carrier liquid.
Fig. l of the drawings is a perspective view of a
popcorn bag or similar food container.
Fig. 2 of the drawings is a perspective view of the
opposite side of the bag illustrated in Fig. l.
Fig. 3 is a perspective view of an empty bag
illustrating a preferred embodiment with portions cut
away to show its interior construction and printed
microwave heater element.
Fig. 4 is an elevational view in cross section through
one end of a paperboard carton containing filled bags
of the type illustrated in Fig. l.
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WogO/16138 PCT/US90/Ot774
Fig. 5 is an illustration of a preferred pattern for a
printed microwave reactive heater element as employed
in the microwavable food containers of this invention.
Fig. 6 is a graphic illustration of typical heater
responses for heating elements made according to this
invention.
With reference to Figs. 1 and 2 of the drawings, a
filled food `container, e.g. a popcorn bag 5, of our
invention is illustrated. The bag comprises a front
side 6 and a back portlon 8 with a sèaled side seam 10.
The top end portion of the front side 6 of bag S may be
sealed to the back side 8 of the bag. The bag is
provided with gussetted side panels 15 which extend
from the top of the bag to a conventional bottom
section 17. As illustrated, the popcorn and oil occupy
a mid section of the bag between fold lines 18 and 19.
A printed heater 27, described in more detail
hereinafter, is provided in the area of the package
adjacent the popcorn.
Fig. 4. illustrates a package 20 of paperboard with
three filled food container bags packaged for
distribution and sale to consumers. As illustrated, a
load of food e.g. popcorn and oil, 21 occupies the mid
- section of the bag and the two end sections of the bag
comprising top portion 12 and bottom portion 17 fold
over the food containing portion to form a compact unit
for packaging.
Fig. 3 illustrates in more detail the construction of a
preferred popcorn bag embodiment of our invention. The
container is made up of a paper outer bag 25 and a
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W090/l613X PCT/US90/02774
greaseproof paper or polyester inner bag or liner 26
which may be fused or adhesively laminated to the outer
bag.
In this embodiment, the microwave heater 27 is printed
as a solid pattern on the inner surface of the paper
outer bag 25. A preferred alternate heater pattern is
illustrated in Fig. 5. As illustrated in Fig. 5, the
printed heater is in the form of a pattern. A
lQ preferred grid pattern, illustrated in Fig. 5, has an
open unprinted area approximately equal to the printed
area. This pattern has been demonstrated to produce a
uniform distribution of heat response to microwave
radiation. Other continuous patterns, such as
contiguous polygons, interlocking circles, lace
patterns, and the like, also may be employed.
While we have shown in the figures, representative
illustrative embodiments of microwave food packages
included in this invention, it will be obvious that the
invention is not limited to the specific structures
illustrated and described herein. For example, the
heating element need not necessarily be applied
directly to one of the surfaces of the container itself
but may be applied to a suitable substrate of paper,
polyester, or the like and then inserted into the
package or attached to the desired area of the
container. As another example, not illustrated, the
heating element may be contained in or printed on the
bottom 17 of a bag of the type illustrated in Figs. 1
to 3.
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In one speclfic example, a preferred printed heater for .
~ ` a popcorn bag containing 70`grams of corn kernels which -~
35 ` normally` yièld a popped volume of 2500- cubic
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WO90/t6138 PCT/US90/02774
centimeters is one in the form of a square or rectangle
having a total area of about 25 to 35 square inches
(about 160 to 225 square centimeters). The heater is
preferably printed in the form of a grid pattern as
illustrated in Fig. 5 with a coating weight in the
range of from about 0.5 to about 8 pounds per 3000
sq.ft. ream. Alternatively, the heater may be printed
as a solid patch as illustrated in Fig. 3 at a coating
weight in the range o~ from about 0~5 to about 8 pounds
per ream. Tests results indicate that a more uniform
temperature response to microwave energy radiatlon is
produced with a discontinuous pattern, e.g., the grid
pattern, than with a solid patch coating.
Heaters printed as a solid patch coating as illustrated
in Fig. 3 by a gravure press at 85 lines, 100 lines,
135 lines and 175 lines per inch produced satisfactory
temperature response for popping corn in a 700 watt
microwave oven. Grid pattern heaters as illustrated in
Fig. 5 printed on a gravure press at 85 lines and 100
lines per inch exhibited better performance than the
heaters with a solid printed pattern with less tendency
to form "hot spots".
. ,~
Other patterns, including a pattern of interconnected
concentric circles, a basket weàve pattern, and the
like, not illustrated, are also possible variations of ~ -
- the illustrated grid pattern and are within the scope
of this invention.
As disclosed in commonly assigned copending patent
application, Serial No. 07/239,544, the preferred `
printing compositions are composed of carbon black,
finely divided-flake aluminum, clay, and a synthetic
resin binder in a suitable carrier vehicle. Preferred
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~ O ~ n ~oso/t6138 PCT/US90/02774
binders include aqueous or non-aqueous solutions or
dispersions of a polymer precursor that serve as both
binder and vehicle for the remaining solid components.
Those binders which are suitable for use in printing
inks are suitable for use as binder and vehicle for the
carbon black, aluminum and clay components of the
printable composition from which the heater is formed.
Generally available latex formulations marketed for
that purpose are preferred. While latex formulations
are preferred as binders, a non-aqueous solvent
formulation of a binder, for example, the product
marketed by Morton Chemical Company under the trade
name Morez 100, also has been found suitable for this
purpose.
