Note: Descriptions are shown in the official language in which they were submitted.
1~6~78
BACKGROUND OF THE INVENTION
This invention relates to packages which are opened by
peeling one packaging member away from another. It pertains
more particularly to packaging which comprises a generally
rigid formed tray and a generally more flexible sheet
material adhered to the tray to form a closing and sealing
lid. In such packages, the packages typically open by
peeling the lid away from the formed tray.
The use of formed trays and peelable lids for those
trays has, in general, been known for some time. For
example, jellies, jams, and dressings are commonly packaged
in individual servings for institutional use in formed trays
with peelable sheet material used for lids sealed to the
trays. More recently, larger trays and accompanying lids
have been used in the packaging of complete servings of
prepared food, or individually packaged complete meals.
Other uses for such packaging can, of course, be found.
This invention pertains most particularly to those
packages which are to be subjected to relatively severe
processing conditions either at the point where the packages
is being filled and sealed or at the point where the package
and/or its contents is to be used. Such packaging typically
encounters substantial heat at the point where the package is
filled and sealed, and may be subjected to significant heat
at the point where the package is used. At the point of end
use, the package may be reheated. At the point of packaging,
1~36878
the package may be subjected to, for example, hot filled
product at approximately 190F. Processing at the point of
packaging may also include processing of the package at, for
example, 250F or more.
To the extent packaging can be designed which will
withstand such processing and use conditions at the point of
packaging and/or the point of use, these packages find
substantial applications, particularly in the packaging of
food products.
In the packaging of food products it is known to
provide, in the packaging, certain materials which will
protect the contained food products from the permeation of
substances into the package from the outside environment.
Substances which can have undesirable affects on the
contained product are, for example, oxygen, moisture,
contaminants, and light. It is especially important, in some
applications, to desirably protect the package contents from
oxygen, moisture and light, in addition to typical containing
and protecting functions common to most packaging.
Sheet materials for providing these desired protective
properties are known and available, and are commonly used in
some applications of packaging materials. For example,
moisture penetration may be prevented by the use of certain
of the olefin polymers such as the polyethylenes, ethylene
copolymers, and polypropylenes. These are only exemplary of
the known moisture barrier materials. Oxygen penetration can
be successfully impeded through the use of various of the
1336878
vinylidene chloride copolymers, the amide polymers, and the
vinyl alcohol polymers.
Light penetration may be prevented by the use of, for
example, metal foils, papers, and pigmented polymers.
It should be understood that the description herein of
the various ways of preventing the entrance of unwanted
factors from the outside environment is only indicative of
the conventional art. The recitation herein is not seen to
be, and is not intended to be, exhaustive, but rather is
exemplary of the existing technology.
As used herein the word "polymer" generally refers to
homopolymers and copolymers unless otherwise specifically
stated.
A particular problem is encountered in providing
packaging for the severe process conditions of, for example,
retort processing or the end use environments, in that there
are only a limited number of packaging materials which can be
used to provide the combined properties of (i) barrier
functions, (ii) the ability to survive the severe processing
and use conditions, (iii) safety for use in the packaging of
food, and (iv) holding the package closed and containing the
product.
of special concern is providing a heat sealable layer on
the exterior surface of the flexible sheet material which
forms the lid of the package, and which can be heat sealed
to, for example, the peripheral flange of the tray and
subsequently be removed with a moderate amount of force. The
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lid cannot, however, be so loosely sealed to the tray that it
is inadvertently or otherwise undesirably released from the
tray prior to an intentional opening of the package. Thus
are there the requirements to be able to seal the package
s tightly closed in combination with providing a readily
openable package. The closure must provide the necessary
protection from the outside environment while the package is
intended to be closed. The pacXage should be susceptible to
being opened with a moderate amount of force by a smooth and
constant pulling. The opening should be smooth and steady
when a constant opening force is applied, such that no jerky
motions or sudden releases of adhesion are encountered.
Susceptibility to leaking through the seal area is
unacceptable.
It is well known, particularly from the technological
work done on retort packaging, that polypropylene is useful
as a sealant material where a high temperature environment is
contemplated, such as retort processing. Applicants are
aware of the use of sheet materials which employ
polypropylene and propylene ethylene copolymers as the
sealant layer in tray lids. A problem with using
polypropylene or its copolymers as the sealant material is
that the peel force required to remove the lid from the tray
is excessively high. This problem has been recognized in the
industry and certain modifications of the polypropylene have
been made in attempts to reduce the high level of adhesion
between the polypropylene sealant layer and the flange of the
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tray. In one case, ethylene vinyl acetate (EVA) has been
added to the polypropylene in a quantity estimated at 3 to 5
percent by weight. Japanese Patent Open number 77465; 1982
teaches a blend of a propylene ethylene copolymer, a high
density polyethylene, and EVA. That document teaches that
polypropylene and high density polyethylene have no
compatibility, and need the addition of the ethylene vinyl
acetate.
Japanese patent open 1672; 1983 teaches a blend of
polypropylene and polyethylene as the sealant layer. The
polyethylene is defined as having a density below .935.
Another reference which teaches blending of material into
polypropylene is U.S. Patent 4,189,519 Ticknor, which teaches
the blending of an ethylene ester copolymer into the
polybutylene to modify its peel properties.
U.S. Patent 4,183,845 teaches blends of polypropylene
and polystyrene as a coating composition for use on
paperboard.
It is an object of this invention to provide novel
polymeric compositions, novel sheet materials, and novel
packaging made from those compositions and sheet materials.
