Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
COMPOSITE MATÆRIAL CONTAINING MICROWAVE
SUSCEPTOR MATERIALS
BACKGROUND OF THE INVENTION
This invention relates to novel composites
useful for controlled generation of heat by absorption
of microwave energy.
Food preparation and cooking by means of
microwave energy has, in recent years, become widely
practiced as convenient and energy efficient. Along
with the growth in the use of microwave cooking has
been a growth in the sale and use of foods specially
packaged for microwave cooking. Such special
microwavable packages attempt to alleviate some of
the problems inherent in microwave cooking, for
example, lack of browning or crispening of the surface
of a cooked food item or uneven cooking due to
development of "hot spots" in the food item. Examples
of packaging materials developed for use in microwave
cooking are those disclosed in U.S. Patents 4,518,651,
4,267,420, 4,434,197, 4,190,757, 4,706,108, UK Patent
Application No. 2,046,060A tpublished 1980 November
05) and European Patent Application Publication No.
63,108 (published 1982 October 20).
U.S. 4,518,651 to Wolfe discloses composite
materials exhibiting controlled absorption of
microwave energy based on the presence of electrically
conductive particles such as particulate carbon in a
polymeric matrix bouncl to a porous substrate. The
resulting composite materials are to have a surface
resistivity of 100 to 1000 ohms per square. Wolfe
teaches that it is critical that at least some of the
polymeric matrix be beneath the surface of the
substrate, be substantially free of electrically
conductive particles and be intermingled with the
~ fl~
substxateO This is achieved by a lamination process
at certain temperaJ,ures and pressures.
U.S. 4,267,420 to Brastad discloses a
packaging material which is ~ plastic film or other
dielectric substrate having a thin semiconducting
coating. The semiconducting coating generally has a
surface resistance of 1 to 300 ohms per sguare, and
the preferred coating is evaporated aluminum. Similar
materials, i.e., films with a continuous layer of
electrically conductive ~aterial deposited thereon,
are also disclosed in UK Patent Application
2,046,060A.
U.S. 4,434,197 to Petriello et al~ discloses
a multi-layer structure having five layers including
outside layers of polytetrafluoroethylene, two
intermediate layers of pigmented
polytetrafluoroethylene and a central layer of
polytetrafluoroethylene containing an energy absorber.
The energy absorber can be a material such as
colloidal graphite, ferric oxide and carbon and should
have a particle size such that it will uniformly
disperse with particles of polytetrafluoroethylene to
form a co-dispersion.
U.S. 4,190,757 to Turpin et al. discloses a
microwaveable package composed of a non-lossy
dielectric sheet material defining a container body
and a lossy microwave absorptive heating body
connected thereto, the heating body possibly
comprising a multiplicity of particles of microwave
absorptive material of different particle sizes and a
binder bonding those particles together~ Absorptive
materials include zinc oxide, germanium oxide, iron
oxide, alloys such as one of manganese, aluminum and
copper, oxides, carbon and graphite. The binders for
these materials are ceramic type materials such as
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cement, plaster of paris or sodium silicate, and the
resulting materials are therefore not flexible. The
package also requires a shield, for example, a metal
foil sheet adapted to reduce by a controlled amount
the direct transmission of microwave energy into the
food product. A ~omewhat similar disclosure is found
in U.S. 4,706,108 to Anderson et al. This patent also
discloses a microwave heatin~ device comprising a
microwave reflective ~ember positioned adjacent to a
magnetic microwave absorbing material.
European Patent Application Publication No.
63,108 disclose~ a packaging material such that at
least a region of one side thereof is provided with a
coating comprising heat reflecting particles in a
predetermined pattern, in ~or instance flake or
particle shape. The heat reflecting particles
preferably consist of metal particles of aluminum or
another food-stuff inert metal and are preferably
included within a layer of polyester,
polymethylp~ntene or another material having
corresponding heat resistance characteristics. The
, content of heat reflecting particles amounts to
0.01-1% by weight of the surface weight of the
coating, and the heat resistant layer has a surface
weight of 15 to 30 grams per square meter.
Despite the many developments tc date in the
field of microwaveable packaging, certain needs gtill
exist. Many existing materials function in one way or
another to convert a portion of the microwave energy
into heat, but the materials offer little control to
the packager in terms of how much heat is generated
and how quickly. For example, some of the materials
tend to heat uncontrollably in a microwave oven,
leading to charring or even arcing, ignition and
burning of the packaging material. Other available
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materials are simply not capable of generating enough
heat quickly enough to be of use in certain
applications (e.g., providing fast heat-up and high
bag temperatures to provide ef~icient popping of
popcorn in a microwave oven). And many of the
available materials are simply not ~uitable for the
mass disposable-packaging market because they are
~imply too expensive to produce.
SUMl~ARY OF THE IN~IENTION
New packaging materials for microwave u~e
have now been found which ~olve some of the problems
inherent in prior art materials. Specifically, this
invention relates to composite materials for
controlled generation of heat by absorption of
15 microwave energy comprising ~a) a dielectric substrate
substantially transparent to microwave radiation and
(b) at least one coating on at least one surface of
the substrate, the coating comprising (i) about 5 to
80% by weight of a susceptor material in flake form
capable of converting microwave energy to heat, and
(ii) about 95 to 20% by weight of a thermoplastic
dielectric matrix, wherein the surface weight of said
coating on the substrate is în the range of about 2.5
to 100 y/m2. The D.C. surface resistance of the
resulting composite material is generally at least 1 x
106 ohms per square. These new materials offer the
advantages of being economical to produce and of being
easily adaptable so as to ma~ch the degree of heat
generated to the requirements of the food which is
packaged in it. The materials can be adapted to heat
to very high temperatures within a very short time and
thus find utility as packaging materials for food
items for which bxowning is desired but which are
cooked for relatively short periods of time (e.g.,
breadstuffs or pizza) and also for food items for
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which high temperatures and rapid heat-up are needed
to insure efficient microwave cooking (e.g., popcorn).
Despite the high degree of heat which these
materials axe capable ~f ge~erating, the amount of
i 5 susceptor material and thermoplastic matrix can be
adapted to avoid charxing, arcing or burning of the
packaging materials ~s often results from use of prior
art materials.
