Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTAINER HAVING A METAL END TO WHICH A
MEMBRANE IS SEALED
BACKGROUND OF THE INVENTION
This invention relates to containers for foods or other consumer products,
wherein a metal ring is affixed about the opening of the container and the
container
is sealed by a flexible membrane lid removably attached to the ring.
S In such containers, the metal ring, also referred to as a metal end, is
attached to the top of the container, typically by double-seaming the metal
end to a
flange at the top of the container, and the flexible membrane lid is sealed to
the
metal end so as to cover an opening defined therein. The lid generally
includes
one or more layers providing strength and/or burner properties, and a sealant
layer
defining the lower surface of the lid. The sealant layer's function is to seal
to the
metal end, typically by heat sealing.
It is desirable for the lid to be sealed to the metal end with sufficient
strength to prevent inadvertent detachment of the lid during shipping and
handling
prior to the consumer opening the container. On the other hand, it is also
desirable
for the consumer to be able to peel the lid off the metal end with relatively
little
force, so that people of diverse ages and abilities can open the container,
and so
that the lid comes off cleanly in one piece rather than tearing. These
objectives of
high seal strength and low peel force tend to be at odds with each other, but
various
container closures that strike a balance between these dual objectives have
been
developed. Some such closures have employed an ionomer resin such as
SURLYN~ as the sealant layer of the membrane lid. This sealant material is
readily heat-sealable to the spot-coated and non-spot-coated electro-tin-
plated
(ETP) steel ends that are commonly employed. However, this material is one of
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the most expensive sealants on the market, and also suffers from the "age-up"
phenomenon whereby the strength of the seal to the metal end tends to increase
with age, which is undesirable. SURLYN~ also has a relatively narrow heat-
'sealing temperature window; if the temperature is too high, the seal strength
is too
great for easy peelability of the lid, and if the temperature is too low, the
seal
strength can be too low to keep the lid from inadvertently detaching from the
metal
end. It can be difficult to control the processing conditions so as to stay
within the
desired temperature window. Further complicating this situation is the fact
that
SURLYN~ seals more readily to non-spot-coated ETP steel than to spot-coated
ETP steel. Thus, a lower sealing temperature must be used for non-spot-coated
metal ends than is used for spot-coated metal ends. In a container processing
plant
that employs both types of metal ends, it is thus necessary to have two
different
processes.
Additionally, even for a given type of metal end, there can be substantial
variability in the surface characteristic of the metal, which can affect the
strength
of the seal. Such variability is not readily predicted or accounted for, and
hence
the strength of the seals with the membrane lids tends to have substantial
variability.
Accordingly, an alternative sealant and closure system has been sought. It
is desired to provide a sealant and closure system affording a wider
temperature
window for the heat-sealing operation. It is also desirable for the closure
system to
be able to use the same heat-sealing process conditions for both spot-coated
and
non-spot-coated metal ends, and to produce more-consistent seal strengths even
under variable surface characteristics of the metal ends.
BRIEF SUMMARY OF THE INVENTION
The present invention addresses the above needs and achieves other
advantages, by providing container closure systems having an ethylene vinyl
acetate-based or ethylene methyl-acrylate-based sealant as the sealant layer
for a
flexible membrane lid. In some embodiments, the sealant comprises ethylene
vinyl
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acetate blended with a polyolefin. The polyolefin can comprise a polyethylene
such as low-density polyethylene. The metal end can have a spot-coated surface
or
an uncoated (i.e., bare metal) surface. A coating of a polypropylene-used
composition is provided on the metal end. The polypropylene-based composition
can comprise, as one example, epoxy having polypropylene dispersed therein.
The
sealant layer of the lid is heat-sealed to the polypropylene-based coating on
the
metal end to form a peelable seal. The resulting seal allows the lid to be
peeled off
the metal end by detachment of the sealant layer from the polypropylene-based
coating. This closure system is less-sensitive to the heat-sealing temperature
than
the prior SURLYN~-based closure and the seal strength is substantially
unaffected
by the metal surface characteristics, since the seal's failure mechanism is
adhesive
delamination between the lid and the coating on the metal end.
In an alternative embodiment of the invention, the coating on the metal end
comprises an epoxy without any polypropylene dispersed therein.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS)
Having thus described the invention in general terms, reference will now be
made to the accompanying drawings, which are not necessarily drawn to scale,
and
wherein:
FIG. 1 is a perspective view of a container in accordance with an
embodiment of the invention;
FIG. 2 is a cross-sectional view along line 2-2 in FiG. 1, showing the
closure system; and
FIG. 3 is a magnified, exploded cross-sectional view of the membrane and
metal end in accordance with one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present inventions now will be described more fully hereinaf3er with
reference to the accompanying drawings, in which some, but not all embodiments
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of the invention are shown. Indeed, these inventions may be embodied in many
different forms and should not be construed as limited to the embodiments set
forth
herein; rather, these embodiments are provided so that this disclosure will
satisfy
applicable legal requirements. Like numbers refer to like elements throughout.
