Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOW CRYSTALLINITY SUSCEPTOR FILMS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Patent Application No.
12/846,159, filed
July 29, 2010, which is a continuation-in-part of U.S. Patent Application No.
12/709.628. filed
February 22. 2010, which claims the benefit of U.S. Provisional Application
No. 61/208,379, filed
February 23, 2009, U.S. Provisional Application No. 61/273.090, filed July 30,
2009, and U.S.
Provisional Application No. 61/236,925, filed August 26, 2009. Each of the
above-referenced
applications is incorporated by reference herein in its entirety.
BACKGROUND
It is known to use a susceptor in microwave heating packages for enhancing the
browning
and/or crisping of an adjacent food item. In one embodiment, a susceptor is a
thin layer of
microwave energy interactive material that tends to absorb at least a portion
of impinging
microwave energy and convert it to thermal energy (i.e., heat) through
resistive losses in the layer
of microwave energy interactive material. The remainder of the microwave
energy is either
reflected by or transmitted through the susceptor. Typical susceptors comprise
aluminum,
generally less than about 500 angstroms in thickness, for example, from about
60 to about 100
angstroms in thickness, and having an optical density of from about 0.15 to
about 0.35, for
example, about 0.17 to about 0.28.
As shown schematically in FIG. 1, the layer of microwave energy' interactive
material
(i.e., susceptor) 102 is typically supported on a polymer film 104 to define a
susceptor film 106.
In most conventional susceptor films, the polymer film comprises biaxially
oriented, heat set
polyethylene terephthalate, but other films may be suitable. The susceptor
film is typically joined
(e.g., laminated) to a support layer 108, for example, paper or paperboard,
using an adhesive or
otherwise, to impart dimensional stability to the susceptor film and to
protect the layer of metal
from being damaged. The resulting structure 110 may be referred to as a
"susceptor structure".
The first commercial microwave susceptor films, and subsequently introduced
and
commercially used susceptor packages have relied upon the use of highly
oriented, highly
crystallized, biaxially oriented and heat set films produced from polyethylene
terephthalate
polymer, or PET. (As used herein, PET will refer to such biaxially oriented
films, unless specified
otherwise.) Typically, such films are highly oriented, that is, the degree of
stretch during the
orienting process is from about 3.5:1 to about 4:1 in the machine direction
(MD) and from about
3.5:1 to about 4:1 in the cross-machine direction (CD). Biaxially oriented PET
films made from
this polymer are commonly used in a wide variety of packaging and non-
packaging uses where
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combinations of some or all of clarity, gloss, smoothness, a good combination
of moisture vapor
and oxygen barrier, good mechanical strength and modest dimensional heat
stability are useful.
Commercially available films that are used in standard susceptor structures
typically comprise
films of this general description, whose properties have generally been
optimized for high volume
applications other than for use in microwave susceptors.
There are several key elements of this standard construction that may limit
the heating
performance of these films, and hence of microwave susceptor packages,
packaging components
or composite susceptor and field modification or shielding packages or
components. There is a
wide understanding of those skilled in the art of making microwave susceptor
packaging of the
phenomenon of self-limiting heating, the visual evidence of which is commonly
referred to as
crazing. During heating induced in the susceptor layer itself resulting from
interaction of the
susceptor material with either or both the electrical or magnetic components
of the
electromagnetic microwave energy. the temperature of the susceptor substrate
film is raised.
While not wishing to be bound by theory, and for this discussion, using the
example of a
vaporized metal vapor deposited on a biaxially oriented film (vacuum
metallization) and
interacting principally with the electrical component of the microwave energy,
it is believed that
when the residual shrink forces in the susceptor substrate film exceed the
ability of the
adhesive/support substrate to hold the susceptor substrate film in its
original and desired
dimensional configuration (particularly in the plane of the film parallel to
its width and length
directions), cracks appear in the susceptor substrate film, causing
discontinuities in the microwave
interactive susceptor material that interrupt the flow of electric current in
the metal layer. As the
crazing progresses and the cracks intersect one another, the network of
intersecting lines
subdivides the plane of the susceptor into progressively smaller conductive
islands. As a result,
the overall reflectance of the susceptor decreases the overall transmission of
the susceptor
increases, and the amount of energy converted by the susceptor into sensible
heat decreases.
When this self-limiting behavior occurs prematurely (i.e., too early in the
heating cycle),
the susceptor may not be able to generate the necessary amount of heat for a
particular food
heating application. In contrast, in some instances, this self-limiting
behavior may be
advantageous where runaway (i.e., uncontrolled) heating of the susceptor might
otherwise cause
excessive charring or scorching of the adjacent food item and/or any
supporting structures or
substrates, for example, paper or paperboard. Thus, for each application, the
need for sufficient
heating must be balanced with the desire to prevent undesirable overheating.
Unfortunately, with
a conventional highly oriented PET susceptor. the temperature at which crazing
occurs can only
be slightly controlled, for example, by modifying the thickness of the metal
layer, the type and
amount of adhesive, and the uniformity of the adhesive application.
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Delaying the onset of crazing in susceptor structures has been the subject of
significant
efforts, but little or no meaningful improvements have been achieved using
traditional thinking.
However, significant and previously unanticipated changes to the base
polymeric materials used
and/or the way they are processed into films are shown in this invention to
result in higher
temperature or delayed onset of crazing and controllably higher microwave
heating potential.
To understand the uniqueness of the approaches taken in the present
disclosure, it is
instructive to first review previous attempts to create improved heating
susceptor films through
modifications to way's in which these films are produced. A discussion of film
characterization
also reveals new understanding that supports the uniqueness of the aspects of
the current
invention.
Since the biaxial orientation hardware used to make standard PET susceptor
substrate
films provide the ability to heat condition, or heat set the highly oriented
film at controlled
temperatures, it has been an objective of some investigators to increase heat
setting temperatures
and dwell times to achieve films with more dimensional stability at elevated
temperatures. For
example, commonly' assigned U.S. Patent Nos. 4,851,632, 4,991526, 5,003,142,
and 5,177,332
and U.S. Patent Application Publication No. 2007/0084860 Al disclose the use
of "heat-stabilized
PET" film, which is defined in U.S. Patent No. 5,177,632 as -PET which is
treated so as to shrink
less than about 2% when heated to 150 C. for thirty minutes. Preferably it
will shrink less than
about 1.5% or 1% when so heated, and most preferably about 0.6% or less."
According to U.S.
Patent No. 5,177,132, -[h]eat stabilized PET is made from a regular grade of
PET film by a
stabilization process involving a series of heat treatment and relaxation
steps, and is well known to
those skilled in the art. A heat stabilization process for PET is more fully
described in Bulletin E-
50542, -Thermal Stabilization of Mylare," from E. I. Du Pont de Nemours and
Company."
Although this bulletin is not publically available for examination, it is
clear from the context of its
description, particularly in U.S. Patent No. 5,177,332, that the heat
stabilization process includes
supplemental heat treatment beyond that normally provided in the production of
'standard food
packaging PET film', and that in all cases PET or PET film refers to film that
is biaxially oriented.
Other investigators have attempted to utilize higher melting point (i.e.,
melting
temperature) polymers to achieve higher heating performance (see, for example,
U.S. Patent No.
5,571,627, which teaches the use of biaxially oriented susceptor substrate
films having onset of
melting in a range of approximately 260 C to 300 C, or U.S. Patent No.
5,126,519, which teaches
the use of films made from PCTA copolyester with melting point greater than
500 C).
Polyethylene naphthalate and certain copolyesters such as polycyclohexylene-
dimethylene
terephthalate (PCDMT), which have inherently higher melting points, have also
been disclosed
(see, for example, U.S. Patent Nos. 5,527,413 and 5,571,627). Despite claims
of improved
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heating performance, it is believed that susceptors using these films have not
proven suitable for
commercial use. Difficulties encountered have included excessive additional
cost, fabrication
problems, and uncontrolled heating leading to unacceptable scorching or
burning of the food, the
package or package components, or both. For example, U.S. Patent No. 5.527,413
discloses that
PCDMT becomes so hot that it can bum or char the paper in the susceptor
structure or bum food
items in contact with the susceptor. Accordingly, it is believed that the use
of higher melting point
copolyesters has not proven satisfactory in creating commercially useful
susceptor films.
Thus, there is a need for a susceptor structure that is capable of
controllably achieving a
greater heat flux and/or higher temperature than conventional susceptor
structures. thereby
permitting better browning and/or crisping of a food item without the danger
of excessive
charring.
In related U.S. Patent Application No. 12/709,628 (U.S. Patent Application No.
2010/0213191 A 1 ), it was recognized that unoriented films, for example,
largely or essentially
amorphous polyethylene terephthalate (APET) films (e.g., between about 1 to
about 25%
crystallinity), could be used in susceptor films that have heating properties
that are superior to
those of conventional, biaxially oriented PET film susceptors. However, it was
also recognized
that the APET film may lack sufficient strength for processing. Accordingly,
the inventors
proposed various means of increasing the strength of the APET film.
Nonetheless, there remains a
need for alternate susceptor base films and susceptor structures formed
therefrom.
SUMMARY
This disclosure is directed generally to a polymer film (or film) for use as a
base film or
substrate in a susceptor film, a method of making such a polymer film, and a
susceptor film
including the polymer film. The susceptor film may be joined to a support
layer to form a
susceptor stnicture. The susceptor film and/or susceptor structure may be used
to form countless
microwave energy interactive structures, microwave heating packages, or other
microwave energy
interactive constructs.
The susceptor structure may generally have a browning reaction rate that
exceeds the
browning reaction rate of a susceptor structure made from a conventional
biaxially oriented PET
film. Accordingly, the present susceptor structure may provide a noticeable
improvement in
browning and/or crisping of a food item heated using the susceptor structure.
The polymer film may be unoriented or oriented to varying degrees. Where the
film is
oriented, the orientation and heat setting conditions may be customized to
provide a desired level
of crystallinity, residual orientation, and therefore, desired heating
performance for a particular
susceptor film application. The film may be characterized as having one or
more of the following:
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a refractive index (nz) of less than about 1.64; a birefringence (n-n) of less
than about 0.15; and a
crystallinity of less than about 50% (where all crystallinity values herein
are derived from density
measurements, as described below).
However, other crystallinities, refractive indexes,
birefringences, and ranges of each may be suitable in some cases. By'
comparison, commercial
biaxially oriented homopolymer films typically used in susceptors may have a
refractive index (nz)
of from about 1.6447 to about 1.6639, and a birefringence (n-n5) of from about
0.1500 to about
0.1700.
It has been discovered that polymer films having a relatively low
crystallinity, orientation,
and/or residual shrink forces may be used in susceptor films and susceptor
structures to achieve a
greater heat flux and/or higher temperature than conventional susceptor
structures. Furthermore,
the present inventors have gained an appreciation for the relationship between
various base
materials, process conditions. and the crystallinity of the resulting film. As
a result, the degrees of
freedom in designing base films and the resulting susceptor structures have
been dramatically'
increased.
In some exemplary embodiments, the polymer film may comprise amorphous
polyethylene terephthalate (APET), amorphous nylon, various copolyesters, or
any combination
thereof. As will be understood by those skilled in the art of polymer film
manufacture, when the
word amorphous is used to describe one or more of these films, it means a film
fabricated in such
a manner that its morphology is predominated by its amorphous portion; films
of this type will
typically have crystallinity of from about 1% to about 25%. These are films
fabricated with high
rates of quenching from the melt, thus preventing appreciable levels of
crystallization from
forming. The copolyester may generally' have a melting temperature similar to
or lower than that
of standard PET polymer, for example, less than about 250 C to 260 C. However,
numerous
other polymers are contemplated.
If desired, one or more additives (i.e., polymers) may be incorporated into
the polymer
film to enhance the strength and/or processability of the polymer film.
Additionally' or
alternatively, the strength and/or processability of the polymer film may be
enhanced by using a
multilayer polymer film, where one or more of such layers provide the desired
level of robustness
for the polymer film. Accordingly, the multilayer film may feature enhanced
tear strength,
toughness, and improved dimensional tolerance so that the film may be
processed (e.g.,
metallized, chemically etched, laminated, and/or printed) and converted into
various susceptor
structures and/or packages using high speed converting operations.
If desired, additional functional characteristics can be imparted to the
multilayer film by
selecting polymers having the desired attributes. For example, the multilayer
film may have
barrier characteristics that may render the polymer film suitable for numerous
applications, for
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example. for packages for refrigerated microwavable food items that require an
extended shelf
life.
Other features, aspects, and embodiments of the invention will be apparent
from the
following description.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic cross-sectional view of an exemplary microwave energy
interactive
structure;
FIG. 2 presents the refractive index (n2) and rise in temperature (AT) in C
for various
exemplary susceptor base films and susceptor structures;
FIG. 3 presents the relative browning reaction rate and pizza browning pixel
count for
various exemplary susceptor structures;
FIG. 4 presents the refractive index (n2) and rise in temperature (AT) in C
for various
exemplary susceptor base films and susceptor structures;
FIG. 5 presents the relative browning reaction rate and pizza browning pixel
count for
various exemplary susceptor structures;
FIG. 6 presents the dynamic dimensional temperature response for susceptor
base film 6-
FIG. 7 presents the dynamic dimensional temperature response for susceptor
base film 1-
6;
FIG. 8 presents the dynamic dimensional temperature response for susceptor
base film 7-
1;
FIG. 9 presents the dynamic dimensional temperature response for susceptor
base film 6-
9;
FIG. 10 presents the dynamic dimensional temperature response for susceptor
base film
6-11; and
FIG. 11 presents the dynamic dimensional temperature response for susceptor
base Min
7-8.