DETAILED DESCRIPTION OF THE INVENTION
The substrate material used in this
invention is a carrier web or film which has
sufficient thermal and dimensional stability to be
useful as a packaging material at the high
temperatures which may be desired for browning or
rapidly heating foods in a microwave oven (generally,
as high as 150 degrees C and above, preferably 220
~ degrees C and above.) Polymeric films, including
i polyester films ~uch as polyethylene terephthalate
films and polymethylpentene films, and films of other
thermally stable polymers such as polyarylates,
polyamides, polycarbonates, polyetherimides,
polyimides and the like can be used. Porous
structures such as paper or non-woven materials can
also be used as ~ubstrates 60 long as the required
thermal and dimensional ~tability is satisfied. For
flexible packaging, the ~ubstrate is preferably about
8 to 50 micrometers thick. Thicker, non-flexible
materials~ ~uch as found in trays, lidding, bowls and
the like, could also be used. The preferred substrate
is biaxially oriented polyethylene terephthalate which
is preferably about 12 micrometers thick.
As previously indicated, the substrate must
have ~ufficient dimensional stability at the elevated
temperature~ involved in microwave cooking to prevent
distortion of the substrate which may result in
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non~uniform cooking from loss of intimate contact of
the packaging material with the food to be cooked.
Substrates lacking such high temperature dimensional
~tability can be used if they are laminated with yet
another substrate layer meeting the thermal ~tability
requirements of the original ~ubstrate. The
lamination can be accompli~hed either by taking
advantage of the adhesive properties of the
thermoplastic ~atrix coating on the original substrate
or by using any number of conventional adhesives to
aid in forming a ~table laminate. For example, a
composite of this invention ~uch as a polyester
copolymer coated polyethylene terephthalate film can
be thermally ~ealed to another polyester film or to
paper or heavier ovenable paperboard. Alternatively,
another adhesive can be applied from ~olution prior to
lamination to increase the strength of the laminate.
These supplemental adhesives can be selected from a
number of commercially available candidates with
required thermal stability. These include
copolyesters, copolyester-polyurethanes and
cyanoacrylates.
The thermoplastic dielectric matrix used in
the composite of this invention can be made from a
variety of polymeric materials with 6uffi~ient thermal
stability to allow for dimensional integrity of the
final packaging material at the elevated temperatures
associated with microwave cooking of food. The
dielectrical properties at 915 megahertz and 2450
megahertz of the matrix is ~180 an important variable
in terms of the heat generated in unit time at 2450
MHz. The dielectric matrix has a relative dielectric
constant o~ about 2.0 to 10 with a preferred value of
2.1 to 5.0, and a relative dielectric loss index of
about 0.001 to 2.5, preferably 0.01 to 0.6. The
~L~7~ 6
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matrix ~150 preferably displays adhesive
characteristics to the substrate in the composite and
any additional cubstrate to which the composite may be
laminated to increase dimensional tability. For best
results, the peel strength of the matrix to
substrate(s) ~eal should be at least 400 to 600 g/in.
A variety of polymeric materials known in the art meet
these reguirements. Examples include but are not
limited to: polyester~, polyester copolymers, curable
resins such as copolyester-polyurethanes and epoxy
resins, polycarbonates, polyethersulfones,
polyarylsulfones, polyamide-imides t polyimides
polyetheretherketones, poly 4'4-isopropylidene
diphenylene carbonate, imidazoles, oxazoles, and
thiazoles. These materials may be crystalline or
amorphous. The preferred matrix is a polyester
copolymer. These are reaction products of a glycol
and a dibasic acid. Suitable glycols include ethylene
glycol, neopentyol, mixtures of 1,4-butane diol,
diethylene glycol, glycerin, trimethylethanediol and
trim~thylpropanediol. Suitable dibasic acids include
azealic, ~ebacic, adipic, iso-, tere- and
ortho-phthalic, and dodecanoic acids. The preferred
polyester copolymer is a copolymer or mixture of
copolymers, of ethylene glycol with terephthalic and
azealic acid or with terephthalic and isophthalic
acid~
The susceptor materials used in this
invention are metals and metal alloys which are
capable of absorbing the electric or magnetic portion
of the microwave field energy to convert that energy
to heat. Suitable such materials include nickel,
antimony, copper, molybdenum, bronze, iron, chromium,
tin, zinc, silver~ gold, and the preferred material,
aluminum. Other conductive materials such as graphite
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and semiconductive ~aterials such as silicon carbides
and magnetic ~aterial such as metal oxides, if
available in *lake form, may also be operable
~usceptor materials and are deemed equivalent to the
susceptor materials claimed herein.
The ~usceptor ~aterial must be in flake
form. For the purpose o~ this invention, a particle
is in flake form if its aspect ratio, defined as the
ratio of the largest dimension of its face to its
~ thickness is at least about 10. Generally speaking,
the conductive materials use~ul as ~usceptors in this
invention will have an aspect ratio in the range cf 10
to 300. The preferred aluminum ~aterials will
generally have an aspect ratio in the range of 20 to
200. Those preferred aluminum materials also
generally have a largest dimension of 1 to 48
micrometers and a thickness of 0.1 to 0.5 micrometersO
As variables, the amount and the physical
size, shape and surface characteristics of the
susceptor flakes used in the coating and the amount of
that coating applied to the substrate depend on the
type and portion size of the food to be cooked. It is
by altering these variables that one may control the
generation of heat exhibited by the material when it
is used in a microwave oven. An advantage of the
composites of this invention is that they can be
tailored to heat to high temperatures in relatively
short periods of time in conventional microwave ovens,
e.g., to temperatures of about 150C or above,
preferably 190C or above, in 120 ~econds when
subjected to microwave energy of 550 watts at 2450
megahertz.
The susceptor level in the thermoplastic
matrix will generally range from about 5 to 80~ by
35 weight of the combined susceptor/matrix. The optimum
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.
_ g _
level will vary according to the particular susceptor
material selected, its size and shape. It has been
found that *or aluminum flakes, the pre~erred amount
is 20 to 70 weight % of the ~usceptor/matrix. The
amount of ~usceptor/ma~rix applied to the substrate
will generally range from about 2.5 to as high as 100
gfm2. This will lead to a dry coating thickness in
the range of as low as 1 to as high as 75 micrometers.