A container 10 in accordance with an embodiment of the invention is
depicted in FIGS. 1-3. The container 10 includes a receptacle 12 having a side
wall 14 that encircles the interior of the receptacle. The wall 14 can have
various
configurations and cross-sectional shapes. A circular cross-section is shown,
but
alternatively the receptacle can have other shapes such as oval, square or
rectangular, oblong, etc. The receptacle can be made of various materials and
by
various processes. For instance, the receptacle can be formed of plastic by
thermoforming, blow-molding, or injection-molding, or can be formed of
composite materials (e.g., paperboard with a liner of impervious material such
as
polymer film and/or foil) by spiral-winding or convolute-wrapping processes.
The
particular details of the receptacle are not important to the present
invention.
The container 10 also includes a closure system comprising a metal end 16
and a flexible membrane lid 18. The metal end 16 is ring-shaped so as to
define a
central opening 20 therethrough. The opening 20 is encircled by a radially
inner
edge 22 of the metal end; the inner edge 22 advantageously is curled or rolled
as
shown. The metal end is affixed to the upper edge of the receptacle wall 14.
The
affixing of the metal end can be accomplished, for example, by seaming
together a
curled radially outer edge of the metal end and the upper edge of the
receptacle
wall as shown. A sealing composition (not shown) can be disposed between the
metal end and receptacle wall in the seam region to make the seam hermetic.
The
membrane lid 18 is sealed to the upper surface of the metal end 16 to seal the
container closed at its top end. The lid advantageously includes a pull tab 24
that
is not sealed to the metal end so that it can be readily grasped and pulled to
remove
the lid from the metal end.
The membrane lid 18 comprises a substrate 26 and a sealant layer 28. The
sealant layer 28 forms the lower surface of the lid. The substrate 26 can
comprise
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various numbers of layers of various materials such as metal foil, polymer
film,
metallized polymer film (i.e., film having a very thin coating of
substantially pure
metal deposited thereon), paper, etc. When the container contents must be
sealed
against moisture and/or oxygen infiltration from the outside environment, at
least
one layer of the substrate comprises a barrier layer. Various materials can be
used
as barners, including metal foil, metallized polyester, non-metallized polymer
film
(e.g., EVOH), and others. A combination of two or more such barrier layers can
be used. The particular structure of the substrate 26 is not important to the
present
invention.
The sealant layer 28 comprises a heat-sealable material. The heat-sealable
material can comprise an ethylene vinyl acetate-based (EVA-based) resin or an
ethylene methyl-acrylate-based (EMA-based) resin. Examples of suitable EVA-
based and EMA-based resins include various resins available from DuPont under
the trademark APPEEL~.
In one embodiment, the sealant layer 28 comprises a blend of an EVA-
based or EMA-based resin with a polyolefin. The polyolefin can comprise a
polyethylene such as low-density polyethylene (LDPE). As an example, the
sealant layer can comprise a blend of EVA and LDPE, or a blend of EVA,
ethylene
acrylic acid (EAA), and LDPE. A suitable composition, for instance, can
comprise
about 80% by weight of EVA having vinyl acetate content of about 28%, and
about 20% by weight LDPE.
In accordance with the invention, the metal end 16 is coated on its upper
surface with a coating 30 of a material that forms a peelable heat 5Ea1 with
the
sealant layer 28 of the lid. In one embodiment of the invention, the coating
30 can
comprise an epoxy. An example of a suitable epoxy is available from Valspar as
product number 88X003C.
In another embodiment, the coating 30 can comprise an epoxy in which
polypropylene is dispersed. When this composition is coated onto the metal end
and cured, the surface of the resulting coating 30 is characterized by a
multitude of
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microscopic spaced "islands" of polypropylene that are dispersed within the
matrix
of the epoxy. These polypropylene islands comprise bonding sites to which the
membrane lid 18 can be sealed. The peel strength of the bond between the lid
and
the metal end can be controlled by varying the fraction of the total surface
area of
the coating 30 that is made up of the polypropylene bonding sites or islands.
This
fraction is a function primarily of the relative proportions of the epoxy and
polypropylene making up the coating 30.
The closure system of the invention can be employed with various types of
metal ends, including bare (i.e., uncoated) metal as well as coated metal. As
examples, the invention is applicable to spot-coated electro-tin-plated (ETP)
steel
as well as uncoated ETP steel. The invention is advantageous in that the blow-
off
strength and peel-force characteristics of the seal between the lid and the
metal end
are not affected to a substantial extent by the surface characteristics of the
metal
end (i.e., whether it is uncoated or coated, etc.), because the failure
mechanism of
the seal is adhesive failure between the lid sealant layer and the epoxy-based
coating on the metal end. Furthermore, the EVA-based or EMA-based sealant 28
has a relatively wide heat-sealing temperature window. Consequently, the
strengths of the seals to the membrane lids tend to be more consistent than in
prior
closure systems.
Many modifications and other embodiments of the inventions set forth
herein will come to mind to one skilled in the art to which these inventions
pertain
having the benefit of the teachings presented in the foregoing descriptions
and the
associated drawings. Therefore, it is to be understood that the inventions are
not to
be limited to the specific embodiments disclosed and that modifications and
other
embodiments are intended to be included within the scope of the appended
claims.
Although specific terms are employed herein, they are used in a generic and
descriptive sense only and not for purposes of limitation.
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