Note: Descriptions are shown in the official language in which they were submitted.
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CAN WITH PEELABLY BONDED CLOSURE
Technical Field
This invention relates to cans, and more
particularly to metal cans having an apertured lid with
a heat-sealed, peelable closure for the aperture. In an
important specific aspect it is directed to heat-sealed-
closure type cans for holding carbonated beverages or
like contents that exert a positive internal pressure on
the closure, and also to lids for such cans, carbonated
beverage-containing packages including such cans, and
methods of producing such cans containing carbonated
beverages.
Background Art
Heat sealable containers are widely used for a
variety of high quality food products. Non-retorted
products packaged with heat sealable foil lidding
include many types of jams, preserves, yogurt and dairy
products, peanuts and snack foods. A wide variety of
retortable fish and meat products (including many
varieties of pet food) are also packaged using heat
sealed foil lidding. In some instances, the entire lid
of a can or like container may be removably bonded by
heat sealing to a flange formed at an open upper end of
the container body, so as to enable the lid to be
completely removed, for access to the contents of the
container. Other containers, exemplified by cans of
tomato or like quiescent fruit juices, have a lid
permanently secured to the container body and formed
with an aperture (for pouring out the contents) covered
by a heat sealed closure or, more commonly, by a closure
bonded with a pressure sensitive adhesive. Such a
closure is commonly a thin, flexible element, e.g. an
aluminum foil-polymer laminate, peripherally bonded by
heat sealing to a flange defining the aperture, and has
a tab that enables the closure to be peeled manually
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from the flange: the flange may be a flat portion of the
can lid surrounding the aperture and coplanar with the
aperture edge.
For easy opening, typical peel forces (at 90° to
the flange) for a heat sealed closure are in a range
between about 10 and about 20 Newtons (preferably about
12 Newtons). The retort process involves pressure
differentials (from inside to outside) of up to 207 kPa
(30 psi), although for many applications, a counter
pressure system is used to prevent the lid or closure
from bursting off the container. This is necessary
because of the reduction in bond strength which
generally occurs at the elevated retort temperatures.
Moreover, in the case of containers with a lid or
closure heat sealed to a flange which is coplanar with
the container aperture, internal pressure will cause the
lid or closure to bulge over the aperture and, in turn,
this bulging exerts a peel force on the heat seal.
Carbonated soft drinks require a container capable
of withstanding internal pressures of 620 kPa or higher.
Such pressures, or even substantially lesser positive
internal pressures, would exert on a conventional heat
sealable closure a peeling force more than sufficient to
cause burst failure. Increasing the strength of the
heat seal bond sufficiently to withstand such forces
would make manual peeling of the closure difficult or
virtually impossible for many consumers. Consequently,
heat sealable closures have not had wide commercial use
with canned carbonated beverages. In present-day
commercially available carbonated soft drink cans,
having a so-called drawn-and-ironed aluminum alloy can
body and an aluminum alloy can lid peripherally secured
to the open upper end of the body, the can end is
commonly formed with a scored area and provided with a
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riveted tab system which, when lifted, creates a lever
action and exerts a downward force that generates a
fracture along a scored line thereby creating an
aperture. The region of the lid~that lies within the
scored area is simultaneously bent down into the top of
the container.
Alternative structures have heretofore been
proposed or produced with the objective of enabling use
of heat sealable closures with containers for carbonated
beverages or other substances that create elevated
internal pressure. For instance, it has been proposed
to provide a spherically domed (rather than planar) lid
having an aperture covered by a similarly spherically
curved closure member bonded thereto, or to provide a
container in which the entire lid is heat-sealed to an
angled (rather than planar) flange around the container
periphery. In a further alternative, a can lid has been
provided with plural small holes (rather than a single
aperture) covered by a single foil laminate seal with a
pull tab. These alternatives, however, have various
limitations or drawbacks.
U.S. Patent No. 3,889,844 describes a can closure
in which a can end is shaped to impart a frustoconical
area around a pour hole sealed with an adhesive tape tab
so that the forces acting on the tape (exerted by can
contents under pressure, such as carbonated beverages)
tend to place the adhesive in shear instead of in peel.
The size of the pour hole descrilaed in this patent
provides a pour rate which is low as compared to
present-day conventional carbonated beverage cans with
scored can ends, and the attainment of long shelf life
at pressures as high as 620 kPa is not shown.
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Disclosure of the Invention
The present invention, in a first aspect, broadly
contemplates the provision of a can comprising a metal
can body having an open upper end; a substantially rigid
metal can lid peripherally secured to and closing the
can body end, the lid having an upper surface; a
frustoconical annular flange formed in a portion of the
lid and projecting upwardly from the lid upper surface,
the flange having an upwardly sloping outer surface and
an annular inner edge lying substantially in a plane and
defining an aperture with an average diameter between
about 16 mm and about 25 mm (0.625 - 1 inch), the flange
outer surface being oriented at an angle of slope
between about 12.5° and about 30° to the plane; and a
flexible closure member of a material comprising a metal
foil, extending entirely over the aperture and peelably
bonded by a heat seal to the flange outer surface
entirely around the aperture.
In currently preferred embodiments of the
invention, the lid has a substantially flat upper
surface. It is also strongly currently preferred that
the aperture be circular, because in noncircular
apertures there are locations around the perimeter where
the tendency of the closure member to peel (burst) is
enhanced. The "average diameter" in the case of a
circular aperture is, of course, simply the diameter of
the aperture.
It will be understood that directions such as
"upper" or "upwardly" are used herein with reference to
a can standing upright with the lid at the top. The
term "angle of slope" refers to the acute angle formed
between the plane of the aperture edge and the line
representing the flange outer surface as seen in a
vertical plane intersecting the aperture edge at a point
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at which the line tangent to the aperture edge in the
plane of the aperture edge is perpendicular to the
vertical plane.
GJhen the can is filled with a carbonated beverage,
5 the closure member is subjected to a differential
pressure (hereinafter sometimes designated DP), i.e. a
positive difference between the pressure within the can
and ambient pressure outside the can, in some
circumstances as high as 620 kPa or even more. This
differential pressure exerts, on the closure member and
heat seal, a force having a tear/shear component (i.e.,
tending to tear the closure member and shear the heat
seal, such component being hereinafter referred to as
the tear/shear force and being sometimes designated y),
and in some cases also a peel component.