The amount of susceptor/ matrix coating used will, of
10 course, vary with the end use of the packaging
material. For applications where browning and
crispening o~ a ~ood product is desired, e.g., cooking
pizza, the amount of coating might be 50 to 75 g/m2.
For other applications where high temperatures and
15 rapid heat-up are desired, e.g., cooking popcorn, the
amount might be 2.5 to 15 g/m2.
The composite of this invention can be made
by a number of methods. In one method, the dielectric
matrix is dissolved in any number of common organic
20 solvents such as tetrahydrofuran, methylene chloride,
ethyl acetate, methyl ethyl ketone or similar
solvents, and then the susceptor is dispersed in this
~olution. The solution is then applied to the carrier
film or web by ~ny number of coating processes 6uch as
25 metered doctor roll coating, gravure coating, reverse
roll coating or ~lot die coating. The solvent is
driven off after application of the coating by
conventional oven drying techniques. A second
technique is useful for melt stable matrices. The
30 matrix material is melted in conventional eguipment
and the susceptor particles blended with the melt.
This mixtuxe is then extrusion or melt coated on the
~ilm or web substrate. In either case, the
application of the susceptor/matrix is a well
35 controlled process that can be readily altered to vary
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the temperature range of the composite material when
used in a microwave ~ven. This contrul is ~uperior to
that used in prior art vacuum metallizing processes
and the coating process can operate at much higher
5 speeds since no vacuum is reguired. Conceptually, the
susceptor/matrix can be applied in patterns that would
allow a variety o~ temperature properties in a single
sheet of composite material.
Ideally, packaging materials of the type
disclosed herein should have reproducible heating
performance. A consumer should be able to rely on a
~pecific mat~rial heating to a specific temperature
range within a specific time frame whenevex exposed to
microwaYe radiation in his microwave oven. In the
absence of such reproducible heating performance, a
packaging material would lac~ wide commercial utility.
To achieve heating performance
reproducibility, it has been found that the susceptor
coating should be uniform and isotropic. The term
isotropic as used herein means that the composite with
the susceptor/matrix coating will exhibit
substantially the same properties (i.e., heat to
substantially the same temperature) when exposed to
the electric field component of microwave radiation in
any direction. Tests indicate that an oblong flake of
susceptor material capable of coupling with the
electric field, for example, will couple better when
the incident electric field is parallel to the flake's
largest dimension. Therefore, the heat generated from
an oblong flake will vary from a maximum when the
incident electric field is parallel to the largest
dimension to a minimum when the incident electric
field is perpendicular to the largest dimension. If
the susceptor/matrix coating is isotropic, then,
regardless of the fact that the susceptor material is
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an oblong flake, the degree nf coupling of the
susceptor material with the incident electric field,
and, thus, the heat generated from the susceptor
coating, will not vary ~ubstantially with the
direction of the incldent electric field.
(For simplicity, this discussion is limited
to ~usceptors which couple with the electric portion
of the microwave field energy. Susceptors which
couple with the magnetic portion of the microwave
field energy are deemed to be equivalent, and the
principles disclosed herein apply equally to the
incident magnetic field in such cases.)
A substantially isotropic coating can be
achieved using oblong flakes of susceptor materials if
at least two coating layers are provided, the
direction of alignment of the flakes (i.e., the
direction Df the longest surface dimension of the
flakes) in one layer being oxiented at about ninety
degrees to the direction of alignment of flakes in the
second layer. To illustrate, when a coating of oblong
flake susceptor/matrix is applied to the substrate,
the flakes tend to be aligned lengthwise in one
direction, e.g., the direction in which the coating
was stroked onto the 6ubstrate. To achieve an
isotropic coating, a second layer of coating is
6troked on in a direction perpendicular to the
direction in which the first layer was applied.
Multiple successive cross-passes of coating may be
applied in this manner. One possible way in which the
multiple layers of coating may be applied to achieve
isotropy i6 by 45 degree opposing gravure printing.
The preferred way to achieve a substantially
isotropic coating i6 to use circular flakes of
~usceptor material. These flakes tend to be flatter
and have smoother edges than other commercially
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available flak~s and are substantially round; it is
believed that their ellipticity (ratio of largest to
smallest ~urface dimensions) is in the range of about
1:1 to 1:2, preferably about 1:1 to 1:1. 5 . This is in
contrast to other commercially available aluminum
flakes which are oblong, and generally have
e~lipticities greater than 1:2, ~ometimes as high as
1:4. Circular aluminum flakes are available
commercially from Kansai Paint Company, Hiratsuka,
Japan, under the designations "Aluminum Y" and
"Aluminum X". Cir~ular flakes will provide an
isotropic coating 60 long as they are applied so as to
be parallel to the ~ilm surface and in a manner which
avoids fragmentation of the flakes which can lead not
only to irregularly shaped ~lakes but also to their
random agglomeration.
To achieve best results, the manner in which
the susceptor/matrix is applied to the substrate has
~een found to be important. First, it has been found
that the susceptor/matrix should be applied to the
substrate in such a way that the plane of the large
dimension of the flake is ~ubstantially parallel to
the surface of the substrate. Second, the ~lakes
~hould be dispersed in the thermoplastic matrix so
that they are ~ubstantially insulated from each other.
A number of factors can be controlled to
achieve these goals. The selection of the flake
susceptor material can greatly affect the ability to
achie~e a uniform and isotropic coating with properly
aligned flakes. Our work indicates that the smoother
and flatter the flakes are, the easier they will be to
disperse in the thermoplastic matrix, thus reducing
agglomeration. The smaller the aspect ratio (largest
dimension to thickness) of the flakes, the less
mechanical damage the ~lakes will encounter during the
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coating process and, thus, the less fragmented debris,
capable of agglomerating, will result. The circular
flakes described above have ~any of these desired
features, e.g., smooth ~dges, flat surfaces and low
aspect ratioO
Apart from the selection of th2 flake
susceptor itself, the manner in which the susceptor
coating is ~pplied to th substrate plays a ~ajor role
in achieving the flake orientation that will lead to
heating performance reproducibility. While the
susceptor thermoplastic matrix coating can be applied
in a single coating layer, it has been found that the
desired flake orientation can more easily be achieved
by application of a plurality of thin, dilute coats Df
the material. Each coating layer is applied from a
dilute (e.g., about 15-35% total solids) dispersion of
susceptor and matrix in solvent. The ideal amount of
susceptor in the coating layers varies according to
the susceptor material selected. Generally, it has
been found that good results are achieved when
coatings are used in which circular aluminum flakes
comprise about 40-70% o~ the total solids (susceptor
and th~rmoplastic matrix), or in which oblong aluminum
flaXes comprise about 20-60% of the total solids, or
in which non-aluminum flakes comprise about 10-40% of
the total solids.
