Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OVENWARE FOR MICROWAVE OVEN
FIELD OF THE INVENTION
Compositions that contain polymers that have good high temperature
resistance, a susceptor for microwave energy generated by a microwave oven
and which has a certain Curie point temperature, and that have relatively high
thermal conductivities, are useful as ovenware in microwave ovens. Also de-
scribed are novel structures for such ovenware.
TECHNICAL BACKGROUND
Common cooking containers such as pots, frying pans, and baking tins
are commonly fabricated from metal. People have become used to cooking in
metal containers, both for the methods of cookirig used and the taste and tex-
ture of the foods produced. More recently the use of microwave ovens has
become popular, and because of the nature of microwaves, metal containers
generally can't be used in such ovens.
In the last 20 years or so, as thermoplastic polymers (TPs) having bet-
ter high temperature resistance have been developed, the use of these poly-
mers for ovenware has been proposed, see for instance U.S. Patents
4,626,557, 4,503,168, 4,585,823, 5,308,913, and 5,141,985, and European
,Patent Application 846,419, all of which are hereby included by reference.
Similar items have been made from thermosetting polymers. These polymeric
cooking containers can be used in thermal and/or microwave ovens and often
can withstand the highest temperatures usually used in these ovens, for ex-
ample about 290 C (-550 F) or more. These containers have several advan-
tages. They can be molded into practically any shape so that they may be
easily sealed and the contents can be refrigerated or frozen. Also, they are
relatively difficult to break, and are relatively low in weight. However, when
cooking food in these containers, particularly in a microwave oven, the cook-
ing method (time and/or temperature for example) may have to be varied from
the method used for a metal container, or the food will not normally have the
same taste and/or texture. For example, bread or a casserole cooked in a
plastic container in a microwave oven may not be browned on the outside sur-
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surface. This is primarily due to the fact that in microwave ovens the heat is
relatively uniformly transmitted into the depth of the items being cooked, as
opposed to being conducted in from the surface. Also, in a microwave oven
there are usually no hot surfaces to impart browning to the item being cooked.
In order to overcome the lack of browning in microwave ovens, as
reported in World Patent Application 01/34720, ceramics containing
susceptors have been incorporated into ovenware. Often, the susceptor-
containing ceramic is in the form of a plate that is part of a ceramic piece
of
ovenware. This ovenware has the drawbacks of being heavy and brittle.
Also, the materials for this ovenware are expensive and hard to form.
In Japanese Patent Application 63-141591 and World Patent
Application 01/34702 it has been suggested that polymers resistant to high
temperatures, especially liquid crystalline polymers (LCPs), be filled with
materials that are susceptors. Materials that have been used include barium
titanate and small amounts of carbon fibers. The resulting composition, when
formed into ovenware and used in microwave ovens, is said to cause
browning of the surfaces of the items being cooked that are in contact with
the
ovenware. These surfaces are heated because the susceptors in the polymer
absorb microwave energy.
The use of various susceptor-containing compositions for use in
microwave ovens is known. See for instance U.S. Patents 5,021,293,
5,049,714, 4,518,651, 4,851,632 and 4,933,526. In the examples in these
patents, the layer containing the susceptor is typically very thin.
The use of susceptors with specific Curie points in microwave
ovenware has been reported, see for instance U.S. Patents 2,830,162,
4,362,917, 4,454,403, 5,268,546, 5,665,819 and 6,077,454. None of these
references report that a polymeric composition containing the susceptor has a
high thermal conductivity. In all of these patents the susceptor or susceptor
containing composition does not contact the food or drink, but is separated
from the food or drink by metal.
SUMMARY OF THE INVENTION
The present invention is directed to a piece of ovenware adapted for
use in a microwave oven. The ovenware, or a portion thereof, is fabricated
from a composition that comprises a mixture of a thermoplastic polymer
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having a melting point and/or glass transition point of about 250 C or higher,
or a thermoset polymer having a softening point that is about 250 C or more
and a heating-effective amount of a microwave susceptor having a Curie
temperature of about 100 C to about 300 C, and optionally with the proviso
that the composition has a thermal conductivity of about 0.70 W/m K or more
when measured through a plane of the composition.
This invention also concerns, a piece of microwaveable ovenware,
comprising a composition which comprises a mixture of a thermoplastic
polymer whose melting point and/or glass transition point is about 250 C or
more or a thermoset polymer whose softening point is about 250 C or more,
and a heating effective amount of a microwave susceptor having a Curie
temperature of about 100 C to about 300 C, wherein at least part of said
composition is in the form of an insert.
The present invention is also directed to a piece of ovenware adapted
for use in a microwave oven. The ovenware, or a portion thereof, is fabricated
from a composition that comprises a mixture of a heating-effective amount of
a microwave susceptor consisting essentially of only one or more susceptors
having a Curie temperature of about 100 C to about 300 C, and a
thermoplastic polymer having a melting point and/or glass transition point of
about 20 C or more above a highest Curie point of said susceptors or about
140 C or more, whichever is higher, or a thermoset polymer having a
softening point that is about 20 C or more above a highest Curie point of said
susceptors or about 140 C or more whichever is higher, and optionally with
the proviso that the composition has a thermal conductivity of about 0.70
W/m K or more when measured through a plane of the composition.