The sheet materials in general have the capability to be heat
sealed to preformed generally rigid trays, and to withstand
retort processing conditions of up to 250 degrees Fahrenheit
for approximately 30 minutes. The package may subsequently
be opened by peeling of the novel sheet material from the
tray, accompanied by cohesive failure of a layer within the
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sheet material, and wherein the cohesive failure is limited
to an area generally confined to that area of the sheet
material which functions to form the heat seal about the
tray. Thus this invention is not directed at nor concerned
with cohesive failures which propagate to areas of the
package lid which do not in general form a portion of the
seal between the sheet material and the remainder of the
package such as the tray.
It is a specific object of the invention to provide
novel compositions of matter which are two component blends
of polyethylene having a density of at least .940 and
propylene polymers, either homopolymers or copolymers.
It is another specific objective of the invention to
provide novel sheet materials, using as a layer thereof,
novel blends of high density polyethylene and a polypropylene
polymer.
It is yet another specific object of the invention to
provide novel packaging made from the sheet materials of the
invention, the packaging having retort processing capability
in combination with peelable seals whose peeling is
accompanied by cohesive failure of one of the layers of the
sheet material, the layer exhibiting the cohesive failure
being a blend of high density polyethylene and a propylene
polymer.
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1336878
SUMMARY OF THE INVENTION
Certain of the objects of the invention are attained in
a composition of matter which is a polymeric blend consisting
essentially of 65% to 95% by weight of a propylene polymer
and conversely 35% to 5% of an ethylene polymer having a
density of at least .940.
one preferred family of the blend compositions consists
essentially of 85% to 95% by weight of the propylene polymer
and 15% to 5% of the ethylene polymer. In this family the
propylene polymer comprises a copolymer of 75 to 85 mole
percent propylene and conversely 25 to 15 mole percent
ethylene.
Another preferred family of the blend compositions of
the invention consists essentially of 65% to 75% by weight of
the propylene polymer and conversely 35% to 25% by weight of
the ethylene polymer. In this family of compositions the
propylene polymer comprises a copolymer of about 95 to 98
mole percent propylene and conversely about 5 to 2 mole
percent ethylene.
Others of the objectives of the invention are seen in a
multiple layer sheet material comprising a first layer of a
blend of a propylene polymer and an ethylene polymer having a
density of at least about .940, a second layer of an adhesive
promoting material, and a third of layer of a barrier
material selected from the group consisting of metal foil,
vinylidene chloride copolymer, vinyl alcohol copolymer,
1~3~i878
ethylene polymers, and propylene polymers.
In one preferred family of these films there is a layer
of adhesive, preferably a propylene polymer, between the
first and second layers.
In another family of these sheet materials there is a
layer of a propylene polymer on the one surface of the first
layer which is on the opposite side of the first layer from
the second layer. The sheet material may comprise both the
adhesive layer between the first and second layers and the
propylene layer which is on the other side of the blend layer
from the second layer. The combination of the first blend
layer and the one or two of the above recited layers on
either surface of the blend layer comprises in general that
portion of the sheet material which participates most
actively in the formation of the heat seal between the sheet
material and the tray. Thus, those layers are referred to
singly and in combination herein as the "sealant layer".
The family of sealant layers of this invention is seen
to have application for use with a substantial variety of
sheet material substructures, which substructures provide
other desirable properties for the package.
In one such substructure, the second layer is a primer
comprising a carboxy modified olefin polymer, the third layer
is metal foil, and a fourth layer of adhesive joins the third
layer to an abuse resistant layer.
In another such substructure, the second layer is a
catalyzed urethane adhesive, the third layer is metal foil,
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and a fourth layer of adhesive joins the third layer to an
abuse resistant layer.
In yet another substructure, the second layer is either
a carboxy modified olefin polymer, or a catalyzed urethane
adhesive or primer; the third layer is a vinylidene chloride
copolymer, and the fourth layer of an adhesive joins the
third layer to an abuse resistant layer.
In still another substructure, the second layer is
carboxy modified olefin polymer, the third layer is a vinyl
alcohol copolymer, and a fourth layer of carboxy modified
olefin polymer adhesive joins the third layer to an abuse
resistant layer. In another application which is closely
related, layers of amide polymer are interposed on either
side of the vinyl alcohol copolymer between the vinyl alcohol
copolymer and the respective adhesive layers. In either
case, the second and fourth adhesive layers can alternatively
be conventional laminating-type adhesives such as catalyzed
urethane adhesives.
The blends of propylene polymer and high density
polyethylene recited as the novel compositions herein are, in
general, useful in the sheet materials of the invention.
Specifically, the blend compositions are 65% to 95% by weight
of the propylene polymer and 35% to 5% of the high density
ethylene polymer. Where the propylene polymer contains a
higher amount of ethylene, such as over 15 mole percent, it
is preferred that thè blend ratio be in the range of 85% to
95% of the propylene polymer and 15% to 5% by weight of the
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ethylene polymer. Copolymers of propylene and ethylene
comprising about 75 to about 95 mole percent propylene and
conversely about 25 to 5 mole percent ethylene are typically
block copolymers. The copolymers used in this invention
which contain the higher amounts of ethylene are typically,
though not necessarily, block copolymers. The more preferred
of the higher ethylene content copolymers are 75 to 85 mole
percent propylene and 25 to 15 mole percent ethylene.
Where the propylene polymer contains a lesser amount of
ethylene, it is preferred that the blend composition be 65%
to 75% by weight of the propylene polymer and 35% to 25% of
the ethylene polymer. Copolymers of polypropylene and
ethylene comprising at least about 95 mole percent propylene
and conversely no more than about 5 mole percent ethylene are
typically random copolymers. The copolymers used in this
invention which contain the lesser amounts of ethylene are
typically, though not necessarily, random copolymers.