The susceptor/matrix coating can be applied
in a single coating layer if coating methods which
insure laminar flow are utilized, e.q., slot coating
with a small gap and a long land length. When only a
single coating layer is to be applied, a high solids
dispersion of the susceptor/matrix should be used, and
the amount o~ ~usceptor in the solids should also be
high.
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As previ~usly mentioned, a uniform and
isotrspic usceptor/matrix coating is desired because
the heating performance of a composite so ooated will
have superior reproducibility. For the purpose of
measuring and quantifying heating performance
reproduciblity, the following test can be used.
Test for Heatinq Performance Re~roducibility
Six l-cm by 2-cm pieces taken from a
sample composite are heated in a 2450 MHz
microwave electric field of 243 V/cm. (This
simulates the hot spot electric field in a
typical 700 watt microwave oven.3 The
samples are divided into two groups.
Samples in Group 1 are oriented so that the
electric field is parallel to the
longitudinal or machine direction (MD) of
the sample, and samples in Group 2 are
oriented so that the electric field is
parallel to the cross or transverse
direction (TD) of the sample. The
temperature of each composite ~ample is
measured after exposure to the microwave
electric field ~or four minutes. The mean
temperatures ~or ~ach group of samples as
well as for all six samples taken as a whole
are determined. With this test, the sample
composite is deemed to possess heating
performance reproducibility if:
(1) MD and ~D are each within Temp + 5%,
(2~ Each MD temperature is within MD + 10%,
3S
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and
(3~ Each TD temperature is within TD + 10%;
S where ~D is the mean temperature for the
samples of Group 1,
TD is the mean temperature for thP
samples of ~roup 2,
Temp is the mean temperature for all
six ~amples,
MD temperature is the temperature for
any ~ample in Group 1, and
TD temperature is the temperature for
any sample in Group 2,
all temperatures being in degrees
Centigrade.
A non-resonant 2450 MHz wavequide system,
~uch as described below, can be used to obtain the
25 data required for the Reating Performance
Reproducibility Test. The system comprifies a
microwave generator feeding 254 watts through a
microwave circulator into a 6ection of WR284
rectangular waveguide terminated with a shorting
30 plate. (WR284 is a rectangular waveguide with an
interior cross-section of 7.2 cm. by 3.4 cm.). The
reflected wave from the ~hort circuit establishes a
243 V/cm pure electric ~ield at the standing wave
maxima in the waveguide section as long as the sample
35 perturbation is small and the reflected energy is
- 15 ~
7~
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dissipated by the matched termination connected to the
third port of the microwave circulator before it can
make a third pass through the sample assembly. Th~
microwave heating of the l-cm by 2-cm 6ample is
measurPd by recording the temperature reading of a
Luxtron Fluoroptic temperature probe which was
s~ndwiched between the l-cm by 2~cm ilm sample and a
5 milimeter diamater Teflon tR)
p~lytetrafluoroethylene (E.I. du Pont de Nemours and
Co., Wilmington, Delaware) rod. The probe-film
assembly is secured to the rod by a Teflon (R)
polytetra1uoroethylene tape. The whole tape-film-
probe-rod assembly is inserted through an aperture
into the ~ample holder position in the waveguide,
located at a distance of (n/4)(23.1 cm), where n is an
odd integer, from the end of the end shorting plate.
(23.1 cm is the full wavelength.) A waveguide phase
shifter and an electric field probe is used to shift
the electric field maximum to the ~ample position.
The temperature versus time heating profile was
recorded for each sample piece over a period of at
least four minutes.
The composite materials of this invention
are further illustrated by the following examples. In
~5 each of these examples, the surface D.C. resistances
of the exemplified composite materials are greater
than 1 x 10~ ohms per square. D.C. 6urface
resistances can be measured by methods known in the
art (e.g., ASTM D257-78) using conventional,
commercially available instruments. All temperatures
are in degrees Centigrade.
The samples prepared in Examples 1-8 and
Comparative Example A were tested in a commercial
microwave appliance rated at 550 watts at a fre~lency
of 2450 megahertz. Tests in the microwave oven of the
~4~6
17 -
invention wer~ run both in the pr~sence and ~bsence of
~ood. Two types of temperature monitor~ were used.
One was a ~inglQ optioal pyrometer probe used wi~h a
Vanzetti Optical Pyrometer. Thi~ non-cont~ct
probe which i5 dependent on the ~mmissivity of the
~rticle who~e temperature ~ being ~easured. The
~econd temperature ~onitsr used was a Luxtron
Fluoroptic four channel device with contact thermo
probes. Temperature measurement6 made in the ~bsence
of food were carried ou~ by ~uspending ~ ~wo-inch
~quare of test material (either the coated film or the
coated film laminated to paper or paperboard) in the
microwave oven in generally the geometri~ center of
the cavity. The test ite~ i~ attached to the Luxtron
thermo probe ~nd to a 6tring which enters the cavity
~rom ~ ~ole drilled throu~h the exterior cabinet and
into the interior cavity. ~he ~tring $t~elf $s the
~uspending ~gent with the test item att~ched t~ it
with a piece of non-lossy ~dhesive tape. Temperature
is recorded at ~ifteen 6econd intervals over the
course of 3 minutes and 15 ~econds. The oven is
cooled to room temperature between tests.
~eE~
E~L
This example ~hows the heat gener~ting
capabilities of the ~ombined ~etal flAke/dielectric
~atrix with 6upp~rt film compared to the ~upport film
itself or the support film coated with the dielectric
matrix but in the absence of the ~etal flake.