This invention also concerns, a piece of microwaveable ovenware,
comprising a composition which comprises a mixture of a heating-effective
amount of a microwave susceptor consisting essentially of only one or more
susceptors having a Curie temperature of about 100 C to about 300 C, and a
thermoplastic polymer having a melting point and/or glass transition point of
about 20 C or more above a highest Curie point of said susceptors or about
140 C or more whichever is higher, or a thermoset polymer having a softening
point that is about 20 C or more above a highest Curie point of said
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susceptors or about 140 C or more whichever is higher, wherein at least part
of said composition is in the form of an insert.
Also described is a piece of ovenware, comprising, a composition that
comprises a mixture of a thermoplastic polymer having melting point and/or
glass transition point of about 250 C or higher, or a thermoset polymer having
a softening point that is about 250 C or more and a heating-effective amount
of a microwave susceptor having a Curie temperature of about 100 C to about
300 C,
provided that one or both of the following conditions is met:
when said cookware is in use said composition is in direct contact
with the food or drink to be heated and/or cooked; and
said composition has a thermal conductivity of about 0.70 W/m K
or more when measured through a plane of said composition.
The invention includes a piece of ovenware, comprising, a composition
that comprises a mixture of a heating-effective amount of a microwave
susceptor consisting essentially of only one or more susceptors having a
Curie temperature of about 100 C to about 300 C, and a thermoplastic
polymer having a melting point and/or glass transition point of about 20 C or
more above a highest Curie point of said susceptors or about 140 C or more,
whichever is higher, or a thermoset polymer having a softening point that is
about 20 C or more above a highest Curie point of said susceptors or about
140 C or more whichever is higher;
provided that one or both of the following conditions is met:
when said cookware is in use said composition is in direct contact
with the food or drink to be heated and/or cooked; and
said composition has a thermal conductivity of about 0.70 W/m K
or more when measured through a plane of said composition.
This present invention also includes a process for cooking in a
microwave oven, comprising contacting an item to be cooked with the
compositions and ovenware described above and exposing the food and the
composition to microwave radiation.
In all of the above microwaveable ovenware and in processes for its
use it is preferred that the ovenware is configured so that the food or drink
which is being heated and/or cooked is in direct contact with the composition
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which comprises the susceptor having a certain Curie point temperature
range.
BRIEF DESCRIPTION OF THE FIGURES
Figures 1-6 illustrate various pieces of microwave ovenware containing
inserts that contain susceptors and have relatively high thermal conductivity.
Figure 7 illustrates an ovenware top of similar construction.
DETAILS OF THE INVENTION
As used herein, the following terms shall have the following meanings.
By a "susceptor" or "microwave susceptor" (which includes CTSs) is
meant a substance that absorbs microwave radiation (MR) of the frequencies
that are used in the microwave ovens. Typically, such frequencies are about
2450 MHz in ovens that are used to cook and/or heat food. Alternatively, the
frequency may be 950 MHz or 896 MHz, particularly in commercial microwave
ovens. Susceptors may vary in their efficiency in absorbing such MR (see
below). When the susceptor absorbs microwave radiation, the energy of the
MR is converted to heat.
By "a heating-effective amount of a (microwave) susceptor" is meant
an amount such that when an ovenware part containing the susceptor is
subjected to MR that part can be heated by the MR such that the food or drink
in contact with the part will be heated, preferably cooked, and more
preferably
browned, seared, or undergo a similar process (collectively herein, browned).
By "ovenware" herein is meant an apparatus that is in contact with the
food or drink while it is being cooked and/or heated in an oven, preferably a
microwave oven. It may be a "container" such as a bowl, pan (with sides),
cylindrical (i.e., the shape of drinking glass), or it may be flat, similar in
shape
to a flat stone for cooking, for example pizza. In some instances the
apparatus may have a cover which may or may not absorb MR. In one
preferred form the ovenware is reusable, that is its design and durability are
such that it can be reused multiple times, for example much like a metal
frying
pan can be reused many times. Preferably the cookware is used in a
cooking/heating process at least 5 times, more preferably at least 10 times.
By an "insert" herein is meant a part of a larger apparatus that is
usually a piece of ovenware and that is different in composition than the
remainder of the piece of ovenware. The insert may be permanently attached
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to the piece of ovenware or may be detachable or not attached at all to the
rest of the ovenware part. For example, if the rest of the piece of ovenware
is
also a composition which contains thermoplastic polymer, a round cooking
pan may be formed by first forming a disk of the composition that contains the
susceptor, and then overmolding the disk with a second composition that
does not contain a susceptor so that the disk forms the bottom interior
surface
of the pan. The edge of the disk may be beveled to that it is locked in place
when the disk is overmolded by the second composition (see Fig. 1). Another
example is a round disk of the susceptor-containing material may be molded
and simply placed on top of a disk of the second composition, and the food or
drink to be cooked placed on top of the disk that contains the susceptor (see
Fig. 2). In both instances the parts that contain the susceptor are considered
to be inserts.
By a "mixture" herein is meant a mixture of ingredients (polymer,
susceptor, filler, for instance) that is mixed, preferably reasonably
uniformly.
It does not include an item that contains layers of one or more, but not all,
of
the ingredients.
By "food" herein is meant cooked or uncooked food and/or drink that is
desirable to cook and/or heat.
