Note: Descriptions are shown in the official language in which they were submitted.
POLYESTER COMPOSITION WHICH IS PARTICULARLY SUITABLE
FOR USE IN THERMOFORMING DUAL-OVEN~BLE TRAYS
Background of the Invention
The wide spread popularity of microwave ovens for
home use has initiated interest in food trays which can
be used in either microwave ovens or convection ovens.
Such food trays must be able to withstand oven
tempera~ures which approach 200C. Such trays are of
particular value as containers for frozen prepared
foods. It is accordingly necessary for such trays to
have good impact strength at freezer temperatures and
dimensional stability at oven temperatures. It is, of
course, also important for such trays to be capable of
withstanding rapid heating from freezer temperatures of
about -30C to oven temperatures of about 175C or even
higher.
Containers which are capable of being heated in
either convection ovens or microwave ovens are
sometimes described as being dual-ovenable. Polyesters
are highly suitable for use in making such
dual-ovenable containers. However, it is important for
the polyester to be in the crystalline state rather
than the amorphous state in order to achieve
satisfactory high temperature stability. ~ormally,
polyesters will undergo crystallization by heat
treatment at elevated temperatures and the crystallites
formed will remain substantially stable up to near the
melting point o~ the polyester. As a general rule,
dual-ovenable containers which are comprised of
polyester will be heat treated to attain a
crystallinity of higher than about 25%.
Injection molding and thermoforming are widely
known methods for forming thermoplastic polyester
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articles. In injection molding, the polyester is
heated above its melting point and injected under
sufficient pressure to force the molten polyester to
fill the mold cavity. The molten polyester is cooled
in the mold until it is rigid enough to be removed. ~ ;
The injection molding of a polyester composition ~ -
containing 0.5% to 10~ by weight isotactic polybutene-l
is described in U.S. Patent 3,839,499. However, the
injection molding method is generally not satisfactory
for the production of thin walled articles, such as
dual-ovenable trays, due to flow lines and layering ;~
which develop during the filling of the mold which lead
to non-uniform properties 9 surface irregularities, and
warping of the finished article. Very high filling
pressures are also required in the injec~ion molding of
thin walled articles due to high melt viscosities.
Thermoforming is another process which is used
commercially in the production of polyester articles.
It is a particularly valuable technique for use in
producing thin walled articles, such as dual-ovenable
food trays, on a commercial basis. In thermoforming, a
preformed polyester sheet is preheated to a temperature
sufficient to allow the deformation of the sheet. The ` :
sheet is then made to conform to the contours of a mold
by such means as vacuum assist, air pressure assist, or
matched mold assist. The thermoformed article produced
is normally heat treated in the mold in order to attain
a crystallinity of at least about 25%.
Crystallization rates can generally be improved by
including a small amount of a nucleating agent in
polyester compositions. For example, United States
Patent 3,960,~07 discloses a process for thermoforming
articles from a polyester composition which is
comprised of (1) a crystallizable polyester, (2) a
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crack stopping agent9 preferably a polyolefin, and (3)
a nucleating agent. Polyester articles which are made
utilizing such compositions generally have improved
mold release characteristics and improved impact
strength. Additionally, the utilization of such
modified polyester compositions results in faster
thermoforming cycle times due ~o the faster rate of
crystallization which is attained.
United States Patent 4,572,852 discloses a `
polyester molding composition which consists of (1)
polyethylene terephthalate, (2) a polyolefin containing
from 2 to 6 carbon atoms, and (3) an effective amount ~;
of a heat stabilizer. Thin walled thermoformed
articles which are prepared utilizing such compositions
exhibit improved impact strength and high temperature
stability. For this reason dual-ovenable trays which
are comprised of polyester/polyolefin blends are widely
utilized commercially. Polyethylene terephthalate
having an intrinsic viscosity of at least about 0.65 is
widely utilized in such applications. It is important
for the polyethylene terephthalate used in
dual-ovenable trays to have an intrinsic viscosity of
at least about 0.65 dl/g in order for the article to `
have acceptable impact strength at low temperatures,
such as those experienced in a freezer.
It would be desirable to improve the low
temperature impact strength of dual-ovena~le trays.
This is because a certain amount of tray breakage
occurs during transporting of frozen prepared foods
which are packed utilizing such trays. Such trays have
also been known to break upon being dropped after
taking them out of home freezers. Thus, it would be
highly beneficial to manufacture dual-ovenable trays
utilizing a material which provides improved low
temperature impact strength.