DESCRIPTION
This disclosure is directed generally to polymer films (or films) for use as a
base film or
substrate in susceptor films, a method of making such polymer films, and
susceptor films and
structures including the polymer film.
In one aspect. the base film may generally have a crystallinity of from
greater than 0% to
about 50%. for example, from about 1% to about 50%, prior to exposing the film
to microwave
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energy. In some examples, the crystallinity may be or from about 1% to about
49%, from about
1% to about 48%, from about 1% to about 47%, from about 1% to about 46%, from
about 1% to
about 45%, from about 1% to about 44%, from about 1% to about 43%, from about
1% to about
42%, from about 1% to about 41%, from about 1% to about 40%, from about 1% to
about 39%,
from about 1% to about 38%, from about 1% to about 37%, from about 1% to about
36%, from
about 1% to about 35%, from about 1% to about 34%, from about 1% to about 33%,
from about
1% to about 32%, from about 1% to about 31%, from about 1% to about 30%, from
about 1% to
about 29%, from about 1% to about 28%, from about 1% to about 27%, from about
1% to about
26%, from about 1% to about 25%, from about 1% to about 24%, from about 1% to
about 23%,
from about 1% to about 22%, from about 1% to about 21%, from about 1% to about
20%, from
about 1% to about 19%, from about 1% to about 18%, from about 1% to about 17%,
from about
1% to about 16%, from about 1% to about 15%, from about 1% to about 14%, from
about 1% to
about 13%, from about 1% to about 12%, from about 1% to about 11%, from about
1% to about
10%, from about 1% to about 9%, from about 1% to about 8%, from about 1% to
about 7%, from
about 1% to about 6%, from about 1% to about 5%, from about 1% to about 4%,
from about 1%
to about 3%, or from about 1% to about 2%, prior to exposing the film to
microwave energy.
Other ranges of crystallinities within and/or overlapping with the above
ranges are contemplated,
for example, from about 4% to about 35%, from about 10% to about 40%, from
about 37% to
about 50%, and so on.
The present inventors have determined that low crystallinity films made in a
variety of
ways may be superior base films when compared to their highly crystalline
counterparts.
Specifically, it has been discovered that lower crystallinity and lower
residual orientation levels
generally correspond to higher heating capability in the resulting susceptor
film and/or susceptor
structure, even where the base film has a high absolute orientation level
similar to a conventional
base film. This presents a significant departure from the conventional use of
highly oriented,
highly crystalline susceptor films. While some attempts to understand the self-
limiting behavior
of susceptors have been made, it is believed that the relationship between the
crystallinity and
dynamic dimensional temperature response characteristics of oriented films
used for microwave
susceptor films and the resulting susceptor performance has generally not been
explored or
appreciated by others.
As stated above, the crystallinity values disclosed herein are derived from
density
measurements unless otherwise noted. As will be understood by those of skill
in the art, there are
different techniques for determining the percent crystallinity of a polymer
film, and the same
sample tested using different methods will yield different results for
crystallinity. Some methods
(e.g., density or differential scanning calorimetry (DSC)) are considered to
be indirect methods
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because they involve the use of physical constants in calculations to arrive
at a crystallinity value,
while other methods (e.g., x-ray diffraction or similar techniques) are
considered to be direct
methods because they do not rely on such constants. Thus, while each test
method provides a
suitable means of comparing relative values determined using the same method,
absolute percent
crystallinity values cannot be readily compared across different test methods.
Specifically, the percent crystallinity values presented herein were
calculated using
Equation (1), in which p, is the measured density of the sample, pa is the
density of 0% crystalline
material, and pc is the density of 100% crystalline material:
(P Ps)
%Crystallinity of Samp s ¨ te = 100 (1)
(Pc ¨ Pa)
Values of 1.333 and 1.455 g/cm3 were chosen for pa and põ respectively (as
published in J.
Brandrup, E. H. hnmergut, E. A. Grulke, Editors. Polymer Handbook, 41)
Edition, John Wiley &
Sons, (1999), p. V/113), to yield Equation (1.1):
(9- ¨ 1.333)
% Crystallinity of sample =190 x ________________________ (1.1)
(145S ¨1.333)
In another aspect, it has also been discovered that, in many cases, the
refractive index
and/or the birefringence of the polymer film may be more indicative of
perfonnance than the
criteria set forth in the prior art. The refractive index of a material is the
ratio of the velocity of
light in a vacuum to the velocity of light in that material. By polarizing
light in a particular
direction of a material, one can measure the refractive index in that
direction. For amorphous
materials with no molecular ordering, a single value can be used to optically
define that material.
For materials capable of molecular orientation, such as semi-crystalline
polymers. the absolute
value of refractive index increases with increasing crystallinity and
orientation, and the different
values of refractive indexes that will be measured in different directions may
be used to
characterize anisotropy of a structure such as a film. The difference between
refractive indexes in
two directions of a polymer film is defined as the birefringence between those
two directions and
can be used to understand differences in orientation between these directions.
Since it is defined
as a difference between the values of refractive index between two directions,
birefringence may
be a positive or negative number depending the morphology of a particular
sample and the
directions chosen for the refractive indexes; higher absolute values of
birefringence are widely
acknowledged to be associated with greater differences in orientation in those
two directions.
Some polymer films useful for forming susceptor structures according to the
disclosure
may generally have a refractive index (nz) (where nz refers to the refractive
index in the machine
direction of the film) of less than about 1.64, for example, from about 1.57
to about 1.62. in each
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of various independent examples, the polymer film may have a refractive index
(n2) of less than
about 1.63, less than about 1.62, less than about 1.61, less than about 1.60,
less than about 1.59. or
less than about 1.58. Additionally or alternatively, the polymer films may
generally have a
birefringence (n-n) (where n, refers to the refractive index in the thickness
of the film) of less
than about 0.15 (including negative birefringence values). In each of various
independent
examples, the birefringence (ni-n) may be less than about 0.14, less than
about 0.13, less than
about 0.12, less than about 0.11, less than about 0.10, less than about 0.090,
less than about 0.080,
less than about 0.070, less than about 0.060, less than about 0.050, less than
about 0.040, less than
about 0.030, less than about 0.020, less than about 0.015, less than about
0.010, or less than about
0.0050. However, other suitable refractive indexes. birefringence values, and
ranges thereof are
contemplated.
As discussed above, prior art films have typically been characterized based on
their static
shrink properties (e.g., '1% shrink at 150 C for 30 min'). However. the
present inventors have
determined that this traditional performance definition may be inadequate and
misleading. and that
specifying and producing susceptor substrate films using this definition as a
primary criteria for
selecting materials and processes has possibly limited the development of
superior performing
microwave susceptor substrate films, susceptor components, susceptor packages
and susceptor
package/field modification or shielding packages and components.
The present inventors have discovered that to realistically characterize the
microwave
heating behavior of susceptor structures, one must understand the dimensional
response of
susceptor substrate films to dynamic temperature exposure, which is more
representative of the
actual conditions experienced during microwave heating using susceptor
structures. This
understanding has been discovered to be particularly useful for designing
superior susceptor
substrate films and useful microwave heating structures for food applications
or other industrial
uses.
For heating of food, this may be of great significance, since the browning and
crisping
changes in the food to be microwave heated in these susceptor packages, and
which are highly
desired quality parameters by consumers, often rely heavily on the Maillard
browning reaction to
achieve natural and appealing surface color change as well as aroma and flavor
development, and
on surface moisture content changes to achieve crisper texture and mouth feel.
The progression of
the MaiHard reaction is also associated with lower water activity in the food
than is typically
present at the start of cooking. Since the kinetics of the Maillard reaction
begin to reach reaction
rates useful to achieve browning in time frames of interest for susceptor
packages above 155 C
and increase as a power function with increasing temperature, for the desired
short cooking times
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in the microwave oven, increasing the temperature is highly beneficial to both
aspects of the
Maillard reaction, lower water activity and increased kinetics.
Notably, as will be discussed further in connection with the examples, many of
the films
described in the prior art as being superior (e.g., as meeting the definition
of 'heat stabilized PET'
in U.S. Patent No. 5,177,132) show little if any improvement over standard
susceptor structures
using standard biaxially oriented PET film, and are eclipsed by the
performance of the susceptor
structures made from the base films of the present disclosure. For example,
films that exhibit
shrink on the order of 1% at 150 C for 30 minutes yield susceptor films that
exhibit typical heat
limiting and crazing behavior that the present films overcome. The inventors
have measured the
dynamic dimensional temperature response of example susceptor structures
produced from 'heat
stabilized PET' claimed to offer improved susceptor performance and show it to
be inferior in
stability at useful microwave heating conditions compared to those of the
present disclosure.
Thus, the typical method of characterizing heating potential by long term
exposure to a
temperature that is at best at the low end of temperatures of interest for
microwave susceptor
heating is inconsistent with developing an understanding of the actual heating
performance
achieved during the actual event of interest, the heating of microwave foods
in microwave ovens,
which is a dynamic process in which it is very often desired that susceptor
package components
reach temperatures significantly in excess of the typical 150 C static test
condition.
In view of the above discoveries, the present inventors have determined that a
vast array
of materials and processes can be used to form susceptor base films having
properties that result in
superior heating performance under microwave heating conditions. For example.
susceptor
structures formed from the base films may generally have a browning reaction
rate that exceeds
the browning reaction rate of a susceptor structure made from a conventional
biaxially oriented
PET film, for example, such that a discernible difference in browning and/or
crisping of food
items may be observed.
In one aspect, the susceptor film may comprise a minimally oriented film.
Minimally
oriented films may be unoriented (i.e., non-oriented) or slightly oriented
(i.e., from greater than
0% to about 20% orientation, as will be discussed below). Unoriented polymer
films are films
that are not subjected to stretching in either or both the machine direction
(MD) and cross
directions (CD) at temperatures below the melting point of the polymer.
Unoriented polymer
films can be quenched rapidly, which results in a low crystallinity. for
example, less than about
25%, which may generally be attributed to the residual melt orientation
associated with drawing
down the melt to the desired final film thickness. In contrast, highly
oriented films of the type
used in conventional susceptor films and structures have high levels of
orientation and/or strain
induced crystallinity and possess high levels of residual shrink forces.
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In each of various examples. minimally oriented films according to the present
disclosure
may be characterized as having one or more of the following:
a refractive index (n2) of less than about 1.59, for example. from about 1.57
to about 1.58,
or from about 1.5727 to about 1.58, for example, about 1.575 (see, e.g.,
Polymer Handbook, J.
Brandrup, E. H. hnmergut, and E. A. Grulke, 4th ed., John Wiley & Sons, Inc.,
1999, ISBN 0-
471-16628-6), or from about 1.5723 to about 1.5727, for example, about 1.5725;
a birefringence (n2-n8) of less than about 0.005, less than about 0.004, less
than about
0.0035, or less than about 0.0028, for example, from about 0.0012 to about
0.0022, for example,
about 0.0016; and
a crystallinity of greater than 0%, for example, at least about 1%, and less
than about
25%, less than about 22%, less than about 20%, less than about 18%, less than
about 15%. less
than about 12%, less than about 10%, less than about 7%, or less than about
5%. In one particular
embodiment, the film may have a crystallinity of about 4%. However, other
crystallinities,
refractive indexes, birefiingences, and ranges of each may' be suitable.
Any suitable polymer may be used to form the susceptor base film or substrate.
In one
example, the film may comprise amorphous PET (APET), for example, APET film
commercially
available from Pure-Stat Technologies, Inc. (Lewiston. Maine). However, other
suitable APET
films and/or other polymer films may be used.
The present inventors have discovered that susceptor films including lower
orientation
levels may tend to resist crazing to a greater extent than conventional,
highly oriented, highly
crystalline biaxially oriented PET films. While not wishing to be bound by
theory, it is believed
that high residual shrink forces may have a significant role in the onset and
propagation of crazing
of susceptor structures. Since less oriented films exhibit much lower heat
induced dimensional
shrinkage forces than highly oriented films, minimally oriented films may'
tend to resist crazing
more than highly oriented polymer films. Thus, a minimally oriented film with
inherently' low,
very low, or even no shrinkage forces, for example, APET, may tend to resist
crazing to a greater
extent than a conventional, highly oriented film with inherently high shrink
forces, for example, a
highly oriented PET. This is believed to be a clear departure from the
conventional approach to
designing susceptor films as taught in the art.
Alternatively or additionally, and while not wishing to be bound by' theory,
it also is
believed that the crystallinity of the minimally' oriented polymer film may'
increase during the
heating cycle, thereby' rendering the polymer more resistant to heat, and
therefore, more heat
stable. As a result, the stability of the susceptor film may increase during
the heating cycle.
In another aspect, the susceptor film may comprise a moderately oriented
polymer film,
that is, a polymer film that has been subject to an orientation process by
which at least one
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dimension of the film is increased from 20% to about 200%, for example, from
about 20% to
about 150%, for example, from about 30% to about 70%, for example, about 50%.
This
corresponds to respective draw ratios of from about 1.2:1 to about 3:1, for
example, from about
1.2:1 to about 2.5:1, for example, from about 1.3:1 to about 1.7:1, for
example, about 1.5:1 (where
the draw ratio equals the percent stretch divided by 100, plus 1). Such an
orientation process may
be useful where the unoriented film lacks sufficient strength for a particular
application.