By "a microwave susceptor consisting essentially of only one or more
susceptors having a Curie temperature of about 100 C to about 300 C" is
meant that the only susceptor(s) present are those that have a Curie
temperature of about 100 C to about 300 C. No susceptors that have a
higher or lower Curie temperature or have no Curie temperature before they
decompose, melt or vaporize are present in the composition.
By "the food or drink which is being heated and/or cooked is in direct
contact with the composition which comprises the susceptor having a certain
Curie point temperature range" is meant that the food or drink is in contact
with this composition either directly or through a polymeric "membrane" or
"film" which is not more than 0.3 mm, preferably no more than 0.2 mm thick.
Such a polymeric film may be a release coating such as a fluorocarbon or
silicone release material, or a film for the physical protection of the of the
susceptor containing, or a film with a pleasing color for better visual
appearance, etc. Not all of the food must contact all of the surface of the
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composition. For example, when cooking a pizza the bottom of the pizza will
often become uneven as it cook and the bottom browns, so not all of the pizza
bottom will be in contact with the composition.
The composition herein that contains the susceptor also contains a
thermoplastic (TP) or thermoset polymer (TSP).
TPs can be reformed by melting the thermoplastic and then cooling it
below its melting point and/or glass transition temperature. Such polymers
are not crosslinked. Except as otherwise noted, the TPs have a melting point
and/or glass transition temperature above about 250 C, preferably above
about 300 C, more preferably above about 340 C, and especially preferably
above about 370 C, when measured by differential scanning calorimetry, with
the melting point being taken as the peak of the melting endotherm, and the
glass transition temperature as the middle of the transition. Such
measurements can be done following ASTM method D3418, at a heating rate
of 10 C/min. If the polymer has no melting point (if amorphous) and no glass
transition temperature, then its decomposition point shall be used.
The TPs useful in the present invention should preferably have
sufficient thermal resistance so that they will not melt or flow when exposed
to
MR in a microwave oven, when they contain food and/or drink, as they are
designed to do. More preferably they should not melt or flow when the
ovenware is exposed to MR in a microwave oven and food or drink is not
present. Typically such ovens for household use have a maximum output of
microwave energy of about 1500 watts.
Useful thermoplastics include polyolefins; polyesters such as
poly(ethylene terephthalate) and poly(ethylene 2,6-napthalate); polyamides
such as nylon-6,6 and a polyamide derived from hexamethylene diamine and
isophthalic acid; polyethers such as poly(phenylene oxides); poly(ether-
sulfones); poly(ether-imides); polysulfides such as poly(p-phenylene sulfide);
liquid crystalline polymers (LCPs) such as aromatic polyesters, poly(ester-
imides), and poly(ester-amides); poly(ether-ether-ketones); poly(ether-
ketones); fluoropolymers such as polytetrafluoroethylene, a copolymer of
tetrafluoroethylene and perfluoro(methyl vinyl ether), and a copolymer of
tetrafluoroethylene and hexafluoropropylene; and mixtures and blends
thereof.
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A preferred type of TP is an LCP. By a "liquid crystalline polymer" is
meant a polymer that is anisotropic when tested using the TOT test or any
reasonable variation thereof, as described in U.S. Patent 4,118,372, which is
hereby included by reference. Useful LCPs include polyesters, poly(ester-
amides), and poly(ester-imides). One preferred form of polymer is "all
aromatic", that is all of the groups in the polymer main chain are aromatic
(except for the linking groups such as ester groups), but side groups which
are not aromatic may be present.
TSPs may be used in place of TPs in the susceptor-containing
compositions. The TSPs should have a softening temperature of about 250 C
or more, preferably above about 300 C, more preferably above about 340 C,
and especially preferably above about 370 C, when measured by ASTM
method D648 (Heat Deflection Temperature), Method A, at a load of 1.82
MPa. Useful TSPs include epoxy resins meant for high temperature use, and
bis(maleimide)triazines.
TPs are preferred types of polymers for use in the present invention.
The thermal conductivities of virtually all TPs and TSPs, including
those types listed above, is generally <<1 W/m K. Because microwaves are
absorbed within the depth of the susceptor-containing material, it is possible
to have substantial temperature gradients through the thickness of the
material. These gradients may be large enough that the interior of the
susceptor-containing material melts while the outer surface stays solid. To
avoid such situations, the susceptor-containing material should have a
thermal conductivity of about 0.7 W/m K or more.
The thermal conductivity of the susceptor-containing composition can
be raised by mixing the TP or TSP with a particulate material (filler) which
itself has a relatively high thermal conductivity, such as about 10 W/m K or
more, more preferably about 20 W/m K or more. Useful fillers are reported
below with their approximate thermal conductivity at about 273 K in
parentheses, as reported by Y. S. Touloukian, et al., in Thermophysical
Properties of Matter, Vol. 2, IFI/Plenum, New York, 1970.) Useful fillers
include graphite (including carbon black and carbon fibers) (50-200, varies
widely), MgO (60), BeO (200), alumina (45-150), zinc oxide (28), CaF2 (700),
boron nitride (125-300, not in Touloukian) and SiC (-100-500). Preferred
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thermally conductive fillers are graphite, MgO, and alumina, carbon black and
carbon fibers. Especially preferred thermally conductive fillers are graphite,
carbon black, boron nitride and carbon fibers.
