Note: Descriptions are shown in the official language in which they were submitted.
BACK~,ROUND OF THE I~VENTION
This invention relates ~o the field of contoured
molded pulp products, such as containers for food or the like,
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I which have a plastic liner on at least one surface, usually
i the upper food containing surface, and provides for the first
time, it is believed, such a product which can be subjected
without damage to temperatures of up to 400F when empty for
a short time, and for as long as 45 minutes when the contained
food or other substance is at room temperature or below at the
outset.
Prior to the present invention, it has long been known
that various thermoplastic materials, including polyesters,
can be bonded by heat and pressure to a pre-formed and contoured
body molded of fibrous pulp material. This provides an
attractive and highly moisture resistant container, suitable
for purposes such as plates, bowls and the like or serving
food. These products as previously known are not suitable,
however, for use as ovenable containers involving exposure to
high temperatures for long times because, among other defi-
ciencies, the plastic liner either melts or shrink-separates
away from the molded pulp base when exposed to high temperatures,
and the pulp exhibits significant undesirable browning or even
outright charring when exposed to such temperatures for long
times. Representative examples of prior art paten~ disclosing
such containers are as follows.
Stevens U. S. patent 2,590,221 (Mar 1952) discloses a
method of fusing or bonding a sheet of thermoplastic material,
such as polyethylene, to a contoured molded pulp body by heat-
ing the face of the ~hermoplastic material to be bonded with
~he pulp to the fusion point while allegedly maintaining the
opposi~e face at a temperature below the fusion point.
Amberg U. S. paten~ 3,616,197 (Oct 1971) discloses a
method of adhering a liner of polypropylene film to a contoured
molded pulp plate by mear.s of an intermediate adhesive layer,
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su~h as a pigmented polyamide ink or a pigmented resin
emulsion.
Singer U. S. patent 3~657,044 (Apr 1972) discloses a
method of bonding a sheet of thermoplastic material, including
polyester, to a contoured molded pulp lbody by the use of super-
atmospheric pressure to force the thermoplastic against the
molded pulp.
Amberg U. S. patent 3,697,36g (Qct 1972) discloses a
method of fusing or bonding a liner of polypropelene film to
a contoured molded pulp plate by means o an intermediate layer
of polyethylene which secures the bond with the molded pulp.
South African patent 73/2988 (May 1972) discloses a
method of bonding a previously coextruded laminate of poly-
propylene and polyethylene to a contoured molded pulp body.
More recently it has been discovered that ovenable
con~ainers suitable for use in the re-heating and/or further
cooking of food in microwave and conventional ovens can be
made by either press-forming them, or folding and adhering
them, from a sheet of flat pap~rboard having a coating of
substantially amorphous polyethylene terephthalate bonded by
its own substance to at least one surace thereof. These food
containers present several problems, however, such as the
fact that the press-forming or the folding and adhering o the
container from the flat sheet is expensive, and there is a
danger that the coating will rupture at the bend or fold
lines. Such containers usually are neither as strong-nor as
attractive as molded containers. Also, the press-formed or
folded-up container necessarily has pleats or folds and/or
corner gaps which are completely unacceptable in certain
containers, such as muffin baking trays, and they preclude
hermetic seallng o a plastic overwrap. Representative examples
of prior art patents disclosing such containers are as follows.
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I
¦. Kane U. S. patent 3,924,013 (Dec 1975) discloses the
; formation of an ovenable food container from a laminate which
includes nonbrowning paperboard with a coating of substantially
¦ amorphous polyethylene terephthalate extruded as a hot resin
'i1 directly thereon and bonded thereto by :its own substance.
1. Middleton U. S. patent 4,147,836 (Apr 1979) discloses
i the formation of an ovenable food container from a laminate
. which includes paperboard coated, by a process which includes
subjecting the paperboard to a corona discharge, with poly-
ethylene terephthalate, noting that the crystal~inity of the
latter does not substantially afect the adhesi~n of the
coating to the paperboard. .