The matrix coating was prepared in the
following manner. The matrix polymer, in this case
15.8 weight p~rt~ of the copolymer condensation
product of 1.0 ~ol of ethylene glycol with 0.53 ~ol of
terephthalic ~cid and 0.47 mol of azelaic ~cid was
combined with 0.5 weight part6 of eruc~mide ~nd 58
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7~
weight parts of tetrahydrofur~n in ~ heated glass
reactor vessel equipped with paddle ~tirrer. After
dissolution of th~ ~olid~ ~t 5S-C, 0.5 weight parts of
~agnesium ~ilicate and 25 weight parts of toluene were
blended in. Finally 35 weight parts of dxy aluminum
flaXe ~Alcoa Alumirlite*flake, grade 16633 W2S blended
in. These flakes have a diame~er distribution of 1 to
48 micrometers (88% in the 4 to 24 micrometer range),
a thickness in the 0.1 to 0.5 ~icrometer r~nqe, and a
6urface ~rea in the range of 1 to 15 ~2/gram.
A second ~atrix coating was prepared in the
same fashion ~s that above except that no aluminum
flake was added. ~ach of these coating dispersions
were cast, in ~eparate experiments, on 12 ~icrometer
thickness, biaxially oriented polyethylene
terephthalate film to a wet coating thickness of 230
~icrometers. The wet coated films were allowed to
dry. The dry coating weight of the dispersion
containing ~luminum Plake was 54 grams per square
meter with the aluminum comprising 67% by weight of
the dried coating. In the ~econd coatin~ dispersion
without aluminum flake added, the dried coating weight
was 19 grams per 6quare ~eter. Coating weiyht ~s
determined by ~tripping the film of the dried coating
and gravimetricnlly deter~ning unit weight of coated
nnd stripped film. In these two cases the ~m~unt of
copolymer matrix i~ approximately equal.
Samples of each coated film and an uncoated
piece of the carrier film detailed above were cut to
two-inch ~guare6. ~emperature measurements were
carried out in the microwave oven as described
earlier~ Results of the heating test are ~et out
below in Table I.
*denotes trade mark
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.. .
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Temp. ~-r) ~fter microwave exposure
_ _or -
Sample 30 sec. 60 sec. 90 ~ec. 195 sec.
Vncoated
carrier film 56 65 68 77
Coated film
without Aluminum
Flake 58 65 70 78
Coated film with
Aluminum Flake 190 213 87* --
*Film has melted.
TABLE I (Continued)
Total
Coating Wet Coating Weight
Weight Thickness Thermoplastic
Sam~le a/m2_ ~micrometers~ ~atrix (q~m2
Uncoated
carrier film -~
Coated film
without Aluminum
Flake 19 230 19
Coated film with
Aluminum Flake 54 230 18**
2S **Thermoplastic matrix comprises 33% by weight of dry
coating.
-- 19 --
~4~
- 20 -
ExamPle 2
This example shows the effect of the amount
of aluminum flake on a weight basis in the dried
coating on the te~perature reached ~y the c~mposite of
the aluminum/matrix coating on carrier film. Thi~
example also shows the effect of the total aluminum/
matrix unit weight on the carrier film on the
temperature generated.
Dispersions of aluminum flake in the matrix
binder dispersion were made in the same ~ashion from
the same materials as given in Example 1. In three
~eparate experiments the coating dispersion without
aluminum flak~ was prepared as in Example 1. To one
dispersion 1.1 weight parts of aluminum flake was
added. Likewise 5.6 weight parts of aluminum flake
was added to the second dispersion and 11~2 weight
parts to the third dispersion. Each of these
dispersions were used to prepare coated film on 12 mil
biaxially oriented polyethylene terephthalate as
described in Example 1. Wet coating thicknesses at
100, lS0 and 200 ~icrometers were cast with coating
knives from each of the aluminum ~lake dispersions
above. Coated samples were allowed to dry. Each of
the dispersions will give, on a dry ~olids basis, 10,
25 and 40 weight percent ~luminum flake, respectively.
Temperature measurements were carried out in
the microwave oven as described in earlier. Results
of these heating tests are ~et out below in Table II.
- 20 -
- 21 -
~AB~LE II
Weight % Wet Coatiny Temp. (-C) after microwave
Al/CoatingThickness, exposure for -
Weiqht a/m2 mm *0 sec.120 sec. 195 sec.
**1~/6100 75 78 81
**10/12 15~ 7~ 75 80
~*10/17 200 79 85 89
25/5100 75 B0 84
- 10 25/16 lS0 89 91 94
25/21 200 109 119 127
40/9100 11~ 126 133
40/20 lS0 166 180 194a
40/25 200 181 191 201a
a-Film shrin~s
*Weight % Al, dry basis based on total Al/
thermoplastic matrix
**Comparative examples
- 21 -
~ 2~
. ~
. - 22 -
Example 3
This example shows that aluminum flake as a
high ~olids paste in mineral ~pirits or high flash
naptha, in the presence or absence of leafing agents,
can be substituted for the dry aluminum ~lake used in
Example 1 ~nd 2.
Copolymer matrix dispersions were prepared
as described in Example 1. Successive aluminum flake
coating dispersions were ~ade substantially as
described in Example 1. In Test A 52.5 weight parts
aluminum paste (Alcoa leafing paste grade 6205, 65
weight ~ non-volatiles in Rule 66 mineral ~pirits) was
used. In Test B 52.1 weight parts of aluminum paste
(Alcoa leafing paste grade HF905, 65.5 weight %
non-volatiles in high ~lash naptha) was used. In Test
C 52.1 weight parts aluminum paste (Alcoa non-leafing
paste grade HF925, 65.5 weight %~non-volatiles in high
flash naptha) was used.
The above aluminum dispersions were used in
coating of 12 micrometer thick biaxially oriented
polyethylene terephthalate using a coating knife to
give a wet coating thickness of 230 micrometers as
described in Example 1.
Heating tests on the dried films were
carried out in the microwave oven as described earlier
with the results set forth below in Table III.
- 22 -
-- 23
TAE~LE I I I
~Temperature ~-CL aft_
Coating Wt. _ exposure for - _
Test Jm215 sec.30 6ec.45 sec.~20 sec.
A 31 151 149 146 156a
B 54 163 --b
~: 47 161 196 221 52
10 a-Film ignited
b-Film arced 2Ind melted at 19 seconds
c-Film melted
-- 23 --
~7~
i~ .
- 24 -
~xample 4
This example illustrates that aluminum flake
with different surface area, as expressed in covering
range in sguare centimeters per gram of flake, can be
substituted for that given in the first example.