In each of various examples, moderately oriented films according to the
present
disclosure may be characterized as having one or more of the following:
a refractive index (nz) of less than about 1.62, less than about 1.61, less
than about 1.60,
or less than about 1.59, for example, from about 1.57 to about 1.59, for
example, from about
1.5733 to about 1.5848, for example, about 1.5791;
a birefringence (111-n,) of less than about 0.05, less than about 0.035, less
than about 0.01,
or less than about 0.0024, for example, from about -0.013 to about 0.0024, for
example. about
-0.0029; and
a crystallinity of greater than 0%, for example, at least about 1%, and less
than about
50%, less than about 48%, less than about 45%, less than about 42%, less than
about 40%, less
than about 38%, less than about 37%, less than about 36%, less than about 35%,
less than about
34%, less than about 30%, less than about 25%, less than about 20%, less than
about 17%, less
than about 15%, less than about 12%, less than about 10%, less than about 7%,
or less than about
5%. In one specific example, the crystallinity of the film may be about 4%.
However, other
crystallinities, refractive indexes, birefringences, and ranges of each may be
suitable.
The orientation may be biaxial or bidirectional (i.e., in both the machine
direction (MD)
and cross-machine direction (CD)), or may be unia_xial or unidirectional
(i.e., in either the MD or
CD). Surprisingly, the present inventors have determined that under certain
process conditions,
strength may be imparted to the film without driving the level of
crystallinity to the levels
observed in conventional biaxially oriented PET. Specifically, the conditions
may be selected to
minimize the strain induced crystallization and thermal induced
crystallization commonly
associated with typical orienting processes. Accordingly, the orientation
process conditions may
be customized to provide a desired level of crystallinity, and therefore,
desired heating
performance for a particular susceptor film application.
As will be understood by those familiar with orientation processes, all other
things being
equal, stretching closer to Tg (approximately 80 C for polyethylene
terephthalate) requires more
force to be exerted to effect a given stretch ratio and greater stress is
imparted to the polymer; this
is the typical route pursued for developing high crystallinity, high residual
orientation and high
mechanical strength properties for standard highly biaxiaily oriented PET
polymer films. The
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resulting films often have a crystallinity of greater than 50%. However, the
present inventors have
found that it may be useful to depart from the conventional practice of
seeking high levels of
crystallinity and residual orientation. Instead, the films of the present
disclosure may generally be
stretched at temperatures well above Tg to minimize strain induced
crystallization. By way of
example, in laboratory testing, films that were stretched at temperatures of
at least about 105 C
and annealed at a temperature of at least about 170 C achieved a significant
improvement in
strength without sacrificing the heating performance of an unoriented film, as
will be discussed
further below in connection with the examples. It will be noted that these
temperatures are
exemplary only and were obtained on laboratory scale equipment (see the
Examples), and such
temperatures may or may not apply on commercial scale equipment for a given
polymer system.
In another aspect, the present inventors have also discovered that if the
polymer is chosen
properly, higher levels of orientation may still result in relatively low
crystallinity, and therefore,
may produce susceptor films having a higher heating potential than
conventional films. By way of
example, it has been discovered that copolyesters having a melting point at or
below that of
standard PET homopolymer can produce susceptor films that have excellent
heating
characteristics. While not wishing to be bound by theory, it is believed that
steric hindrance
considerations of copolyesters of interest in this disclosure retard
crystallinity development and
even highly oriented films made from these materials are typically incapable
of reaching absolute
crystallinity levels that are commonly achieved with PET homopolymers. Even
though these
films may achieve close to their potential maximum orientation crystallinity
during processing. the
copolymer is not capable of reaching the absolute crystallinity level possible
with homopolymers.
Accordingly, films produced from copolyesters of the type disclosed herein,
whether unoriented,
slightly oriented, moderately oriented, or highly' oriented have lower
refractive indexes and
birefringence values than typical in biaxially oriented standard and heat
stabilized standard PET
films.
More particularly, in each of various examples, highly oriented films
according to the
present disclosure may be characterized as having one or more of the
following:
a refractive index (nz) of less than about 1.64, for example, from 1.56 to
about 1.63, for
example, from about 1.58 to about 1.61, for example, from about 1.5769 to
about 1.6124, for
example, about 1.5920;
a birefringence (n1-n9) of less than about 0.15, less than about 0.14, less
than about 0.125,
or less than about 0.11, for example, from about 0.0030 to about 0.10, for
example, from about
0.0029 to about 0.1022, for example, about 0.0437; and
a crystallinity of greater than 0%, for example, at least about 1%, and less
than about
50%, less than about 48%, less than about 45%, less than about 40%, less than
about 38%, less
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than about 37%, less than about 36%, less than about 35%, less than about 33%,
less than about
30%, less than about 28%, less than about 26%, less than about 25%, less than
about 22%, less
than about 20%, less than about 17%, less than about 15%, less than about 12%,
or less than about
10%. In one particular example, the crystallinity may be about 7%. However.
other
crystallinities, refractive indexes, birefringences, and ranges of each may be
suitable. These
values reflect the avoidance of significant amounts of strain induced or
thermally induced
crystallinity in the films during the orientation process. Accordingly',
highly oriented susceptor
films and structures including base films comprising a copolyester have been
shown to have
excellent heating properties. comparable to the unoriented, slightly oriented,
and/or moderately
oriented base films described above.
These results are unexpected and in sharp contrast to the prior art, in which
copolyesters
with melt points higher than homopolymer PET were used in an attempt to create
'eater
temperature resistance. Others have attempted to create higher heat capability
susceptor base
films by heat setting the biaxially oriented films at higher temperatures for
longer times. These
have generated mixed results. The present inventors have discovered that
slight increases in the
temperature of onset of the release of the significant shrink forces present
in highly oriented
homopolymer PET films do little to increase resistance to crazing and
resultant deterioration in
heating and limited heat generating capability. However, the present inventors
have learned that
by' minimizing crystallinity and residual orientation, even in highly oriented
films, it is possible to
create mechanically' strong. thin films that have superior heating performance
to conventionally
used susceptor films.
Any suitable copolyester may be used. The copolyester may' generally have a
melting
point of less than about 260 C, for example. from about 200 C to about 260 C,
for example, from
about 220 C to about 260 C. In some examples, the copolyester may have a
melting point of less
than about 250 C, for example, from about 200 C to about 250 C. for example,
from about 220 C
to about 250 C. The melting point may be similar to or lower than the melting
point of standard
PET polymers, which have peak crystalline melt points when determined by
second DSC heating
in the range of 250 C to 260 C, although some references report PET
homopolymer melt points as
high as 265 C (see "Polymer Chemistry, An Introduction 3rd Edition" by Malcolm
Stevens
published 1999 by Oxford University Press, p. 344).
The copolyester also may generally be resistant to the development of the very
high
(>50%) levels of crystallinity commonly associated with standard biaxially
oriented PET or heat
stabilized PET films. In some examples, the base film may have a crystallinity
of less than about
37%, despite having undergone high degrees of biaxial orientation (similar to
stretching ratios
common for biaxially oriented standard or heat stabilized PET polymer films).
It is anticipated
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that similarly low or even lower crystallinity values would be observed for
unoriented, slightly
oriented, or moderately oriented films produced from copolyesters.
One specific example of a copolymer that may be suitable for use in a
susceptor base film
is SKYPET-BR (SK Chemicals of Seoul. Korea), which falls in the broad
classification of
substances described by CA Index Name 1,4-benzenedicarboxylic, dimethyl ester,
polymer with
1,4-cyclohexanedimethanol and 1.2-ethanediol under CAS Registry Number 25640-
14-6.
SKYPET-BR has a melting point of 236+/- 2 C, well below the typical melting
point of standard
PET polymer. An alternative polymer with similar properties is Eastman PET
9921 produced by
Eastman Chemical Company. Kingsport, TN. These materials are part of a broad
class of
copolyester materials based on 1,4-cyclohexanedimethanol (commonly abbreviated
as CRDM),
which may be formed through the modification of poly(ethylene terephthalate)
with 1.4-
cyclohexanedimethanol or the modification of poly(I,4-cyclohexylenedimethylene
terephthalate)
with ethylene glycol or isophthalic acid.
Other potentially suitable materials include polyethylene terephthalate
copolyesters
(diethylene glycol-isophthalate modified) prepared by the condensation of
dimethyl terephthalate
or terephthalic acid and ethylene glycol with one or more of the following:
dimethyl isophthalate,
isophthalic acid, and diethylene glycol. The resulting polymers may' vary in
terms of modification
or co-polymerization levels, yielding different properties, which can be
exploited in the context of
modifying performance of substrate susceptor films produced from these
polymers.
It will be appreciated that while several examples are provided herein,
countless other
possibilities and combinations thereof are contemplated, including
copolyesters that are developed
after the date of this disclosure.
In still another aspect, the present inventors have observed that higher
orientation
temperatures (all else being equal) may lead to higher heating capability.
While not wishing to be
bound by theory, it is believed that this is the result of higher polymer
chain mobility at higher
temperatures and resulting greater ease of the chains slipping past one
another during orientation.
This reduces the stress required to achieve a given degree of stretching, and
can act to reduce
strain induced crystallinity, the minimization of which is believed to be
advantageous. Two
rotational isomers of polyester exist, gauche and trans. Gauche isomers can be
generally
characterized as in a relaxed state. while trans isomers are in an extended,
higher energy state.
Significantly, the gauche isomer is only found in the amorphous domains or
regions of the
structure, while trans can exist in both crystalline and amorphous regions.
Only trans exists in
crystalline regions.
During strain induced crystallization, a portion of the gauche isomer content
is
understood to be converted to trans in the amorphous regions, further
increasing total crystallinity
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as well as residual orientation. In some process situations, for example heat
setting that does not
induce further crystallization, it may also be possible to reverse some of the
conversion, lowering
crystallinity as a result in potential advantageous ways. While not wishing to
be bound by theory,
it is believed that minimizing (or even some modest reversing of) the gauche-
trans conversion
contributes to the performance of the susceptor films and structures of the
present disclosure, and
while gauche and trans content of the amorphous and crystalline domains of
films of this
disclosure have not been directly measured, they can be inferred from the
ability of the material
and process choices disclosed herein to minimize the overall crystallinity
level of the resulting
susceptor substrate films compared to the typically >50% crystallinity levels
that characterize the
standard and heat stabilized films that represent past practice. Higher gauche
isomer content in
films of this disclosure is also consistent with the lower refractive indexes
and birefringence
values reported herein.
In yet another aspect, depending on the film, it may also be advantageous,
beyond what
can be achieved with homopolymer films, to gain some additional heating
capability by annealing
at higher temperatures, as discussed in connection with the Examples.
It will be appreciated that the copolyesters described above may likewise be
used to
produce unoriented films, slightly oriented films, and moderately oriented
films. In any of such
cases, and for highly oriented films, the copolyester can be used either in
films of homogenous
structure made solely of the copolymers or in coextrusions combining discrete
layers of
copolymer and homopolymer. Blends of co- and homopolymer polyester may also be
used
advantageously compared to 100% homopolymer structures. Incorporation of
copolyester, even
with homopolymer present, serves to result in films with properties as
susceptor base films
superior to 100% homopolymer comprised base films. It will be noted that where
the copolyester
is used in a moderately oriented film, alone or in combination with one or
more other polymers, a
greater degree of orientation may be used without driving the level of
crystallinity above 50%, as
compared with susceptor base films comprising only' PET homopolymer. In each
of various
examples, films including copolyester and PET homopolymer may be characterized
as having one
or more of the following:
a refractive index (nz) of less than about 1.64, for example, from about 1.60
to about 1.63
or from about 1.5975 to about 1.6280, for example, about 1.6123;
a birefringence (n1-n) of less than about 0.15, less than about 0.14, less
than about 0.125,
or less than about 0.11, for example, from about 0.065 to about 0.14, for
example, from about
0.0654 to about 0.1355, for example, about 0.1031; and
a crystallinity of greater than 0%, for example, at least about 1%. and less
than about
50%, less than about 48%, less than about 45%, less than about 42%, less than
about 40%, less
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than about 38%, less than about 37%, less than about 36%, less than about 35%,
less than about
34%, less than about 30%, less than about 25%, less than about 20%, less than
about 17%, less
than about 15%, less than about 12%, or less than about 10%,. However, other
crystallinities,
refractive indexes, birefringences, and ranges of each may be suitable. It is
also contemplated that
other polymers may be used alone or any combination to form various susceptor
base films having
the properties described herein in connection with the present base films.
For any of the susceptor base films described herein or contemplated hereby,
the kinetics
of crystallization of the polymer film may be manipulated to achieve the
desired level of
crystallinity at various points in the heating cycle, with time, temperature,
and the use of
nucleating agents being variables that may be adjusted as needed to attain the
desired susceptor
film performance. Further, since different food products require different
heating cycles for
optimum preparation, it is anticipated that the additional degrees of freedom
associated with
controlling initial crystallinity levels and the kinetics of further
crystallinity increases during
heating will permit expanded customization capabilities, which may' further
enhance the utility
and uniqueness of the susceptor films described herein.
It is also contemplated that in some instances, the susceptor film may be
intended to be
used more than once. In such instances, the crystallinity of the polymer film
may be higher upon
the second use and any subsequent use.
The polymer film may be formed in any suitable manner. In one example, the
polymer
film substrate may be a water quenched film, a cast film, or any other type of
polymer film that is
formed using a rapid quenching process. However, numerous other processes and
systems may be
used. When such films do not undergo a conventional post-extrusion orientation
process, it will
be appreciated that, in some instances, the film may be difficult to handle
and/or convert into a
susceptor structure. Thus, it is contemplated that the film may be subject to
a minimal orienting
process to orient (i.e., stretch) the film slightly (e.g., up to 20%, for
example, from about 5% to
20%) to improve processability of the film. Since such orienting is relatively
minor as compared
with standard highly oriented films that are stretched about 350-450% in each
direction, such
slightly' oriented films may be considered herein to be substantially
unoriented.