Thus, the problem heretoore unresolved by the prior
art is the ability to provide an inexpensive, di~posable
container which is three-dimensionally contoured without
pleats or folds, and which is capable, among other things, of
holding food during exposure to high temperatures for long
times in either microwave or conventional ovens without
having the plastic liner melt or shrink separate away from the
contoured base, without any significant undesirable browning
of the base7 and without any other detrimental ef~ect to the
container or the food.
SUMMARY OF THE INVENTION
The product of this invention comprises a pre-formed
contoured base molded of fibrous pulp with a liner directly
bonded to at least one eontoured por~ion of at least one side
of the base, ~he liner consisting essentially of polyethylene
terephthalate which will not shrink-separate away from the
eontoured base as a result of exposure to a temperature of
about 400F. The process of ~his invention comprises the steps
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of (1) pre-heating a pre-formed base which has been obtained by molding
fibrous pulp from an a~ueous slurry ~hereof against an open-face suction
mold to substantially finished contoured shape which includes horizontal and
sloping but no substantially vertical wall portions, (2) pre-
heating a film of polyes-ter having a thickness less than about
2.0 mil and (3) then pressing the pre-heated film into contact
with at least one contoured portion of at leas-t one surface of
the pre-heated base so that the film is s-tretched into direct
contact with the contoured base and becomes directly bonded to
the base to form a contiguous and integral liner thereon, charac-
terized in that the polyester fi].m is (a) substantially amor-
phous, substantially unoriented polyethylene terphthalate and
it (b) is pre-heated to a thermoforming temperature within the
range in which it will subs-tant.ially crystallize in time but
then (c) is rapidly pressed :into contact with -the base while it
is still substantially amorphous and before enough time has passed
in that temperature range to crystallize it to the stage at
which it will not directly bond to the base, whereby (d) after
the lined product is cooled to room temperature the liner will
not shrink-separate away from the contoured base as a result of
subsequent exposure to a temperature of about ~00F.
Desirably -the film is pre-heated to a thermoforming
-temperature within -the range of from about 250"F -to about ~00F
but then is rapidly pressed into con-tact with the base before
the film has been at that thermoforming temperature for longer
than about 2 seconds.
The present invention will be further illustrated by
way of the accompanying drawings wherein similar reference charac-
ters refer to similar parts, and in which:
Fig, 1 is a pictorial view of a relatively simply and
shallowly cont~ured food serving container of a type with which
this invention is useful;
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Fig. 2 is a pictorial view of a relatively intricately
and deeply contoured muffin baking container of a type with
which this invention is useful;
Fig. 3 is a fragmentary sectional elevational view
show~ng a plurality of representative emp-ty containers nested
one within another in a stack for shipment and storage;
Fig. 4 is an enlarged fragmentary sectional elevational
view, with the scale greatly exaggerated for purposed of clarity,
showing a contoured molded pu].p base with a polyester liner
bonded by its own substance to the upper surface thereof; and,
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i Fi~. 5 is a highly schematic sectional elevational
¦ view, again with the scale exag$erated, showing the essential
¦ el~ments of the apparatus for practi~ing the process of bonding
i together the pre-formed base and-film according to this
invention.
DETAILED DESCRIPTION OF T~E INVEMTION
Il
The present invention will be described in detail by
explainin~ first the pre-formed molded pulp base, second the
polyethylene terephthalate fi~m, third the process for bonding
the film to the base, and fourth the polyester lined molded
pulp product attained thereby.
First, t pre formed molded pulp base determines the
shape and size of the product of this invention, which is
dimensioned as dictated by the end use for which the product
is intended. The base may be obtained according to any one of
several well-known pulp molding techniques.
For high quality products, such as ovenable containers
for merchandizing food products according to the presently
preferred embodiment of this invention, the molded pulp base
is obtained by the "precision molding" process, which is also
~ariously known as a "die-drying" or "close-dry-lng" process.
Precision molded pulp articles are obtained by molding fibrous
pulp from an aqueous slurry thereof against a screen-covered
open-face suction mold to substantially finished contoured
~hape, and then drying the damp pre-form under strong pressure
imposed by a mating pair of heated dies. This may be done,
or instance, by apparatus and according to me~hods such as
those described in Randall U. S. patent 2,183,869 ~Dec 1939).