More than one such filler may be used. Generally speaking, the higher
the amount of thermally conductive filler used, the higher the thermal
conductivity of the TP or TSP composition will be. The upper limit of
thermally
conductive filler that can be used may be determined more by its effect on the
physical strength and toughness of the composition than by the actual thermal
conductivity achieved. These thermally conductive fillers, especially those
that
are not also susceptors, may optionally be added to the susceptor containing
compositions to increase the thermal conductivity to the desired level.
Useful susceptors are known in the art. Materials that are useful
susceptors include selected inorganic compounds, semiconductors and poor
electrical conductors such as carbon and metals. Specific materials include
aluminum (powder or dust), carbon (in various forms such as carbon black,
graphite powder, and carbon fiber), barium titanate, and metal oxides such as
zinc oxide, and iron oxides such as magnetite. In some instances metals may
not be a favored form of susceptor.
A favored form of susceptor comprises a susceptor that has a minimum
Curie temperature of about 100 C, more preferably about 120 C, and
especially preferably about 150 C and a maximum Curie temperature of about
300 C, more preferably about 275 C, and especially preferably about 250 C.
It is to be understood that any such minimum temperature may be combined
with any such maximum temperature to form a preferred Curie temperature
range. Such susceptors are termed herein "Curie temperature susceptors",
CTS. Preferably the susceptor in the composition contains only one or more
CTSs.
By a Curie point temperature is meant that the susceptor has a
temperature above which it is not a susceptor, i.e., it does not absorb a
significant amount microwave radiation above that temperature. Although not
bound by theory, it is believed that this is due to a phase change which
results
in the material changing from ferromagnetic or ferrimagnetic to paramagnetic
material around the Curie temperature, see McGraw-Hill Encyclopedia of
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Science and Technology, 7 th Ed., Vol. 4, McGraw-Hill Book Co., New York,
1992, p. 623, which is hereby included by reference.
Preferably the CTSs are the only susceptors in the composition. When
this is the case, the thermoplastic polymer having a melting point and/or
glass
transition point of about 20 C or more above a highest Curie temperature of
said susceptors or about 140 C or more whichever is higher, or a thermoset
polymer having a softening point that is about 20 C or more above a highest
Curie temperature of said susceptors or about 140 C or more whichever is
higher. Preferably these melting, glass transition or softening temperatures
are about 40 C or more above the highest Curie temperature, more preferably
about 60 C or more above the highest Curie temperature, and especially
preferably about 100 C or more above the highest Curie Temperature. If only
one such susceptor is present, then its Curie temperature is used for the
purpose of calculating the minimum melting, glass transition or softening
temperature, as appropriate.
If the compositions containing the CTS(s) are present in the cookware
in the form of inserts, or the cookware otherwise contains polymeric
compositions that do not contain susceptors, it is preferred that at least
those
polymeric compositions in direct contact with the compositions containing the
CTS(s) have the same thermal properties, i.e., melting, glass transition or
softening points, as are required or preferred for the CTS containing
compositions. Preferably the CTS containing compositions are in (at least
partial) direct contact with the food being heated and/or cooked while being
exposed to the MR.
Useful CTSs are known in the art, for example ferrite materials, see for
instance U.S. Patent 5,665,819, 6,077,454, both of which are hereby include
by reference, and ferromagnetic alloys and metals. Preferably the CTS is not
a metal or metal alloy. Preferably the CTS is a ferrite, a magnetic oxide, see
for instance McGraw-Hill Encyclopedia of Science and Technology, 7th Ed.,
Vol. 7, McGraw-Hill Book Co., New York, 1992, p. 53-54, which is hereby
included by reference.
If the cookware being exposed to the MR is empty, or part is not in
contact with the food, or the food/drink is dried out by heating too long, it
is
possible the melting, softening and/or decomposition temperature of the TP or
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TS resin may be exceeded, thereby causing damage to the cookware.
Indeed if the temperature of the cookware or its contents gets too high, a
fire
may result. If the susceptor is a CTS, then when the cookware (actually the
CTS) reaches its Curie point it will cease (or at least diminish) the
conversion
of the MR generated by the microwave oven to thermal energy and a further
temperature rise will be slowed or not occur. This is a safety feature which
prevents overheating of the cookware to protect the cookware and also to
prevent a fire. In some cases, particularly when the cookware has a specific
purpose, for example is a "pizza stone" to heat a pizza, the Curie point may
be chosen so that the bottom surface of the pizza is not overheated by the
cookware, and an optimum browning (for instance in the case of pizza crust)
occurs.
Indeed, different inserts containing CTSs having different Curie
temperatures may be used to optimize heating, browning or crisping in
different parts of the food item, or to make up for uneven distribution of the
MR in the oven. For example when cooking a round pizza which is a bit
smaller in diameter than the length and/or depth of the microwave oven,
oftentimes less microwave radiation reaches the bottom of the crust near the
center of the pizza compared to the periphery of the pizza. In that case an
insert underneath the center area of the pizza may contain relatively more
CTS and or use a CTS with a higher Curie temperature than an insert which
contacts the bottom of the pizza more towards the periphery of the pizza.
Other such combinations will be evident to the skilled person.