The sheet materials of the invention can be used
particularly advantageously as heat sealable lid sheet
material for covering formed trays, and for forming seals to
the formed trays to form and seal the packages. The seals
are formed by the sealant layer, thus joining the sheet
material to the formed trays in closing and sealing the
packages.
In packages made with sheet material of the invention
the lid is peelable from the surface to which it is sealed
within the sealant layer, thus joining context that the lid
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1336878
peels away from that surface by means of cohesive failure of
the sealant material in that area of the lid which forms the
seal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE l is a pictorial view of a package made with
compositions and sheet materials of the invention, showing
the package with the lid partially peeled open.
FIGURE 2 is a cross-section of the simplest embodiment
of the sheet materials of the invention.
FIGURES 3, 4 and 5 are cross-sections as in FIGURE 2 and
showing alternate sealant layer constructions.
FIGURES 6, 7, and 8 show sheet materials of the
invention wherein additional layer substructures are added to
the sealant layers as disclosed and described with respect to
FIGURES 2-5.
FIGURE 9 shows, in cross-section, the detail of the
structuring of the layers and sublayers of a more complex
sheet material of the invention.
FIGURE 10 is a cross-section of a portion of the package
of FIGURE l taken at 10-10 of FIGURE l.
FIGURE 10A is a fragmentary cross-section of a package
similar to the package in FIGURE 10 and showing, in general,
alternate modes of peeling of lid material.
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DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
The invention is illustrated in FIGURE 1 as a package
comprising a tray 2 having an outwardly directed flange 3
surrounding its open top, and a multiple layer sheet material
10 sealed to the flange 3 of tray 2 at seal area 4 to thereby
provide a closure to the open top of the tray. The pattern
of seal 4 is so selected as to provide a closing and sealing
function to the package when the seal is fabricated. Access
to the interior portion 5 of the package is then desirably
obtained by removing the lid as by peeling it off the package
-- and, in the process, breaking the seal and removing the
entire cross-section of the lid sheet material from over the
opening in the top of the tray.
FIGURE 1 illustrates tray 2 having the flange 3 and seal
area 4. One corner of the sheet material 10 has been raised
and peeled away from flange 3 to expose a small portion of
the interior 5 of the tray to the outside environment.
Continued pulling and peeling is affective to remove the lid
entirely from the tray.
As indicated in FIGURE 1 the seal area 4 has been
separated into two portions 4A and 4B in that portion of the
sheet material which has been peeled away from the tray.
FIGURE 2 illustrates the simplest embodiments of the
sheet materials of the invention. Layer 12 is the sealant
layer. Layer 16 is a barrier layer. Layer 14 is an adhesion
promoting layer which promotes adhesion of layers 12 and 16
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L336878
to each other. The composition of layer 12 is a blend of a
propylene polymer and an ethylene polymer having a density of
at least .940. The sealant layer 12 may be used with any of
a broad range of subcombinations of other layers for
providing the other properties desirable in the sheet
material. Thus, it will be seen hereinafter, that the
sealant layer 12 is combined with a plurality of exemplary
layer subcombinatiGns to make multipie layer sheet materials
having substantially varying properties. In some cases it is
combined with layers which provide an oxygen barrier
capability. It may be combined with layers which provide a
light barrier capability. Similarly it may be combined with
layers which provide a moisture barrier capability. It may
also be combined with layers which provide a combination of
the above, as well as barriers to other gases, liquids, etc.
As seen in FIGURE 2, layer 16 represents a broad range
of barrier layers, such as metal foil, the vinylidene
chloride copolymers, vinyl alcohol copolymers, and olefin
polymers. The selection of the composition for layer 16 is
limited only by the ability to provide an adhesive promoting
material 14 which will provide good adhesion between layers
12 and 16. Additional layers may be provided between layers
12 and 16, especially to assist layer 14 in promotion of the
desired adhesion. It is, of course, possible to provide
additional layers on that side of layer 16 which forms the
apparent exterior surface of the sheet material in FIGURE 2.
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13~687~
Reference is now made to the composition of layer 12.
In addressing the technical issues surrounding the
composition of layer 12, it is seen that heat tolerance and
heat stability are critical. For those applications
requiring these parameters, propylene polymers are seen to be
excellent for tolerating the severe processing conditions.
Also, the sealability of propylene polymers to for example,
formed trays having a propylene-based surface, is excellent.
Unfortunately, to the extent that propylene homopolymer or
propylene copolymer is used by itself as the sealant layer 12
for sheet material 10, the adhesion between sealant layer 12
and the flange 3 of the tray, and the cohesive strength of
layer 12, are so strong that the ability to open the package
is substantially impeded.
The applicants herein have found a particularly
advantageous capability to control the peeling of the sheet
material, while providing an acceptably strong seal for
protecting the contents on the interior of the package. This
capability is achieved by providing a special family of blend
compositions of propylene for layer 12.
In general, the composition of layer 12 is a polymeric
blend of 65% to 95% by weight of a propylene homopolymer or
copolymer and conversely 35% to 5% of an ethylene polymer
having a density of at least .940, and generally known as a
high density polyethylene (HDPE). To the extent the
propylene polymer is a homopolymer or a copolymer containing
up to about 5% ethylene, the larger fractions of HDPE are
1336878
preferred in the blend composition, up to about 30% HDPE. To
the extent the propylene polymer contains more ethylene, such
as containing 20% ethylene and 80% propylene, then smaller
fractions of HDPE are preferred in the blend composition,
such as 10%.
