U.S. patent number 3,865,302 [Application Number 05/305,250] was granted by the patent office on 1975-02-11 for container for cooking food therein.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to William P. Kane.
United States Patent |
3,865,302 |
Kane |
February 11, 1975 |
CONTAINER FOR COOKING FOOD THEREIN
Abstract
A container is provided consisting of a polyethylene
terephthalate tray with a polyethylene terephthalate lid bonded, by
a novel adhesive, to an upper surface of the tray. The adhesive is
coated only onto this upper surface, which surface is in the form
of a peripheral flange. The adhesive is free from flow at cooking
temperatures of up to about 400.degree.F. and the bond strength at
these cooking temperatures is sufficient to prevent leakage from
the tray, yet, nonetheless, permit peeling of the lid from the tray
without tearing, after cooking.
Inventors: |
Kane; William P. (Bon Air,
VA) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
23180019 |
Appl.
No.: |
05/305,250 |
Filed: |
November 10, 1972 |
Current U.S.
Class: |
229/123.1;
229/125.35; 156/69; 229/903; 426/396; 156/334; 426/106; 426/113;
220/359.4 |
Current CPC
Class: |
B65D
77/20 (20130101); Y10S 229/903 (20130101) |
Current International
Class: |
B65D
77/10 (20060101); B65D 77/20 (20060101); A47j
027/00 (); B65b 007/28 (); B65d 001/00 (); B65d
005/64 (); B65b 029/08 () |
Field of
Search: |
;426/106,113,114,123,396,410,412,413,415 ;206/46F ;156/69,334
;161/125,231 ;260/28.5AV,75T ;229/3.5R,43,48T |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lutter; Frank W.
Assistant Examiner: Weinstein; Steven L.
Claims
I claim:
1. A container for cooking food including:
the combination of a dish-like tray of polyethylene terephthalate,
the tray having a bottom part and a body part and an outwardly
projecting peripheral flange having an upper surface on the upper
extremity of the body part;
an adhesive of an ethylene copolymer and petroleum wax having a
composition consisting essentially of (a) 30 percent to 40 percent
by weight of petroleum wax and (b) 70 percent to 60 percent by
weight of such ethylene copolymer, positioned only on the upper
surface of the flange, the adhesive having a melt flow of less than
0.2 grams at 320.degree.F. (160.degree.C.) for 10 minutes and a
surface free energy of less than 30 dynes; and
a substantially flat cover part of polyethylene terephthalate
having lower surface parts adapted to be bonded to the flange by
the adhesive, wherein said adhesive is free from flow at cooking
temperatures up to about 400.degree.F and said combination exhibits
a bond strength greater than 100 grams/inch at 400.degree.F and
less than 500 grams/inch at room temperature when said cover part
is bonded to said flange.
2. The container of claim 1 wherein the polyethylene terephthalate
in the tray has an intrinsic viscosity of at least 0.7.
3. The container of claim 1 wherein the ethylene copolymer is of an
acid terpolymer resin consisting of an interpolymer of ethylene,
vinyl acetate, and an organic acid.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is a container consisting of a tray having a lid
secured to a flange of the tray by a novel adhesive.
This invention further is a method of using this novel container,
both the lid and tray of which are polyethylene terephthalate, with
an adherent bond while foods are heated at temperatures up to about
400.degree.F. and, after cooking and cooling, the lid of which may
be peeled with ease from the tray to permit access to the cooked
contents therein.
2. Description of the Prior Art
U.S. Pat. No. 3,547,388 to Hernnes is exemplary of the prior art.
Such patent discloses a food package including a one piece
container and a two-layer laminated film cover joined to a flange
thereof. The upper layer of the cover film laminate has properties
similar to the material from which the container body is made while
the lower layer of the film cover laminate permits bonding, but
easy separation from the container body.
Other patents of the prior art of interest to the container and
method of using such container (including cooking in it and opening
of it, after cooking) also of this invention include Pat. Nos.:
3,547,338; 3,491,935; 3,616,898; 3,202,271; 3,615,707; 3,655,503;
3,656,210; 3,530,917; 3,561,668; and 3,498,525.
None of these patents show the thermally stable container of the
instant invention in which only the upper surface of the flange of
the tray is coated with an adhesive, to minimize the exposure of
the adhesive to the contents in the tray, and in which food may be
cooked or reheated at normal cooking temperatures up to about
400.degree. F. and the lid of which may be peeled easily from the
tray, after cooking.