The concentration of the susceptor in the TP- or TSP-based
composition (along with the mass of the composition and the efficiency of the
susceptor) will determine what fraction of the MR will be absorbed. The higher
the fraction absorbed by the susceptor-containing composition, the less is
available to be directly absorbed by the food or drink in any particular
cooking
situation. The higher the fraction of MR that is absorbed by the susceptor-
containing composition, the hotter that composition will get. In some
instances the concentration of the susceptor in the TP- or TSP-based
composition may also affect the temperature which the composition can reach
in the microwave oven. If this does happen, generally speaking the higher the
concentration of susceptor, the higher the temperature that can be reached,
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unless the susceptor stops absorbing microwave radiation because it is above
its Curie point.
One material may serve to function as both a susceptor and thermally
conductive filler. For example various forms of carbon are susceptors and
have high thermal conductivities. Carbon, for instance in the form of graphite
powder, carbon fiber or carbon black is a preferred combined susceptor and
thermally conductive filler. Metal in the form of powders or dusts also are
susceptors and have high thermal conductivities.
Preferably the filler(s) and susceptors should be relatively small
particles. Typically the largest dimension (on average)for a particulate
material should be less than about 500 m, and if fibrous material is used the
length(on average) should be less than 1 mm. The fillers and susceptors are
preferably uniformly dispersed in the TP or TSP. They may be mixed into the
TP using standard melt mixing techniques and equipment, such as single or
twin screw extruders. They may be mixed into the TSP before the TSP is
crosslinked by standard mixing methods used for uncrosslinked TSPs.
The thermal conductivity of the compositions in some cases must be
about 0.7 W/m K or more, preferably about 1.0 W/m K or more, more
preferably about 2.0 W/m K or more, very preferably about 3.0 W/m K or
more, and especially preferably about 5.0 W/m K or more. However, inserts
and other ovenware types which contain susceptors, as described herein,
need not have relatively high thermal conductivities in all instances, for
example where the insert is relatively thin and or is efficiently cooled in
the
ovenware of which it is a part. The TP or TSP preferably are present as a
continuous phase in the composition. Typically the high thermal conductivity
filler (or susceptor if the susceptor also has a high thermal conductivity) is
about 5 to about 85% by weight of the composition. The thermal conductivity
of the composition is measured through a plane (thinnest cross section) of a
test part or piece of ovenware, using ASTM Method E1530.
The thermal conductivity of polymer compositions (not containing
substantial amounts of susceptors) previously described for ovenware is
typically quite low. For instance, using an LCP with the same composition as
LCP-4 of U.S. Patent 5,110,896, which is hereby included by reference, and
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which had the composition (in molar parts) of 50/50/70/30/320
hydroquinone/4,4'-biphenol/terephthalic acid/2,6-napthalene dicarboxylic
acid/4-hydroxybenzoic acid, a composition containing 51.6% LCP, 13% of a
blue pigment concentrate in the LCP, 35% talc and 0.56% Ultranox
antioxidant (all percentages are by weight of the total composition) was made
and molded into a disc. At 100 C the through the plane of the disc heat
conductivity was 0.40 W/m K.
One important consideration in the design of ovenware containing a
susceptor is transferring the heat that is generated by the absorption of the
MR by the susceptor to the food being cooked. This is particularly true when
the susceptor is contained in a part whose thickness (at least in a portion of
the part) is about 50 m or more, more particularly when the thickness is
about 100 m or more, and very particularly when the thickness is about 200
m or more. Polymers in general have poor thermal conductivity. If the heat
within the susceptor-containing material is not conducted out of that
material,
its temperature, especially internally, will rise, and heat will not be
efficiently
transmitted to the food being cooked. This, of course, negates some of the
benefits of using a susceptor-containing material. Perhaps just as important,
if the temperature of the susceptor-containing material (particularly a
susceptor that does not have a Curie temperature below the melting or
softening point of the polymer) rises to the melting point or glass transition
temperature (whichever is higher) of the TP or TSP of the composition, the
susceptor-containing material may melt, decompose, or even catch fire, or the
food being cooked may be ruined and/or catch fire. The same may be true for
a composition that is in contact with the susceptor-containing material, for
example the material in contact with the inserts as shown in Figures 1 to 6.
Therefore, when the susceptor-containing material has thicknesses as
described above, it is advantageous that the susceptor-containing
composition also have a relatively high thermal conductivity.
Figures 1-6 show various pieces of ovenware constructed according to
the present invention. Fig. 1 shows a frying or cooking pan from the top (Fig.
1 a) and in cross section (Fig. 1 b). In Fig. 1 a, 1 is an insert containing a
susceptor which has an upper surface 5. 2 is the body of the frying pan that
is
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made of a thermoplastic composition that has no susceptor and a relatively
low thermal conductivity. The body 2 is overmolded over the edge of 1, and
both of 3 are (optional) molded-in handles. Fig. lb shows 1, 2, and 3 in cross
section, and in particular shows the beveled edge 4 of 1, and how 2 is
overmolded over this beveled edge to hold I in place. In addition the pan of
Fig. 1 may also have feet (not shown, and which may be molded as part of 2)
of the composition of 2 to hold the pan above the bottom of the microwave
oven, thereby allowing the MR to readily heat the bottom center of I by
bouncing off a metal bottom of the microwave oven. Because handles 3 are
made from a composition not containing a susceptor and have relatively low
thermal conductivity, they remain relatively cool through the cooking process,
and often allow the pan to be picked up without burning the cook's fingers.