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Summary of the Invention
It has been unexpectedly found that polyethylene
terephthalate/polyethylene ionomer blends offer an
outstanding combination of properties, including
improved low temperature impact strength, for
utilization in thermoforming heat set, thin walled
articles such as dual-ovenable trays. The subject
invention accordingly relates to a thermoformed,
non-oriented, heat set, thin walled article,
comprising: (a) from about 92 to about 99 weight
percent polyethylene terephthalate having an intrinsic
viscosity of at least about 0.7 dl/g as measured in a :~
60:40 phenol/tetrachloroethane mixed solvent system at
30C; and (b) from about l to about 8 weight percent of
a polyethylene ionomer having a melt flow index as
measured using ASTM Method D-1238 of less than about 2 ;
g/lO minutes; said article having a total crystallinity
of from about lOZ to about 40%~
The present invention further reveals a process for
making a heat set, partially crystalline, thin walled
article which comprises thermoforming a substan~ially
amorphous sheet which is comprised of ~a) from about 92
to about 39 weight percent polyethylene terephthalate ~ .
having an intrinsic viscosity of at least about 0.7 : .
dl/g as measured in a 60:40 phenol/tetrachloroethane ~ :
mixed solvent system at 30C; and (b) from about l ~o
about 8 weight percent of a polyethylene ionomer having . ~
a melt flow inde~ as measured using ASTM Method D-1238
of less than 2 g/10 minutes; wherein the thermoforming
is carried out in a heated mold for a time sufficient
to achieve a crystallinity in said article which is ;
within the range of about 10~ to about 40Z~ :
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Detailed ~escription of the Invention
The thermoplastic resin compositions of this
invention are comprised of polyethylene terephthalate
(PET) and at least one polyethylene ionomer. Such
compositions will normally contain from about 32 to
about 99 weight percent PET and from about 1 to about 8
weight percent polyethylene ionomer. It is generally
preferred for the thermoplastic resin compositions of
this invention to contain from about 94 to about 98.5
weight percent PET and from about 1.5 to about 6 weigh~
percent polyethylene ionomer with the most preferred
compositions containing from about 2 to about 4 weight
percent polyethylene ionomer and from about 96 to about
98 weight percent PET.
PET is comprised of repeat units which are derived
from terephthalic acid or a diester thereof and
ethylene glycol. The PET utilized in the thermoplastic
resin compositions of this invention can be a modified
PET. Such modified PET can contain small amounts of
repeat units which are derived from diacids other than
terephthalic acid and/or glycols in addition to
ethylene glycol. For instance, small amounts of
isophthalic acid or a naphthalene dicarboxylic acid can
be used in the diacid component utilized in preparing
the PET. PET which has been modified with a small
amount o~ a diol containing from 3 to 8 carbon atoms is
also representative of a modified PET which can be
used. For instance, a small amount of 1,4-butane diol
can be utilized in the glycol component used in
preparing the modified PET. Normally, no more than
about 5 weight percent of the repeat units in such
modified PET will be comprised of diacids or diols
other than a terephthalic acid and ethylene glycol. It
is, of course, contemplated that diesters of such
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dicarboxylic acids and diols can also be used. In most
cases, such modified PET will contain less than about
3% diacids other than terephthalic acid and less than
3% diols other than ethylene glycol. It will normally
be preferred for such modified polyesters to contain
only abou~ 1% dicarboxylic acids other than
terephthalic acid and/or less than 1~ glycols other -~
than ethylene glycol. In any case PET homopolymer is
an excellent choice for utilization in the
thermoplastic resin compositions of this invention.
The PET utilized in the thermoplastic resin
compositions of this invention will normally have an
intrinsic viscosity (I.V.) of at least about 0.7 dl/g.
In most cases, the PET will have an I.V. which is
within the range of about 0.8 dl/g to about 1.4 dl/g.
It is generally preferred for the PET to have an
intrinsic viscosity of at least 0.9 dl/g with it being
more preferred for the PET to have an intrinsic
viscosity of about 0.95 dl/g. Intrinsic viscosity is
defined as the limit of the fraction ln (v)/C as C, the
concentration of the polymer solution, approaches 0,
wherein v is the relative viscosity which is measured
at several different concentrations in a 60/40 mixed
solvent of phenol and tetrachloroethane at 30C.
The polyethylene ionomers which can be utilized in
~he practice of this invention are generally copolymers -~
of ethylene and at least one ~ ethylenically
unsaturated carboxylic acid wherein from about 5
percent to about 90 percent of the carboxylic acid
groups are ionized by neutralization with metal ions.