In currently preferred embodiments of the
invention, the closure member material is deformable,
and the average diameter of the aperture, the angle of
slope of the flange, and the deformability of the
material are mutually selected such that the closure
member, when subjected to differential pressures up to
at least about 620 kPa (preferably up to at least about
687 kPa) in the can, bulges upwardly with an arc of
curvature such that a line tangent to the arc at the
inner edge of the flange lies at an angle (to the plane
of the flange inner edge) not substantially greater than
the angle of slope of the flange outer surface, thereby
to eliminate any peel component of the force exerted by
the differential pressure on the closure member and heat
seal.
Also, in some currently preferred embodiments, the
closure member and heat seal have a tear/shear force
resistance of at least about 13.4 kg/cm (75 lb./in.),
and the average diameter of the aperture and the angle
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of slope of the flange are mutually selected such that
when the closure member is subjected to differential
pressure of up to at least about 620 kPa (preferably up
to at least about 687 kPa) within the can, the
tear/shear force exerted on the closure member and heat
seal does not exceed the aforesaid tear/shear force
resistance.
As a further particular feature of the invention,
in currently preferred embodiments, the annular inner
edge of the flange is formed with a reverse bead curl,
which may be substantially tangent to the upwardly
sloping outer surface of the flange.
Conveniently and advantageously, in at least many
instances, the metal foil of the closure member is
aluminum alloy foil, e.g. having a thickness between
about 75 ~ and about 100 ~ (0.003 - 0.004 inch). Also
advantageously, the heat seal may be formed as an
annulus surrounding the aperture and having a width
between about 2 to 3 mm. This width of heat seal is
found to be sufficient to withstand tear/shear forces
encountered in use, and at the same time it facilitates
manual peeling of the closure member to open the
aperture. To enable such peeling without difficulty,
the 90° peel strength of the heat seal is between about
8 and about 20 N, preferably between about 10 and about
16 N. The closure may be provided with a tab portion
having a manually graspable free end.
In contrast to the riveted tab structure of
conventional carbonated beverage cans, a heat-sealable
closure member may become completely separated from the
can upon opening, and may then be separately discarded,
creating environmental problems. To avoid this
consequence, and further in accordance with the
invention, the closure may be provided with an extension
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overlying the lid in opposed relation to the
aforementioned tab portion, and the heat seal may
include both an annulus surrounding the aperture as
described above and a further seal portion bonding the
extension to the lid such that the peel force required
to separate the extension from the lid is greater than
that required to separate the closure member from the
lid at the annulus, the aperture being easily opened by
peeling back the closure member from the flange while
the closure member remains secured to the lid by the
further seal portion. This promotes retention of the
closure member on the lid, as desired for environmental
reasons. Moreover, the peeled but retained metal foil
closure member can be folded over the aperture to
provide a measure of coverage and protection for the
contents of a can which has been only partially emptied.
Additionally, a body of fragrance-providing
material may be disposed between the closure member and
the lid and surrounded by the heat seal such that when
the closure member is subjected to a peel force
effective to open the aperture, the body of fragrance-
providing material becomes exposed. The fragrance
thereby released, in proximate relation to the nostrils
of a person drinking from the can, enhances the
effective flavor sensed by the drinker.
The can body may be a drawn and ironed metal can
body for holding a carbonated beverage. The lid may be
formed with a peripheral rim engaging the open upper end
of the can body and projecting upwardly above the upper
surface of the lid, the body being formed with an
outwardly concave lower end, and the rim and body lower
end being mutually shaped and dimensioned to permit
stable vertical stacking of the can with other
identically shaped and dimensioned cans. In such a
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structure, although the flexible closure member (bulging
because of the internal pressure) is domed so as to rise
to a height above the annular flange, the height of the
rim, the concavity of the body lower end, and the height
to which the closure rises above the annular flange are
such that there is sufficient clearance between the lid
upper surface of the can and the concave bottom of
another identical can stacked above it to accommodate
the domed closure.
Metal foil as used fox the closure (e. g. in a foil-
polymer laminate) has the advantage of affording
excellent gas barrier properties, so that shelf life and
quality are improved with foil-based closures. Aluminum
foil, for instance, is an effectively perfect barrier
for oxygen (important for beer to prevent development of
off-flavors owing to oxidation) and for carbon dioxide
(important where carbonation levels need to be
maintained).
The aperture defined by the flange preferably
extends over a minor fraction of the area of the open
end of the can body. Especially for holding contents
such as carbonated beverages, in cans wherein the open
end of the can body has a center of symmetry (e. g. being
circular), the annular flange and the aperture are
disposed eccentrically of the can body open end so as to
be relatively close to the periphery of the lid, for
ease of pouring or drinking. That is to say, the flange
is disposed in a portion of the lid eccentric to the
geometric axis of the.can, i.e., close to a side of the
can.
Although the shape of the aperture can take
different forms, noncircular apertures are nonpreferred,
and, in particular, angular apertures or aperture shapes
with very small radii of curvature are not suitable for
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the present invention. If, instead of a circular
aperture, an elliptical or irregularly shaped aperture
is provided, e.g. having an aspect ratio between about
1.1 and 1.5, the flange is not strictly frustoconical;
it will be understood that the term "frustoconical" is
used broadly herein to define an upwardly convergently
sloping flange continuously surrounding an aperture,
whether the aperture is circular or not.
In further aspects, the invention embraces a can
lid member as described above, mountable on a metal can
body having an open upper end so as to be peripherally
secured to and to close the can body end; the
combination of this lid member with a flexible closure
member extending entirely over the aperture and peelably
bonded to the flange outer surface around the aperture;
a carbonated beverage package comprising a can as
described above in combination with a body of a
carbonated beverage contained within the can; and a
method of producing a can containing a carbonated
beverage, comprising filling a drawn and ironed metal
can body, having an open upper end, with a carbonated
beverage, and closing the open upper end of the can body
by peripherally securing thereto a metal can lid member
as described above having a flexible closure member
extending entirely over the aperture defined by its
annular flange and peelably bonded to the flange outer
surface around the aperture.