If desired, the crystallinity of minimally oriented films can be controllably
increased
through post-extrusion heat treatment or conditioning. Crystallization kinetic
modifying additives
may also be used, as described above.
Additionally or alternatively, additives may be incorporated into the film to
modify its
properties to facilitate processing or to provide more robust microwave
heating performance. As
an example, a strength enhancing additive (e.g., a polymer) may be used to
make more robust an
otherwise somewhat fragile low gauge cast APET film. Examples of additives
that may be
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suitable include an ethylene methyl acrylate copolymer, an ethylene-octene
copolymer, or any'
other suitable polymer or material that improves the strength and/or
processability of the polymer
film. Other additives providing different functions or benefits may also be
used. Any of such
additives may' be added in any suitable amount, for example, up to about 15%
by weight of the
polymer film, up to about 10% by weight of the polymer film, up to about 5% by
weight of the
polymer film, or in any other suitable amount. In other examples, the
additives may be used in an
amount of from about 1% to about 10%, from about 2% to about 8%, or from 3% to
about 5% by
weight of the polymer film, or in any' suitable amount or range of amounts.
Any of the susceptor base films may comprise a multilayer film including at
least two
distinct layers. each of which may comprise one or more polymers and,
optionally, one or more
additives. The layers may' be coextruded or may be formed separately and
joined to one another
using an adhesive, a tie layer, thermal bonding. or using any other suitable
technique. Other
suitable techniques may include extrusion coating and coextrusion coating.
If desired, each layer of the multilayer film may be a rapidly' quenched film,
i.e., a film
formed under conditions that provide very fast freezing of the polymer melt
after it has exited the
opening of the extrusion die. This rapid freezing and further lowering of the
temperature of the
solidified polymer film minimizes the development of crystalline micro or
macro structures. As
stated above, it is believed that when films with low crystallinity are used
to form a susceptor film,
the susceptor film is capable of achieving higher temperatures and heat flux
during microwave
heating, as compared with conventional susceptors made from biaxially oriented
polyethylene
terephthalate.
If desired, additional functional characteristics can be imparted to the
multilayer film by
selecting polymers having the desired attributes. For example, ethylene vinyl
alcohol (EVOH)
may be used to impart oxygen barrier properties. Polypropylene (PP) may be
used to impart water
vapor barrier properties. Such properties may render the film useful for
controlled or modified
atmosphere packaging, and in particular, for chilled or shelf stable foods,
where higher oxygen
and moisture barriers are typically required than for frozen foods. Numerous
other possibilities
are contemplated.
Numerous multilayer films are contemplated by the disclosure. By way of
illustration
and not limitation, some exemplary structures include: (a) APET/olefin; (b)
APET/tie layer/olefin;
(c) APET/tie layer/olefin/tie layer/APET; (d) APET/tie layer/PP/tie
layer/APET; (e) APET/tie
layer/PP/tic layer/amorphous nylon 6 or nylon 6,6; (f) APET/tie layer/APET;
(g) APET/tie
layer/EVOH/tie layer/APET; (h) APET/tie layer; (i) APET/tie layer/regrind of
all layers/tie
layer/EVOH/tie layer/APET; (j) APET/tie layer/EVOH/tie layer/amorphous nylon 6
or nylon 6,6;
(k) APET/tie layer/olefin/tie layer/EVOH/tie layer/APET; (1) APET/tie
layer/olefin/tie
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layer/EVOH/tie layer/nylon 6.6; (m) PET homopolymer/ copolyester/PET
homopolymer; and (n)
copolyester/PET homopolymer/copolyester.
In examples a-c and k-1 and in any other multilayer film contemplated by this
disclosure,
the olefin layer may' comprise any suitable polyolefin, for example, low
density polyethylene
(LDPE), linear low density polyethylene (LLDPE), medium density polyethylene
(IVIDPE), high
density polyethylene (HDPE), polypropylene (PP), copolymers of any of such
polymers, and/or
metallocene catalyzed versions of these polymers or copolymers.
In example i and in any other multilayer film contemplated by the disclosure,
the regrind
layer may include the film edge scrap and any other recyclable material,
according to conventional
practice. Any of the various other examples (examples a-h or j-/) or other
films contemplated by
this disclosure may contain such a regrind layer. In some cases. regrind
layers may require a tie
layer to bond them satisfactorily to the adjacent film layers.
In examples b-j, and in any other multilayer film contemplated by this
disclosure, the tie
layer may comprise any' suitable material that provides the desired level of
adhesion between the
adjacent layers. In some exemplary embodiments, the tie layer may comprise
Bynel from
DuPont, Plexar from Equistar, a LyondellBasell company, or ExxlorTM from
Exxon. The
precise selection of the tie layer depends on the adjacent polymers it is
intended to join and
rheological properties that ensure even distribution of layers in the
coextrusion process. For
example, DuPont Bynel 21E781 is part of the Bynel 2100 Series of anhydride
modified ethylene
acrylate resins that are most often used to adhere to PET, nylon, EVOH,
polyethylene (PE), PP,
and ethylene copolymers. Plexar PX1007 is one of a class of ethylene vinyl
acetate copolymers
that can be used to bond a similar range of materials as the Bynel resin
mentioned previously.
Exxlonk grades may be used to enhance the impact performance of various nylon
polymers. In
addition, the tie layers and other resins may be selected for their prior
sanctioned use in high
temperature films for applications such as retort pouches, where minimal resin
extractables into
food are allowed.
It is contemplated that either amorphous nylon 6 or nylon 6,6 could be
substituted for
APET in any of the above multilayer film structures or any other structure
within the scope of the
disclosure. Countless other structures are contemplated.
Numerous techniques may be used to form a multilayer film. While film casting
is a
commonly used rapid quench film production technique, adaptations of the air-
cooled blown film
process may also create quench rates suitable for the creation of the
multilayer films of this
disclosure. The use of chilled air applied to the outside of the blown film
"bubble" can increase
the quench rate compared to the use of room temperature air directed only on
the exterior surface
of the bubble. Additionally, the use of chilled air exchange for internal
bubble cooling can boost
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output rates. Higher quench rates can be achieved through the use of water
cooled mandrels that
contact the interior of the bubble, but this process is relatively inflexible
in the width of film that
can be produced, as the higher quench rates are only achieved from intimate
contact between the
polymer bubble and the mandrel, and different mandrels are required to produce
different film
widths.
Another approach for the tubular film blowing process is the tubular water
quench
process (TWQ). TWQ entails the direct contact of cooling water with the
exterior of the polymer
bubble, which results in extremely high heat transfer rates and very rapid
quenching of the
extruded polymer film. Sonic TWQ processes combine direct water contact with
the exterior of
the bubble with an internal mandrel for support and further cooling. Another
TWQ process may
solely utilize direct water contact on the external surface of the bubble,
sometimes supplemented
with chilled air exchange in the interior of the bubble. In some
circumstances, the latter TWQ
process may be more advantageous to use because equipment without internal
mandrels is less
costly to build and operate and provides more flexibility in film width
changes. Such TWQ
extrusion lines are available, for example, from Brampton Engineering of
Canada under the trade
name AquaFrost systems. However, numerous other processes and systems may be
used.
The basis weight and/or caliper of the polymer film, whether single layer or
multilayer,
may vary for each application. In some embodiments, the film may be from about
12 to about 50
microns thick, for example, from about 12 to about 35 microns thick, for
example, about 12 to 20
microns thick. However, other calipers are contemplated.
A layer of microwave energy interactive material (i.e., a susceptor or
microwave
susceptible coating) may be deposited on one or both sides of the polymer film
to form a susceptor
film. The microwave energy interactive material may comprise an
electroconductive or
semiconductive material, for example, a vacuum deposited metal or metal alloy,
or a metallic ink,
an organic ink, an inorganic ink, a metallic paste, an organic paste, an
inorganic paste, or any
combination thereof. Examples of metals and metal alloys that may be suitable
include, but are
not limited to, aluminum, chromium, copper, inconel alloys (nickel-chromium-
molybdenum alloy
with niobium), iron, magnesium, nickel, stainless steel, tin, titanium,
tungsten, and any
combination or alloy thereof.
Alternatively, the microwave energy interactive material may comprise a metal
oxide, for
example, oxides of aluminum, iron, and tin, optionally used in conjunction
with an electrically
conductive material. Another metal oxide that may be suitable is indium tin
oxide (ITO). 1TO has
a more uniform crystal structure and, therefore, is clear at most coating
thicknesses.
Alternatively still, the microwave energy interactive material may comprise a
suitable
electroconductive, semiconductive, or non-conductive artificial dielectric or
ferroelectric.
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Artificial dielectrics comprise conductive, subdivided material in a polymeric
or other suitable
matrix or binder, and may include flakes of an electroconductive metal, for
example, aluminum.
In other embodiments, the microwave energy interactive material may be carbon-
based,
for example. as disclosed in U.S. Patent Nos. 4,943,456, 5,002,826, 5,118,747,
and 5,410,135.
In still other embodiments, the microwave energy interactive material may
interact with
the magnetic portion of the electromagnetic energy in the microwave oven.
Correctly chosen
materials of this type can self-limit based on the loss of interaction when
the Curie temperature of
the material is reached. An example of such an interactive coating is
described in U.S. Patent No.
4,283,427.
Other microwave energy interactive materials capable of being combined with
the films
of the present invention to create microwave susceptor structures will
represent other
embodiments of this invention.
The susceptor film may then be laminated or otherwise joined to another
material to
produce a susceptor structure or package. In one example, the susceptor film
may be laminated or
otherwise joined to paper or paperboard to make a susceptor structure having a
higher thermal flux
output than conventional paper or paperboard based susceptor structures. The
paper may' have a
basis weight of from about 15 to about 60 lb/ream (16/3000 sq. ft.), for
example, from about 20 to
about 40 lb/ream, for example, about 25 lb/ream. The paperboard may have a
basis weight of
from about 60 to about 330 lb/ream, for example, from about 80 to about 140
lb/ream. The
paperboard generally may have a thickness of from about 6 to about 30 mils,
for example, from
about 12 to about 28 mils. In one particular example, the paperboard has a
thickness of about 14
mils (0.014 inches). Any suitable paperboard may be used, for example. a solid
bleached sulfate
board, for example, Fortress board, commercially available from International
Paper Company,
Memphis, TN, or solid unbleached sulfate board, such as SUS board.
commercially available
from Graphic Packaging International.
Alternatively, the susceptor film may be laminated or otherwise joined to
another polymer
film. It is contemplated that the polymer film would exhibit little or no
shrink, similar to its base
film counterpart. such that the performance attributes of the susceptor film
are not adversely
affected. It is also contemplated that such polymer films may be clear,
translucent, or opaque, as
needed for a particular application. It is further contemplated that the
laminated (or otherwise
joined) structures may be capable of being thermoformable. It is anticipated
that shallow draw
shapes could preserve susceptor functionality in all but the highest stretch
areas during
thermofonuing, and one could advantageously use die and or plug design to
tailor local stretch
ratios to customize degree of susceptor functionality. The inherently lower
crystallinity of the
films of this disclosure lend themselves advantageously to formability, as
high crystalline
21
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WO 2014/142887 PCT/US2013/031420
materials do not forin easily, particularly on in-line form-fill-seal
packaging machinery. Post
crystallization of formed structures may be induced through methods common to
those skilled in
the art.
If desired, the susceptor base film may undergo one or more treatments to
modify the
surface prior to depositing the microwave energy interactive material onto the
polymer film. By
way of example, and not limitation, the polymer film may undergo a plasma
treatment to modify
the roughness of the surface of the polymer film. While not wishing to be
bound by theory. it is
believed that such surface treatments may provide a more uniform surface for
receiving the
microwave energy interactive material, which in turn, may increase the heat
flux and maximum
temperature of the resulting susceptor structure. Such treatments are
discussed in U.S. Patent
Application Publication No. 2010/0213192A1 and U.S. Patent Application No.
13/804.673, filed
March 14, 2013, both of which are incorporated by reference herein in its
entirety.
Also, if desired, the susceptor film may' be used in conjunction with other
microwave
energy interactive elements ancUor structures. Structures including multiple
susceptor layers are
also contemplated. It will be appreciated that the use of the present
susceptor film and/or structure
with such elements and/or structures may provide enhanced results as compared
with a
conventional susceptor.
By way of example, the susceptor film may be used with a foil or high optical
density
evaporated material having a thickness sufficient to reflect a substantial
portion of impinging
microwave energy. Such elements typically are formed from a conductive,
reflective metal or
metal alloy, for example, aluminum, copper, or stainless steel, in the form of
a solid patch
generally having a thickness of from about 0.000285 inches to about 0.005
inches, for example,
from about 0.0003 inches to about 0.003 inches. Other such elements may have a
thickness of
from about 0.00035 inches to about 0.002 inches, for example, 0.0016 inches.
In some cases, microwave energy reflecting (or reflective) elements may be
used as
shielding elements where the food item is prone to scorching or drying out
during heating. In
other cases, smaller microwave energy reflecting elements may be used to
diffuse or lessen the
intensity of microwave energy. One example of a material utilizing such
microwave energy
reflecting elements is commercially available from Graphic Packaging
International, Inc.