The ~ ethylenically unsaturated carboxylic acid can
be a monocarboxylic acid, or have more than one
carboxylic group attached to it. The carboxylic acid
groups are neutralized with at least one cation from
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the group consisting of metallic cations having a
valence of 1 to 3. The polyethylene ionomers used in
this invention will have a melt ~low index as measured
using ASTM Method D-1238 after being dried for 16 hours
in a vacuum oven at 63C of less than about 2 g/10
minutes. It is preferred for the polyethylene ionomer
to have a melt flow index of less than about 1.5 g/10
minutes with it being most preferred for the
polyethylene ionomer to have a melt flow index of less
than about 1.2 g/10 minutes.
The a,~ethylenically unsaturated carboxylic acids
which can be copolymerized with the ethylene monomer
preferably have 3 to 8 carbon atoms. Examples of such
acids include acrylic acid, methacrylic acid,
ethacrylic acid, itaconic acid, maleic acid, fumaric
acid and monoesters of other dicarboxylic acids, such
as methyl hydrogen maleate, methyl hydrogen fumarate,
ethyl hydrogen fumarate, and maleic anhydride, which is
considered to behave like an acid and be an acid in the
present invention.
The polyethylene ionomer will generally contain
from about 2 to about 40 weight percent
,~-ethylenically unsaturated carboxylic acids and from
about 60 to about 98 weight percent ethylene. The
polyethylene ionomer will more typically contain from
about 3 to about 20 weight percent ,~-ethylenically
unsaturated carboxylic acids and from about 80 to about
97 weight percent ethylene.
A preferred polyethylene ionomer is a copolymer of
ethylene and an a,~-ethylenically unsaturated
monocarboxylic acid having 3 to 6 carbon atoms. A most
preferred a,~-ethylenically unsaturated monocarboxylic
acid is acrylic acid. Methacrylic acid is another
highly preferred a, ~-ethylenically unsaturated
monocarboxylic acid. ~ -
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The polyethylene ionomers used in this invention
will normally have an impact strength as measured at
23C using ASTM Method D-1822S of at least 1100 KJ/m2.
It is preferred for the polyethylene ionomer to have an
impact strength of at least 1150 KJ/m2 with it being
most preferred for the polyethylene ionomer to have an
impact strength of at least 1200 KJ/m2.
United States Patent 4,248,990, which is
incorporated herein by reference, discloses
polyethylene ionomers and a process for making
polyethylene ionomers in greater detail. Polyethylene
ionomers which can be used in the practice of this
invention are commercially available from E. I. du Pont
de Nemours & Company, Inc. and are sold under the '~
tradename Surlyn~. For example, Surlyn~ 1605 is a
polyethylene ionomer which contains approximately 10
acrylic acid and approximately 5% sodium acrylate.
Surlyn~ 9721 is a polyethylene ionomer which contains
ethylene and methacrylic acid.
The thermoplastic resin composition of this
invention will preferably contain one or more heat
stabilizers. The inclusion of one or more heat
stabilizers has particular utility when the finished
article being made from the resin will be subjected to
high service temperature conditions for long periods of
time. The retentlon of adequate physical properties,
especially impact strength, is very important in
applications such as food trays for use in
dual-ovenable applications. Heat stabilizers as used
herein are compounds which demonstrate antioxidant
properties, the most important of which is the capacity
of inhibiting oxidation. An effective heat stabilizer
in the practice of this invention must be capable of
protecting the thermoformed article during exposure to
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elevated temperatures. The following compounds are
representative examples of useful heat stabilizers
which can be incorporated into the thermoplastic resin
compositions of this invention: alkylated substituted
phenols, bisphenols, thiobisacrylates, aromatic amines,
organic phosphites, and polyphosphites. The particular
aromatic amines which demonstrate specific heat
stabilizing capabilities include: primary polyamines,
diarylamines, bisdiarylamines, alkylated diarylamines,
ketone-diarylamine condensation products,
aldehyde-amine condensation products, and aldehyde
imines. Conditions which would be considered severe
would be those in which the thermoformed article would
be exposed to temperatures near 200C for periods
exceeding about 30 minutes. Preferred heat stabilizers
~or such severe high temperature applications,
particularly where any staining or discoloration from
the heat stabilizer is undesirable, are the polyphenols
which contain more than two phenol ring structures.