In the can of the invention, the °provision of the
frustoconical annular flange defining the can aperture,
and the securing of the flexible closure member by
peelable bonding to the upwardly sloping outer surface
of this flange, enable the use of a peelably bonded
closure member on an otherwise conventional carbonated
beverage can, despite the high differential pressure
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(positive internal pressure) acting on the closure
through the aperture and the resultant outward bulging
or doming of the flexible closure member. This is
because the angle of slope of the flange can be made
5 steep enough so that a line tangent to the arc of
curvature of the domed closure member at the inner edge
of the flange lies at an angle (to the plane of the
flange inner edge) which is not substantially greater
than, and is preferably less than, the angle of slope of
10 the flange outer surface. In such case, the internal
pressure acting on the closure member does not exert any
significant component of peeling force that would tend
to separate the closure member from the flange by
peeling. Instead, the forces acting on the peelably
bonded flange area owing to tension in the closure
member are predominantly shear in character. Heat seal
bonds, for instance, are strong under shear loading,
especially at ambient temperature; the inability of
conventional heat sealed closures to withstand internal
pressure in carbonated beverage cans has been caused by
the substantial peeling forces exerted on such closures
when the closures bulge, under the elevated pressure
within a can of carbonated beverage, at a substantial
angle to a planar horizontal flange surrounding an
aperture.
For a given internal pressure condition, aperture
dimension, and closure member, the minimization or
elimination of peeling force exerted on a closure bond
by elevated pressure within the can is dependent on the
angle of slope of the flange. Stated generally, the
greater the angle of slope, the easier it is to provide
a bonded closure that will not burst from internal
pressure yet can be easily manually peeled by a
consumer, having regard to the extent of doming of
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practicable flexible foil closure members under the
pressures within a carbonated beverage can. With the
flat lid surface and upwardly projecting frustoconical
flange of the present invention, any desired angle of
slope can readily be provided, in contrast to the range
of angles permitted by other geometries such as a
uniformly spherically domed lid having an aperture
therein. Moreover, the arrangement of flange, aperture,
and domed closure of the invention, occupying only a
portion of the area of the can end, enables the height
of the closure to be restricted to an extent compatible
with convenient vertical stacking of cans.
Further features and advantages of the invention
will be apparent from the detailed disclosure
hereinbelow set forth, together with the accompanying
drawings.
Brief Description of the Drawings
FIG. 1 is a perspective view of a can embodying the
present invention in a particular form;
FIG. 2A is an enlarged and somewhat simplified
fragmentary elevational sectional view of a portion of
the lid member of the can of FIG. 1, including the
aperture-defining flange and closure member;
FIG. 2B is a highly simplified and schematic
representation of the same view as FIG. 2A;
FIG. 3 is a view similar to FIG. 2A of a flexible
closure member bonded to a conventional planar flange
defining an aperture;
FIG. 4 is a fragmentary view similar to FIG. 3 of a
portion of the flange and closure member of the
embodiment of the invention shown in FIGS. 2A and 2B;
FIG. 5 is a simplified and somewhat schematic top
plan view of the can of FIG. 1;
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FIG. 6 is an exploded diagrammatic elevational
sectional view of the can lid and closure member of
FIG. 5;
FIG. 7 is a plan view of the closure member of
FIG. 5;
FIG. 8 is a side elevational view, partly broken
away, of two cans having the structure shown in FIG. 1,
illustrating the ability of the cans to be stacked
vertically;
FIG. 9 is a view similar to FIG. 2B illustrating a
condition of excessive bulging of the closure member;
FIG. 10 is a graph representing the relationship
between sealing temperature and peel strength in Example
2 described below;
FIG. 11 is a graph representing the relationship
between heat seal temperature and burst pressure in the
same example;
FIG. 12 is an enlarged fragmentary sectional
elevational view of a portion of a lid member embodying
the present invention;
FIG. 13 is a schematic fragmentary sectional
elevational view of a lid member embodying the
invention;
FIG. 14 is a graph showing bulge height of an
exemplary closure member as a function of pressure
within the can (i.e., differential pressure OP);
FIG. 15 is a schematic plan view of a can lid
embodying the invention and having a "stay-on" closure
member;
FIG. 16 is a graph showing 90° peel force as a
function of displacement of the closure member of
FIG. 15;
FIGS. 17A and 17B are highly schematic fragmentary
elevational sectional views in illustration of a further
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embodiment of the invention including a fragrance
reservoir;
FIG. 18 is a sectional elevational view of one form
of can lid embodying the invention and including a
fragrance reservoir; and
FIGS. 19 and 20 are views similar to FIG. 15 of two
can lids embodying the invention and including both a
stay-on closure member and a fragrance reservoir.
Best Modes for Carrying Out the Invention
The container of the invention will be described,
with reference to the drawings, as embodied in a metal
can 10 for holding a carbonated beverage such as soda or
beer. The can 10 includes a one-piece can body 11
constituting the bottom 12 and continuous, upright,
° axially elongated, generally cylindrical side wall 14 of
the can, and a lid 16 which, after the can has been
filled with the beverage, is peripherally secured to the
open top end of the can body to provide a complete,
liquid-tight container.
In this embodiment, the body 11 may be an entirely
conventional drawn-and-ironed aluminum alloy can body,
identical in structure, alloy composition, method of
fabrication, configuration, gauge, dimensions and
surface coatings to can bodies currently commercially
used for carbonated and other beverages (alternatively,
for example, the body may be a steel can body, such as
are in common use in Europe). In particular, and in
common with known can bodies, the bottom 12 of the body
11 is externally concave and the open top end of the
body has a circular edge 18 lying in a plane
perpendicular to the vertical geometric axis of the side
wall 14. The terms "aluminum" and "aluminum alloy" are
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used interchangeably herein to designate aluminum metal
and aluminum-based alloys.
Except as hereinafter described, the lid 16 may
also be a generally conventional aluminum alloy lid
member of the type currently commercially used for
beverage cans having drawn and ironed one-piece can
bodies such as the body 11. Thus, the alloy of which it
is constituted, the steps and procedures employed in its
fabrication (with the exceptions noted below), and its
general overall configuration, dimensions, gauge and
surface coatings as well as the manner in which it is
secured to the top edge 18 of the can body 11, may all
be the same as in the case of present day can lids well-
known in the art.
It should be noted, however, that since the can lid
of the present invention is not subjected to the rivet-
forming and scoring operations that must be performed on
currently conventional can lids, the invention may
permit the use of nonconventional can lid alloys and
materials. For example, coated steel can lids, which
are normally too difficult to open by the conventional
scoring mechanisms, could be used in the practice of the
invention. Similarly, AA 3104 alloy, commonly used for
can bodies (but not, heretofore, for can lids), when
used at an appropriate gauge, may have sufficient
strength for the lid structure of the present invention;
it could offer the advantages of lower cost as compared
to the AA 5128 alloy currently used for can lids and
would also afford benefits for recycling, in that the
can lid and body would be made of the same alloy.