In general, as increasing amounts of propylene are used
in the blend, the force required to peel the package open
becomes commensurately greater. To the extent that the sheet
material 10 has excellent interlayer adhesion and appropriate
layer cohesive strengths, these greater peel strengths are
acceptable, and thus up to about 90% propylene polymer can be
used where the propylene contains up to 5% ethylene. To the
extent the interlayer adhesion within sheet material 10 is of
a lesser degree, using high amounts of propylene (for example
over 80% of a copolymer having 95% or more propylene) can
result in delamination within the sheet material 10 when an
attempt is made to peel sheet material 10 from the closed and
sealed package. Thus where interlayer adhesions are more
moderate, it is desirable to use less propylene polymer in
the blend, and respectively more polyethylene. Preferred
compositions in these embodiments range between 70% and 80%
by weight polypropylene and 30% to 20% HDPE. As the fraction
of propylene polymer decreases below 70%, the seal strength
between layer 12 and flange 3 is reduced to the point where
shock resistance of the sealed package is reduced and the
preferred seal strength is not achieved.
The adhesion between the layers in sheet material 10 is
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1336878
affected by the tendency of the sheet material to elongate
under elongation stresses. It can also be affected by
compression of one or more of the layers during the heat
sealing process. To the extent the sheet material can be
elongated, the elongation puts stresses on the interfaces
between the several layers, as each of the differing
individual layers responds to the stress according to the
properties of its composition. This tends to weaken the
adhesion at those respective interfaces. Layer compression
has a similar affect, in applying lateral and longitudinal
stresses at the layer interfaces. Thus those sheet
structures 10 which can be elongated, or undergo significant
compression during heat sealing, generally work best when
they are combined with a layer 12 which comprises a blend of
propylene and ethylene polymers in the lower end of the range
of propylene, wherein the lid can be peeled off with more
modest forces. On the other hand, since higher fractions of
propylene do yield packages having stronger seals and
stronger cohesion on layer 12, higher fractions of propylene
are preferred where their use can be tolerated. Thus, the
amount of propylene in the blend of layer 12 is generally in
the higher end of the range for those sheet materials which
have a stabilizing layer such as paper or metal foil.
The amount of the HDPE used in the blend of layer 12 is
preferably selected with reference to the nature of the
propylene polymer which is contemplated for use in the blend
and the adhesive and cohesive strengths in and between the
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1336878
several layers in the sheet material. A relatively larger
amount of HDPE is used in the blend where the propylene
content of the propylene polymer is in the upper portion of
its range. To the extent the amount of propylene in the
copolymer is lessened, lesser amounts of HDPE are used in the
blend. In most cases, the propylene polymer is a copolymer
having at least a small amount of ethylene, ie. 2%, in its
composition. Amounts of up to about 5% ethylene are
generally considered as random copolymers. As the amount of
ethylene in the copolymer is increased over 5%, the structure
of the polymer usually becomes more like that of a block
copolymer and the material takes on the amorphous
characteristics of a block copolymer. There is of course,
the common knowledge within the industry that there is a
transition range between approximately 5% ethylene and
approximately 10% ethylene wherein the properties of the
resulting copolymer may primarily be those of the random
copolymer or those of the block copolymer, depending
primarily on the process of forming and recovering the
copolymer. Further, the copolymer may exhibit a combination
of the properties of both random copolymer and block
copolymer.
Applicants herein have tested many compositions for use
in layer 12 and have identified two specific compositions for
use in layer 12 which compositions are seen to be preferred
for those embodiments which are disclosed hereinafter. The
specific preferred composition depends on a number of
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1336878
factors. A primary factor in selecting that composition
resides in the identification and characteristics of the
other layers in the sheet material. For those sheet
materials which use a layer of metal foil, it is generally
preferred that layer 12 comprise 85% to 95% by weight of
propylene polymer and conversely 15% to 5% by weight HDPE.
The propylene polymer in this case is a copolymer of 75 to 85
percent propylene and conversely 25 to 15 percent ethylene.
In those sheet materials 10 which do not include a
stabilizing layer such as paper or foil -- and herein
addressing primarily those sheet materials wherein all the
layers are polymeric or adhesives, a preferred composition
for layer 12 is 65% to 85% propylene polymer and conversely
35% to 15% ethylene polymer. The most preferred range is
about 70% to about 80% propylene polymer and conversely about
30% to about 20% HDPE. In these blend compositions, the
propylene polymer is preferably a copolymer of 95 to 98
percent propylene and conversely 5 to 2 percent ethylene.
For purposes of this invention, preferred copolymers
having larger fractions of ethylene are those comprising 75%
to 85% propylene and conversely 25% to 15% ethylene.
Copolymer composition percentages, in all cases herein, are
mole percentages.
FIGURES 3, 4, and 5 show alternate structures for the
sealant layer 12, in which the sealant layer comprises a
combination of two or three layers. In all of FIGURES 3, 4,
and 5, the blend composition disclosed for FIGURE 2 is
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1336878
indicated as layer 12A. Layer 12Bl serves as an adhesive
between layer 12A and layers 14 and 16. Layer 12B2 in
FIGURES 3, 4, and 5 represents a layer of propylene
homopolymer or propylene copolymer, and wherein the propylene
copolymer preferably contain a lesser amount of ethylene.
Acceptable, but less preferred material for layer 12B2 is a
copolymer of propylene containing a greater amount of
ethylene.