Dispersions o~ the matrix copolymer were
prepared as described in Example 1. Successive
dispersions were then prepared as described in Example
1 using aluminum flake with differing cov~ring power.
Test A employed the very same dispersion as described
in Example 1 using 35 weight parts of the Alcoa
dedusted Aluminite flake grade 1663 with a covering
range of 20,000 squarP centimeters per gram. Test B
employed 34.1 weight parts of aluminum flake (Alcoa
15 dedusted Aluminite flake grade 1651 with a covering
range of 12,000 square centimeters per gram3. Test C
employed 47.7 weight parts of aluminum paste ~Alcoa
leafing paste grade 6678, 71.5 weight % non-volatiles
in Rule 66 mineral spirits and a covering range of
28,000 to 30,000 square centimeters per gram) was
used.
These aluminum flake dispersions were cast
on 12 micrometer khick biaxially oriented polyethylene
terephthalate with a coating knife to give a 230
25 micrometer wet coating thickness as described in
Example 1.
The dried films were tested in the microwave
oven as described earlier and the results are set
forth below in Table IV.
- 24 -
~ ~7~;26
- 25 -
~ABLE IV
DryTemp. (-C) aEter microwave
Coating Wt._exposure for ~
~ q/m~ _ 30 sec. 45 sec. 195 sec.
A 54 213 87a
B 61 - b
C 84 153 172 67a
a-Film melted
b-Ignited in 7 seconds
- 25 -
` ~2~L2~
26 -
Example 5
This example will illustrate the
substitution o~ a higher softening point matrix
copolymer for the copolymer described in Example 1.
A dispersion of the ~ame copolymer was
prepared as described in Example 1 with the addition
of 1.8 weight parts o~ a copolymer made by reacting
1.0 mol of ethylene glycol with 0.~5 ~ol of
terephthalic acid and 0.45 ~ol of isophthalic acid.
lD To this mixed copolymer dispersion is added 5.6 weight
parts of aluminum flake (Alcoa dedusted Aluminite
flake grade 1663~ as described in Example 1.
This coating dispersion is cast on 12
micrometer biaxially oriented ethylene terephthalate
film using a coating knife to ~chieve a 200 micrometer
wet coating thickness as described in Example 1.
Testing of a dried example of this coated
film is carried out in a microwave oven as described
earlier. For comparison, a coated film sample with
nearly the ~ame aluminum content, on a dry basis, as
prepared in Example 2 was tested. The results are
presented in Table V.
- 26
~7~
-- 27 --
Weight % Temp . ( D C) after microwave
Al/Dry exposure for -
Copol,vmer Coati~a Wt. ~O ~ec. 120 sec~ 195 sec.
Single
(Example 2) 25/21 g/m2 109 119 127
Mixed
(Example 5) ?3/27 g/m2 118 131 139
-- 27
- 28 -
xample 6
This example illustr~tes the use of a
~econdary ~uppoxt web to promot~ dimensional ~tability
of the primary ~truc~ure of ~he invention ~ described
in Example 1.
Samples of film ~oated with the aluminum
flake/polyester cop~lymer dispersiGn ~s described in
Example 1 or 2 ~s treated with an ~dhesive colution on
t~e uncoate~ 6ide of ~aid 6tructure. The ~dhesive
used was a 601ution of ~ ~isture curable, isocyanate
ended copolyester (~orton Chemicals Adcote*76FS93, 3
weight parts of the adhesive diluted with ~ weight
parts o~ ethyl acetate ~s recommended by the
manufacturer) ~nd was applied by ~ typical laboratory
aerosol ~pray device. The adhesive as applied was
dried briefly with ~id of ~ ~ot air gun and then a
6uitably ~ized piece of bleached white paper ~160
micrometer thic~ness) applied with the aid of a rubber
roller. The laminate was ~tored under a weighted
glass plate ~r a minimum of 1~ hours prior to use.
The laminates as described above were tested
in a microwave oven ~s described earlier. In these
tests the ~uspension ~tring was attached to the paper
6ide of the laminate nnd the fiber optic probe to the
coated side of the film. The results of these tests
are presented below in Table VI.
* denotes trade mark
3~
-- 28 --
d q' ~
~9 --
TABLE VI
Weight %
Al/D~yWet Temp (-C) after microwave
CoatingCoating . exposure for -
5 Test Sample Wt. hickness* 60 sec. 120 sec. 195 sec.
Unlaminated
(see40/25
Example 2) ~/m2 200 ~81191 201a
L~minated 40/25
g/m2 200 167 178180
a-Film ~hrinks
* micrometers
- 29
~;~7~
- 30
Example 7
This example and the following Example 8
will illustrate the utility of this invention in the
preparation of foods in a microwave oven. These
examples illustrate the range Df heat generating
capability of the artirles of this invention in
preparation of foods requiring additional heat to
improve cooking food performance or to improve visual
appearance or textural consiskency of the cooked food.
~ In this example, it will be shown that popping
performance of commercial microwave popcorn packages
can be improved by incorporation of the article of
invention as part of the microwave popcorn packagev
A laminate of the primary ~tructure as
described in Example 1 and a paper 6econdary ~upport
web as described in Example 6 are used. ~he primary
6tructure before lamination consisted of 40 weight %
of aluminum flake dispersed in the polyester c~polymer
matrix (dry solids basis) and applied to 12 micrometer
thick biaxially oriented polyethylene terephthalate at
a wet cast thickness such as to achieve a dry coating
weight of 11 grams per square meter of which 3.6 grams
per square meter was aluminum flake. The dry coating
weight was determined by gravimetric techniques
wherein a convenient ~ized piece of the coated film is
soaked in tetrahydrofuran until the coating is
stripped. After rinsing with additional
tetrahydrofuran, the stripped support film is oven
dried and weighed. The aluminum flake composition o~
the coating is readily determined on the coated film
either directly by x-ray fluorescence techniques or by
pre-digestion of a ~ample in strong mineral acid
~ollowed by determination of aluminum using standard
atomic absorption techniques.