The high thermal conductivity of the insert 1 allows heat to flow readily to
the
upper surface 5 and hence to the food being cooked. This prevents
overheating of I and/or 2. The pan of Fig. 1 may be used to brown the
bottom of food while cooking, as a frying pan (with or without added oil or
grease), or may be used much as a pot on a surface cooktop. Most of the
comments about the pan of Fig. 1 are applicable to the ovenware shown in
the other Figures.
Figure 2 shows a cross section of a pan similar to that in Fig. 1, except
insert 6 has an edge 7 which is "reverse beveled", allowing 6 to be removed
from the body 8 for easy cleaning of 6 and/or 8, and/or easy replacement of 6
and/or 8.
Figure 3 shows a top view (Fig. 3a) and a cross section (Fig. 3b) of a
"pizza stone" in which an insert 9 merely rests upon the base 10 that has
handles 11. The base may optionally have molded in feet (not shown) to hold
the stone above the bottom of the microwave oven as is also optional in the
pan. The base 10 may have a slight recess slightly larger than the diameter
of 9 so 9 does not readily slide off 10 if the pizza stone is tilted during
transport (to the table). Fig. 3c shows an alternate construction of 9 that
has
raised edges 12. These raised edges may prevent the pizza from slipping off
9, and/or aid in browning the edges of the pizza (not shown).
Figure 4 shows top views of several rectangular pans 13, with each
pan having two or more inserts 14 present in various patterns. These inserts
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may be overmolded much like the inserts of the pan of Fig. 1, and each of 14
may have beveled edges similar to 4.
Figure 5 shows a cross section of a microwave wok, with the insert 16
being at the bottom of the generally spherical wok (the thermal heat source
for
a wok is usually underneath the wok) being overmolded with a body 17 that
has handles 18.
Fig. 6 shows a top view (Fig. 6a) and a cross section (Fig. 6b) of a
cylindrical piece of ovenware having an insert 19 inside a body 20 that has
handles 21. The insert may cover the entire bottom and most of the interior
sides as shown in Fig. 6, or. may just cover part of the interior bottom
and/or
sides (not shown). By using the microwave oven at a low power level this
type of ovenware may simulate a so-called slow cooker or crock cooker,
which not only heats the food being cooked but also tends to brown the food
in contact with the cooking vessel sides and/or bottom.
The ovenware item (such as those in Figures 1-6) may also have a top
(see Figure 7) which fits on top of the ovenware item or directly on top of
the
food in the ovenware item. This top may simply be used to close off the top of
the ovenware item, much as a top is used on a conventional metal pot.
However the top, particularly when in direct contact with the food, may also
be
the composition as described herein containing a susceptor and having a
certain minimum thermal conductivity. The susceptor-containing composition
may be present in the top as an insert, analogous to the inserts shown in
Figures 1-6. In some instances only the top may contain the susceptor-
containing material, particularly when the object is to brown the top of the
food
item being cooked. In that instance the ovenware item may be an ordinary
ovenware item as is used today. Figure 7 shows such a top with the
susceptor-containing composition present as insert 22 overmolded with 23,
another composition which does not contain a susceptor, the overmolding
composition having handle 24. In some instances it may be preferable that 25
be smaller than the container holding the food (not shown), so that the top
directly contacts the top of the food (not shown). Tops containing the
susceptor-containing compositions described herein are also considered
pieces of ovenware herein.
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It is noted that in all of the above Figures (and generally) where
multiple inserts are present in a piece of ovenware each of the inserts may
have a different susceptor or combination of susceptors with different Curie
temperatures. In this way different parts of the food in the ovenware may be
heated differentially. Also, single inserts shown in the Figures may be
divided
into two or more contiguous inserts containing different susceptors or
combinations of susceptors with different Curie temperatures.
In another type of ovenware, especially for commercial applications,
the susceptor-containing material may be a conveyor belt, especially the
surface of the conveyor belt which contacts the food resting upon the
conveyor belt. This would therefore be a mechanism for browning the surface
of the food that was in contact with the surface of the conveyor belt.
In some instances it may be preferable that the surface of the food
being cooked and/or heated in contact with the susceptor containing polymer
be crisp when the cooking/heating is completed. For instance pizza crusts
and many baked goods often preferably have a crispy surface. If the surface
of the susceptor containing polymer in contact with the food is smooth, water
vapor formed during the cooking/heating process may not be readily able to
escape, thereby resulting in a soft (mushy/soggy) textured food surface. It
has been found that if the surface contains "water vapor escape channels" the
surface of the food is often much crispier. By these channels is meant
grooves, surface irregularities, knurled pattern channels, holes or other
"channels" through which the water vapor formed in the cooking/heating
process may escape from between the food's surface and the susceptor
containing polymer's surface. Such channels may be formed by convention
means, for example they may be machined or embossed into the surface of
the susceptor containing polymer, but it is preferred that these channels be
formed during the molding process for insert or other surface of the susceptor
in contact with the food surface.
A preferred type of food using the cookware described herein and/or in
the cooking processes described herein is pizza.