Some representative examples of suitable polyphenols
include tetrakis(methylene-3(3,5-di-t-butyl-4-hydroxy
phenyl)proprionate~methane and 1,3,5-trimethyl~2~4,6-
tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene.
Persons skilled in the art will be able to easily
ascertain the effective amount of heat stabilizer
needed, with this amount generally being within the
range of about 0.005 to about 2% by weight based upon
the total weight of the thermoplastic resin
composition. It will normally be preferred for the `~
amount of heat stabilizer utilized to be within the
range of 0.01 to 0.5% by weight, based upon the total
weight o~ the thermoplastic resin composition. The
amount of heat stabilizer used will vary with such
factors as the degree of protection required, the
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severity of heat exposure, solubility limitations of
the heat stabilizer chosen in the thermoplastic resin
composition, and the overall effectiveness of the heat
stabilizer.
One or more pigments or colorants can also be added
to the thermoplastic resin composition in order to
provide it with a desired color. For instance,
titanium dioxide can be included in the thermoplastic
resin composition in order to provide it with a
brilliant white color. One or more colorants can also
be added to the thermoplastic resin composition in
order to provide it with any of a multitude of colors.
Such colorants will normally not act as nucleating
agents. Some representative examples of non-nucleating
organic colorants include: phthalocyanine blue,
solvent red 135, and disperse yellow 64 (CAS No. ;
10319-14-9), Many other dyes of the solvent and
disperse groups are also useful for coloring the
thermoplastic resin compositions of this invention.
The amount of colorant or combination of colorants
needed to obtain a specific desired color can be easily
ascertained by persons skilled in the art.
The thermoplastic resin compositionq of this
invention can be prepared by simply melt blending the
PET with the polyethylene ionomer, the he~t stabilizer -~
and optionally a colorant. Such melt blending is done
at a temperature at which the PET is in the liquid
state. PET homopolymer has a melting point of about
260C. Since such a melt blending procedure must be ~ ;
carried out above the melting point of the PET, it will ~
normally be done at a temperature within the range of -
about 260C to 350C. Normally, it is preferred for
the melt blending procedure to be carried out at a
temperature within the range of about 280C to 320~C.
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In such a melt blending procedure, the polyethylene
ionomer is simply dispersed throughout the molten PET.
Sufficient mixing action will be applied so as to
result in the formation of a homogeneous system. In
other words, the polyethylene ionomer and heat
stabilizers or colorants added should be unifor~ly
dispersed throughout the PET in order to produce
optimal thermoplastic resin compositions. Such a melt
blending procedure can commercially be carried out in
extruders which provide sufficient shearing forces so
as to result in adequate mi~ing.
After the thermoplastic resin compositions of this
invention have been prepared, they can be utilized in ~-
making a wide variety of useful articles of
1~ manufacture. The thermoplastic resin compositions of
this invention have particular value or use as
thermoforming compositions from which thin walled
articles such as dual-ovenable trays can be made. The
articles of manufacture to which this invention relates ~
are thin walled thermoformed articles. Thin walled as ~;
used herein means articles having wall thicknesses of
less than about 1 mm.
Since a partially crystalline ~inished article is
necessary for good dimensional stability at high
temperatures, knowledge of the degree of crystallinity
or percent o~ crystallinity is of considerable
importance. Density is a convenient method of
determining the percent of crystallinity since there is
a direct relationship between the two for a given
polyester composition. A calibrated gradient column
can be used for determining density a~ a par~icular
temperature. The density value can then be converted
into a percent of crystallinity.
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The terms crystallization temperature and
crystallization onset are used interchangeably to mean
the temperature or temperature range in which a
regularly repeating morphology, brought about by a
combination of molecular mobility and secondary bonding
forces, is induced in a polymer over a molecular
distance of at least several hundred angstroms. The ~ `~
crystallization temperature or crystallization onset
can be visually observed as the point at which a
substantially amorphous, unoriented sheet of
PET/polyethylene ionomer changes from a translucent, ;
hazy appearance to a white appearance.
As used throughout this specification and the
appended claims, the term glass transition temperature
means that temperature or temperature range at which a
change in slope appears in the volume versus
temperature curve for said polymer and defining a
temperature region below which the polymer exhibits a ~ -
glassy characteristic and above which the polymer
exhibits a rubbery characteristic. The glass
transition temperature (Tg) of polyethylene
terephthalate is about 70C.
Another aspect of this invention relates to a
process for producing heat set, thin-walled articles
from the thermoplastic resin compositions of this
invention using conventional thermoforming equipment.