In particular, the lid 16 in this illustrated
embodiment is substantially rigid, and has a
substantially flat upper surface 20 with a circular
periphery, around which is formed a raised annular rim
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22 projecting upwardly above the plane of the flat upper
surface 20. When the lid is mounted on the open upper
end of a beverage-filled can body, in known manner, the
rim 22 engages the upper edge 18 of the can body; the
5 circular flat surface 20 lies substantially in a
horizontal plane, perpendicular to the vertical
geometric axis of the cylindrical side wall 14, and is
centered with respect to the latter axis.
The lower end 14a of the side wall 14 of the can 10
10 is shaped (tapered) to interfit with the rim 22 of the
lid of another identical can 10a, when the can 10 is
stacked vertically on top of the can 10a as shown in
FIG. 8. A multiplicity of the cans may thus be stably
vertically stacked, one on another, as is true of
15 present-day conventional cans of the same general type.
The elevation of the lid rim 22 above the flat upper
surface 20 of the lid, together with the concavity of
the can bottom 14, cooperatively define a central gap or
space between the lid of one can and the bottom of the
next can above it, in such a stacked arrangement.
Also in common with present-day conventional lid
members used with one-piece drawn-and-ironed aluminum
alloy beverage can bodies, the lid 16, when secured to
the beverage-filled can body, provides therewith a
complete sealed enclosure holding the beverage. The lid
is thus subjected to elevated internal pressure within
the can (i.e., pressure higher than ambient atmospheric
pressure) if the beverage is carbonated. However, the
formed aluminum alloy lid is substantially rigid, so
that it undergoes at most only a small deflection of its
upper surface as a result of this pressure condition,
and the upper surface 20 remains substantially flat
notwithstanding the internal pressure acting on the lid.
The lid 16 is arranged to provide an aperture
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through which the beverage contained in the can may be
poured or removed by drinking directly from the can,
either with a straw inserted through the aperture or by
juxtaposition of the consumer's mouth to the aperture.
Heretofore, in cans for holding carbonated beverages or
other such contents at elevated pressure, the aperture-
providing feature has conventionally included a scored
portion of the metal of the lid member and a riveted
pull tab system for parting the lid metal along the
score line to open the aperture.
The present invention, in contrast, provides a pre-
formed open aperture 24 in the lid, and a peelable,
flexible closure member 28 covering the aperture. In
order to achieve adequate burst resistance without
requiring excessive force to peel the closure member, a
shallow frustoconical annular flange 30 is formed in the
lid within the area of the flat upper surface 20, to
surround and define the aperture 24 and to provide a
seat for the closure member.
More particularly, the flange 30 projects upwardly
from the upper surface 20 of the lid, and has an
upwardly sloping outer flange surface 32 and an annular
inner edge 34 defining the aperture 24, which is
illustrated as being of circular configuration but is
not limited to a circular shape. The inner edge 34, as
shown in FIGS. 2A and 2B, is preferably formed as a bead
36 with a reverse curl, which is tangent to a horizontal
plane represented by line P (FIGS. 2A and 2B) and to the
line of slope of the outer flange surface 32 so that,
once the closure member 28 is heat-sealed to the flange
surface, the cut metal (typically an aluminum alloy) at
edge 34 cannot come into contact with the contained
beverage. This is advantageous because the cut metal at
the edge (unlike the major surfaces of the lid) has no
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protective coating, and would be attacked by acidic or
salt-containing beverages if it were exposed thereto.
The reverse curl of bead 36 also prevents a drinker's
lips from touching and being injured by the cut metal at
edge 34, and avoids any possibility of damage to the
closure member by contact with the cut metal. However,
the invention may also be embodied in a can wherein the
aperture has a standard (not reverse) bead curl, which
also affords such benefits as safety for the consumer,
it being noted that where the cut edge of the metal is
not kept from contact with the contained liquid by a
reverse curl, it may be protected by application to the
cut edge of a lacquer.
The flexible closure member 28 is constituted of a
sheet material comprising metal foil, e.g. aluminum
foil; in the described embodiment of the invention, the
closure member is fabricated of an aluminum foil-polymer
laminate sheet. Stated more broadly, materials that may
be used for the closure member include, without
limitation, lacquer coated foil (where the lacquer is a
suitable heat seal formulation); extrusion coated foil
(where the polymer is applied by a standard or other
extrusion coating process); the aforementioned foil-
polymer laminate, wherein the foil is laminated to a
polymer film using an adhesive tie layer; and foil-
paper-lacquer combinations such as have heretofore been
used for some low-cost packaging applications.
The closure member extends entirely over the
aperture 24 and is secured to the flange outer surface
32 by a heat seal extending at least throughout the area
of an annulus entirely surrounding the aperture. Since
the reverse curl bead 36 does not project beyond the
slope of the flange outer surface, the closure member
smoothly overlies this bead as well as the flange outer
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surface, affording good sealing contact between the
closure member and the flange.
The closure member, in the described embodiment of
the invention, is bonded by heat sealing to the flange
30, covering and closing the aperture 24, before the lid
member 16 is secured to a can body 11 filled with a
carbonated beverage. Once the lid has been mounted on
the body to complete the enclosure of the beverage,
elevated pressure generated by the beverage acts on the
inner surface portion of closure member 28 which is
exposed through the aperture to the interior of the can,
causing the flexible closure member to bulge outwardly.
Further in accordance with the invention, however, the
angle ~ (FIG. 2A) of slope of the flange outer surface
relative to the plane of the annular edge 34 (i.e.,
plane P) is selected to be such that a line tangent to
the arc of curvature of the bulged closure member at the
inner edge of the flange lies at an angle to plane P not
substantially greater than the angle 8 of slope of the
flange outer surface. As indicated in FIG. 2B, since
the upper surface 20 of the lid member 16 is flat and
horizontal (and thus parallel to plane P), B may
alternatively be defined as the angle of slope of the
flange outer surface to the flat lid surface 20.
Preferably the angle ~ is between about 12.5° and
about 30° to the plane P, and more preferably at least
15°. In currently particularly preferred embodiments,
the angle ~ of slope is between about 18° and about 25°
to the plane P.
In FIGS. 2A and 2B, A is the diameter of the
aperture 24 in plane P, R is the radius of curvature of
the bulged or domed closure member 28, and h is the
maximum vertical height of the domed closure member
above the aperture plane P. In these figures, the foil
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19
closure is shown domed to the point at which the flange
is tangential to the arc of the domed foil closure
member 28, i.e., at which the line of slope of the
flange surface 32 as seen in a vertical plane is tangent
to the arc of curvature of the closure 28 (as seen in
the same vertical plane) at the edge of aperture 24.