The manufacture of the above cookware items can be carried out by
conventional melt forming techniques, for example injection molding. When
an insert is "locked into" the ovenware item, for instance as in Fig. 1, the
insert
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may be overmolded by a polymeric composition that does not contain a
susceptor. In this type of situation it is preferred that the TP or TSP of the
insert and of the overmolding composition have the same or nearly the same
composition (the polymer itself, not the fillers and other materials, if any
mixed
with the polymer). This helps to avoid differential shrinkage and subsequent
cracking and/or loosening of the insert. If LCPs are used for both parts, it
is
preferred that the overall orientation in these parts from the molding
operation
be in the same direction in the final ovenware item. If the melting points or
glass transition points of polymers of the insert and overmolding composition
are the same or similar, one must be cautious not to significantly melt or
otherwise deform the insert during the overmolding operation.
All of the polymeric compositions described herein may contain other
ingredients typically added to thermoplastics (or thermosets), such as
fillers,
reinforcing agents, plasticizers, flame retardants, pigments, antioxidants,
antiozonants, and lubricants, in the amounts usually used for such
compositions. These additives may somewhat affect the thermal conductivity,
and any thermal conductivity limitations must still be met.
The ovenware items may be coated fully or partially (including coating
of compositions containing a susceptor) with various types of release coatings
which prevent the food being cooked from sticking to the ovenware and/or
allowing easier cleaning of the ovenware. For example various types of
fluoropolymer-containing coatings may be used, such as those available
under the Teflon and Silverstone brand names. Silicone release coating
may also be used.
For the susceptor-containing compositions (such as the inserts in
Figures 1-6) to have a desirable level of durability in a reusable piece of
ovenware, it should preferably be at least about 0.25 mm thick, more
preferably at least about 0.50 mm thick.
The ovenware items described herein are especially useful in
microwave ovens wherein the usual mode of cooking (cooking herein includes
both initial cooking and simple (re)heating of food and drink) is absorption
of
MR by the food or drink being cooked or heated. Most microwave ovens use
just MR to provide thermal energy, but some also have a convection (thermal)
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heating source. The ovenware described herein may be used in both of these
types of microwave ovens. The ovenware items may often also be used in
"ordinary" convection ovens since these items often have good thermal
resistance.
When cooking in a convection oven or a microwave oven one wants to
balance the overall thermal history of the mass of food with the amount of
browning that takes place, usually on one or more surfaces. For example in a
convection oven if the temperature of the oven is too high the outside surface
of the food item may brown too much, i.e. be burned, before the interior of
the
item is fully cooked. Thus a loaf of bread could be burned on the outside
before the interior of the loaf is fully baked. In order to control such
factors in
a convection oven, through experience and experimentation cooks have
learned to adjust food recipes, the cooking temperature, the cooking container
(material), and the shape and mass of the food being cooked to control the
relative amounts of cooking vs. browning. Similar considerations come into
play in cooking in a microwave oven with the present ovenware. To some
extent one should preferably balance the amount of MR which is directly
absorbed by the food being cooked with the amount of MR which is being
absorbed by the susceptor and converted to heat. This heat from the
susceptor is conducted by the (relatively high heat conduction) composition to
the surface of the food being cooked, where the heat is transferred to the
food's surFace. The relative amounts of MR absorbed directly by the food and
the susceptor are influenced by the relative masses, shapes and
configurations of the susceptor and the food, as well as the pattern of MR in
the oven. In turn the mass, shape and configuration of the susceptor is
determined by concentration of the susceptor in the susceptor-containing
composition, the volume and especially the thickness of this composition, the
thermal conductivity of the susceptor-containing composition, and the relative
position of this composition versus the position of the food in the oven.
Another variable that can affect the amount of browning vs. the degree of
cooking is the recipe for the food itself. Some experimentation may be
needed, but this is a normal process when designing new cooking containers
and/or recipes for foods.
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In order to obtain the desired balance of properties, particularly MR
absorption efficiency and thermal conductivity, in the susceptor-containing
compositions, one can use separate materials to act as a susceptor and as a
thermally conductive filler, and balance each property separately (the
presence of these two materials will affect the performance of the other
somewhat because each occupies a volume percentage of the composition),
or it may be possible to use a single material which is a susceptor and is
highly thermally conductive, or any combination of these.
Curie temperatures may be determined by an adaptation of ASTM
Method E1582-00, Procedure C. This method is designed to calibrate
instruments for thermogravimetric analysis using, in this procedure, a Curie
temperature standard. Instead of using a standard, the analysis is run with
the material whose Curie temperature is to be determined on a previously
calibrated instrument. The heating rate is 5 C/min. As shown in Figure 1 of
the method, the Curie temperature is taken as the inflection point of the
temperature vs. apparent weight chart which is generated.
In the Examples, the following materials were used:
Boron Nitride (BN) - PolarTherm PT160 from General Electric
Advanced Materials, USA
Ferrite 1- A Mn/Zn ferrite reportedly having the formula
(MnO*ZnO-Fe2O3)(FeO-Fe2O3) and a reported Curie temperature of 125 C and
obtained from Steward, Inc., Chattanooga, TN 37407, USA, and sold by them
as #74000. This ferrite also reportedly has a density of 5.15 g/cm3, and an
average particle size of 10 m (Coulter).
LCP A - A liquid crystalline polyester containing repeat units
derived from hydroquinone/terephthalic acid/2,6-naphthalene dicarboxylic
acid/4-hyroxybenzoic acid in the molar ratio 100/5/95/100, and had a melting
point of 359 C.