As seen in FIGURE 3, layer 12Bl is interposed between
blend layer 12A and adhesion promoting layer 14. Layer 12Bl
in general serves to improve the adhesion of layer 12A to
layer 16 through adhesion promoting layer 14. Where layer 14
is a primer such as a carboxy modified olefin polymer and
layer 16 is a metal foil, layer 12Bl is preferred to be a
propylene polymer and to have lesser amounts of ethylene. In
some cases a homopolymer propylene is preferred, to improve
the adhesion through layer 14 to foil layer 16. Other
adhesive compositions may be used for layer 12B1 so long as
they serve the adhesive function. Exemplary materials are
catalyzed urethane adhesives and olefin polymers.
FIGURE 4 is seen to be a derivative of FIGURE 3 in that
an additional layer 1252 has been added to the exterior of
the surface of layer 12A to form a covering thereover. Layer
12B2 is, in general. relatively thin.
Referring to FIGURE 5, the sealant layer 12 is a
combination of blend layer 12A and an exterior covering layer
12B2 corresponding to layer 12B2 in FIGURE 4. The difference
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1336878
between FIGURES 4 and 5 is that layer 12Bl of FIGURE 4 has
been omitted in FIGURE 5.
Referring now to the sheet material of FIGURE 3, the
peeling of the sheet structure 10 of FIGURE 3 results in
cohesive failure within layer 12A as seen at layer 12 in
FIGURE 9, and shown in dashed outline in FIGURE 3. Referring
to the sheet material of FIGURES 4 and 5, the tearing of the
sheet material when it is removed from the package comprises
(i) a tearing through of layer 12B2 into layer 12A, (ii)
cohesive failure within layer 12A and (iii) a tearing back
through of layer 12B2 to the outer surface 12-S of the sheet
material. The general paths of the preferred propagations
are shown in dashed outline in each of FIGURES 3, 4, and 5.
FIGURES 6, 7, and 8 generally show exemplary sheet
structures using the sealant structure of the invention. In
all of FIGURES 6, 7, and 8 the sealant layer is shown as a
single layer 12. In these structures, the illustrated
sealant layer 12 represents all of the sealant layer-
structures as shown in FIGURES 2-5.
Finally, FIGURE 9 shows the sheet structure of FIGURE 8
with the more complex three layer sealant material structure
combined with it, to indicate the overall complexity of the
sheet structures of the invention when using the more complex
sealant structure which is seen in FIGURE 4.
Returning now to FIGURE 6, layer 12 is the sealant
layer. Layer 14 is a carboxy modified polypropylene primer.
Layer 16 is metal foil. Layer 18 is a conventionally known
- 20 -
1336878
curing type urethane adhesive. Layer 20 is an abuse
resistant layer such as biaxially oriented nylon or
polyethylene terephthalate.
In those sheet materials incorporating therein a layer
16 of foil, the preferred composition for layer 12 is (i) 85%
to 95% by weight of a copolymer of propylene and ethylene
containing 75% to 85% propylene and 25% to 15% ethylene and
(ii) 15~ to 5% by weight HDPE.
While layer 14 has been indicated as a polyolefin-based
primer, preferably polypropylene, an adhesive may be used in
place of, or in combination with, the primer layer 14.
Exemplary of adhesives which are acceptable, and particularly
for use in contact with food products, are the urethane
adhesives which are catalyzed with aliphatic type catalyst.
In FIGURE 7, layer 12 is the sealant layer. Layer 14 is
an adhesive layer. Layer 16 is a vinylidene chloride
copolymer. Commercially available vinylidene chloride
copolymers are known to be effective oxygen barrier and water
vapor barrier materials, and any of the commercially
available ones of those polymers are acceptable. Exemplary
of those polymers are vinylidene chloride-vinyl chloride
copolymer and vinylidene chloride-methylacrylate copolymer.
Layer 18 is an adhesive. Layer 20 is an abuse resistant
material such as biaxially oriented nylon or polyethylene
terephthalate. The selection of the composition for adhesive
layers 14 and 18 is known to those skilled in the art. In
some cases the preferred adhesives are urethane type
- 21 -
, . .
, ~
~ ~
13~687~
adhesives. Particularly with respect to layer 14 the
adhesive is one which is catalyzed by an aliphatic type
catalyst such that the resulting cured adhesive is acceptable
for use in a food package. Also acceptable for layer 14 are
carboxy modified olefins.
Layers 14 and 18 of FIGURE 7 may alternatively be
extrudable polymeric materials, in which case layers 14, 16,
and 18 may be coextruded, optionally with additional layers
in the coextrusion. The combination of the coextruded layers
may then be adhesively mounted to layers 12 and 20 through
conventional laminating processes, optionally with the use of
additional adhesion promoting layers.
In a specific variation on the embodiment represented by
FIGURE 7, a primer such as a urethane primer is interposed
between the vinylidene chloride copolymer of layer 16 and
layer 14. Layer 14 is a propylene ethylene copolymer.
Formation of the structure includes the process of applying
the primer to the surface of the vinylidene chloride
copolymer followed by extrusion laminating the primed surface
to a preformed film of the blend composition of layer 12,
using the propylene ethylene composition which forms layer 14
as the extrusion laminant.
In FIGURE 8, layer 12 is the sealant layer. Layer 14 is
an adhesive layer, preferably a carboxy modified olefin
adhesive. Preferred adhesive materials are carboxy modified
ethylenes, carboxy modified ethylene vinyl acetates, and
carboxy modified propylenes. Exemplary of these materials
- 22 -
1336878
are the Admers from Mitsui, the CXA polymers for Dupont, and
the Plexars available from Chemplex Company. Layer 22 is
amide polymer such as nylon. Layer 16 is a vinyl alcohol
polymer such as polyvinyl alcohol or ethylene vinyl alcohol
copolymer. Layer 24 is an amide polymer such as nylon. Layer
18 is an adhesive such as a urethane curing type adhesive.