Precision molded pulp articles are dense, hard and boardy, with
an extr~mely smooth, hot-ironed surface finish. Such precision
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! molded pulp articles have been made and sold in the form of
i disposable plates, bowls and fhe likP for many years under the
tradPmark "CHINET" by Keyes Fibre Company of Waterville, Maine,
USA.
In addition to a pulp base obtained by the precision
moldlng process, molded pulp articles produced according ~o
the well-known "free-dried" or "open-dired" process similarly
may be used in ~he practic~ of this invention. The free-dried
process involves molding fibrous pulp from an aqueous slurry
thereof against a screen-covered open-face suction mold to
substantially finished contoured shape, and then drying the
damp pre-form in a free space such as placing it on a conveyor
and moving it 910wly through a heated drying oven. Such
molded pulp articles are characterized by a non~compacted
consistency, resilient softness, and an irregular fibrous feel
and ~ppearance. When using ~ree-dried molded pulp bases in
the practice of this invention, the rougher fibrous surface
thereof requires that the film be pressed against the surface
of ~he pulp base in the bonding process at somewhat lower
temperatures and/or pressures, as can be understood, to insure
that the film retains sufficient strength that it will not be
sub;ected to pinhole punctures by the unpressed surface fibers
of the pulp base.
Molded pulp bases obtained by other well-known pulp
molding processes similarly are useful in the practice of
this invention. This includes molded pulp articles which are
first molded according to the free-dried process, and then
"after-pressed" in the manner disclosed in Randall U. S. paten~
2,704,493 (Mar 1955). This also includes molded pulp articles
which are first molded according to the free-dried process, and
then placed on open-face warpage preventing forms on the con-
veyor which moves the articles through the drying oven, which
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~! simply hold the articles to prevent shape distor~ion from
warpage during the drying ~rocess, as disclosed in Reifers U. S.
patent 3,185,370 (May 1965).
According to the preferred embodiment of this invention, ;
the pulp consists of subs~antially 100% bleached kraft wood
, fibers. This material is preferred because it is nonbrowning
t' in the sense that, when exposed to a temperature of about 400F
for as long as 15 minutes, it will no~ noticably discolor from
its attractive white, beyond perhaps a slight yellowing, to a
significant undesirable brown, and certainly will no~ scorch
or char. When food or other substances at room temperature
or below are in close proximity to provide a heat sink effect,
thls material can be exposed to temperatures of up to 450F
for as long as 45 minutes without exhibiting any significant
browning.
Other types and grades of fibrous pulp can be used,
however, without defeating the broader ob}ects of this
invention, when the end use of the product does not contemplate
exposure to such temperatures for such times.
Irrespective of the molding process or the type or
grade of pulp used to obtain the base, the same may be treated
with fluorocarbon oil repellant sizing and/or reactive
synthetic water sizing or other well known sizing, which
quite surprislngly does not interfere with the bonding step
explained below.
Whether the molded pulp bases are obtained according
: to the precision molding process, or one of the several
~ariations of the free-drying process, they are in any event
molded to substantially finished three-dimensional shape,
which may be contoured simply or intricately, shallowly or
deeply, according to the ultimate use ~o which the polyester
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lined product will be put. In every case, the contours include
horizontal wall portions and sloping wall portions, but no
substantially vertical wall portions.
The absence of substantial vertical wall portions,
other than minor vertical shoulders or lugs of the type which
are well known for anti-jamming or de-nesting purposes, permits
a plurality of like empty bases, and of course the finished
lined products which are made from them as well, to be nested
one within another in a stack of the same for compact,
inexpensive and convenient shipment and storage purposes. By
the same token, the molded pulp bases, and the finished lined
products made from them as well, are of substantially uniform
single thickness throughout, in the sense that every contour of
consequence which pro;ects outwardly from one side o the
product is reflected by a corresponding recess in the other
side of the product.