Example 1
A melt blend of 50 weight percent LCP A and 50 weight percent Ferrite
1 was made by melt mixing in a 30 mm Werner & Pfleiderer (Stuttgart,
Germany) twin screw extruder. The LCP was fed at the back and Ferrite 1
was side fed. There were 9 barrels, and barrel 2 was 148 C, barrel 3 was
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304 C, and the rest of the barrels were 345-357 C. The screw was operated
at 250 rpm, which gave a production rate of about 13.6 kg/h. The resulting
blend was extruded into strands and cut into pellets. These pellets were then
injection molded, in an HPM 6 oz. single screw injection molding machine with
barrel temperatures set to 370 C, into circular 10.2 cm dia. x 0.32 cm thick
discs.
A disc was placed into a Panasonic Model NN-6470A microwave
oven (this oven was made for the consumer cooking market) equipped with a
turntable and which had a power input of 1.54 kW. Also placed into the oven
was a beaker containing 400 ml of water to absorb "excess" microwave
radiation if the ferrite was above the Curie temperature (this water was
changed as needed to prevent excessive boiling). The oven was turned on at
high power for 1 min, and then the disc was removed briefly and its
temperature measured with an infrared pyrometer. The total time of exposure
to microwave radiation and the measured temperature are given in Table 1.
Table I
Time, Temp.,
Min C
0 28.9
1 151.0
2 172.0
3 187.0
4 198.7
223.0
6 233.2
7 235.6
9 245.0
257.9
11 265.0
A 5.08 cm diameter disc was cut from one of the molded discs and its
thermal conductivity through its thickness measured. The composition had a
thermal conductivity of 0.355 W/m K.
Example 2
The thermoplastic composition used was the 10.2 cm diameter discs
described in Example 1. The microwave oven used was also as described in
Example 1, but without the 400 ml of water present.
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The first food to be tested were "Bagel Bites" with a cheese, sausage
and pepperoni topping. These were partially cooked bagel halves (cut
perpendicular to the bagel hole's axis) topped on the flat side with the
meats,
cheese and tomato sauce. After some experimentation with varying times
and power levels, after cooking for 7 min. at low power the topping was nicely
heated and melted, and the bottom of the bagel was nicely browned where it
had contacted (or was in near contact) the disc. At the end of the cooking the
disc temperature was measured as 116 C.
The next food to be tested was a"Celeste " Pizza for One", topped
with cheese, tomato sauce and pepperoni. The pizza was cut into 8 wedges,
and the wedges were individually tested. Again after some experimentation
varying times and cooking levels, a nicely cooked pizza with a browned
bottom crust was obtained after 6 min at low power. At the end of cooking the
disc temperature was 99 C.
Example 3
A melt blend of 1157 g percent LCP A, and 679 g Ferrite 1, and 727 g
of BN was made by melt mixing in a 30 mm Werner & Pfleiderer (Stuttgart,
Germany) twin screw extruder. The materials were powder blended and fed
at the back of the extruder. There were 9 barrels, and barrel 2 was about
223 C, barrel 3 was about 302 C, and the rest of the barrels were 344-358 C.
The screw was operated at 300 rpm, which gave a production rate of about
13.6 kg/h. The resulting blend was extruded into chunks and the chunks
chopped into "pellets". These pellets were then injection molded, in an HPM 6
oz. single screw injection molding machine with barrel temperatures set to
370 C, into circular 10.2 cm dia. x 0.32 cm thick discs.
Using the microwave oven of Example 1 a disc was placed into the
oven. Also placed into the oven was a beaker containing 400 ml of water to
absorb "excess" microwave radiation if the ferrite was above the Curie
temperature (this water was changed as needed to prevent excessive boiling).
The oven was turned on at high power for 1 min, and then the disc was
removed briefly and its temperature measured with an infrared pyrometer.
The total time of exposure to microwave radiation (in minutes) and the
measured temperature ( C) are given in Table 2.
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Table 2
Time, Temp.,
min C
0 23
1 120
2 129
3 133
4 133
134
6 139
7 140
8 143
9 142
A 5.08 cm diameter disc was cut from one of the molded discs and its
thermal conductivity through its thickness measured. The composition had a
thermal conductivity of 0.645 W/m K.
Example 4
The thermoplastic composition used was the 10.2 cm diameter discs
described in Example 4. The microwave oven used was also as described in
Example 1, but without the 400 ml of water present.
A"Celeste Pizza for One", topped with cheese, tomato sauce and
pepperoni was used. The pizza was cut into 8 wedges, and the wedges were
(except as noted below) individually tested by cooking on a disc. The
"normal" microwave cooking time for this pizza was given on the package as
3.5-4.5 minutes at full power. After some experimentation varying times and
cooking levels, a nicely cooked pizza with a browned, crispy bottom crust was
obtained after 30 sec at full (high) power. At the end of cooking the disc
temperature was 127 C (not where the pizza was). A wedge cooked for 30
sec but not on the a disc was hot and the top was cooked, but the bottom was
soft (tasted "doughy") and not browned.
A test was done where two wedges were cooked on the same disc at
the same for 1 min at high power were well cooked on top and browned and
very crispy on the bottom. Since two wedges were about 1/4t" of the pizza, 1
min at high power simulated the recommended cooking time.
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