Layer 20 is an abuse resistant layer such as biaxially
oriented nylon or polyethylene terephthalate. In some
embodiments of the FIGURE 8 structure, layers 22 and 24 may
be omitted such that layers 14 and 18 are in direct
interfacial contact with barrier layer 16.
Particularly with respect to FIGURE 8, the barrier layer
16 may be modified by blending into the vinyl alcohol other
materials which enhance its properties. It is known, for
example, to blend amide polymers into an ethylene vinyl
alcohol composition, with or without plasticizers in order to
improve certain of the physical properties of the ethylene
vinyl alcohol. Amide polymers known to be blended into
ethylene vinyl alcohol are, for example nylon 6, nylon 66,
and nylon copolymers such as the polyether amides. All
recitations herein of vinyl alcohol polymer compositions
include the blend compositions.
FIGURE 9 is a representation of a more complex structure
of FIGURE 8 wherein the sealant layer 12 is shown as the
three layer subcombination of 12A, 12Bl, and 12B2 as seen in
FIGURE 4. FIGURE 4 shows the more complex nature of the
sealant layer 12 while showing a simplified representation of
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the other layers 14 and 16. FIGURE 8, on the other hand,
shows a more complex representation of the layers 14 and 16,
wherein the layers 18, 20, and 22 have been added. FIGURE 9
shows the combination, then, of FIGURES 4 and 8, wherein both
the sealant layer complexity is shown as in FIGURE 4 and the
nonsealant layer complexity is shown as in FIGURE 8. Turning
now directly to FIGURE 9, layer 12A is the blend composition
of the sealant layer. Layer 12Bl is an adhesion promoting
material, and is preferably propylene based. Layer 12B2 is a
propylene composition. Layer 14 is an adhesive. Layer 22 is
an amide polymer. Layer 16 is a vinyl alcohol polymer.
Layer 24 is an amide polymer. Layer 18 is a curing urethane
adhesive. Layer 20 is the abuse resistant layer such as
biaxially oriented nylon or polyethylene terephthalate. In
FIGURE 9, layers 12A, 12Bl, and 12B2 are as represented in
FIGURE 4. Layers 14, 16, 18, 20, 22, and 24 are as
represented in FIGURE 8.
Since the most preferred use contemplated for the sheet
structures and packages of the invention is in use in retort
packages for food products, it is significant to address
those test parameters which are required for government
acceptance of the package for use with food. While other
uses for the packages and sheet materials of the invention
are contemplated, and while the requirements for some of
those other uses are not as stringent, and thus the abuse
testing is less severe, it is desirable herein to address the
specific test for the most preferred use. The significant
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test required by the U.S. Department of Agriculture is an
impact test in which the package is dropped on a 15 degree
slope from the vertical, from such a height as to impact a
barrier, perpendicular to the angle of drop, with 20 inch
pounds of impact on the package flange. The test requires
dropping the package twice onto adjacent edges. The drop
test may acceptably be performed with secondary protective
packaging. In testing of preferred embodiments of this
invention, secondary protective packaging is not used. The
determination of a successful test is based on whether the
package leaks or doesn't leak. After the test is complete,
those packages which appear to have passed the test are
subjected to an internal pressure of 5 psi for 60 seconds
under water and observed for escape of contents from the
package. In those packages where no material escapes, the
package is considered to have passed the test.
Referring now to those sheet structures using metal
foil, as in FIGURE 6, it is preferred that the sealant layer
blend be (i) 85 to 95 percent, most preferably 90%, of a
copolymer comprising 75 to 85 percent, most preferably 80% of
propylene and conversely 25% to 15%, most preferably 20% of
ethylene, and (ii) 15% to 5%, most preferably 10%, high
density polyethylene. Up to 20% high density polyethylene
may be used in the blend with its corresponding preferred
minimum of 80% of the propylene copolymer. Where over 20%
HDPE is used there is a notable increase in the amount of
strings or feathered edges left on the tray flange when the
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sheet material (comprising the lid) of the tray is peeled
off. Between 20% and 30% high density polyethylene can be
used with successful opening of the package, albeit with
increasing amounts of strings and/or feather edges left on
the tray flange by the sheet material. At approximately 30%,
and greater amounts, of high density polyethylene,
delamination of the sheet material occurs as seen in FIGURE
lOA along the exemplary lines 25 26, 27, 31, 33, or 35,
leaving all or a portion of layer 12 as a covering on the
package; such that the package is not effectively opened.
Thus over 30% high density polyethylene in layer 12, blended
with a propylene copolymer having over 15~ ethylene, for use
with a foil based sheet structure is no part of this
invention.
Referring now to FIGURES 7-9, and wherein all of the
layers are of a polymeric and/or adhesive nature, there are
no layers in the sheet material which will prevent
substantial elongation of the sheet material when elongation
forces are applied. For these families of sheet materials,
the composition of the sealant layer blend preferably resists
elongation, and comprises 65% to 85%, most preferably 70% to
80% of the propylene polymer and conversely 35% to 15%, most
preferably 30% to 20% of the HDPE. In this case the
propylene polymer is preferred to have between 2% and 5%
ethylene, and most preferably about 4% ethylene. To the
extent the fraction of propylene in the blend composition is
increased over 85%, the tear propagation on peeling the sheet
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material away from the tray tends to progress from the inner
surface 12-S through all the layers along lines such as those
represented by 31, 33, or 35 in FIGURE lOA, to the outer
surface of abuse layer 20. To the extent the amount of
propylene in the blend composition is less than 70%, adhesion
to the flange is insufficient for the closed and sealed
package to survive the impact test previously described,
without protective secondary packag_ng. While such packages
are thus not preferred for use with food products, they do
have acceptable uses for packaging other products and wherein
the package performance criteria are less demanding.