For the closure configuration illustrated in FIGS.
2A and 2B, the forces acting on the heat sealed flange
area due to the tension in the foil, are predominantly
shear in character, with no significant peel force
component. In this case, the burst resistance will
depend on the shear strength of the heat seal joint or
the bulge strength of the foil or foil laminate itself.
This ensures that the burst resistance of the lid is
enhanced significantly compared to that of a standard
heat sealed container.
Heat seal bonds are strong under shear loading,
especially at ambient temperature, and an annular heat
seal about 2 mm - 3 mm wide is sufficient to resist the
anticipated shear forces which result from the internal
pressure. If the foil is domed to a lesser extent than
shown in FIGS. 2A and 2B, relative to the flange slope
angle 8, the foil laminate mill tend to hold down the
heat seal bond with a corresponding additional
enhancement of the burst resistance. If, however, the
foil were domed to a greater extent than is shown in
FIGS. 2A and 2B, relative to the flange slope angle, a
peel force component would arise at the inner edge of
the aperture, with an increased likelihood of burst
failure.
The frustoconical aperture-defining flange enables
provision of a flange slope angle 0 sufficient to
accommodate the extent of doming or bulging of the
closure member to be used therewith, under the elevated
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internal pressures for which the can is designed, and
thereby enables the burst resistance to be enhanced
without increasing the peel force requirement.
As will therefore be clear, the flange slope angle
5 and the form of the foil closure strongly influence the
burst resistance. In addition to the flange slope angle
and extent of doming of the closure, not only the
resistance of the heat seal bond to shear forces but
also the strength of the foil of the closure member are
10 selected to withstand the forces acting thereon. If the
flange slope angle, in accordance with the invention, is
such as to substantially avoid any substantial peel
force component of forces acting on the heat sealed area
owing to tension in the foil from the internal pressure
15 acting on the closure member, and if the heat seal bond
and the shear resistance of the bond are adequate, burst
failure could occur by failure of the foil itself, The
shear force required to break the heat seal bond can be
adjusted either by increasing the width of the heat
20 sealed region, or by selecting laminates or coating
formulations which achieve a higher shear strength.
Both of these expedients, however, would increase the
peel force required to open the container.
The effect of heat sealing the closure member 28 to
a sloping flange surface rather than a horizontal flange
surface, will be apparent from a comparison of FIGS. 3
and 4. FIG. 3 represents an aperture 40 in a
conventional lid member 41 wherein the flange 42 around
the aperture is simply a flat horizontal portion of the
lid upper surface, coplanar with the aperture edge 43.
A flexible closure member 44 covering the aperture 40
and bonded by heat sealing to the coplanar flange 42
will bulge, in the same manner as the closure member 28
in FIG. 2A, if the lid member 41 is mounted on a can
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body filled with a carbonated beverage or other
pressure-generating contents. Assuming that equal
elevated pressures exist within the cans of FIGS. 2A and
3, that the diameters of apertures 24 and 40 are equal,
and that the same flexible sheet material is used for
the closure members 28 and 44, the extent of bulging of
the closure members (defined by h and R) should be
essentially identical in both cans. In the case of the
planar flange of FIG. 3, the consequent tension force FT
acting on the heat-seal-bonded portion of the closure
member 44 at the edge of the aperture 40 will have a
substantial peeling force component FP acting at 90° to
the plane of the flange surface. In the case of the
sloping flange of the invention, however, as shown in
FIG. 4, owing to the above-described relation of angle 8
to the angle of the tangent to the arc of curvature of
the domed closure member 28 at the aperture edge 34 (in
which, in FIG. 4, the reverse curl is omitted for
simplicity of illustration), the same tension force FT
(which acts in the direction of the aforementioned
tangent at the edge of the aperture) has no significant
peeling force component FP acting in direction D at 90°
to the plane of the (sloping) flange surface 32.
Under the pressures that may obtain within a can of
carbonated beverage, the peeling force component FP
acting on a flange that is coplanar with the aperture
edge can be sufficient to cause the closure member to
progressively separate from the flange by peeling until
it bursts open, at least if the strength of the heat
seal bond is within conventional limits as desired for
ease of peeling by a user. The sloping of the flange
prevents this from happening, and thereby increases the
burst resistance of the heat-sealed closure member
sufficiently to enable its safe use on a carbonated
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beverage can without having to increase the heat seal
bond strength to a point which would make the closure
member difficult to remove by a user.
It will be understood that the extent of bulging of
the closure member under the influence of pressure
within the can, and thus the angle of the tangent
(relative to plane P) to the bulged or domed closure
member at the aperture edge, is dependent on the
pressure within the can and the elastic deformability of
the closure member. Desirably, the slope angle A of the
flange surface 32 should be chosen to be sufficiently
large so as to be compatible with the bulging
characteristic of the chosen closure member material.
The provision of the flange, which serves as a seat for
the heat sealing of the closure member, as a
frustoconical projection from a (preferably
substantially flat) upper surface of a substantially
rigid lid, facilitates this provision of a relatively
large slope angle. At the same time, by making the
aperture area a minor fraction of the total area of the
can open end, the height h of the domed closure may
readily be kept sufficiently small to be accommodated
between the lid of one can and the concave bottom of
another when the cans are stacked vertically as shown in
FIG. 8.
Further, it will be understood that the benefits of
the invention may be realized even if the flexible
closure member bulges slightly beyond the ideal limit of
tangency to the slope of the flange. Tn such a case,
the peel component of force will start to grow, but may
still be insufficient to cause failure of the bond.
FIGS. 5 - 7 illustrate further the configuration
and arrangement of the flange, aperture and closure
member at the top of the can in the embodiment of
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FIG. 1. With a circular can lid member 16 having a
diameter of 48 mm, mountable on a can body having a
correspondingly dimensioned circular open upper end, a
circular aperture 24 having a diameter of 20 mm is
defined by a frustoconical annular flange 30 having a
maximum diameter (in the plane of lid surface 20) of 30
mm. As best seen in FIG. 7, the foil-polymer laminate
closure member 28 has a circular central portion 32 mm
in diameter (large enough to completely overlie the
sloping outer surface of the flange), with a short
projection 28a on one side for overlying part of the
flat upper surface of the lid and an integral tab
portion 28b on the opposite side which, outwardly of the
flange 30, is not heat sealed but is free to be bent and
pulled. The exploded diagrammatic elevational view of
FIG. 6 indicates the relative positions of the can lid
16 and the closure member 28, as well as the folding of
the tab. The closure member is subjected to a
preliminary forming step to impart a frustoconical shape
(also indicated in FIG. 6) to its circular central
portion for proper seating on and sealing to the
flange 30. ,
The aperture 24 is shown in FIG. 5 as being
disposed eccentrically of the geometric center (center
of symmetry) of the can lid 16, i.e., relatively close
to the edge of the lid, so that a user can easily bring
the aperture to his or her mouth for drinking the
contained beverage directly from the can. However,
depending on use and contents, different positions for
the aperture may be employed. Also, if desired,
aperture configurations other than the circular shape
shown may be provided.