SUMMARY OF THE INVENTION
According to the present invention, containers are provided for the
packaging of food and for use in the subsequent cooking and
reheating of the food in preparation for human consumption.
The invention comprises a versatile container consisting of a
thermally stable tray and a lid secured to a flange of the tray by
an adhesive on the flange which provides a secure seal at heating
temperatures and a seal peelable after heating and cooling to room
temperature.
Trays of various materials have been widely employed for packaging
and direct consumption therefrom of preprocessed foods. Limitations
of materials from which trays were formed as well as limitations in
other components of the package have resulted in restrictions in
use of such packages.
Previously it has been the practice to form trays of heat sealable
polymers, such as vinyls and olefins, to facilitate securing lids
thereon, but these in general do not possess the thermal stability
required for reheating foods. Formed trays of aluminum foil have
been employed, but these lack versatility in that they are
unsuitable for use in microwave ovens.
The requirements for a satisfactory, versatile tray to which the
present invention is directed are that it should have a closure,
i.e., a lid, that is bonded to the tray body so that the bond does
not rupture during reheating the container and its contents up to
about 400.degree.F., and yet with the sealed lid readily peelable
to gain access to the contents after such heating and cooling to
temperatures suitable for direct consumption, i.e., 70.degree. to
150.degree.F. Further, the combination should have thermal
stability in that the tray or lid should not embrittle or be prone
to cracking upon heating. Likewise, it should not become distorted
upon heating. Finally, the lid should be adhered in a manner to
minimize exposure of the contents by the adhesive.
The present invention provides a tray and lid combination capable
of being sealed with an adhesive critically located and having
these particular characteristics.
The container is formed from a tray of polyethylene terephthalate
to which is secured or bonded a polyethylene terephthalate lid.
This bonding of the lid to the tray is accomplished by use of a
novel adhesive that is coated only onto a flange of the tray. By
doing this, undue exposure of the adhesive to the contents of the
tray is precluded or prevented. The food in the tray may be
reheated or heated to temperatures up to about 400.degree.F.
without having any problems as far as the bond is concerned; it
remains a tight seal. After cooking, this bond is easily broken
when the lid is peeled from the tray.
In short, then, this invention is a container in the form of a
lidded tray that can be effectively used for cooking, or reheating
at oven temperatures and which after cooking and cooling may be
easily opened, by peeling the lid from the tray to reach the food
contained therein, without damage to the tray or contents.
More specifically, these requirements are provided by a container
comprising a tray of polyethylene terephthalate and a lid of
polyethylene terephthalate bonded thereto with an adhesive applied
only to an upper surface of the tray, the adhesive being free from
flow at cooking temperatures up to about 400.degree.F. and having a
bond strength at such cooking temperatures sufficient to prevent
leakage from the tray and the lid being peelable from the tray
without tearing after heating at such cooking temperatures and
cooling to room temperatures. The container as above described
wherein the bond strength is greater than 100 grams/inch at
400.degree.F. and less than 500 grams/inch at room temperature, and
also wherein the polyethylene terephthalate in the tray has an
intrinsic viscosity of at least 0.65 in a 60/40 mixture of
tetrachloroethane/phenol at 25.degree.C also provides these
requirements.
The invention also comprehends a container comprising: a dish-like
tray of polyethylene terephthalate having an intrinsic viscosity of
at least 0.65 in 60/40 tetrachloroethane/phenol, the tray having a
bottom part and a body part and an outwardly projecting peripheral
flange having an upper surface on the upper extremity of the body
part; an adhesive only on the upper surface of the flange, the
adhesive having a bond strength at least 100 g./in. at
400.degree.F. and less than 500 g./in. at 70.degree.F., a melt flow
of less than 0.2 grams at 320.degree.F. (160.degree.C.) for 10
minutes and a surface free energy of less than 30 dynes; and, a
substantially flat cover having its lower surface substantially
coextensive with and bonded to the flange by the adhesive.
Also comprehended in the foregoing structures is the aforesaid
polyethylene terephthalate containing up to 1.0 percent talc and up
to 2 percent TiO.sub.2.
The specific combination of adhesive used in sealing the lid to the
tray, the tray and the location of the adhesive of the present
invention permits these strong bonds at higher cooking temperatures
which are, nonetheless, easily peelable after cooking, and do not
contaminate the contents. The adhesive is coated only onto the
flange of the tray, thus sealing of the lid is simplified, and
there is a minimum of contact potential with the contents. For
example, if the adhesive is coated onto only selected parts of the
lid, registration problems may be presented in making certain that
none of it is applied so as to overlie the food when the lid is
placed on the tray.