FIGURE 10 illustrates in cross-section a portion of the
tray of FIGURE 1 where the lid sheet material 10 has been
peeled slightly away from the flange 3 of tray 2. Referring
now to FIGURE 10, it is seen that the layer of sheet material
10 which is in closest proximity to flange 3 is layer 12, the
sealant layer. It is seen in FIGURE 10 that layer 12
provides the contact between the tray 2 and the sheet
material 10 at seal area 4. As sheet material 10 is peeled
from the tray flange 3 as seen in FIGURES 1 and 10, a tearing
occurs in sealant layer 12 at the seal area 4 to provide
access to the interior 5 of the container. The peeling of
the sheet material 10 away from flange 3 is accompanied by a
tearing in layer 12 which comprises a cohesive failure of
layer 12 in the area of the seal 4. It is seen that the
tearing progresses from the outer surface 12-S of layer 12,
across an interior portion of layer 12, and back to the outer
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surface 12-S of layer 12. Thus the removal of sheet material
10 from the package by way of peeling it from flange 3
results in a separation of layer 12 in the seal area, such
that a first portion 12-0 of layer 12 remains on the flange 3
and a second portion 12-I of the thickness of layer 12 is
removed with the sheet material 10.
It is seen from FIGURE lOA that it is critical that the
progress of the tearing of layer 12 proceed along the
interior of layer 12 across the width of seal 4 and back to
the interior surface 12-S of layer 12 in order to gain direct
access to the interior of the package. It is specifically
critical that the progression of the tear in layer 12 not
progress (i) along an extended interior path in layer 12 as
at 26 in FIGURE lOA or (ii) to the interface of layers 12 and
14 and along that interfaced across the package, as at 27 in
FIGURE lOA. To the extent these paths of propagation (26 or
27) of the tear are followed, a portion of the sheet material
10 comprising at least a portion of the thickness of layer 12
remains on the package after the peeling has taken place. As
seen in FIGURE lOA, a portion of sealant layer 12 remains on
the tray covering the opening and preventing access to the
contents.
An intermediate and marginally acceptable type of tear
is shown at 28 in FIGURE lOA wherein the tear propagates
along an extended path from the interface of cohesive failure
12-I back toward the surface 12-S of layer 12. The length of
extended path 28 determines the amount of material 29 from
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layer 12 that is left on flange 3 after the removal of the
lid. A particular problem addressed in the art of record,
and particularly Ticknor, is that conventional peelable sheet
materials tend to propagate as shown at 28 in FIGURE lOA, and
leave a plurality of strings, or feathered edges 29 at the
tray flange when the sheet material 10 is peeled away.
A more serious problem occurs when the tear propagates
along a path of indeterminate length 26 either in layer 12,
at the interface of layers 12 and 14, or at another interior
path in sheet material 10. In that event, the access to the
tray is not effectively obtained by attempting to peel the
sheet material away from the flange 3. To the extent the
tear is propagated along a shorter path 28 which eventually
terminates at surface 12-S, access is obtained, but with the
accompanying presence of undesirable amounts of sealant layer
12 remaining on flange 3 attached to that portion 12-O of
layer 12 which remains in the seal area 4 of flange 3. That
portion of layer 12 which remains and extends outside the
seal area 4 usually looks like a plurality of small strings,
or feather edges. These strings and feather edges are
undesirable to the extent that they leave an unappealing
appearance, an appearance of unsanitary conditions with
respect to food, and can provide locations for the breeding
of bacteria and the like.
Thus while extended tears as at 28 are marginally
acceptable, the clean tear as seen in FIGURE 10 is much more
desirable, and such propagations of the tear are a particular
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objective of the preferred embodiments of the invention. To
the extent the embodiments of the invention used are the less
preferred embodiments, a certain amount of extended tear
propagation as at 28 may be encountered.
In some embodiments of the invention, the process
forming the seals at 4 between sheet material 10 and flange 3
of tray 2 has a significant affect on the opening peel
characteristics. Referring now especially to FIGURE 7, it
has been observed that layer 16 of vinylidene chloride
copolymer tends to be significantly thinner after formation
of seal 4 than before formation of the seal. For purposes of
illustration, layer 16 in FIGURE lOA has been shown as
polymeric; and the substantial thinning of layer 16 is seen
at the seal area. The seal area herein includes the entire
thickness of sheet material 10 which overlies the area of
- bonding which exists between layer 12 and flange 3. The
process of forming seal 4 includes the combined application
of heat and pressure over the area that becomes seal 4, so
some reduction in thickness is generally experienced by all
the layers. The degree of reduction, however, of the
vinylidene chloride copolymer of layer 16 is greater than the
reduction of the other layers. To the extent this reduction
in thickness occurs, the sheet material has an increased
tendency for propagation of the tear from surface 12-I along
a path toward and through all the layers to the outer surface
of layer 20. These paths are represented in FIGURE lOA by
dashed lines 31 and 33 which tear more or less directly
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through the sheet material, and by line 35 which tears to an
interface of layer 16, along the interface for a distance,
and from there to the outer surface of layer 20.
The occurrence of the above phenomenon is believed to be
related to the thermal fluid flow properties of the
vinylidene chloride copolymer, combined with the thermal
conditions of formation of heat seal area 4. The vinylidene
chloride copolymer becomes somewhat fluid at about 23CF.