The complete technique consists of the following steps:
1. Forming a substantially amorphous sheet from
the homogeneously blended PET/polyethylene ionomer
composition.
2. Preheating the sheet until it softens and
positioning it over the mold.
3. Drawing the preheated sheet onto the heated
mold surface.
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4. Heatsetting the formed sheet by maintaining
sheet contact against the hea~ed mold for a sufficient
time period to partially crystallize the sheet.
5. Stripping the part out of the mold cavity.
The sheeting and film for use in the thermoforming
process can be made by any conventional method. The
most common method being by extrusion through a flat
die. It is important that the sheet or film be
quenched immediately after extrusion in order to
minimize the extent of crystallization developed after
forming.
The term substantially amorphous as used herein
shall mean a sheet having a level of crystallinity low
enough to enable thermoforming of the sheet ~o be
accomplished with satisfactory mold definition and part
formation. In currently available thermoforming
processes, the level of crystallinity of the preformed
sheet should not exceed about 10 percent.
The preheating of the substantially amorphous sheet
prior to positioning over the thermoforming mold is ~
necessary in order to achieve t:he very short molding ~-
times required for a viable commercial process. The
sheet must be heated above its Tg and below the point
at which it sags excessively during positioning over
the mold cavity. In the thermoforming process 9 a sheet :
temperature which is within the range of about 130C to
about 210C and a mold temperature which is within the
range of about 140C to about 220C will normally be
utilized. It is often preferred to use a sheet
temperature which is within the range of about 155C to `~ ;
about 185C and a mold temperature which is within the
range of about 165C to about 195C. ,~
This invention can be practiced by using any of the
known thermoforming methods including vacuum assist,
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air assist, mechanical plug assist or matche~ mold.
The mold should be preheated to a temperature
sufficien~ to achieve ~he degree of crystallinity
desired. Selection of the optimum mold temperature is
dependent upon the type of thermoforming equipment,
configuration and wall thickness of the article being
molded and other factors.
Heatsetting is a term describing the process of
thermally inducing partial crystalli~ation of a
polyester article without appreciable orientation being
present. In the practice of this invention,
heatsetting is achieved by maintaining intimate contact
of the film or sheet with the heated mold surface for a
sufficient time to achieve a level of crystallinity
which gives adequate physical properties to the
finished part. It has been found that desirable levels
of crystallinity should be about 10 to about 40
percent, For containers to be used in high temperature
food application, it was found that levels of ;
crystallinity above 15 percent were necessary for
adequate dimensional stability during demolding ~`
operations. A preferred range of crystallinity is from
25 to 35 percent, this range yields parts with
excellent dimensional stability and impact resistance.
The heat set part can be stripped out of the mold
cavity by known means for removal. One method, blow
back, involves breaking the vacuum established between
the mold and the formed sheet by the introduction of
compressed air. In commercial thermoforming operation,
the part is subsequentlv trimmed and the scrap ground
and recycled. ~-
In the preparation of films or sheeting for
subsequent use in thermoforming processes, it is
extremely important that the polyethylene ionomer be
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homogeneously dispersed throughout the PET to form a
homogeneous blend in order to achieve optimum results.
The film can be produced by conventional extrusion or
casting methods. Depending upon the method employed in
making the film or sheeting, the intrinsic viscosity of
the film or sheeting produced may be nearly the same or
slightly lower than the intrinsic viscosity of the
starting thermoplastic resin composition. In other
words, the intrinsic viscosity of the thermoplastic
resin composition may be reduced slightly by the
casting or extrusion process. The thermoformed
articles made should have intrinsic viscosities which
are similar to the intrinsic viscosities of the film or
sheeting from which they are made.
Throughout the specification and appended claims,
all percent expressions are weight percent based on the
total weight of the composition polymer, sheet or
article. The following examples are intended to be
illustrative of the invention rather than limiting its
scope. ;
Example 1
A PET resin having an I.V. of 1.04 dl/g was ~;
extruder blended with Surlyn~ 9721 (a polyethylene
ionomer). The thermoplastic resin composition made
contained about 96.6% PET and 2.4~ polyethylene
ionomer. The resin was extruded u~ilizing a 1.75 inch
(4.45cm) extruder which was operated at a temperature
within the range of about 285C to about 305C
utilizing an extruder speed of 70 rpm and a die
temperature of about 292C. The extruder screw
produced sufficient shearing force to homogeneously
blend the polyethylene ionomer into the PET. Sheeting
having a thickness of 0.03 inches (0.076cm) was
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prepared utilizing a chill roll temperature of about
63C and a take up speed of 4 feet/minute (121.9 ;~
cm/minutes).