(Marietta, GA) under the trade name MicroRitee packaging material. In other
examples, a
plurality of microwave energy reflecting elements may be arranged to form a
microwave energy
distributing element to direct microwave energy to specific areas of the food
item. If desired, the
loops may be of a length that causes microwave energy to resonate, thereby
enhancing the
distribution effect. Examples of microwave energy distributing elements are
described in U.S.
22
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Patent Nos. 6,204,492, 6,433,322, 6,552,315, and 6,677,563, each of which is
incorporated by
reference in its entirety.
In still another example, the susceptor film and/or structure may be used with
or may be
used to form a microwave energy' interactive insulating material. Examples of
such materials are
provided in U.S. Patent No. 7,019,271, U.S. Patent No. 7,351,942, and U.S.
Patent Application
Publication No. 2008/0078759 Al. published April 3, 2008, each of which is
incorporated by
reference herein in its entirety.
If desired, any of the numerous microwave energy interactive elements
described herein
or contemplated hereby may be substantially continuous, that is, without
substantial breaks or
interruptions, or may be discontinuous, for example, by including one or more
breaks or apertures
that transmit microwave energy. The breaks or apertures may extend through the
entire structure,
or only through one or more layers. The number, shape, size, and positioning
of such breaks or
apertures may' vary for a particular application depending on the type of
construct being formed.
the food item to be heated therein or thereon, the desired degree of heating,
browning, and/or
crisping, whether direct exposure to microwave energy is needed or desired to
attain uniform
heating of the food item, the need for regulating the change in temperature of
the food item
through direct heating, and whether and to what extent there is a need for
venting.
By way of illustration, a microwave energy interactive element may include one
or more
transparent areas to effect dielectric heating of the food item. However,
where the microwave
energy interactive element comprises a susceptor, such apertures decrease the
total microwave
energy interactive area, and therefore, decrease the amount of microwave
energy interactive
material available for heating, browning, and/or crisping the surface of the
food item. Thus, the
relative amounts of microwave energy interactive areas and microwave energy
transparent areas
must be balanced to attain the desired overall heating characteristics for the
particular food item.
As another example, one or more portions of a susceptor may be designed to be
microwave energy inactive to ensure that the microwave energy is focused
efficiently on the areas
to be heated, browned, and/or crisped, rather than being lost to portions of
the food item not
intended to be browned and/or crisped or to heating the environment.
Additionally or
alternatively, it may be beneficial to create one or more discontinuities or
inactive regions to
prevent overheating or charring of the food item and/or the construct
including the susceptor.
As still another example, a susceptor may incorporate one or more "fuse"
elements that
limit the propagation of cracks in the susceptor, and thereby control
overheating, in areas of the
susceptor where heat transfer to the food is low and the susceptor might tend
to become too hot.
The size and shape of the fuses may be varied as needed. Examples of
susceptors including such
fuses are provided, for example, in U.S. Patent No. 5,412,187, U.S. Patent No.
5,530,231, U.S.
23
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Patent Application Publication No. 2008/0035634A1, published February 14,
2008, and PCT
Application Publication No. WO 2007/127371, published November 8, 2007, each
of which is
incorporated by reference herein in its entirety.
It will be noted that any of such discontinuities or apertures in a susceptor
may comprise a
physical aperture or void in one or more layers or materials used to form the
structure or construct,
or may be a non-physical "aperture". A non-physical aperture is a microwave
energy transparent
area that allows microwave energy to pass through the structure without an
actual void or hole cut
through the structure. Such areas may be formed by simply not applying
microwave energy
interactive material to the particular area, by removing microwave energy
interactive material
from the particular area, or by mechanically deactivating the particular area
(rendering the area
electrically discontinuous). Alternatively, the areas may' be formed by
chemically deactivating the
microwave energy interactive material in the particular area, thereby
transforming the microwave
energy interactive material in the area into a substance that is transparent
to microwave energy
(i.e., so that the microwave energy transparent or inactive area comprises the
microwave energy
interactive material in an inactivated condition). While both physical and non-
physical apertures
allow the food item to be heated directly by the microwave energy, a physical
aperture also
provides a venting function to allow steam or other vapors or liquid released
from the food item to
be carried away from the food item.
The present invention may be understood further in view of the following
examples,
which are not intended to be limiting in any manner. All of the information
provided represents
approximate values, unless otherwise specified.
EXAMPLE 1
A calorimetry test was conducted to determine the thermal flux produced by and
maximum temperature reached by various susceptor structures. Various polymer
films were used
to form the susceptor structures, as set forth in Table 1. The polymer films
included DuPont
Mylar 800C biaxially oriented PET (DuPont Teijin FilmsTm, Hopewell, VA), Pure-
Stat APET
(Pure-Stat Technologies, Inc., Lewiston, Maine), DuPont HS2 biaxially oriented
PET (DuPont
Teijin FilmsTM, Hopewell, VA), and Toray Lumirror F65 biaxially oriented PET
(Toray Films
USA, Kingstown, RI). Physical properties of the raw films (some of which were
obtained from
the manufacturer data sheets) are set forth in Table 1.
24
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Table 1
DSC Initial Heating % Shrink
Thickness Density 150 C 190 C 190 C
Crystallization Melting
Film Crvstallinity
microns g/cm3 by Density Exothenn Endotherm 30 min 5 min 20 min
Jig AH Jig MD/TD MD/TD MD/TD
DuPont 800C 12 1.398 53 0 41 1.25/1.25
DuPont HS2 17 1.400 55 0 3.6/-0.2
bray F 65 17 1.400 55 0 46
4.0/0.5
PureStat APET 12 1.338 4.1 -36 36 -
None/None
The control films (samples 1-1, 1-6, and 1-7) were determined to have a
percent
crystallinity of 52-55%, which is consistent with information provided by
manufacturers of
BOPET films commonly used to form susceptor films, as set forth in Table 2.
Table 2
Commercially available Datasheet Measured % Crystallinity
Calculated from
12 micron (0.00048 in) thick Nominal Density, Density, g/cm3 Measured Density
using Eqn. (1.1)
3 cm
PET Film g/cm3 = 1.333 gicm3
pc= 1.455 g/cm3
DuPont Teijin Mylar 800 1.4 1.398 53
Mitsubishi Hostaphan RD 1.4 1.397 53
Nuroll PXE 1.395 1.398 53
Toray Lumirror 10.12 1.4 1.399 54
1() Each susceptor structure was made by joining a susceptor film to a
paperboard support
layer using from about 1.5 to about 2.0 lb/ream of one of the following
adhesives: Royal 20469
(Royal Adhesives & Sealants, South Bend, IN), Royal 20123 (Royal Adhesives &
Sealants, South
Bend, IN). or Henkel 5T-5380M5 (Henkel Adhesives, Elgin, IL). However, other
suitable
adhesives may be used.
Refractive index measurements were taken using a Metricon 2010 Prism Coupler
(Metricon Corporation, Pennington, NJ) at 633 nm. Results for the films are
listed as (iii)
(machine direction, MD), (ny) (cross or transverse direction, CD), and (r),)
(thickness) directions.
The birefringence (nz-n) was calculated from the refractive indexes and.
throughout this
specification, represents the difference in refractive indexes in the MD and
thickness of the films.
All these films were without added colorants or pigmentation, and thus were
clear.
Samples 1-1 and 1-6, made using commercial standard and heat stabilized PET
homopolymer films, respectively, exhibited high refractive indexes in both MD
and CD (II, and n,
respectively) that are characteristic of the highly oriented, highly
crystalline films of the prior art.
The superior performing cast film based structures samples 1-3 and 1-4 exhibit
much lower (nz)
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and (nv) values that in fact are quite close to reported values for amorphous
homopolymer PET
polymer. Combining this data with crystallinity >50% for samples I -1 and 1-6
and only about 5%
for samples 1-3 and 1-4 confirms that these samples represent largely
amorphous films with little
if any residual orientation. The differences between (nz) and (nO values in
samples 1-1 and 1-6
represent small differences in MD and CD orientation or heat setting, but
these are within the
range of what one could expect to encounter with films possessing reasonably
balanced MD/CD
orientation.
The calorimetry data was collected using a FISO MWS Microwave Work Station
fiber
optic temperature sensing device (FISO, Quebec, Canada) with eight (8)
channels mounted onto a
Panasonic 1300 watt consumer microwave oven model NN-S760WA. A sample having a
diameter of about 5 in. was positioned between two circular Pyrex plates,
each having a
thickness of about 0.25 in. and a diameter of about 5 in. An about 250 g water
load in a plastic
bowl resting on an about 1 in. thick expanded polystyrene insulating sheet was
placed above the
plates (so that radiant heat from the water did not affect the plates). The
bottom plate was raised
about 1 in. above the glass turntable using three substantially triangular
ceramic stands. Thermo¨
optic probes were affixed to the top surface of the top plate to measure the
surface temperature of
the plate. After heating the sample at full power for about 5 minutes in an
about 1300W
microwave oven, the average maximum temperature rise from initial ambient
temperature in
degrees C of the top plate surface was recorded. (Finite element analysis
modeling of the
calorimetry test method has shown that the average maximum temperature rise is
proportional to
the thermal flux generated by the susceptor structure.) The conductivity a
(mmho/sq) of each
sample was measured using a Delcom 717 conductance monitor (Delcom
Instruments, Inc..
Prescott, WI) prior to conducting the calorimetry test, with five data points
being collected and
averaged. The results are presented in Table 3, where AT is the rise in
temperature for the
sample, and where AAT is the difference between the rise in temperature for
the sample and the
rise in temperature for the control sample (structure 1-1, standard biaxially
oriented, heat set PET
film). A visual assessment of the level of crazing of the susceptor surface
after microwave
cooking exposure was made and is noted. It will be noted that the absolute
temperature reached
for each sample was about 22 C higher than the reported AT since all samples
had an initial
temperature of about 22 C (ambient temperature).
In general, susceptor structures 1-3 and 1-4 provided the most heating power
and no
visible crazing was evidentõ while structure 1-1 exhibited a lower heating
power than structures 1-
3 and 1-4 and exhibited the expected amount of crazing for standard commercial
susceptors.
Structure 1-6 had somewhat less crazing than structure 1-1 and provided a
moderate heating
power. Structure 1-7 was intermediate to 1-1 and 1-6 in both heating power and
crazing.
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Notably, structure 1-5, which had already' been heated once, exhibited a
greater power
output than structure 1-1. Although no visible crazing was observed, the
sample still exhibited
some degree of self-limiting behavior (as evidenced by ATmax). While not
wishing to be bound
by theory, it is believed that this self-limiting behavior is at least
partially the result of a change in
density of the polymer film during the microwave heating cycle. Specifically,
it is known that the
density of a polymer film may decrease as the polymer film heats. However, as
the polymer film
heats, there is also an increase in crystallinity and an accompanying increase
in density. It is
believed that the magnitude of this increase in density exceeds the magnitude
of the initial density
decrease, such that there is an overall increase in density during the heating
cycle. It is further
believed that this increase in density may cause disruptions or microcrazing
in the susceptor
structure that create electrical discontinuities on an atomic scale.
27
0
Table 3
n.)
o
1¨,
Sample/ Polymer film Board 6/0 Degree of n, (MD)
nz-n Degree of AT max AAT (3, before 0, after Visible .6.
1--,
structure (0.5 mil) (pt) Crystallinity post- ny (CD)
heat ( C) (OC) (mmho/sq.) (minho/sq.)
crazing .6.
n.)
oe
(initial) extrusion setting
oe
¨.1
orientation .
DuPont Mylar 800C 1.6644 142.9
Yes, typical
1-1 18 53 High 0.1707 Medium 0
20 0
PET 1.6488 +4.5
level
DuPont Mylar 800C 1110
Yes, high
1-, 18 - High - - Medium . -
31.9 0 0
PET, second heating +18
level
Pure-Stat APET, 15734
1-3 .
12 4.1 None
0.0014 None 164.7 21.8 13 1 1 0 No
metallized on first side 1.5735
Pure-Stat APET,
1.5733 P
1-4 metallized on second 12 4.1 None 0.0012 None
166.8 23.9 14 1 1 0 No
1.5737 .
r.,
side
.
Pure-Stat APET,
.
u.,
1-5 second heating on first 12 - None - - None
149.0 6.1 1 0 No .3
r.,
.
side
,
u.,
,
Yes, .
1.6587 .3
,
1-6 DuPont HS2 14 55 High 1.6568 0.1662 High
152.0 9.2 5 0 moderate .
level
1-7 Toray F65 12 55 High - High
149.6 6.7 8 0 Yes, between
+3.8 1-1 & 1-6
,-
IV
n
c 4
=
. 6 .
=
28
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EXAMPLE 2
The microwave reflection, absorption, and transmission (RAT) properties of a
conventional susceptor structure (structure 1-1) were compared with an
experimental susceptor
structure (stnicture 1-3) using the calorimetry test described in Example 1
with various heating =
times. Each sample evaluated for low power RAT was placed into an HP8753A
Network
Analyzer. The output is used to calculate the reflection (R), absorption (A),
and transmission (T)
(collectively "RAT") characteristics of the sample. A merit factor was also
calculated at each
heating time, where:
Merit Factor = Absorbance (A)/(1- Reflectance (R)).
The merit factor is a useful measure of the ability of a microwave susceptor
structure to resist the
development of high transmission fraction, which detracts from good browning
perfonnance. A
high merit factor maintained during a cook cycle indicates a susceptor retains
the ability to both
convert microwave energy' to sensible heat to create effective surface
browning and to reflect
energy' so as to avoid excessive direct microwave heating of the interior of
the food product.