Second, the polyethylene terephthalate film from which
the liner of the molded pulp base is obtained is a ~hin film
of thermoformable, substantially amorphous, substantially
unoriented polyethylene terephthalate, which ls understood to
have a relatively high molecular weight. While such poly-
ethylene terephthalate is available in sheet form with a
thickness exceeding about 7.0 mil, it is preferred to practice
this invention with such material in thinner film form, having
a thickness of less than about 7.0 mil. Such material is
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commercially available under the ~4~u~ ae "PETRAt' from
Allied Chemical Corporation of Morristown, New Jersey, USA,
and many of its below discussed properties are explained, it
is believed, in ~erman patent 2125978 (Dec 1971).
According to the preferred embodiment of ~his invention,
where a film of such polyethylene terephthalate is bonded to
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a base having a smooth surface obtained by the precision
molding process, a film with a thickness of between about 0.5
mil and about 2.0 mil is preferred, and films with a thickness
of approximately l.0 mil, namely 0.9 mi:L and 1.25 mil, have
proved very satisfactory. When practic:ing this invention to
produce extremely intrica~ely and deeply contoured containers,
such as muffin baking trays with a plurality of deep, closely-
spaced pockets or recesses, or with molded pulp bases obtained
by the free-drying process with relatively rough surfaces,
however, then somewhat thicker ilms may be advisable to
accommodate severe stretching or to safeguard against undesirable
pinholeing, as can be understood.
The polyethylene ~erephthala~e fllm useful according
to this in~ention is thermoformable in the sense that it becomes
thermoformable to the extent that it can be stretched and formed
into conformity with the shallower contoured molded pulp bases
at and above a temperature of about 170F. This material melts
to a liquid at temperatures in excess of about 480F to about
490F. .
This film is substantially amorphous, being practically
transparent and having l~ess than about 5% crystallinity~ Above
the lowest thermoforming temperature and below the melting
temperature, however, this material will spontaneously
crystallize. A film with a thickness of about 1 mil, for
example, will crystallize substantially when held for 2 to 15
seconds in the temperature range between about ?50F and about
400F. When held for such times within the crystallizing
temperature range, the film turns from substantially trans-
parent to translucent white, evidencing increased crystallinity,
and becomes brittle and not readily bondable with mo:Lded pulp
at any temperature below about 400F. It is noteworthy,
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however, that exposure of the film to the crystallizing
temperature range for less than a few seconds will not result
in a detri~ental increase in crystallinity which has a material
adverse affect on either the formable or the bondable
characteristics of the film. This is an attribute which, it
is now beli~ved, may be responsible for many of the improved
results attained by this invention, as explained in greater
detail below.
The polyethylene terephthalate film useful according
to this invention is ubstantially unoriented, which means
that the film is not deliberately stretched during manufacture.
The film is understood to be ~ormed by extruding polyethylene
terephthalate resin through a slit die at a temperature of
between about 510F and about 540F, and cooling the ~lat thin
extrudate on a polished roll at a temperature of between about
60F and about 200F. Some small degree of longitudinal
orientation undoubtedly occurs during extrusion of the resin
into the film, but it is understood that the degree thereof is
minimized.by extruding through the slit die and cooling on a
polished roll.
Use of polyethylene terephthalate having a relatively
high molecular weight is now believed to be an important,
although perhaps not critical, feature of the present invention.
In actual practice, material with a molecular weight which is
understood to be in excess of about 15,000, namely in the range
of 15,000 to 30,000, has been found to be useful, and is now
preferred. This is in contrast with polyethylene terephthalate
having a relatively low molecular weight, such as a molecular
weight not exceeding about 10,000. It is contemplated, however,
that molecular weights as low as about 12,000 may still prove
useul for the production of lined containers or other products
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in which the contours are quite shallow requiring minimal
stretching of the film prior to bonding, and the contemplated
end use of the product is such that a minimal mechanical bond
between the liner and the base can be tolerated.
Third, the 2rocess for bondin~ the film to the base
comprises the steps of pre-heating the pre-formed base obtained
in the manner explained above, rapidly pre-heating a film of
polyethylene terephthalate having the characteristics described
above and then quickly hot pressing the pre-heated film into
contact with the surface of the pre-heated base which is to have
a liner ~ormed thereon, the hot pressing being done at a
temperature and for a time which insures that the ilm is
stretched into direct con~act with the contoured base, and
becomes directly bonded by its own substance ~o the base so as
to form a contiguous and integral liner thereon, and fina~ly
cooling the base and liner to room temperature.