Preferred components of the heater printing composition
include carbon in the form of carbon black or graphite,
and a finely divided metal component, e.g. aluminum,
tin, bronze, nickel, and the like, which are conductive
or semiconductive or ferromagnetic materials capable of
converting microwave radiation energy to heat. The
inert powdered solid temperature moderators suitable
for use in these formulations include clays, e.g.
kaolin and English china clays, alumina, alumina
hydrate (aluminum hydroxide), aluminosilicates, silica,
calcium carbonate, titanium dioxide, and the like. The
temperature moderator should be essentially inert and
substantially unresponsive or only mildly responsive ~o
microwave radiation. -~ Preferred binders comprise
synthetic resins -in a suitable vehicle: especially
preferred binders include polymer latex formulations
marketed for this purpose.
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: The relative proportions by weight of carbon to metal
35 . in the composition-may ~be within the range of:$orm
.
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WOgO/16138 PCT/US90/02774
about 1:2 to about 2:1 with a preferred range of from
about 1:1.5 to 1.5:1. The preferred ration of carbon
black to aluminum flake is about 0.6. The content of
the inert temperature moderator ingredient, e.g. clay,
in the composition may range from about 10 percent by
weight of the total ~dry basis) weight to about 35
percent.
The relative proportions of binder solids to the
remainin~ solids making up the heater components may be
in the range of from about 0.3:1 to 1:1. A binder
solids content in the range o~ from about 30 to about
40 weight percent of the total composition welght is
generally pref~rred. Preferably only enough binder is
used to adequately bond the solid coating components to
one another and to the substrate.
In a preferred embodimentr wherein the microwave
reactive material is a mixture of carbon black and
aluminum flake with alay as a moderator, collectively
referred to as pigment, and the binder is an acrylic
emulsion, the pigment to binder weight ratio should be
about 2:1 to or higher. The weight ratio of carbon
black to aluminum flake can be varied from about 2:1 to
1:2 without having a major effect on temperature
response.
Other materials can be included in the coating
composition, such ~as surfactants, dispersion aids and
other conventional additives- used in coating and
printing compositions to facilitate application of the
coating composition to the substrate by rotogravure or
other suitable printing or coating methods. The
coating can be applied using conventional printing and
- 35 coating processes, e.g., rotogravure, silk screen,
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WO90/16138 ~- PCT/US~/02774
flexography, air knife, rolls, blade, etc. After the
coating composition has been applied it can be dried
using conventional drying ovens normally provided in
web printing and coating processes.
The following examples of test results demonstrate
particular embodiments of this invention and some of
the possible variations in compositions and coatings
which may be adapted to varying consumer product needs.
In the following examples, all coating formulations
were applied to a 40 lb/3000 sq~ ft. uncoated, bleached
kraft paper with a Bird applicator and dried on a photo
drier at 200F. Unless otherwisè specified, the biner
used was a combination of Rhoplex B-15, an acrylate
latex supplied by Rohm & Haas Company and Lucidene 602,
a styrene/acrylic latex supplied by Morton Chem~cal
Company and commonly used in aqueous printing inks.
In all cases the carbon black was dispersed (using a
shot mill) into part or all of the Lucidene 602 binder
used in the formulation with additional water added as
necessary to obtain the desired viscosity. After the
carbon black was uniformly dispersed, it was
transferred to a container equipped with a propeller
type mixer. The remaining binder (Lucidene 602 and/or
Rhoplex B-15), as well as the remaining components,
were gradually added along with additional water as
needed. In each case, agitation was continued until a
uniform mixture was obtained. The heater response of
these coatings was determined by placing a printed bag
in a Litton -microwavè oven (Model 2238, 700 watt
-- rating). The sample-was supported 2 inches off the
- bottom of the oven with a `glass ring and the
- 35 temperature response was-measured with a Hughes Probeye
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WO90/16138 PCT/US90/02774
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Thermal Video System.
In typical applications, such as popcorn bags, printed
heaters made up with our printable microwave
interactive coatings utiliæing the partial area
coverage patterns, for example, the grid pattern, are
capable of providing the desired level of temperature
response for a given load requirement. It is easy, for
example, to select a coating weight with a temperature
response required to yield a high volume of popped corn
with a minimum number o unpopped kernels while at the
same time minimizing the tendency to cause scorching of
the paper bag.
lS Pattern coatings, e.g. the grid pattern, covering 3S to
80 percent of the heater surface area will usually
provide the desired temperature response over the
entire printed area. Preferably, the area covered by
the coating forming the pattern is within the range of
40 to 7S percent of the heater area.
EXAMPLE
A microwave reactive coating composition was prepared
with l9 weight percent carbon black, 27 weight percent
aluminum flake, l8 percent kaolin, and 34 weight
percent binder from a 1:2 mixture of Lucidene 602 and
Rhoplex B-15. ~-
- The solids, viscosity and surface tension of the
formulation were adjusted by the addition of water,
alcohol and carboxymethylcellulose as necessary for
excellent runnability on a full scale gravure printing
press at 250 ftlmin. Heater test specimens were
3S prepared by rotogravure printing of a 25 square inch
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~3ll~O90/1~138 PCT/US90/02774
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heater on paper using the pattern illustrated in Fig. 5
with print lines 1.8 mm wide and 4.5 mm square
openings, thus providing an area coverage by the heater
composition of approximately 50 percent.
The test specimens are identified as follows.
Coating Weight
10 SPecimen Printina A~paratus lb/3000 sq ft
X 65 line screen gravure 7.l
cylinder
Y 100 line screen gravure 3.8
cylinder
Z 133 line screen gravure 2.5
cylinder
Heater response tests were carried out in a 700 watt
Litton microwave oven with and without an added load.
The test results are illustrated graphically in Fig. 6
wherein X, Y, and Z designate tests of the specimens
under-no load conditions and X', Y', and Z' designate
test results with the same heaters with a 200g. water
load. The test results demonstrate the uniformity of
temperature over relatively long periods of time
obtainable with these heaters.
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