Further, a new parameter, craze perimeter divided by field area (P/A, mm/mm2),
was
determined for some heating times of structure 1-1. The perimeter length of
each craze of each
sample was measured under magnification using image analysis to examine the
respective samples
after heating. The total craze perimeter was divided by the filed area to
arrive at P/A. The results
are presented in Tables 4 and 5.
Notably, at longer heating times, structure 1-3 provided greater heating than
structure 1-1.
Susceptor structures with larger merit factors generally exhibit greater food
surface browning and
crisping because they' limit the amount of direct microwave heating of the
food while maximizing
the susceptor absorbance. Therefore, as a practical matter, a structure using
a low crystallinity
polymer film may be able to advantageously provide a greater level of surface
browning and/or
crisping while minimizing dielectric heating of the food item.
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Table 4: Structure 1-1
Time of R A T Merit Factor Delta T P/A
.. Heating (sec) A/(1-R) Max ( C)
(mm/mm2)
._ -
1 0.43 0.47 0.10 0.82 -
2 0.42 0.47 0.11 0.81 0 -
0.43 0.46 0.10 0.81 -
0.41 0.48 0.12 0.81 5 -
0.33 0.46 0.21 0.69 16 -
40 0.28 0.44 0.28 0.61 32 1.32
60 0.26 0.37 0.37 0.50 51 1.00
80 n/a n/a n/a n/a 64
100 0.27 0.38 0.35 0.52 67 -
140 0.20 0.23 0.57 0.29 91 1.03
160 0.24 0.28 0.48 0.37 93 ..
180 0.18 0.19 0.63 0.23 111 -
180 0.18 0.18 0.64 0.21 110 1.97
200 0.18 0.20 0.62 0.24 120
220 0.-y) 0.21 0.57 0.27 118 -
240 0.20 0.24 0.56 0.30 114 -
260 0.17 0.16 0.67 0.19 127 1.64
280 0.16 0.15 0.69 0.18 133 -
300 nia n/a n/a n/a 141 2.77
Table 5: Structure 1-3
Time of R A T Merit Factor Delta T Max P/A
Heating (sec) A/(1-R) ( C)
(mm/mm2)
0 0.42 0.47 0.11 0.81 0 0
5 0.43 0.46 0.11 0.81 1.0 0
10 0.41 0.46 0.13 0.78 5.2 0
20 0.42 0.46 0.11 0.79 17.6 0
40 0.40 0.43 0.17 0.72 34.4 0
80 0.40 0.47 0.13 0.78 64.9 0
160 0.30 0.49 0.20 0.70 120.7 0
320 0.12 0.48 0.40 0.55 178.8 0
5 EXAMPLE 3
Image analysis was used to determine the extent of browning of a food item
using various
susceptor structures. In each example, a Stouffer's microwavable flatbread
melt was heated on the
susceptor structure for about 2.5 minutes in a 1000W microwave oven. When the
heating cycle
was complete, the food item was inverted and the side of the food item heated
adjacent to the
10 susceptor was photographed. Adobe Photoshop was used to evaluate the
images. For this
browning level evaluation, the analysis technique used entailed selecting an
RGB color set point
of R=79, G=30, B=13 as most representative of the hue this food product
developed when
browned by the test susceptor structures. No single lightness/darkness value
of this hue, however,
was able to accurately characterize the browning as qualitatively assessed by
experienced
15 observers, so three lightness mean values of 33, 82-85, and 109
corresponding to three key
lightness regimes were selected and individual pixel counts made at each of
those values, where
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higher mean lightness values correspond to less development of the chosen hue
during cooking.
Since three mean lightness values were used, a fairly tight pixel selection
tolerance level of 20%
was utilized around each lightness value. The results are presented in Table
6.
Although both structures provided some degree of browning and/or crisping, the
significantly higher pixel counts for lightness values corresponding to darker
areas clearly
indicated that structure 1-3 provided the greatest degree of browning and
crisping without burning
the food item or susceptor structure. Replicate testing of structure 1-3
confirmed the superior
browning performance of the structure.
Table 6
Test Structure No. of pixels No. of pixels No. of
pixels
Lightness = 33 Lightness ¨ 82-85 Lightness = 109
4-1 Structure 1-1 984 3619 6330
(0.5 mil DuPont 800C susceptor
film joined to 18 pt paperboard)
4-7 Structure 1-3 (replicate 1) 8591 10976 1764
(0.5 mil Pure-Stat APET
susceptor film joined to 12 pt
paperboard)
4-3 Structure 1-3 (replicate 2) 9023 7099 1907
(0.5 mil Pure-Stat APET
susceptor film joined to 12 pt
paperboard)
EXAMPLE 4
Various films and susceptor structures were prepared for evaluation. Two film
producers
were used to prepare APET films: SML Maschinengesellschaft mbH (Lenzing,
Austria) (-SML-)
(sample 5-3) and Pure-Stat Technologies, Inc. (Lewiston. ME) ("Pure-Stat")
(samples 5-4 through
5-15). Additionally, Dartek N201 nylon 6,6 (Liqui-box Canada, Whitby,
Ontario. Canada) was
evaluated (sample 5-2). Mylar 800 biaxially oriented PET (-BOPET) (DuPont
TeijanTm Films,
Hopewell, VA) (sample 5-1) was evaluated as a control material. The films were
then metallized
with aluminum and joined to 14 pt (0.014 inches thick) Fortress board
(International Paper
Company, Memphis, TN) using a substantially continuous layer of from about 1
to about 2
lb/ream (as needed) Royal Hydra Fast-en 20123 adhesive (Royal Adhesives,
South Bend, IN) to
form various susceptor structures.
Various strength enhancing additives were also evaluated, including OptemaTm
TC 120
and OptemaTm TC 220 ExCo (ethylene methyl acrylate copolymer resins,
ExxonMobil Chemical),
Sukano im F535 (ethylene methyl acrylate copolymer resin, Sukano Polymers
Corporation,
Duncan, SC), EngageTM 8401 (ethylene-octene copolymer, Dow Plastics), and
Americhem 60461-
CD1 (composition unknown) (Americhem Cuyahoga Falls, OH).
31
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The process for forming the APET film used by Pure-Stat Technologies, Inc. was
as
follows. Traytuft 9506 PET resin pellets (M&G Polymers USA, LLC, Houston, TX)
were
desiccant dried and conveyed to a cast film line extruder hopper. The additive
pellets were
metered into the extruder throat, combined with the dry PET pellets, melted,
mixed, and extruded
through a slot die to form a flat molten film. The molten film was cast onto a
cooling drum,
rapidly quenched into a largely amorphous solid state, and conveyed over
rollers to a windup
where the film was wound into a roll for further processing. The film was
about 0.0008 inches or
about 80 gauge in thickness. It will be noted that thicker or thinner films
can be produced by
varying the extruder output and cooling drum surface speed. The process used
by SML
Maschinengesellschaft mbH was similar.
DSC data was obtained for each film sample by heating the sample in a Perkin-
Elmer
differential scanning calorimeter (DSC-7) (Perkin-Elmer, Inc., Waltham, MA) at
10 C/minute.
with a nitrogen purge to prevent degradation. Values were measured for samples
heated to 300 C
and cooled to 40 C. The results are presented in Table 7. It is important to
note that the DSC
data was taken from an initial heating of the test specimens. Therefore, the
values reflect the
impact of any post-extrusion orientation and the specific thermal heat history
each specimen
experienced due to processing on the crystallinity of the specimen. The
negative enthalpy change
associated with crystallization is proportional to the amount of non-
crystalline polymer present in
the specimen. The positive enthalpy change associated with melting is a
measure of the degree of
crystallinity attained by the specimen during the DSC measurement. The more
equal the absolute
values of these enthalpy values the more amorphous the specimen. Therefore,
the values confirm
that the highly oriented film, sample 5-1, possessed very high levels of
orientation and
crystallinity and the cast APET films 5-3 through 5-15, films possessed low
levels of crystallinity.
The somewhat larger differences in enthalpy noted for samples 5-6 through 5-15
reflect the impact
of the non-PET strengthening additives present, but still are indicative of
low levels of
crystallinity in these films.
The apparent roughness of the surface (PEL) of each film was evaluated before
and after
treatment. Images of the surface of the film were acquired using atomic force
microscopy (AFM)
at 0 to 100 nm full scale. A gray level histogram was generated using a gray
scale from 0 to 256
units full scale light to dark using an image analysis system developed by
Integrated Paper
Services (IFS), Appleton, WI. A binary image was produced at a gray scale of
120, which is
equivalent to a plane intersecting the Z direction of the AFM image at
120/256*100 nm = 46.9 nm
or 469 angstroms in height. The perimeter of the detected region was measured
and normalized
by the linear size of the image to form a dimensionless ratio, perimeter
divided by edge length, or
PEL, with greater PEL values indicating a rougher surface. In general, the PEL
data indicate that
lower PEL levels (smoother film surface) are associated higher calorimetry and
browning results.
32
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
Peak load before break was measured according to TAPPI T-494 om-01. The values
indicate that the strengthening additives in samples 5-6 through 5-15 were
successful in increasing
the robustness of the films. This was borne out in trials on commercial
production equipment,
where strengthening additive modified films processed without difficulties,
while unmodified
films of the type represented by samples 5-3 through 5-5 were more fragile in
converting
operations, and required adjustments to normal process parameters such as
tension, and were
converted less efficiently.
The haze of each polymer film was measured according to ASTM D1003 using a BYK
Gardner Haze-Gard plus 4725 haze meter (BYK-Gardner, USA, Columbia, MD). In
all cases, the
incorporation of strengthening additives increased the haze of the films. In
some instances, the
most preferable additives may be those which exhibit lower levels of haze
while providing the
desired increase in strength for processing, and result in beneficially
increased heating
performance when made into susceptor films and structures.
Each susceptor structure was then evaluated using the calorimetry test
described in
Example 1. The results are presented in Table 7, where AT is the rise in
temperature for the
sample, and where AAT is the difference between the rise in temperature for
the sample and the
rise in temperature for the control sample (structure 5-1, standard biaxially
oriented, heat set PET
film).
Additionally, each structure was evaluated using the food browning test
described in
Example I. except that an RGB (red/green/blue) setpoint of 104 was used (RGB =
104 /60/25,
which visually corresponds to a brown hue generally associated with a browned,
crisped pizza
crust). The maximum pixel selection tolerance was chosen as most
representative of visual
browning assessments made by experienced observers for pizza crusts. In this
case it was judged
to not be necessary (as it was in Example 1) to utilize multiple mean levels
of lightness values of
this RGB hue setpoint to quantify browning in a way that accurately
discriminates browning
performance of the test variables. Additionally, a Kraft DiGiorno microwavable
pizza was used.
The number of pixels having that hue was recorded, such that a greater number
of pixels indicated
that more browning was present.
It will be noted that prior to evaluating structure 5-1 (control), the
unheated pi77a crust
was examined to determine a baseline pixel count of 24313 pixels having the
color associated with
the RGB value 104/60/25. This baseline value was used to calculate the results
presented in
Table 7, where:
AUB is the number of pixels for a given sample minus the baseline value for an
unbrowned crust (24313); and
A% Imp is the percent improvement over the results obtained by the control
sample (structure 5-1).
33
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
The calorimetry results and pizza browning results both show significant
increases over
control for all the unoriented, low crystallinity films, whether they
incorporated additives or not.
Visual observations of the cooked pizza crusts confirmed much more desirable
levels of browning
than were achieved with the standard control sample made from highly biaxially
oriented, high
crystallinity film. Thus, strengthening additives can improve film robustness
with no detriment to
performance when incorporated into microwave susceptor films and structures.
The relative reaction browning rate (RBRR) was then calculated using a
simplified
Arrhenius equation relationship for browning kinetics of a rough doubling of
browning reaction
rate for each temperature increase of 10 C, as follows:
relative reaction browning rate = 2 7' .
34
Table 7
0
t.)
Sample/ Film Thickness Wt Additive Tg
Crystallization Melting Peak Haze PEL QAT
Pixels ALM % A RBRR =
1-,
Structure (microns) (lb/ ( C) exothenn endothenn load
120 ( C) Imp .6.
1-,
ream) Peak AEI Peak Afl MD/CD
.6.
t.)
T ( C) (J/g) T ( C) (J/g)
(lbf/in) oew
--4
5-1 BOPET 12 10.4 None 75 None None 252 41
3.97 3.6 16.9 0.0 43577 19264 n/a 1.0
4.19
5-2 Nylon 6,6 25 17.6 None - None None 261
70 9.50 <8.0 - 16.2 60798 36485 89.4 3.1
9.00
5-3 APET 13 11.2 None 74 135 -37 247 37 - -
- - - .. - -
5-4 APET 25 22.4 None 78 141 -36 250 36 3.97
2.0 9.0 24.8 56248 31935 65.8 5.6
4.19
5-5 APET 12 10.4 None 77 130,136 -36
251 36 - - - - - - - -
P
r.,
5-6 APET 20 17.0 3% Optema TC120 78 129 -28 251 34
5.61 13.1 17.5 16.7 56958 32645 69.5 3.2
'
5.45
.
u,
.3
5-7 APET 20 18.9 5 /0 Opterna TC120 63,79 129
-28 251 36 6.30 15.0 11.9 20.3 69477 45164 134.4 4.1
5.77
,
u,
,
5-8 APET 20 14.0 3% Optema TC220 63,79 130
-32 251 33 5.56 6.7 18.0 16.7 65890
41577 115.8 3.2
.3
,
5.06
5-9 APET 20 16.7 5% OptemaTC220 - - -
- - 5.39 14.8 13.5 25.8 62745 38432 99.5
6.0
4.74
5-10 APET 20 19.2 3% Engage 8401 62,79 131 -
28 252 36 5.96 11.2 20.5 17.9 78926 54613
183.5 3.5
5.34
5-11 APET 20 17.8 5% Engage 8401 - - - -
- 5.83 21.7 3.6 29.3 66470 42157 118.8 7.6
4.73
5-12 APET 20 16.2 3% Sukano F35 60,78 127 -26
252 35 5.24 7.7 10.1 35.8 79637 55324 187.2 12.0
4.79
Iv
n
5-13 APET 20 15.7 5% Sukano F35 - - - -
- 5.27 10.9 8.6 30.4 62952 38639 100.6
8.2 1-3
4.21
cp
5-14 APET 20 17.8 3% Americhem 64,80 134 -28
252 34 5.41 7.3 3.7 26.8 85485 61172 217.5
6.4 tµ.)
o
5-15 APET 20 15.9 5% Amen chem - - - - -
5.16 12.8 6.0 21.8 75940 51627 168.0 4.5
.6.