More precisely, and with reference to Fig. 5, the
presently preferred process is aceomplished as follows. The
pre-forme~ precision molded pulp base L0 is placed in an open-
face die or mold 12, which is dimensioned to provide full
support for ~he base by supporting the Lower sur~ace thereof
which is not to be coated with the polyester liner.
The die 12 is made with suction ports 14 eonnected
with a source of vacuum (not shown~ so that ~acuum can be
applied through the base during the bonding steps of the
process. Preferably, the vacuum applied is in the range of
about 20 inches of mercury, although lower ~alues will ~e
suitable for the shallower contoured articles and/or thinner
films and higher values may be advisable for the deeper
contoured articles and/or thicker films.
-12-
The die 12 is maintained by conventional hea~ing means
(not shown) at an elevated temperature in the range between
about 300F and about 600F. The appropriate temperature is
selected within this range to insure thaLt the molded pulp base,
after being placed therein, will be pre-heated, in the time
l available prior to the bonding step, to a temperature such
that the surface to receive the film is in the range between
¦l about 300F and about 375F, which means that the opposite
surface in contact with the heated die may attain higher
temperatures for a short time. When the molded pulp base is
relatively thick, and/or when it has a relatively intricately
and/or deeply contoured shape, then pre-heating to higher
temperatures will be required, as can be understood, whereas
thinner bases, with relatlvely simply and shallowly con~oured
shapes, or where minimal bonding can be tolerated, will permit
pre-heating temperatures in the lower end of the range. In
any event, the base is pre-heated to insure that the surface to
which the film will be bonded is at the desired temperature,
and preferably although certainly not necessarily by the simple
expedient of holding the base or a short time in a heated die.
The film of polyethylene terephthalate 16 also is
pre-heated, and placed in position closely opposing the exposed
surfaces of ~he molded pulp base 10 against which it is to be
applied. The manner in which the film is positioned is not
an essential feature of the present invention, since it can be
accomplished in a number of ways. Because the film should be
pre-heated rapidly and then quickly pressed against the base,
it is recommended that the film be positioned with respect
to the base before or at least as part of the pre-heating step.
The film can be pre-heated in any sui~able manner which
will insure that it is raised from room temperature past and
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substantially above both the temperature at which i~ becomes
thermoformable and the temperature at which it starts to
crystallize in a relatively short time. According to the
preferred process, the film L6 is placed in contact with a
plate 18, which may be coated with a malterial such as Teflon
simply to insure that the film 16 does not stick to the plate
18. The plate 18 is maintained by suitable hea~ing means (not
shown~ at an elevated.temperature in the range between about
325F and about 400F, the temperature within this range
being selected as requirPd by the thickness of the film and its
molecular weight. Thicker film with a thick~ess up to about
7.0 mil will require that the plate 18 be maintai~ed at a
higher temperature within the range, or that the film be held
in contact with the plate for a slightly longer period of time,
as can be understood, whereas thinner film with a thickness
close to approximately 1 mil can be pre^heated rapidly to the
requisite bonding temperature by contact with a plate maintained
at a lower temperature within the range.
The film should be pre-heated rapidly, at least during
that portion of the pre~heating which is in the temperature
range in which the polyethylene terephthaLate crystallizes.
For ins~ance, with film having a thickness of between about
0.5 mil and about 2.0 mil, the film is pre-heated from room
temperature to the bonding temperature in a period of time
which does not e~ceed about 2 seconds.
The base having been appropriately pre-heated, as soon
as the film has been pre-heated to the desired elevated
temperature, vacuum quickly is applied through the base 10 to
suction-press the film into contact with the surfac~ or
surfaces of the base which are desired to be coated. The film
at its pre-heated elevated temperature is s~retched into direct
ll ~L3 3~
contact with the contoured base, and the film becomes thinned in
areas and to extents as determined by the shapes and depths of
the contours of the base. Continued application of pressure
induced by the vacuum insures that the film becomes directly
I bonded, by its own substance, to the exposed surface or surfaces
ii of the molded pulp base so as to form a con~iguous and integral
liner on the base.