The manufacture of the can of the invention,
including particularly the lid and closure, may (as
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24
stated) be in many respects generally conventional.
However, certain modifications of conventional practice
and equipment, now to be described, are employed to
achieve the novel flange shape and the heat sealing of
the closure member thereto.
Illustratively, but without in any way limiting the
invention thereto, the foil laminate closure stock may
be a suitable aluminum foil (e. g. made of the aluminum
alloy identified by Aluminum Association registration
No. AA3104, with a foil gauge of 75 ~ - 100 ~ laminated
on one side with a suitable heat sealable polymer film
(e.g., polyethylene or polypropylene, 25 a - 50
thick). The other (outwardly exposed) side should have
a suitable protective lacquer coating. It may be
desirable to print onto the foil using rotogravure,
flexographic or another known printing method. It may
also be desirable to emboss the laminate to provide an
attractive surface texture which enhances the appearance
of the closure and assists in opening by making the
closure easier to grip.
In order to seal to the aperture, the closure
members 28 with their described. integral pull tabs are
formed and stamped out from the foil laminate stock
using a suitable press (standard presses can be used
with tooling specifically designed for these closure
members), and are shaped (by a drawing process) so that
they will fit over the raised aperture of the lid.
A heat sealing machine with tooling designed to
conform to the frustoconical flange shape is used to
heat seal the closures to the can lid. That is to say,
the tooling is angled to match the flange (and the
formed closure). The exact heat sealing conditions are
dependent on the polymer and heat seal coating
formulation used. Since the inside coating of the can
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lid member 16 should not be significantly softened or
melted during the heat sealing operation, the bottom
heat seal tool should be held at a relatively low
temperature (<50°C). The upper tool temperature is set
5 to ensure that the heat seal bond is achieved in an
acceptably short time. Typical commercial heat sealing
machines have dwell times of 0.3 sec. The dwell time,
pressure and temperatures may be optimized for the
particular heat seal application. Heat sealing the
10 closure to the lid involves use of a customized heat
sealing line (such as those built by Hans Rychiger AG,
Steffisburg, Switzerland), with appropriately
constructed heat seal tooling provided to bond the
closure to the angled aperture.
15 The forming of the can lid member 16 itself with
the frustoconical flange 30 and aperture 24 as described
is relatively straightforward, using modified can end
forming tooling, with provision for forming the reverse
curl bead 36. The can lids of the invention do not
20 require the formation of a rivet or tab.
The lids, complete with heat sealed closures, are
substantially compatible with existing can filling lines
and will be a direct replacement for the currently
commercially used lids for cans for carbonated beverages
25 and the like. Modifications may be made in the lid
handling equipment to minimize or eliminate the
possibility of damaging the raised aperture and closure.
Alternatively, the can lid may initially be
provided with the aperture 28 and reverse curl bead 36
around the edge thereof, and the closure member 28 may
be heat sealed to the upper surface of the lid in
covering relation to the aperture, before the upwardly
sloping frustoconical configuration is imparted to the
flange portion of the lid immediately surrounding the
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26
aperture. Forming of the frustoconical flange 30 then
proceeds, with concomitant deformation of the already
heat sealed foil closure member, followed by mounting of
the lid on a can body already filled with carbonated
beverage.
As initially applied to the can lid, the portion of
the closure member 28 extending across the aperture may
be substantially planar as indicated at 28c in FIG. 12,
which shows a frustoconical flange 30 having an angle of
slope A of 23°, When the lid is mounted on a can body
filled with a carbonated beverage, so as to completely
enclose the beverage, the resultant pressure within the
can creates a positive differential pressure DP causing
the deformable closure member to bulge upwardly. FIG.
13 illustrates the location of the heat seal annulus 46
on the sloping outer surface of the frustoconical flange
30.
A particular feature of the present invention is
the dimension of the aperture 24. There is a consumer
preference for cans with good pouring characteristics
(good pour rate with a smooth, streamlined flow). Cans
with large opening ends (LOEs) have been introduced in
recent years and have been successful, especially for
beverages with lower carbonation levels (e. g. lemonade
and iced tea), although in the case of highly carbonated
beverages, problems with score line failure and burst
resistance have been encountered. A conventional shape
of apertures for beverage cans is approximately oval
with an aspect ratio between about 1.1 and about 1.5. A
standard aperture is 17.8 mm in diameter and an LOE is
25.4 mm x 17.8 mm; thus, the current aperture size for a
carbonated beverage container, expressed as average
diameter, is from about 17.8 mm to about 22.2 mm.
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Some can designs have also provided a separate vent
hole in the lid to improve pouring and drinking
characteristics, but the inclusion of the vent hole adds
to manufacturing cost and may complicate the opening
process for the consumer.
The aperture size and shape are important in
determining pouring and drinking characteristics. In
general, larger aperture sizes give better flow rates
with a more even flow. The relation between aperture
and flow rates is illustrated by the following test data
obtained in experimental pouring tests with the can lid
oriented downwardly at an angle of 30° to the
horizontal:
TABLE 1
Aperture Pour Rate (g./sec.)
Standard can aperture 56
LOE 70
14.3 mm, flat flange 1S
15.9 mm, flat flange 31
19.0 mm, flat flange 50
22.2 mm, flat flange 75
14.3 mm, angled flange 24
15.9 mm, angled flange 35
19.0 mm, angled flange 56
22.2 mm, angled flange 93
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In the above table, "angled flange" means an
upwardly sloping frustoconical flange as provided in the
present invention; "flat flange" means that the portion
of the lid surrounding the aperture is substantially
coplanar with the aperture edge, as in conventional can
lids.
As will be apparent from Table l, for equivalent
hole sizes, the pour rate for "angled flange" apertures
is higher by about 10 to 15o at a 30° tilt than that for
"flat flange" apertures. The 19.0 mm angled flange
aperture has a pouring rate at 30° tilt approximately
the same as that of the current standard can aperture.