To gain access to the contents of the container the lid is peeled
from the upper margin of the container thereby exposing the
container contents.
In containers in which components are hermetically sealed to one
another, there have been several approaches in facilitating the
opening of the container and thus exposing the container contents.
For example, the film covering or lid has been made from a
different plastic material than that from which the container is
made. By selecting two such different plastic materials, a weld or
bond can be obtained between the film covering and container body
which is also properly hermetically sealed, and further permits
easy removal of the film covering from the container body.
Such prior art containers have the disadvantage, however, that the
container body or covering film, in most cases the covering film,
consists of a plastic material having a low softening range or
melting point, and as such they are not well suited for the
packaging of food substances which are to be externally heated,
such as in a hot air chamber, whether it be for the purpose of
sterilization or whether it be for the purpose of heating the
contents of the package prior to consuming the same. It has
generally been the experience with such prior art containers that
the part which is formed or made from the plastic having the low
temperature stability softens during heating and deforms, thus
making the container potentially undesirable from a sanitary, as a
serving tray for edible materials, and/or aesthetic viewpoint.
Accordingly, the instant invention provides a novel container which
exhibits unique packaging versatility and which overcomes
disadvantages, such as loss of bondng strength at high cooking
temperatures or lack of ease of opening after cooking or which
minimizes the exposure of the contents to bonding adhesives, of
currently used containers. In its broadest terms, this invention is
a disposable container which comprises a lid of polyethylene
terephthalate bonded to a tray of the same material by a novel
adhesive critically positioned only on a flange of the tray. This
container, and method of using it, will now be described in greater
detail .
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a side elevational view, with parts broken away for
clarity, of a container of this invention consisting of a tray
having a lid secured to an upper flange thereof by a novel
adhesive.
FIG. 2 is an enlarged cross-sectional view of parts of the
container of FIG. 1.
FIG. 3 is a perspective, exploded, view of the container of FIG. 1
showing the adhesive only on the flange of the tray ready to bond
the lid thereto.
FIGS. 4 and 5 are graphs showing relationships of surface free
energy (in dynes) to the ethylene copolymer component of the
adhesive and the relative peel strength to this surface free
energy.
FIG. 6 is a schematic illustration of an apparatus for applying an
adhesive to a flange of a tray.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is a container of a thermoplastic material
(polyethylene terephthalate) which may be tightly sealed and which,
nevertheless, may be opened easily.
In recent years various products of perishable and durable nature
have been packaged in containers formed from plastic materials and
particularly thermoplastic materials which can be heat sealed. One
type of container employed for packaging such items, such as frozen
foods and the like, is a tray container which can be made from
heat-sealable thermoplastic materials such as polyethylene,
polypropylene, polyvinyl chloride and the like. The tray is
provided with a lid formed of a heat-sealable thermoplastic
material generally the same material as in the tray itself. Sealing
of the container is conveniently accomplished by fusing the lid and
tray together by means of heat and pressure after the contents of
the container are placed therewithin. This type of container, after
being properly heat-sealed, is substantially impervious to vapors,
moisture and satisfactorily protects the contents thereof against
contamination from external sources. These containers can be
produced relatively inexpensively in a variety of shapes and sizes
but generally are unsuitable for certain uses when being subjected
to temperatures of up to 400.degree.F. whereby the contents can be
heated and cooked or sterilized as is sometimes desired when
certain food items are packaged.
With the thermoplastic materials used to form the containers, it is
extremely difficult to peel the lid from the tray when attempting
to remove the cover or lid from the tray to reach the contents.
Moreover, with a tight bond or fusion between the lid and the tray
it is virtually impossible to initiate a tear therebetween. The
difficulties in opening these containers are further increased when
the containers are subjected to heat as when the contents are
heated while in the container.
The container of this invention solves these and other
problems.
Referring to the drawings and FIG. 1 in particular, there is shown
a typical novel container C of the present invention.
Such container C is formed of polyethylene terephthalate and
consists of a tray 10 having a lid 11 secured to it by a novel
adhesive 13.
The tray 10 includes a bottom part 14 with a body part or sides 15
connected thereto and further includes a peripheral flange 16
having an upper surface 17 on the upper extremity of the body part
15.