3S
- 30 -
- 31 -
~ commerci~l ~icrowave popcorn bag paper
made ~rom a copolye~ter-coated polyethylene
terephthalate laminate was altered for use in this
test. A three by five inc~ reot~ngle o~ a laminate as
described above was a~fixed to the bottom of the bag
using a cy~noacrylate adhesive, The 6aid piece was
affixed with the coated ~ide on the in~ide bottom of
the bay and the paper ~ide upward. A 100 gram plug of
the combined popcorn and oil from a purchased bag of
microwave popcorn was tr~nsferred to the bag with
heater pad ~ffixed. The 100 gram plug of popcorn and
oil was found to contain 554 kernel~ of popcorn. The
test bag was then 6ealed at its top opening using a
bar sealer (at 125-C ~nd 35 kilopascal~ for ~ne
~econd).
The test bag and ~ control bag (commercial
bag as described above) were then tested in t~e 550
watt microwave oven as described in Example 1. A
fiber optic probe for the Luxtron Fluoroptic*
thermometer described in Example 1 was inserted in the
exterior bottom flap of the package so that the sensor
end was located below the approximate geometric center
of the test pad ~nd separated from ~t by ~ust ~ne
layer of the bng. In thefie tests the bag (test or
25 control) was x~ised ~rom the met~l floor of the
interior cavity of the ~icrowave ~ven with the ~se of
~n inverted paperbo~rd tray 15 centimeters ~quare by 3
centimeters in height (the tray is fabricated from
unbleached, pressed, ovenable paperboard of 50
30 ~icrometer ~h$ckness).
The time for popping ~3 minutes, 15 6econds)
was within the range recommended on the commercial
package. Once each bag had been popped, the bags were
cooled and opened. The bag content~ were poured into
35 a graduated 2~00 cubic centimeter beaker nnd its
* denotes trade mark
- 31 -
\
- 32 -
volume measured. ~he pGpp~d 2nd unpopped kernels are
then separated ~nd a count ~ade of the unpopped
kernels from which the percentage of the unpopped
kernels out of the total content was calculated.
5 These results ~re set forth in Table VII.
TABLE VII
Pop Volume Count of
~ax. Temp. C (Cubic Unpopped %
Baa at ba~ bottom (Centimeters) kernels Unpopped
10 control 236 1875 15~ 29
Test 257 2000 143 26
- 32 -
4~%1E;
- 33 -
Example 8
In this example the utility of the invention
in providing sufficient heat in a microwave oven to
effect brownin~ and crispening of microwave pizza is
illustrated.
An article of this invention as described in
Example 1 is used to prepare a tray for cooking of
commercially ~vailable microwaveable pizza. In this
example the primaxy structure consisted of a 12
micrometer thickness film of biaxially oriented
polyethylene terephthalate to which was applied,
according to the description to Example 1, a
dispersion of aluminum flake (Alcoa dedusted Aluminite
flake grade 1651) in the polyester copolymer binder
eolution as described in Examples 1 and 4, applied to
a dry coating weight o~ 61 g~m2. The aluminum flake
content of the liquid matrix dispersion is 67 weight %
on a dry solids basis and the wet coating thickness
used was 230 micrometers. The coated side of the
dried film was affixed to the top cide of an inverted
paperboard tray using a cyanoacrylate adhesive. The
20 centimeter square by 3 centimeter height tray was
constructed of pressed ovenable paperboard with a
thickness of 50 micrometers.
A commercial microwaveable pizza (255 gram
cheese pizza) was removed from its freezer package and
centered on the tray described above. The tray with
pizza was then placed on the floor of the 550 watt
microwave oven described in Example 1 and cooked for
two minutes. The top of the pizza was bubbling hot
with aesthetically pleasing appearance judged from
cheese melted but retaining its ~hredded appearance.
The bottom of the pizza crust immediately after
removal from the microwave oven was dxy to the touch
and had no visible moisture. The bottom crust was
- 33 -
~7~6
~ 34 - _
browned with a few small areas beginning to show signs
of charring which is the expected appearance of pizza
crust. The crust was noticeably crisp when a knife
was scraped across it and was definitely crisp when
cut with the knife. A control pizza was cooked using
the tray incorporated in a commercial package, a tray
lined with lightly metallized polyethylene
terephthalate film. It too gave atisfactory
appearance ~f the top and crust but this was achieved
only after the recommended cooking time of 3 minutes
and 30 seconds.
Com~arative ExamPle
This example illustrates the importance of
the flake structure for optimum performance in terms
~f temperatures generated.
A copolymer dispersion is prepared as
described in Example 1 using 11.2 weight parts of
powdered aluminum (less than 75 micrometer particle
size). This dispersion is cast on 12 micrometer thick
biaxially oriented polyethylene terephthalate film
with a coating knife to achieve a wet coating
thickness of 200 micrometers as described in Example
1. .
The dried coated film was tested in A
microwave oven as described earlier. The test
results, and the results for a comparable film in
which aluminum flake was used as the susceptor
material ~from Example 2) are set forth in Table A.
- 34 -
``\
-- 35 --
TABLE A
Weight % Wet Coating Temp ( ^ C~ after microwave
Al/Dry thk., _ exPosure for - _
~ Coatinq Wt Micrc~meters 60 sec.120. sec. 195 sec.
Powdered 40/28 g/m2 200 78 84 90
Flake 40/25 g~m2 200 181 191 ~Ola
a Film ~hrunk
~0
~5
~.274~
.
- 36 -
Exam~les 9-27
Numerous film samples were prepared to
investigate the factors important for providing
reproducible heating performance. Each of the samples
listed in Table VIII was prepared by hand-coating
polyethylQne terephthalate film with ~ doctor-knife
type draw bar with a coating of aluminum flake in a
polyester copolymer matrix as used in Example 1. The
types of ~luminum flake used were as follows:
C-l: circular flake, average diameter of 10 microns,
~Aluminum X", available from Kansai Paint Company,
Hiratsuka, Japan
C-2: circular flake, average diameter of 20 microns,
~Aluminum Y~, available from Xansai Paint ICompany,
Hiratsuka, Japan
E-l: oblong flake, average diameter of 35 microns,
'~OBP-8410", available from Obron Corporation,
Painesville, Ohio
E-2 : oblong flakef average diameter of 2-5 microns,
~'L-875-AR", available from Silberline Manufacturing
Company, Lansford, Pennsylvania
Circular flakes C-l and C-2 were flatter and had
smoother edges than oblong flakes E-1 and E-2.