The sheeting prepared was thermoformed into trays
utilizing a standard thermoformer. The thermoforming
process was carried out utilizing a preheat time of 45
seconds, a mold time of 8 seconds, a sheet temperature
of 171C, a mold temperature of 182C, a top oven
temperature of 299C, and a bottom oven temperature of
116C. The trays prepared in this experiment were very
satisfactory. In fact, they were determined to have an
impact strength of 9.5 x 104 g . cm at a temperature of
-29C. The trays made were also determined to have a
crystallinity of 28%.
Example 2
A PET resin having an intrinsic viscosity of 1.04
dl/g was extruder blended with Surlyn~ 9720 (a ,
polyethylene ionomer). Surlyn~ 9721 has the same -
physical properties as Surlyn~ 9720. However, Surlyn~
9720 is offered for wire and cable applications and
contains a stabilizer which is not present in Surlyn~
9721. The thermoplastic resin composition made
contained about 97~ PET and 3~ polyethylene ionomer.
The resin was extruded utilizing an extruder which was
operated at a temperature within the range of about
268C to about 288C utilizing an extruder speed of 86
rpm and a die temperature of about 284C. The extruder
screw produced sufficient shearing force to
homogeneously blend the polyethylene ionomer into the
PET. Sheeting having a thickness of 0.076 cm (0.03
inches) was prepared utilizing a chill roll temperature
of about 74C and a take up speed of about 121.9
cm/minutes (4 feet/minute). The sheeting made was then
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thermoformed into trays utilizing a standard
thermoformer. The thermoforming process was carried
out utilizing a preheat time of 45 seconds, a mold time
of 8 seconds, a sheet temperature of 171C, a mold
temperature of 182C, a top oven temperature of 299C
and a bottom oven temperature of 116C. The trays made
were determined to have an impact strength of 7.9 x 104
g cm at a temperature of -29C. The trays made were
also determined to have a crystallinity of about 32%.
Example 3 (Comparative)
In this experiment trays were prepared utilizing
essentially the same procedure as was described in
Example 2, except that linear low density polyethylene
was substituted for the Surlyn~ 9720. The trays made ;
in this experiment were satisfactory. However, the
trays made in this experiment d:id not possess the
outstanding low temperature impact strength exhibited
in the trays made in Examples l and 2. In this
experiment the trays made utili~ing 3~ linear low -
density polyethylene only have a low temperature impact
strength as measured at -29C of 7.1 x 104 g ~ cm.
Accordingly, the trays made in Example 1 utilizing
Surlyn~ 9721 had 35~ more low temperature impact -
strength than did the trays made utilizing linear low
density polyethylene. The trays made in Example 2
utilizing Surlyn~ 9720 had 12% better low temperature
impact strength than did the trays made in Example 3
which utilized linear low density polyethylene. This ~`
experiment shows that the low temperature impact
strength of dual-ovenable trays can be grea~ly improved
by utilizing polyethylene ionomers in the thermoplastic
- composition used in thermoforming the trays.
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In this experiment dual-ovenable trays were
prepared by thermoforming a sheet having a thickness of
0.076 cm which was comprised of a thermoplastic
composition which contained about 97% PET having an
intrinsic viscosity of 0.95 dl/g, about 3~ Surlyn~ 9721
and about 0.6~ Ethanox~ 330 (a stabilizer). The
dual-ovenable trays made in this experiment exhibited a
low temperature impact strength at -29C of 9.7 x 10
g cm. This is a 63~ improvement in impact strength
over dual-ovenable trays which are made utilizing
similar compositions which contain linear low density
polyethylene in lieu of the polyethylene ionomer
utilized in this experiment. More specifically,
dual-ovenable trays which were made utilizing linear
low density polyethylene in lieu of the Surlyn~ 9721
exhibited low temperature impact strengths as measured
at -29C of only 6.0 x 104 g cm. This experiment,
again, shows the superiority in low temperature impact
strength of dual-ovenable trays which are made ;~
utilizing polyethylene ionomers.
While certain representati~e embodiments and
details have been shown for the purpose o~ illustrating
- this invention, it will be apparent to those persons -
skilled i~ this art that various changes and
modi~ications can be made therein without departing
from the scope of this invention. -~
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