With reference to the drawings, the tray 10 is shown to be
dish-like in shape but can be of any desired shape such as
circular, square, triangular and so forth. Tray 10 is formed by
conventional operations from a thermoplastic material, polyethylene
terephthalate.
The lid 11 is made of the same thermoplastic material (polyethylene
terephthalate) as the tray 10 may be provided with a pull-tab which
preferably extends beyond the marginal edge of the associated
flange 14. One or more such pull-tabs can be provided for peeling
the lid from the tray.
After placing the contents in the tray 10, the cover or lid 11 is
placed thereover with its peripheral edges substantially coincident
with those of flanges 16. The cover 11 is sealed to flange 16 by
the adhesive 13 by means of heat and pressure in a conventional
manner to form a seal extending the entire length of the flange 16.
The line of heat seal is disposed in parallel relationship to the
annular surface and toward the peripheral edge of the flange.
In the embodiment illustrated in the drawing, the container C
includes the dish-like tray 10 having the bottom part 14 and body
part 15 which is circumferentially continuous and which extends
upwardly (and outwardly if nesting between containers is desired)
from the bottom and terminates in the radially outwardly directed
circumferentially continuous flange 16 at the open upper end or
upper margin of the container. The container further includes the
covering film or lid 11 bonded to the radially outwardly directed
flange of the container body.
The container body or tray 10 is thermoformed into a one-piece
shape as illustrated from a plastic material having a predetermined
strength and rigidity at elevated temperatures. A material which
can be subjected to elevated temperatures in a hot air oven is
polyethylene terephthalate which does not distort easily at
elevated temperatures and also retains its strength and rigidity at
such temperatures. The lid is also made from a plastic material
having properties similar to the container body or tray such as
polyethylene terephthalate. The lid so made permits bonding to the
radially outwardly directed flange of the tray, but permits easy
separation therefrom.
The container body or tray 10 is thermoformed in a one-piece shape,
as illustrated, by conventional forming apparatus, in particular
apparatus having a heated mold and ram to enhance crystallization
of the formed structure. Starting with unoriented, amorphous
polyethylene terephthalate film (IV=0.65 or greater in 60/40
TCE/phenol) with a nominal thickness of 15 mils (0.015 inch)
containing 0.5 to 1.5 percent titanium dioxide (TiO.sub.2) and 0.25
to 1 percent talc, the former as an opacifier and the latter to
enhance the rate of crystallization, the film is preheated from
about 80.degree.C. (176.degree.F.) to about 150.degree.C.
(302.degree.F.), preferably 100.degree.-110.degree.C. for not more
than 30 seconds. (Preheating should not exceed about 30 seconds or
crystallization may occur, making forming impossible.) The mold and
the ram, (preferably a ram is employed) is heated to
130.degree.-185.degree.C. (266.degree.-465.degree.F.), preferably
150.degree.-155.degree.C. The residence time in the mold may be up
to 30 seconds; however, with the upper range of talc, residence
time as low as 6-7 seconds are sufficient to form and crystallize
the tray, and are preferred. Crystallization may continue
momentarily after the tray is removed from the mold.
Due to the ready separability and bonding characteristics of the
lid 11 relative to the radially outwardly directed flange 16 of the
tray 10 due to the adhesive 13, the lid 11 may be easily stripped
or peeled from the tray 10, through the use of suitable finger tabs
or the like (not shown), as a result of the bond strength which is
provided between the lid and the flange of the tray. The high
temperature stability characteristics of the tray 10 as well as
lid, provides an overall container which can be subjected to
elevated temperatures as in a hot air oven without incurring any of
the disadvantages previously mentioned due to this bond.
An adhesive of this invention may be formed from a composition
consisting essentially of (a) from 60 to 70 percent by weight of an
"ethylene copolymer" of the type defined and disclosed in U.S. Pat.
No. 3,215,657 to Beresniewicz et al. and (b) from 40 to 30 percent
of a petroleum wax.
Minor amounts of other materials such as pigments, dyes,
anti-oxidants and the like may also be present, but, in general,
the "ethylene copolymer" and petroleum wax should constitute at
least 80 percent of the total composition weight.
The critical aspect of this is the percentage of ethylene copolymer
and paraffin wax or petroleum wax in the composition. If the wax
content is by weight less than 30 percent and more than 40 percent,
marginally effective seals, particularly as to peelability, are
produced by the adhesive between the polyethylene terephthalate lid
and tray after heating for 1 hour at 400.degree.C., for example.