The composition of layer 12, however, requires a temperature
of about 320F. in order to. form a strong heat seal at 4. So
the conditions which provide for formation of a strong seal
have a tendency to heat layer 16 to such a temperature that
its thickness is easily reduced.
The instant problem is attenuated, and can be overcome,
by applying a uniform heat and pressure throughout the seal
area during the formation of the seal 4, and applying that
minimum amount of heat and pressure which is required to form
a strong seal, without compressing the sheet material in such
a way or to such a degree as to cause tear propagation along
lines 31, 33, 35, or similar lines.
To the extent the sheet material 10 conforms to the
sheet structure of FIGURE 6, and to the extent the adhesion
promoting layer 14 is a primer, the layer 12 composition is
preferred to ~e a coating grade propylene copolymer which
must be less than about 1.5 mil, preferably leas than 1 mil,
and most preferably about .35 to .75 mil in thickness, with
an ideal thickness being approximately 0.5 mil. To the
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extent the thickness of layer 12 is optimized, the peeling
will be enhanced. To the extent the thickness of layer 12
deviates from the preferred thicknesses, the peel will not be
as clean as the peel of a comparable film which uses a more
preferred thickness for the blend composition of layer 12.
In those sheet structures using pre-formed films of the
blend compositions of layer 12, there is an existing process
limitation in that it is difficult to form ar. unsupported
film of these compositions with a thickness less than 1 mil.
Thus a l mil thickness is a preferred lower limit for the
thickness of the blend composition of layer 12 in those
films. Thinner films may be made by coextrusion of the layer
12 composition with supporting layers which are later
stripped away. While such processes are more expensive, a
thinner layer is effectively formed thereby. Up to 1.5 mils
thickness may be used for layer 12, although the ease of
peeling of the sheet material 10 from tray 2 is impeded as
the thickness is increased. To the extent equipment and
processes are available to fabricate films thinner than 1
mil, thickness of as little as 0.5 mil is preferred.
Referring back now to FIGURES 3-5, layer 12Bl serves as
an adhesive layer and its thickness is typically only great
enough to form a uniform and acceptable adhesive, preferably
on the order of 0.1 to 0.3 mils. Layer 12B2 serves as an
outer covering for the sheet structure. It is important that
layer 12B2 be thin enough that it can be easily torn through
to reach layer 12A in which cohesive failure and propagation
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occurs to break the seal when the sheet material is removed
from the closed and sealed package. Thus layer 12B2 is
generally quite thin, on the order of leas than 0.5 mil,
preferably less than 0.3 mil.
The combination of layers 12A, 12Bl, and 12B2 may
conveniently be formed as a coextrusion, in which case the
thickest portion of that subcombination will normally be
layer 12A. In the FIGURE 4 structure, layer 12A is
preferably about 0.5 to 1.0 mil thick, with layers 12B1 and
12B2 both being on the order on 0.5 mil or less, preferably
less than 0.3 mil. In a typical structure of FIGURE 4, layer
12A is 0.5 mil thick, and layers 12Bl and 12B2 are
approximately 0.25 mil thick. While layer 12B1 may be
thicker, in some embodiments up to 0.5 to 1 mil, layer 12B2
is necessarily thin so that it can be easily torn through to
reach layer 12A with the propagating tear. To the extent
layer 14 is a primer such as a carboxy modified olefin, it is
usually of insignificant thickness and is invisible for most
purposes. To the extent layer 14 is an adhesive layer, its
thickness is generally 0.1 to 1.0 mil, preferably 0.1 to 0.3
mil, depending on whether it is a pre-formed film, an
extrusion laminate, or an extrusion coating resin.
The thickness of layer 16 depends on the material being
used for layer 16. To the extent it is metal foil it
preferably has a thickness of 35 gauge to 200 gauge, most
preferably 100 to 200 gauge. When layer 16 is a vinylidene
chloride copolymer, the thickness of that layer is preferably
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1336878
approximately 1.0-2.0 mils. Where the barrier layer 16 is a
vinyl alcohol, its preferred thickness is typically on the
order of 0.5 to 1.0 mil.
The overall thickness of sheet materials of this
invention ranges from a low of about 2 mils to a high of
about 30 mils, with a preferred thickness of about 2.5 mils
to about 12 mils, most preferably about 3 to about 7 mils.
Ar.other type of barrier material which may be used for
layer 16, either alone or in combination with others of the
barrier materials, is the family of polyolefins, such as
polypropylene or polyethylene, which are used as moisture
barriers. For example it is common to combine a moisture
barrier of high density polyethylene into the same film with
a vinylidene chloride copolymer or an ethylene vinyl alcohol
copolymer which serve as oxygen barriers. Methods for
combining polyolefins and oxygen barriers into the same film
with the use of compatible adhesives are known in the art.
For purposes of this invention, blends of polypropylene
and polyethylene in layer 12A have functional properties
essentially equivalent to those of propylene ethylene
copolymers.
Thus it is seen that the invention provides novel
compositions in the blend layer 12, and particularly 12A.
Novel sheet materials are provided as indicated in FIGURES 2-
9. Novel packages are provided as indicated in thecombination of the sheet structures of FIGURES 2-9 and the
packages illustrated in FIGURES 1, 10, and lOA. The
13~6878
resulting packages exhibit improved capability to protect a
product, through processes exhibiting high thermal stresses,
and to provide, in combination, the capability for the sheet
structure to be peeled away from the package cleanly, in such
a way as to provide complete and open access to the contents
of the package.
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