The 14.3 mm aperture (with both flat and angled flanges)
has a significantly lower pour rate than that of the
current standard can aperture. The 22.2 mm angled
flange aperture provides a higher pour rate than the LOE
design (which, like the standard can, has a flat
flange). For the aperture range of interest, the pour
rate is approximately proportional to aperture area.
As hereinafter further explained, the tear/shear
forces acting on the closure member and seal tend to
increase with aperture size, so that the maximum
aperture diameter is limited by the need to provide a
can with adequately high burst pressure or burst
resistance (i.e., the pressure at which the closure
member and seal rupture or fail). Therefore, the range
of average aperture diameter in accordance with the
present invention is between about 15.9 mm and about
25.4 mm, to afford satisfactory pour rates (without any
separate vent hole) and at the same time to achieve high
burst resistance without sacrifice of other
characteristics such as peelability.
Another important characteristic, for attainment of
adequately high burst resistance, is the tear/shear
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force imposed on the heat seal and closure member by a
given differential pressure. The tear/shear force y
(kg/cm) is determined by the differential pressure OP
(kg/cm~), aperture diameter A (cm) and angle of slope
of the frustoconical flange 30, in accordance with the
relation
__A~~P
y 4sinB (1)
In particular instances, depending (for example) on
the degree of carbonation of the contained beverage and
the consequent magnitude of differential pressure that
the can, closure and seal must be designed to withstand,
the design value of tear/shear force resistance for a
can in accordance with the invention (i.e., the value
that the closure member and heat seal must be able to
withstand) may range from less than (or about) 4.5 kg/cm
(25 lb./in.) to about (or even somewhat more than)
75 lb./in., a tear/shear resistance of about 13.4 kg/cm
being currently preferred in many cases. Typical
filling line pressures for carbonated beverages are
between about 345 and about 414 kPa, though for some
beverages (sports drinks, lemonade, etc.), lower
carbonation levels are used. However, in order to take
account of extreme conditions (temperature, agitation,
etc.) a minimum test burst pressure requirement of 620
kPa is currently specified for many applications, and a
burst resistance of 69.5 kPa would be even more
desirable.
Table 2 sets forth values calculated using relation
(1) above for tear/shear force y (kg/cm) for various
aperture diameters A and flange slope angles 8 at a
differential pressure DP of 7.03 kg/cm2 (100 psi).
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TABZE 2
y (
kg/cm)
A(cm)= 1.27 1.59 1.90 2.22 2.54 2.86 3.17
2.5 51.2 64.0 76.8 89.6 102.4 115.2 127.9
5 25.6 32.0 38.4 44.8 51.2 57.6 64.0
7.5 17.1 21.4 25.6 29.9 34.2 38.5 42.8
10 12.9 16.1 19,3 22.5 25.7 28.9 32.1
12.5 10.3 12.9 15.5 18.1 20.6 23.2 25.8
15 8.6 10.8 12.9 15.1 17.3 19.4 21.6
17.5 7.4 9.3 11.1 13.0 14.8 26.7 18.6
20 6.5 8.2 9.8 11.4 13.1 14.7 16.3
22.5 5.8 7.3 8.7 10.2 11.7 13.1 14.6
25 5.3 6.6 7.9 9.3 10.6 11.9 13.2
These are the minimum strength requirements (kg/cm) for
the closure member and heat seal to withstand a pressure
5 differential OP of 7.03 kg/cm~ without rupture or failure
(bursting), for each specified combination of aperture
diameter A and slope angle 8. As is apparent, for a
given differential pressure, the tear/shear force
strength requirement decreases with increasing flange
10 angle and increases with increasing aperture diameter.
By way of illustration, an aperture diameter of
2.2 cm and a flange angle of about 22.5° would require a
closure foil with a breaking strength in excess of
10.2 kg/cm and an equivalent minimum heat seal shear
15 strength, for burst resistance of 7.03 kg/cm2.
Typical aluminum lidding foils of 75 ~ thickness
can withstand a tear force in excess of13.4 kg/cm.
Practicable heat seals capable of withstanding a shear
force of 75 lb./in. can also readily be provided, in
20 configurations suitable for the heat seal 46.
Therefore, combinations of A and 8 in Table 2 for which
the calculated value of y is13.4 kg/cm or less enable
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satisfactory and practicable attainment of a burst
resistance of 7.03 kg/cm2 in the can of the present
invention.
As already stated, to avoid a peel component in the
force exerted on the closure member and heat seal by the
differential pressure DP, the bulge height h of the
closure member above the plane P of the aperture 24
should not exceed a value hmax at which the slope of the
flange 30 is tangent to the arc of the bulging closure
l0 at the edge of the aperture. This upper limiting value
hmax (in mm) is, again, determined by the angle of slope
8 of the flange and the aperture diameter A (in mm) of
the aperture 24; in the case of a circular aperture,
such limiting value can be calculated using the relation
hmax ' ~ ~ sin B tan 8 ~ ( 2 )
It will be seen that the maximum permitted bulge height,
to achieve the described freedom from any peel
component, increases with aperture diameter and also
increases with flange angle.
The actual bulge height in a closure member 28
produced by a given differential pressure DP is
dependent on the properties of the closure foil related
to deformation, i.e., the deformability of the foil, as
well as on the aperture diameter. FIG, 24 illustrates
the relationship of bulge height h (here given in mm) to
pressure DP for a 22.2 mm aperture diameter and an
exemplary aluminum foil 100 a thick.
Examples of the maximum permitted bulge height (mm)
as defined above, calculated for a circular aperture
using relation (2), for various combinations of A (in
mm) and 8, are set forth in Table 3:
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TABLE 3
hmax
A(mm) 8 17.5 20 22.5 25 27.5 30
-
15.9 1.2 1.4 1.6 1.7 1.9 2.7.
19 1.5 1.7 1.9 2.1 2.3 2.5
22.2 1.7 1.9 2.2 2.5 2.7 3.0
25.4 1.9 2.2 2.5 2.8 3.1 3.4
For an aperture diameter of 22.2 mm with a flange slope
angle of 22.5°, the maximum bulge height should be 2.2
mm to avoid peel force components.
If the bulge height exceeds the critical value,
FIG. 14 can be used to determine the angle of the
tangent to the arc of the bulging closure foil at the
edge of the aperture. If the stress within the foil can
be determined, the peel component of the stress can be
estimated. Provided that this component is less than
the measured peel stress for the closure material,
failure by peeling will not occur. However, it is
preferred that the lid parameters be chosen to ensure
that the bulge height does not exceed the above-defined
limiting value at least for differential pressures up to
620 kPa, more preferably for differential pressures up
to 689 kPa.