Six l-cm by 2-cm pieces taken from each
coated film ~ample were heated in a microwave electric
field of 243 V/cm, using the procedure described
previously, three with the electric field parallel to
the machine direction of the film, and the other three
- 36 -
26
- 37 -
with the electric field parallel to the transverse
direction of the film. (Films were hand coated in the
machine direction of the film.) The temperature of
the film was ~easured over a period of about ~ive
~inutes. Mean temperature data are presented in Table
VIII which also indicates whether the samples passed
the Heating Performance Reproducib.ility Test ~et forth
previously~
.,
-- 38 --
-
~ABLE VI I I
Wet # of ~6 Al of
Elake . ThicknessCoatirJg Dry
Ex. TYPerMIL) * Passes Coat
g C-2 2 3 20
C--2 6 1 20
11 C-l 2 3 60
12 C-2 6 1 60
13 C 2 6 1 33
14 ~ 1 20
C-l 6 ~ 60
1016 ~-2 2 3 33
17 E-2 6 1 6 0
18 C-l 6 1 33
19 C-l 2 3 2 0
E-2 2 3 60
21 E-2 6 1 33
22 E-1 6 1 60
23 C-2 2 3 60
24 ~-2 2 3 33
E-l 2 3 33
2~ C-l 2 3 33
27 E-1 6 1 33
~Per layer of coating
-- 3~ --
: ~7~2~
-- 3~ --
TABLE VIII [Continued~
Passes Heating Perfor-
Ex. 4'_MD 4' TD ~ ance Reproducibility ~est?
943.541.7 42.6 Yes
1043.343.6 43.4 Yes
ll233,6226.6230.1 Ye~
12215.9207.12~1.5 Yes
1353.657.6 55.6 Yes
1445.044.4 44.7 No
15213.0170.8l91.9 No
16184.6168.8176.7 No
10 17205.4194~2199.8 No
1859.468. t) 63.7 No
1951.446.4 48.9 No
20*190.0184.1187.1 No
2181.394.4 87.8 No
22129.2119.0124.1 No
23141.6130.2135.9 No
2473. 264.9 69.1 No
15 25219.3175.3197.4 No
26105.9125.7115.8 No
2798.6133.1 115.9 No
4'MD - Mean temperature of MD samples at 4 ~inutes
4'TD - Mean temperature of TD samples at 4 minutes
20 4~ Temp - Mean temperature of all samples at 4 minutes
*3 minute MD, TD, Temp values used for this
experiment.
. -- 39 --
~ 2~4~
- 40 -
These data show that, in general, the coatings of the
two circular flakes, C-l and C-2, produce
substantially less variation in temperature when
exposed to external E-field of a widely varying
polari2ation angle than coatings of tha two oblong
flakes. As a result, the films coated with the
circular flakes have ~uperior temperature
reproducibility.
To compare data for films attaining
temperatures above 190 degrees C after four minutes,
one may review Examples 11, 12 and 25. Figures 1 and
2 graphically present the temperature data obtained
for the films in respective Examples 11 and 12, bot~
films coated with circular flakes which pass the
Heating Performance Reproducibility Test. In
contrast, Figure 3 presents the temperature data for
the film in Example 25, one coated with oblong flakes
which failed the Heating Performance Reproducibility
Test. Temperature vs. time data for each of the six
pieces of film in each example are presented in the
figures. nE//MD~ indicates that the piece was heated
in the microwave electric field with the electric
fîeld parallel to the machine direction of the film;
~E//TDn indicates that the piece was oriented with the
electric ~ield parallel to the transverse direction of
the ~ilm. rrhe figures ~how that for the film of
Example 25, in which an oblong aluminum flake material
was used as ~usceptor material, the temperature of the
six pieces after four minutes exposure to a microwave
electric field of 243 V/cm varied by as much as 90
degrees C. By comparison, Figures 2 and 3 show that
for the films of Examples 11 ~nd 12, in which a
circular ~luminum flake material was used as susceptor
~ 40 -
.. ~ 2~
- 41 -
material, the temperature o~ the ~ix pieces after four
minutes varied by no more than about 25 degrees C.
1~
. - 41 -
~'~7~6
- 42 -
ExamPles 28- 32
This set of examples ~h~w the improvement
which can be obtained in the temperature
repr~ducibility o~ a film coated with oblong ~lake
~usceptor ~aterial when the material is applied in a
~anner to produce a ~ubstantially isotropic coating.
The susceptor ~aterial utilized in this example is a
~oncircular aluminum flake, designated ~Reynolds
LSB-548, available from Reynolds Aluminum Company,
Louisville, Kentucky. The matrix was prepared as in
Claim 1. Samples of PET ~ilm were hand-coated with
the ~usceptor/matrix coating, the first coating being
applied in the machine direction, the ~econd coating
lS being appliced in the tr~nsverse direction, and
subsequent coatings being applied alternately in the
MD and the TD. Six pieces of each film sample were
exposed to a microwave electric field of 24~ V/cm for
four minutes, three with the electric field parallel
to MD, and the other three with the electric field
parallel to TD. The averaye temperatures for each
~ample, MD and TD, are presented in Table IX.
- 42 -
- ~3 -
~ABLE_IX
# Coating Dry Coating Al in Dry
Passes . Thickness Coating _ _
Ex ~ ~D mils %~2 4'MD 4'TD
28 4 0 1.3-1.5 2~10.079.255.7
29 5 0 1.6-1.7 2011.8104.271.7
30 6 0 1.7-1.9 2012.999.795.3
31 8 0 2.4-2.S 2017.51~3.5121.6
32 2 2 1.4-1.6 2010.7~8.9~0.0
33 3 2 2.3-3.1 2019.3147.4154.0
34 3 3 2.5-~.8 2019.~157.8159.7
10 35 4 4 3.3_3.4 2024.0162.3160.~
36 1 0 0.2 403.346.7 56.3
37 1 1 0.6-~.7 4010.7128.7131.3
38 2 2 1.4-1.7 4025.5162.0167.7
39 4 4 2.4-2.7 4042.0157.01~4.7
- 43 -
.
~74~
- ~4 -
These data ~h~w that by increasing the isotropy of the
coating (by applying layer(6) in which the alignment
of ~lakes is oriented about ninety degrees to the
alignment of flakes in another layer(s), ~s in
Examples 32-35 and 37-39~, t~e temperature
reproducibility ~f the coated film was i~proved.