_
tµ.)
o
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
EXAMPLE 5
Various Pure-Stat APET base films were uniaxially oriented and used to prepare
susceptor structures for evaluation. Mylar 800 biaxially oriented PET (DuPont
TeijanTm Films,
Hopewell. VA) was also evaluated as a control material.
Structure 6-1 was a commercially produced susceptor structure comprising the
commercially metallized, 48 gauge standard biaxially oriented PET film
described in the previous
paragraph laminated to paperboard, as described in Example 4. Structure 6-2
was the same as
structure 6-1. except that structure 6-2 was hand laminated.
Samples 6-3 through 6-17 were unixially oriented in the machine direction on a
BrOckner
Karo IV Laboratory Stretching Machine (lab stretcher) (Bruckner Maschinenbau
GmbH & Co.
KG, Siegsdorf, Germany) using the orientation and heat setting temperatures
set forth in Table 8
and a draw ratio of about 1.5:1. The oriented films were then evaluated for
various properties, as
indicated in Table 8. White pigmented samples do not lend themselves to
crystallinity
determination by density or refractive index or birefringence due to the
inclusion of pigment, but a
comparison of the values for crystallization exotherm and melting endotherm
for white
samples 6-7 and 6-10 with clear sample 6-9 indicates white sample 6-10,
oriented and heat set at
the same conditions as clear sample 6-9 have similar levels of crystallinity
and residual
orientation.
As shown in FIG. 2, moderate uniaxial orientation was capable of creating
films with
improved susceptor heating performance compared to control susceptor films.
Refractive index
values (n2) for these samples indicate them to be largely amorphous in nature,
even after moderate
orientation, with some modest increase in crystallinity for several of the
samples. The much
higher value for (nz) for the control is consistent with highly crystalline
film with high residual
orientation.
The films were then metallized and hand laminated to 14 pt paperboard, as
described in
connection with Example 4 to form various susceptor structures.
The films were then metallized by taping sheets of the lab stretching machine
oriented
samples to a full size roll of 48 gauge standard biaxially oriented PET and
running the roll through
the aluminum metallizer at commercial conditions (within a normal range of
parameters with
target optical density = 0.20). The metallized sheet samples were removed, and
then hand
laminated to form susceptor structures. Hand laminated film samples were
laminated to 0.014"
uncoated SBS paperboard (International Paper, Memphis, TN) using a No. 10
Mayer rod to apply
Royal Hydra Fast-en Bond-Plus 20123 adhesive (Royal Adhesives and Sealants,
South Bend, IN)
and laminated with a Cheminstruments Laboratory Laminator (Cheminstniments,
Inc.. Fairfield,
OH) set at 50 psi nip loading and speed 4.
36
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
Each susceptor structure was then evaluated using the calorimetry test
described in
Example 1. The results are presented in Table 8, where AT is the rise in
temperature for the
sample, and where AAT is the difference between the rise in temperature for
the sample and the
rise in temperature for the control sample (structure 6-2, standard biaxially
oriented, heat set PET
film).
Structure 6-2 exhibited a rise in temperature very similar to that of the
commercial control
structure 6-1. Accordingly, structure 6-2 may be considered to be a reasonable
representation of
commercially available susceptor structures. Several uniaxially oriented
laminated susceptor
samples (6-13 and 6-15) demonstrated lower heating performance compared to the
control sample.
As will be noted in Table 8, these samples were oriented at the lowest
temperatures, resulting in
the highest stretching stress, and received the lowest annealing temperature
exposure, minimizing
relaxation of that strain. Corresponding samples 6-14 and 6-16. while oriented
respectively at the
same temperatures as 6-13 and 6-15, received higher annealing temperature
exposure and
demonstrated significantly increased heating capability compared to the
control sample. Samples
6-5. 6-10 and 6-11 demonstrated the highest achieved AT's and RBRR's,
reinforcing the utility of
the present invention of combining modest orientation with heat setting for
relaxation of the
orientation stress. These comparisons demonstrate that while this disclosure
clearly demonstrates
the potential to create superior susceptor structures from a wider array of
materials and processing
regimes, not all combinations will yield equally performing structures and
care must still be
exercised in choosing specific process conditions. An advantage of this
embodiment is the ability
to adjust heating capability of the resultant susceptor structure through
relatively simple
modification of orientation conditions for uniaxial orientation.
As noted above (Example 1), the absolute temperature reached for each sample
was about
22 C higher than the AT reported since all samples had an initial temperature
of about 22 C
Notably, each of the samples (even ones that performed relatively poorly)
exhibited temperature
rises to absolute temperatures that exceed the 150 C shrink performance
specification commonly
used for films. Further, it is noted that 150 C is below the temperature one
would reasonably
expect Maillard browning reactions to occur at rates of interest for heating
microwavable foods.
This further reinforces that the conventional means of characterizing films
for use as susceptor
base films is inadequate.
The relative reaction browning rate (RBRR) was then calculated, as described
in Example
4. Additionally, each structure was evaluated using the pizza browning test
described in Example
4. The browning test results are presented in FIG. 3. Given variation in pizza
formulation and
resultant different temperature increase dynamics, the relative browning rate
based on AAT
calorimetry normalized to the AT of the control materials provided a
reasonable prediction of
browning pixel count and was confirmed by visual examination of the pizzas.
37
Table 8
0
Sample/ Film Initial Orient Heat Final % Tg Crystallization
Melting n, n,-n1 Peak Peak AT AAT RBRR Pixel
t..)
o
Structure thickness temp set thickness Cryst ( C) exotherrn
endotherm (MD) stress stress ( C) ( C) count
.6.
(microns) ( C) temp (microns) by Peak AH
Peak AN MD CD 1--,
.6.
( C) density 1' ( C) (J/g) T ( C) (J/g)
(1b1/in) (1bi/in) n.)
oe
61 Clear n/a n/a n/a 12 53 - - - - 1.6644
0.1707 - 148.2 1.5 1.1 38,665 Fl
6-2 Clear n/a n/a n/a 12 53 - - - -
- 1.6639 0.1704 17,759 18,536 146.7 0.0 1.0 39,381
6-3 White 28 120 140 19 - - - - - - -
- - - 155.1 8.4 1.8 -
6-4 White 28 120 170 19 - - - - - - - -
- - 161.3 14.6 2.7 -
6-5 White 28 120 220 19 - - - _
-
-
- - -
- 167.1 20.4 4.1 70,768
6-6 Clear 20 120 140 12 5.4 81 135 -32 251 34 1.5741 0.0010
-- 149.2 2.5 1.2 -
6-7 White 20 120 140 12 - 80 132 -29 251 32 - -
- - 154.4 7.7 1.7 - Q
6-8 Clear 20 120 170 12 12.1 80 125 -23 251 33 1.5845 -0.0044
- 154.3 7.6 1.7 -
-
.
6-9 Clear 20 120 200 12 29.9 80 122 -7 239,251 28 1.5848 -0.0130
-- 156.6 9.9 2.0 84,043
-
6-10 White 20 120 200 12 - 82 124 -5 238,251 29 - -
- 167.8 21.1 4.3 92,318
,
6-11 Clear 20 120 230 12 7.3 80 126 -26 251 33 1.5739 0.0003
-- 170.0 23.3 5.0 94,175
- - - - - - -
6-12 Clear 20 120 220 12 -
7,237 7,940 163.4 16.7 3.2 -
6-13 Clear 28 95 140 19 5.3 - 130 -30 251 35 1.5739 0.0022
6,961 6,903 136.4 -10.3 0.5 _
6-14 Clear 28 95 200 19 - - 123 -6
251 29 1.5845 -0.0013 7,419 10,057 156.8 10.1 2.0
74,844
6-15 Clear 28 105 140 19 5.4
- 132 -28 251 34 1.5733 0.0024 7,315 7,433 143.7 -3.0
0.8 -
6-16 Clear 28 105 200 19 - - 122 -6
252 32 1.5844 -0.0045 7,142 9,259 158.9 12.2 2.3
75,434
6-17 Clear 28 120 230 19 - -
123 -12 251 34 1.5786 -0.0090 8,158 7,989 158.1 11.4
2.2 44,611 A
,-i
.
w
=
-
. 6 .
=
38
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
EXAMPLE 6
Various films were biaxially oriented and used to prepare susceptor structures
for
evaluation. Homopolymer (HP) films, copolymer (CP) films, and coextruded (CX)
films were
evaluated. The films for orienting were produced by Pacur (PACUR, Oshkosh, WI)
on standard
sheet extrusion equipment. Homopoiymer films were produced from PQB15-093
homopolymer
PET resin supplied by Polyquest (Polyquest, Inc., Wilmington. NC), the
copolymer films were
produced from SKYPET-BR 8040 copolyester (SKC Chemicals, Seoul. South Korea),
and the
coextruded films were an A/B/A three layer coextruded structure with 2-15%
copolyester skin (A)
layers encapsulating a 70% homopolymer core (B) layer using the resins
described above.
The films were oriented using a using a Karo series lab stretching machine
with the
conditions set forth in Table 9. The films were then metallized by taping
sheets of the lab
stretching machine oriented samples to a full size roll of 48 gauge standard
biaxially oriented PET
and running the roll through the aluminum metallizer at commercial conditions
(within a normal
range of parameters with target optical density = 0.20). The metallized sheet
samples were
removed, and then hand laminated to form susceptor structures. Hand laminated
film samples
were laminated to 0.012" uncoated SBS paperboard (International Paper.
Memphis. TN) using a
No. 10 Mayer rod to apply Royal Hydra Fast-en Bond-Plus 20123 adhesive (Royal
Adhesives and
Sealants, South Bend, 1N) and laminated with a Cheminstruments Laboratory
Laminator
(Cheminstruments, Inc., Fairfield, OH) set at SO psi nip loading and speed 4.
Refractive index and birefringence was obtained for various samples. As shown
in FIG.
4, lower residual orientation and crystallinity provides the ability to enter
a new and superior
susceptor heating regime compared to commercially standard highly oriented
homopolymer PET
films.
Each susceptor structure was evaluated using the calorimetry test described in
Example 1
and pizza browning test described in Example 4, and a relative browning
reaction rate (RBRR)
was calculated as described in Example 5. The results are presented in Table 9
and FIG. 5, in
which samples/susceptor structures 6-1 and 6-2 (from Example 6) are included
for comparison.
Structure 6-2 (the hand laminated commercial control from Example 6) exhibited
a
temperature rise of 146.7 C. This corresponds closely to structure 7-1 (which
was oriented on the
lab scale stretching machine), which exhibited a temperature rise of 145.5 C.
Accordingly, for
purposes of this discussion, structure 7-1 represents a reasonable simulation
of a commercial
susceptor film material. Structure 7-2, which was similar to structure 7-1
(except that the
orientation level was 3.8 x 3.8). exhibited a rise in temperature of only
134.7 C. Thus, there was a
decrease in performance at the lower orientation level for this lab stretched
homopolymer PET
film based susceptor.
39
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
Structure 7-17, which comprised the lower melting point copolyester (with all
other
conditions being the same as structure 7-2), exhibited a rise in temperature
of 154.9 C, which was
greater than any of the structures made from a homopolymer base film,
including the commercial
control (structure 6-2). Structure 7-18. which had an increased degree of
orientation, exhibited an
increase in temperature of 158.8 C. In contrast, structure 7-21, which had a
lower orientation
temperature and a lower degree of orientation, showed only a temperature
increase of 125.4 C. It
will be noted that while this performance may not be acceptable for most
microwave heating
applications, there are other applications in which lower heating capacity may
be desirable.
Finally, structure 7-19, which was made with a greater annealing (i.e., heat
set) temperature than
structure 7-21, the rise in temperature was 144.2 C. It will be noted that the
corresponding value
for a homopolymer susceptor base film made under the same conditions was 138.2
C (structure 7-
6).
Additionally, it will be noted that an increase in orientation temperature
from 120 C to
125 C resulted in a nearly 4 C increase in AT (see structures 7-17 and 7-18)
with copolymer (also
went from 3.8 x 3.8 to 4x4), while the same increase in orientation
temperature results in only
about a 2.3 C increase in AT with homopolymer (see structures 7-1 and 7-3 with
homopolymer (at
4x4 for both temps).