The key to acceptability is a combination of melt flow at
160.degree.C./10 minute period in an oven, surface free energy of
less than 28 dynes. The flow is 0 grams to 0.2 grams at
160.degree.C. for 10 minutes (ASTM 1238).
An important consideration, as previously mentioned, is the
behavior of the polymer from which the trays are formed. In the
forming process the body of the tray is oriented and crystallized,
and as such is not subject to cracking and exhibits a minimum of
dimensional change in the heating-cooling sequence incidental to
food preparation or instrument sterilization. The flange, on the
other hand, receives slight, if any orientation in the forming
process, but crystallizes. Accordingly, if the intrinsic viscosity
of the polyethylene terephthalate in the structure is of the range
customarily used for films and fibers (0.55-0.60 in 60 parts TCE/40
parts phenol), the unoriented crystallized flange can become
brittle and crack, causing leakage and other failure of the bond.
The polyethylene terephthalate in the trays, it has been found,
with an intrinsic viscosity of at least 0.65, is free from such
embrittlement. As is known to those skilled in the art, the polymer
from which the film for the trays is made, should have a viscosity
slightly higher than this level to compensate for losses by
depolymerization in processing. The extent of such loss will, of
course, depend upon process conditions, such as the moisture
content of the starting polymer.
The adhesives effective in the present invention are those which
bond the polyethylene terephthalate without special treatment of
this substrate. Particularly useful are wax-ethylene terpolymer
compositions described in U.S. Pat. No. 3,215,657. The terpolymer
component provides the effective bonding agent and the wax acts as
a diluent to control the peel strength and the physical properties
(e.g., melt index) of the composition.
To determine effective compositions an acid terpolymer resin sold
under the trademark Elvax 4355 an interpolymer of ethylene, vinyl
acetate and an organic acid was used at various ratios in a
composition with a paraffin wax having a 140.degree.C. melting
point. The undiluted terpolymer had a melt index of 6.0 g./10 min.
(ASTM 1238), typically 25 percent vinyl acetate and an acid number
of 6. The melt flow (index) is a function of the percentage
terpolymer.
Effective adhesives may be found by determination of the surface
free energy, a function of the contact angle of the adhesive on the
polyethylene terephthalate substrate. FIG. 4 illustrates this
surface free energy as a function of composition. Peel strength can
be correlated with surface free energy, as shown in FIG. 5.
Inspection of these two figures shows that at about 75 percent
Elvax 4355 the rapid rise in surface free energy, with the
corresponding unwanted increase in peel strength, suggests that a
surface free energy of about 30 dynes is an upper limit.
The adhesive is applied to the rim of the tray, rather than the lid
for significant reasons. First, due to health standards in trays
intended for foodstuffs, as most of them are, it is essential to
present the minimum possible contact between the adhesive and the
contents of the tray. Accordingly, with a lid free of adhesive
except where bonded to the tray at the flange, the only contact of
the contents and the adhesive in lidded tray of this invention is
at the very thin edge of the seal. Applying the adhesive on the
lid, even if only annularly at the edge of the lid, would require a
relatively broad band of the adhesive material to assure contact
with the flange, and would present considerable mechanical
difficulty and expensive machinery for registering the adhesive
with the flange on the tray.
Similarly, lids with an adhesive laminate or fully coated with
adhesive to minimize the problem of registration, could present an
intolerable exposure of the adhesive to the contents. This could
cause difficulty not only from the possible contamination of the
contents with toxic materials, but also may cause adhesion of the
contents, e.g., surgical instruments subsequent to heating, as for
sterilization.
Apparatus for application of the adhesive to the rim of the tray is
schematically illustrated in FIG. 6. Conveyor 20 driven by rolls 21
and 22 engages trays 10, either singly or in groups of two or more
and moves them in kiss contact with applicator roll 23. Adhesive,
as described, is applied to roll 23 by hopper 25 with the coating
thickness controlled by doctor blade 26. The adhesive may be
applied as a melt, in which case roll 23 is heated to the softening
temperature of the adhesive, or quite effectively, it may be
applied from a toluene solution, in which case applicator roll 23
is at ambient temperatures. With the solvent applied adhesive it is
desirable to employ a heated drying tunnel to remove the solvent
from the adhesive.