If desired, the film can be suction-applied against
both sides of the base, such as the undersurface of the marginal
rim or flange of the molded pulp base in the manner explained
in the aforesaid Singer patent, although this is wholly optional.
Similarly, the step of pressing the film into contact with the
base may include super-atmospheric or mechanical pressing
instead of or in addition to ~acuum pressure, su~h as by a
movable die to press the film against the marginal rim or
flange, or other portions, of the base, although this also is
deemed to be a purely optional feature as the present invention
is now understood. By the same token, the film may be pressed
against only a portion of only one surface of the base, such as
in situations where the base includes a contoured bottom
container portion with a hingedly connected closeable cover
portion, and only the inside o~ the bottom container portion
is to be lined with plastic. Also, either the base or the
film or both may contain printing for decorative, informative
and other purposes, and the objects of this invention will not
be defeated if the printing is in the interface between the
base and the film or if the printing media has adhesive
characteristics.
The pressing step of the process is maintained for a
time, such as several seconds or less, ~o insure a proper
physical bond be~ween the film and the base. It has been ound
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~ C!245
that when the base and the film have been pre-heated in the
manner explained above, and the film is pressed in~o contact
with the base by vacuum in the range of about 20 inches of
mercury an extremely strong, mechanically inter-locking bond
be~ween the plastic and the pulp can be attained in less than
about 1 second. A film of polyethylene terephthalate with a
thickness of approximately 1.0 mil can be both rapidly pr -
heated and then quickly pressed into contact with a pre-heated
base in a total time of less than about 1 second.
If mechanical pressing or the use of super-atmospheric
pressure is employed, then the time required to obtain a suitable
bond will be shorter, as can be understoot, which is an
extremely important consideration or mass production techniques.
Thereafter, the base and liner are allowed to cool ta
room temperature. The presently preferred process f'or cooling
the base and liner is to remove the lined base from the heated
vacuum die 12, following which the product cools slowly to room
temperature. After removal from the heated die 12, the molded
pulp base portion of the product retains heat, which 1s dissi-
pated slowly, requiring at least several minutes, depending on
the mass of the article and whether the articles are immediately
nested in a stack'for cooling, to cool from the bonding
temperature to room temperature.
Appropriate trimming o~ excess film and/or molded pulp,
sueh as at the edges of the product or around apertures through
the product, can be performed by any well-known method before,
during or after either the bonding step or the cooling step.
Finally, it is now contemplated that a final heat
annealing step may prove useful to further crystallize the
liner, for improving its properties for certain encl uses. T~is
would involve exposing the finished product to a temperature
in the range between about 250F and about 400F for several
minutes or more.
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Fourth, the polyester lined molded ~ product 20 of
this invention consists es~entially of a pre-formed contoured
base 10 of fibrous pulp, which may or may not contain one or
more sizing ingredients, with a l$ner 22 directly bonded to at
least one contoured portion of at least one side of the base, as
shown in Fig. 4. The liner 22 is substantially impervious, and
consists essentially of polyethylene terephthalate which will
not shrink-separate away fro~ the contoured base 10 as a result
of exposure to a temperature of about 400F.
As explained first above in connection with the detailed
description of the pre-formed base, the base of the product
according to the preferred embodiment of this invention is
obtained by molding fibrous pulp to substantially finished
contoured shape, and then drying the pulp under pressure imposed
by a mating pair of heated dies. The pulp is substantially
100% bleached kràft wood fibers, which will not exh~bit
significant undesirable browning as a result of exposure to a
temperature of about 400F for as long as 15 minutes.
Also as explained above, the polyester terephthalate
liner according to the preferred embodiment of this invention,
if separated from the base, would be found to be not readily
bondable to molded pulp at any temperature below about 400F.
$he liner has a crystallinity of at least about 8%, which may
account for the fact that the material is not readily bondable
to molded pulp in the manner in which the starting film was.
The strong bond obtained initially by the above-described
process, however, is not adversely affected by the subsequent
loss of bonding characteristics.