Coextruded structures appear to be somewhat more sensitive to process
conditions, but
are still capable of superior calorimetry and pizza browning results when
higher stretching
temperatures and higher annealing temperatures are employed. In fact, the best
pizza browning of
this set of samples was achieved by coextruded sample 7-26. Without wishing to
be bound by
theory it is believed that the presence of both homopolymer PET and
copolyester layers of the
structure results in something of a 'mixed mode' for orientation,
crystallization and relaxation and
may result in a somewhat narrower window of process operating conditions.
However, since it
has been shown possible to create superior heating susceptors using this film,
they are
commercially useful as potential cost reductions compared to 100% copolyester
films and may
provide other useful functionality, such as when a surface copolyester layer
is used as a heat seal
material.
FIG. 5 plots calorimetry based relative browning rate vs. pixel count from
actual pizza
cooking tests; these data confirm that within the limitations of pizza
variability and cooking
dynamics a clear positive correlation exists between the calorimetry results
and actual food
browning results.
Table 9
Sample/ Material ; Initial Orient heat set
Stretch Final (.% Cryst Tg Crystalli7ation Melting n, n,-n,
Peak Peak AT MT RB RR Pixel 0
Structure thickness temp temp
ratio thickness by (CC) exotherm endothenn (MD) stress stress (C) CC)
count t..)
o
(microns) CC) ( C) (microns) density Peak All
Peak AH MD CD
T CC) 0/0 T CC) (Jig)
(Ibilin) (Ibt/in) 4=.
-
-
1-,
6-1 HP : - - - 53 - -
1.6644 0.1707 - 148.2 1.5 1.1 38,665 4=.
..
t=-)
6-2 HP , - - - 12 - -
1.6639 0.1704 17,759 18,536 146.7 0.0 1.0 39,381 oe
oe
-
-
-
-
-4
7-1 HP 200 120 140 4 x 4 12.5 36.4 - None
None 253 44 1.6037 0.1015 12,069 18,651 145.5 -1.2
0.9 ,
7-2 HP 200 120 140 3.8x3.8 . 13.9 34.6 -
None None 252 42 1.5957 0.0770 10,475 15,675 134.7 -
12.0 0.4 -
7-3 HP 200 125 140 4 x4 ' 12.5 - 118 -4
)53 41 1.5920 0.0356 6,908 8,249 147.8 1.1 1.1
- -
7-4 I IP . 200 115 140 3.8x3.8 = 13.9 36.8
- None None 153 46 1.6104 0.1055 11,146 20,471 135.6
-11.1 0.5 -
7-5 HP 200 115 140 3.5x3.5 16.3 37.6 - 137 -1 254
45 1.6099 0.1038 10,534 15,586 144.5 -2.2 0.9 57,812
7-6 HP 200 115 200 3.5x3.5 16.3 40.5 - - -
179, 253 2.43 1.6307 0.1253 15,779 25,987 138.2 -8.5
0.6 -
7-7 TIP 200 125 200 4 x 4 12.5 40.2 - - -
210, 252 1,39 1.5931 0.0551 7,280 9,698 134.3 -12.4
0.4 -
7-8 CP 200 120 140 4 x4 ' 12.5 25.8 87 147 -
1 236 32 1 - .5956 0.0480 - 158.1 11.4 2.2
59,071
-
P
7-9 CP 200 120 200 4 x 4 12.5 15.1 77 124 -11
237 29 1.5890 0.0306 - - 149.7 3.0 1.2
o
1.,
7-10 CP 200 120 220 4 x 4 12.5- - - - - - -
- - - 152.2 5.5 1.5 71,018 '
o
o
0.
7-11 CP 172 120 140 3 x 3 19.1 -
- - - 124.1 -2).6 0.2
-
-
-
-
-
-
-
o
7-12 CP 172 120 200 3 x 3 19.1- - - - - -
- - - - - 130.0 -16.7 0.3 "
o
1-
7-13 CP 172 120 220 3 x 3 19.1 -
- - - 149.9 3.2 1.2
-
-
-
-
-
-
-
,
o
7-14 CP 127 120 140 3 x 3 14,1-
- - - 1 - - 147.4 0,7 1.0 0
-
-
0
0
7-15 CP 127 120 200 3 x 3 14.1 6.7 66,77 124 -
18 237 29 1.5833 0.0028 - 134.5 -12.2 0.4
7-16 CP 127 120 140 2 x 2 31.8- - -
- - - 128.9 -17.8 0.3 -
7-17 CP 200 120 140 3.8x3.8 13.9 - None None 235
34 1.5868 0.0575 8,317 10,300 154.9 8.2 1.8
- _
7-18 CP 200 125 140 4 x 4 12.5- 78 124 -26 237
29 1.5769 0.0086 4,841 - 158.8 12.1 2.3 77,367
7-19 CP 200 115 200 3.5x3.5 16.3- - None None
205, 236 1,33 1.6088 0.0849 10,461 17,744 144.2 -2.5
0.8 -
7-20 CP 200 125 200 4 x 4 12.5- 78 122 -14 236
27 1.5836 0.0152 4,389 4,536 158.3 11.6 2.2 80,221
7-21 CP 200 115 140 3.583.5 16.3- - None None
238 37 1.6124 0.1022 15,849 25,161 127.9 -
18.8 0.3 -
00
7-22 CX 200 115 140 3.5x3.5 16.3 - - 120 -2
241.252 34 1.6135 0.1181 10,541 14,085 133.0 -
13.7 0.4 n
-
1-3
7-23 CX I 200 120 140 3.5x3.5 16.3- 79 122 -
20 239, 252 34 1.6050 0.1097 9,425 10,778 138.6 -8.1
0.6 -
7-24 CX 200 115 140 3.583.5 16.3 78 129 -23
240, 252 32 1.6175 0.0869 8,604 8,770 140.7 -6.0
0.7 ci)
-
- t=-
)
o
7-25 CX 100 115 200 3.5x3.5 16.3- - None None 183,
241, 253 1,35 1.6280 0.1355 9,261 13,260 148.7 2.0
1.1 - 1--,
t...)
7-26 CX 200 125 200 3.5x3.5 16.3 - 78 127 -24
241,252 31 1.5975 0.0654 7,494 7,765 157.7 11.0 2.1
82,757 -a-,
,....,
.6.
w
=
41
CA 02900458 2015-08-06
WO 2014/142887 PCT/US2013/031420
EXAMPLE 7
To demonstrate the significantly different dynamic dimensional temperature
responses of
films of this disclosure compared to standard susceptor films of the prior
art, changes in sample
lengths were monitored as functions of temperature using a Perkin-Elmer
(Perkin-Elmer Inc.,
Waltham, MA) DMA 7e. The instrument was used in the constant force, thermal
mechanical
analysis mode. Samples were heated from 40 to 220 C at 2.5 C per minute under
a helium purge,
with a constant static force of 10 mN. An extension analysis measuring system
was used with
samples cut 3.2 mm wide, with thickness depending on the films to be measured
and with gauge
lengths of about 10 mm. An ice/water bath was used to aid with furnace
temperature control.
Measurements were performed on strips of film cut in the machine direction
(MD) or
transverse/cross direction (CD). Examples of the dimensional changes, recorded
while heating the
film samples, are given in FIGS. 6-11. Values were calculated in terms of
percent change
compared to the original sample length before heating as follows:
% change in sample length = (instantaneous length during heating / original
length) x
(100).
A characteristic of this test method that affects displayed results is
important to clarify for
proper data interpretation. A very slight tension is applied to the ends of
the sample to ensure
accuracy in the measurement of instantaneous length used to calculate the %
change in sample
length. Samples show % change values less than 100 (shrinkage) by overcoming
this slight
tensional force and become shorter (shrink) as temperatures are reached at
which residual shrink
energy is released. Samples that indicate expansion (values greater than 100%)
in curves
generated by this test should not be considered to have an intrinsic growth or
expansion
characteristic at increasing temperatures. Above Tg, films with little or no
tendency for
dimensional change at a particular temperature will tend to be susceptible to
slight stretching by
the sample holding tension at that temperature and the curve will erroneously
imply that growth or
expansion would occur under conditions of no tensional load. Films that are
more highly oriented
and/or crystalline possess less remaining elongational capability in the
nibbery state and will be
expected to be more resistant to this stretching and indication of false
expansion than will films
with lower levels of orientation and crystallinity.
Table 10 identifies the films for which dynamic dimensional temperature
response curves
are shown; three films representing conventional highly oriented, highly
crystalline, high
refractive index and birefringence standard PET homopolymer films are compared
with three
films representing several embodiments of the present disclosure, all of which
demonstrate
superior susceptor performance.
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Table 10
Structure Description n, nz-
n, AT AAT RBRR
( C) ( C)
6-2 DuPont Mylark 800C 1.6639 0.1704
146.7 0 1.0
1-6 DuPont HS2 1.6587 0.1662 152.0
9.2
7-1 Homopolymer, biaxially oriented 1.6037 0.1015 145.5 -
1.2 0.9
(commercial simulation)
6-9 Homopolymer, biaxially oriented 1.5848 -0.0130
156.6 9.9 2.0
6-11 Homopolymer, uniaxially oriented 1.5739 0.0003 170.0
23.3 5.0
7-8 Copolymer, biaxially oriented 1.5956 0.0480 158.1
11.4 2.2
Samples 6-2. 1-6, and 7-1 are all homopolymer PET highly oriented films
representing a
common commercial susceptor base film (sample 6-2), a heat stabilized film of
the prior art (1-6)
and a laboratory stretched film targeted to simulate the properties and
perfonnance of a film
similar to sample 6-2 (sample 7-1). Samples 6-9, 6-11, and 7-8 are all lab
stretching machine
oriented samples of the present disclosure. Samples 6-9 and 6-11 are
moderately uniaxially
oriented films of the present disclosure made from homopolymer PET, with
sample 6-11 exposed
to a higher heat set temperature than sample 6-9; sample 7-8 is a highly
oriented film of the
present disclosure made from a copolyester with a melting point below that of
typical
hoinopolymer PET.
FIGS. 6-11 trace the dynamic dimensional temperature responses of these films.
Duplicate runs for each film in MD and CD are shown in each figure. Of
immediate note is that
all 6 samples begin with some response to increased temperature at about 80 C,
roughly Tg for
PET. This is to be expected as the polymer transitions from the glassy state
to the rubbery state.
Notably, as samples 6-2, 1-6, and 7-1 continue to increase in temperature,
meaningful shrinkage
occurs in the MD for all samples and CD for all but sample 1-6, and continues
to increase with
increased temperature, especially at temperatures above 150 C, the static
exposure temperature for
shrinkage measurements typically referenced as representing a good predictor
of susceptor
performance.
The dynamic dimensional temperature response of these films represents
shrinkage
behavior that leads to typical crazing in a laminated susceptor structure and
limited heating
capability; the calorimetry data for these three samples indicates this
limitation in heating. Sample
1-6 appears to grow slightly in the CD, which is likely the result of somewhat
enhanced thermal
stability in that direction compared to the MD and the false expansion
indication discussed above,
but shrinks significantly in the MD at temperatures commonly encountered and
desired in
susceptor heating. While sample 1-6 shows some increased heating capability
compared to
samples 6-2 and 7-1, it is still inferior to susceptors made according to this
disclosure. The greater
magnitude of shrinkage seen in sample 7-1 compared to sample 6-2 is expected
to result from
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differences between commercial machine and lab stretching equipment, but
significantly,
susceptors made from the two films perform quite similarly in heating and cook
tests. This
confirms that past a threshold of craze initiation, susceptor performance is
lost in an essentially
irretrievable manner.
Samples 6-9, 6-11, and 7-8, all of which are experimental films of the present
disclosure
that provide superior heating to the three standard type films, reacted quite
differently to increased
temperature, evidencing no shrinkage in either MD or CD over the same
temperature range where
the standard films shrink. Likewise, all of the superior performing susceptors
of this disclosure
that were tested according to this method (including, for example, samples 5-
6, 5-7, 5-8, 5-10, 5-
12, and 5-14), exhibited similar behavior. Taking into account the false
expansion indication of
the test method, for susceptor use, these films may be considered to be
dimensionally stable and
should resist crazing onset in a superior fashion: calorimetry and browning
tests confirm this
superior performance.
The dramatic difference in dynamic dimensional temperature response is
consistent with
maintenance of low residual orientation as evidenced by reduced levels of
refractive index and
birefringence as well as reduced crystallinity in susceptor base films and
represents strong
evidence of the novelty of the various aspects of this invention and its
reduction to practice in a
highly useful manner.
While the present invention is described herein in detail in relation to
specific aspects and
embodiments, it is to be understood that this detailed description is only
illustrative and exemplary
of the present invention and is made merely for purposes of providing a full
and enabling
disclosure of the present invention and to set forth the best mode of
practicing the invention
known to the inventors at the time the invention was made. The detailed
description set forth
herein is illustrative only and is not intended, nor is to be construed, to
limit the present invention
or otherwise to exclude any such other embodiments, adaptations, variations,
modifications, and
equivalent arrangements of the present invention. All directional references
(e.g., upper, lower,
upward, downward, left, right, leftward, rightward, top, bottom, above, below,
vertical, horizontal,
clockwise, and counterclockwise) are used only for identification purposes to
aid the reader's
understanding of the various embodiments of the present invention, and do not
create limitations,
particularly as to the position, orientation, or use of the invention unless
specifically set forth in
the claims. Joinder references (e.g., joined, attached, coupled, connected,
and the like) are to be
construed broadly and may include intermediate members between a connection of
elements and
relative movement between elements. As such, joinder references do not
necessarily imply that
two elements are connected directly and in fixed relation to each other.
Further, various elements
discussed with reference to the various embodiments may be interchanged to
create entirely new
embodiments coming within the scope of the present invention.
44