Specifically, the instant invention in one embodiment provides a
container for baking and cooking which comprises:
a substantially flat bottom part;
at least four upstanding sides connected to the flat bottom
part;
the container being formed of substantially amorphous polyethylene
terephthalate; and
a lid of polyethylene terephthalate secured thereto.
In another embodiment, this invention is a container for baking and
cooking which comprises:
a dish-like tray of polyethylene terephthalate, the tray having a
bottom part and a body part;
an outwardly projecting peripheral flange having an upper surface
on the upper extremity of the body part;
a substantially flat cover part having lower surface parts
substantially coextensive with the flange and in abutting contact
therewith;
a thin coating of a heat-activated adhesive only on the upper
surface of the flange;
the adhesive having a melt flow of less than 0.2 g. in 10 min. at
160.degree.C. and a surface free energy of less than 30 dynes.
And, a method is provided for cooking in these novel containers,
which cooking method includes the steps of: forming a container
formed from a tray and lid both of polyethylene terephthalate and
bonded thereto by an adhesive by the methods previously described;
placing food in the container; and, cooking the food at a
temperature of up to about 400.degree.F.
This invention further is a method of sealing a polyethylene
terephthalate lid to the flange of a polyethylene terephthalate
tray to provide a critical bond between the lid and tray. This bond
or seal is such that during cooking at temperatures up to at least
400.degree.F. the container remains hermetically sealed and, after
cooking, the lid may be easily peeled from the tray. The tray may
be suitably formed by heating the material from which the tray is
formed (i.e., the container or tray blank) and by pressing the
heated material to form such tray with its bottom part and body
part with the flange connected thereto. The specific type of
polyethylene terephthalate material used in the tray is
critical.
In forming the tray, the portions of it except the flange may be
oriented. Due to lack of orientation the flange, unless it has
specific properties, tends to crack. Such properties for the
polyethylene terephthalate material which prevents this cracking
include an intrinsic viscosity of at least 0.65.
The "intrinsic viscosity" of the polyethylene terephthalate of the
tray of this invention is critical to the practice of such
invention. It has been discovered that such "intrinsic viscosity"
must be above 0.65, for reasons further to be explained.
The intrinsic viscosity of polyethylene terephthalate of the
present invention is determined in a trifluoroacetic acid-methylene
chloride solvent system, since dissolution times are prohibitively
long in the tetrachloroethane-phenol solvent system used for
conventional polyethylene terephthalate. For this determination the
relative viscosity (nr) of the present polymer is determined in a 1
percent solution at 30.degree.C. in a solvent comprising 25 parts
by weight trifluoroacetic acid (TFA) and 75 parts by weight
methylene chloride (Ch.sub.2 C1.sub.2). The relative viscosity in
this solvent (relative viscosity is the flow time of the solution
through a capillary viscometer divided by the flow time of the
solvent) is converted to the relative viscosity which would be
obtained in a conventional solvent of 0.6 parts by weight
1,1,2,2-tetrachloroethane (TCE) and 1 part by weight phenol at
25.degree.C. Using the empirical equation nr(TCE-phenol) = 1.0324
nr (TFA--CH.sub.2 C1.sub.2) - 0.19, the relative viscosity in
TCE-phenol is calculated. From this relative viscosity the
intrinsic viscosity is determined from the experimentally
constructed table: INTRINSIC VS. RELATIVE VISCOSITY POLYETHYLENE
TEREPHTHALATE IN TCE/PHENOL Relative Intrinsic Relative Intrinsic
______________________________________ 1.20 0.19 1.82 0.64 1.30
0.26 1.84 0.66 1.40 0.34 1.86 0.67 1.50 0.42 1.88 0.68 1.52 0.44
1.90 0.70 1.54 0.46 1.92 0.71 1.56 0.48 1.94 0.73 1.58 0.49 1.96
0.73 1.60 0.50 1.98 0.75 1.62 0.51 2.00 0.76 1.64 0.53 2.04 0.78
1.68 0.55 2.08 0.80 1.70 0.56 2.12 0.82 1.72 0.57 2.16 0.85 1.74
0.58 2.20 0.87 1.76 0.60 2.30 0.93 1.78 0.61 2.40 0.98 1.80 0.62
2.42 1.00 ______________________________________ (This table is
extrapolated for relative viscosities above 2.42, intrinsi greater
than 1.00).