Purely representative products according to this
invention are illustrated in Figs. 1 and 7'. Fig. 1 is a
pictorial view of a relatively simply and shallowly contoured
,
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food serving container 30 having horizontal base wall portions
32, a horizontal marginal rim or ~lange wall portion 34,
sloping sidP wall portions 36, and sloping partioning rib wall
portions 38, but no substantially vertical wall portions.
j Fig. 2 is a pictorial view of a relatively intricately
, and deeply contoured muffin baking container 40 which is three-
i, dimensionally contoured ~ith horizontal base wall portions 42,
horizontal upper connecting wall portions 44, and steeply
sloping relatively deep wall portions forming a series of
closely spaced pockets 46, but no substantially vertical wall
portions.
In every case, the container or other product according
to this invention is three-dimensionally contoured without
substantially vertical wall portions to permit a plurality
of like empty containers, such as the containers 50 illustrated
in fragmentary fashion in Fig. 3, to be either deeply or
shallowly nested, one within another, in a stack of such
containers for compact, economical and convenient shipment and
storage p~rposes. Insubstantial vertical wall portions 52
may be provided to form anti-jamming shoulders to facili~ate
de nesting the empty containers from the stack, as is well
known .
The product according to the preferred embodiment of
this invention, such as an ovenable food container, can be
exposed to a temperature of about 400F and the liner will
; neither melt nor shrink-separate away from the contoured base.
Such a product may be held a~ a tempera~ure of about 400F for
as long as 15 minutes and the molded pulp base still will not
exhibi~ significant undesirable browning (although the pulp
base will brown and eventually char if held at such a high
temperature for times longer than about 15 minutes).
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In actual use, a product according to the preferred
embodiment of this invention can withstand higher temperatures
or longer times when food or other substances are heated in
~he corl~ainer product. For ins~ance9 ~he muffin baking
container of Fig. 2, with baking batter in the pocke~s, both
at room temperature, can be exposed for the purpose of baking
muffins in the container to temperatures as high as 450F for
as long as 45 minutes without liner separation or pulp browning.
This is because the cool batter being first warmed and then
baked has a chilling heat sink effect which prevents the con-
tainer itself fr~m attaining a temperature as high as about
400F for more than about 15 minutes, although by the end of
the 45 minute period the edges of the container, remote from
the muffin pockets, may exhibit some yellowing.
Similarly, the food serving container illustrated in
Fig. 1 containing a frozen meal may be subiected to temperatures
as high as about 450F for as long as 45 minutes, for the
purpose of warming the meal with or without substantial
additional cooking, ~ithout liner separation or pulp browning.
The temperatures and times will depend on the mass of the
contalner, and particularly the mass and shape of the food or
other substances heated in it, as well as the starting
temperature of the container and its contents when first
exposed to such high temperatures. In addition, the end use
parameters will depend on ~he manner in which heat is applied
to the con~ainer and its con~ents. A conventional home
convection oven will apply heat in a manner which is different
from a co~mercial contact oven, both of which heat the
co~talner as well as its contents, whereas a microwave oven
directly heats only the cont~nts and the container is heated
only indirectly by the heating contents but not the microwaves.
~.
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Z4S
~1 In the latter regard, the ovenable food container
I' product according to the preferred embodiment of this invention
! has many advantages over food containers made of aluminum, for
¦l instance. Unlike aluminum, the container of this invention is
~ransparent to micro~aves, so that the con~ainer may be used in
~ either microwave or heat ovens. The product of this invention
i is significantly less energy intensive to manufacture than
aluminum containers, and the containers of this invention are
recyclable.
When a produc~ according to this invention is heated in
actual use, it is suspected that the polyethylene terephthalate
liner increases in crystallinity, which decreases its tendency
to shrink-separate away from the base, even as the pulp base
may begin to lose ~ome of its original character:Lstics during
exposure to high temperatures ~or long times. If this under-
stanting is corrPct, then the product of this invention is not
static ln its characteristics when exposed to heat during end
use, but rather the heat changes the characteristics of both
the pulp and the polyester but in a manner which still is
compatible with the requirements for an ovenable food container.
While the above described embodiments constitute the
best mode now known for practicing this invention, other
, embotiments and equivalents are within the scope of the actual
; invention, which is claimed as:
!
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