Alternatively, intrinsic viscosity can be determined in a solution
of polyethylene terephthalate in a trifluoro-acetic-acid-methylene
chloride solvent and converted to the value in
tetrachloroethylene-phenol by an empirical equation obtained from
multi-point viscosity determinations of film of the prior art and
film of the present invention:
[.eta.].sub.TCEP =1.020 [.eta.].sub.TFAMC -0.09
in which
[.eta.].sub.TCEP =the intrinsic viscosity of polyethylene
terephthalate in a solvent comprising 40 parts by weight
tetrachloroethane and 60 parts by weight phenol;
[.eta.].sub.TFAMC =the intrinsic viscosity of polyethylene
terephthalate in a solvent comprising 25 parts by volume
trifluoroacetic acid, 75 parts by volume methylene chloride.
The polyethylene terephthalate material of this invention is formed
from a polymer formed by the condensation reaction of ethyl glycol
and terephthalic acid. Particulars of this material are shown and
described in U.S. Pat. No. 2,465,319 to Whinfield and Dickson. More
specifically, polyethylene terephthalate useful in preparing the
structure of this invention includes (a) polymers wherein at least
about 97 percent of the polymer contains the repeating ethylene
terephthalate units of the formula: ##SPC1##
with the remainder being minor amounts of ester-forming
components.
The polyethylene terephthalate material of this invention also
includes blends of polyethylene terephthalate with a different
polymer compatible therewith wherein the blend contains at least 70
percent polyethylene terephthalate, as defined above.
This material does not get brittle with age, it has a long shelf
life; it provides a permanent, nonyellowing, dimensionally stable
base and has excellent resistance to grease, oxygen and moisture.
Further, it is resistant to initial tear and scratching and can
withstand heavy impact and prolonged flexing. These and other
properties are of great value in packaging applications.
This invention, then, is a container comprising a tray of
polyethylene terephthalate and a lid of polyethylene terephthalate
bonded thereto with an adhesive applied only to an upper surface of
the tray, the adhesive being free from flow at cooking temperatures
up to about 400.degree.F. and having a bond strength at such
cooking temperatures sufficient to prevent leakage from the tray
and the lid being peelable from the tray without tearing after
cooking at such cooking temperatures.
The bond strength is greater than 100 grams/inch at 400.degree.F.
and less than 500 grams/inch at room temperature.
The container of this invention comprises a dish-like tray of
polyethylene terephthalate, the tray having a bottom part and a
body part and an outwardly projecting peripheral flange having an
upper surface on the upper extremity of the body part; an adhesive
only on the upper surface of the flange, the adhesive having a melt
flow of less than 0.2 grams at 320.degree.F. (160.degree.C.) for 10
minutes and a surface free energy of less than 30 dynes; and, a
substantially flat cover part having lower surface parts
substantially coextensive with and bonded to the flange by the
adhesive.
A method of making this container includes the steps of: forming a
tray having a bottom part and a body part with a flange having an
upper surface connected to such body part, the tray being of
polyethylene terephthalate; coating an adhesive onto the upper
surface only of the flange of the tray; placing a lid of
polyethylene terephthalate having lower outer surface parts in
abutting contact with the adhesive and sealing the abutting surface
portions of the lid and tray together.
The adhesive is a composition consisting essentially of (a) 30
percent to 40 percent by weight of a petroleum wax and (b) 70
percent to 60 percent by weight of an ethylene copolymer.
The wax and ethylene copolymer together constitute at least 80
percent of the total weight of such composition.
The ethylene copolymer is of an acid terpolymer resin consisting of
an interpolymer of ethylene, vinyl acetate, and an organic
acid.
The adhesive is a composition consisting essentially of (a) at
least 30 percent by weight of a petroleum wax and (b) an ethylene
copolymer and wherein such wax and ethylene copolymer constitute at
least 80 percent of the total weight of the composition.
This invention further is a method including the steps of: placing
food in a tray formed of polyethylene terephthalate; coating an
adhesive only onto an upper surface of such tray; bonding a lid
formed of polyethylene terephthalate to the upper surface of such
tray by means of such adhesive; cooking the food contained in the
tray at a temperature of up to about 400.degree.F.; and peeling the
lid from the tray after cooking.
The bond between the tray and lid is hermetically tight when
cooking the food contained in the tray at a temperature of up to
about 400.degree.F. and wherein the bond between the tray and lid
is of peelable strength after cooking at up to about such
400.degree.F. temperature and at room temperature whereby the lid
may be peeled from the tray without tearing.
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