Metal foils have comparatively good creep
resistance over the range of temperatures that may be
experienced in service, and therefore afford an
important advantage over polymeric closure member
materials with respect to creep susceptibility and
consequent short shelf life. Since creep is dependent
on applied stress, increasing the thickness of the
closure material can reduce or eliminate creep. For
aluminum foil closure members, a thickness between about
75 - 100 a is sufficient to virtually eliminate creep.
The performance of the bond between the closure
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membrane and the lid flange is dependent on the
properties of the adhesive layer and on the design of
the joint. The flange angle is designed to ensure that
the forces between the closure membrane and the flange
are predominantly shear in character under the fully
pressurized conditions of use. However, the shear
stress in the joint can be affected by the width of the
heat seal; i.e., increasing the width of the bond
spreads the load and thereby reduces the stress
intensity.
It is desirable for the width of the heat seal to
be less than about 3 mm and preferably about 2 mm. If
the width is increased above about 3 mm, the peel force
required to open the container will be increased.
Furthermore, an increased heat seal (and flange) width
would mean that the drinking aperture has to be located
further from the container edge, detracting from the
convenience of the consumer by making the container less
comfortable and more inconvenient to drink from.
Experimentally, it is found that a 2 mm wide heat
seal annulus for the foil closure performs well in the
can of the invention (see Example 4 below). Fully
pressurized cans have been stored at ambient temperature
(20°C) for several weeks, with no detectable sign of
creep in either the foil or in the adhesive bond joint.
In containers for beverages and the like with
manually peelable closures, the peel force required to
open the container should preferably fall within the
range between about 3N and 20N, and still more
preferably within the range between about 10N and 16N as
measured by a 90° peel test. The peel force required is
dependent on the peel strength of the bond and on the
effective width of the seal during the peeling
procedure. In the case of an angled flange, there will
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also be a geometrical factor, which will affect the
final peel force required.
Tn the case of heat seal bonding, the peel strength
is influenced by the particular lacquer formulations on
the two mating surfaces, and on the heat sealing
conditions which are used. For example, in one
preferred embodiment, the outer can end panel surface
has a thin vinyl lacquer coating (Valspar Unicoat°, up
to about 2 p. thick) and the aluminum foil closure
material has a vinyl based heat seal lacquer (Alcan
Rorschach TH388°, between about 5 and 8 a thick).
For this combination of coatings, the peel strength
falls within an acceptable range for peelability. At
the same time, provided the closure foil has sufficient
strength, the heat seal bond can meet the requirements
for shear strength.
Variations in peel strength can be obtained by
changes to the heat sealing temperature, the heat
sealing pressure and/or the dwell time for sealing.
In addition to the aforementioned vinyl based
lacquer systems, various other combinations of can end
lacquer and heat seal coatings have been found to be
suitable for the present invention. These are
exemplified, without limitation, as follows:
Can Iid coating (exposed side) Foil Closure coating
Epoxy coating (solvent based
lacquer)
Polypropylene (extrusion coated) Polypropylene based
heat sealed lacquer
Laminated polypropylene Polypropylene
formulation: extrusion
coated
Polyester coated (e. g. extrusion Polyester compatible
coated) heat seal coating
It should be recognized that the combination of
specific coating formulations on the can lid (exposed
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side) and on the foil closure material (product side)
needs to be carefully selected to provide the desired
combinations of peel strength and shear strength.
Furthermore, the coatings must also provide adequate
5 protection from any corrosive attack of the metal by the
product. The coatings must also comply with applicable
food/beverage contact regulations.
The coatings, at the thicknesses applied, must also
be capable of maintaining integrity during the forming
10 operations to which the components of the lid are
subjected. In particular, the coating on the lid must
survive the bead curl forming operation.
It is found that coating formulations based on the
classes of coatings listed above are able to meet all of
15 these requirements. As will be seen from the above list,
at least one of the two coating formulations (and
preferably both) have a thermoplastic polymer as a major
component (e.g. vinyl, polypropylene, polyester) and
heat sealing is the preferred method of attaching the
20 closure.
It will also be noted that the adhesion between the
lacquers/coatings and the metal surfaces is important
and suitable cleaning and pretreatment of the metal
surface prior to coating is recommended.
25 As already stated, for the peelable closures of the
present invention, it is desirable that the foil closure
be relatively easy for the consumer to peel back from
the pouring/drinking aperture. However, it is also
desirable to design the closure in such a way that the
30 consumer is discouraged from removing the closure foil
completely, since it may then be discarded as litter. A
preferred design of closure for this purpose is
illustrated in FIG. 15, which shows a can lid 116 having
a flat upper surface and an eccentrically disposed
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36
aperture 124 surrounded by an angled flange to which a
foil closure member 128 is bonded by an annular portion
146a of a heat seal. On the side of the aperture
adjacent the lid edge, the closure member has an
integrally formed pull tab 128b (folded back over the
aperture, with its unfolded position indicated at
128b'). The closure member also has an integral "stay-
on" extension 128a positioned in opposed relation to tab
128b (with respect to the aperture) and overlying the
flat upper surface of the lid. Extension 128a is bonded
to the lid by a further heat seal portion 146c, which is
so dimensioned as to require a substantially greater
peeling force (for separating extension 128a from the
lid) than that required by annular heat seal portion
l5 146a (for separating the closure member from the angled
flange around the aperture).
In other words, the closure member 128 of FIG. l5
includes a "stay-on" tab area or extension 128a which is
sealed to the lid panel 116 by portion 146c of the heat
seal that has a size and shape which requires a substan-
tially higher peel force (greater resistance to peeling)
than the annular seal portion 146a surrounding the
aperture 124, thereby discouraging the consumer from
completely removing the closure foil. As a result of
this design, when the consumer peels open the closure,
the peel will initially be within the targeted range for
each opening, e.g. about 10-20N. Then as the aperture
is completely opened, the peel force will fall to a very
low value so that the consumer will sense that the
opening is completed. If the consumer continues to pull
the closure, the required peel force will rise rapidly
to a value which exceeds the normally accepted easy peel
range, i.e. about 25N. An example of the peel
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characteristics of a closure of this invention is given
in FIG. 16.
This variation in peel force requirement can be
achieved most readily by careful design of the seal
region, in particular by appropriately selecting the
dimensions of the heat seal portions 146a and 146c. In
the case of a heat sealed closure, this is easily
achieved by the design of the heat seal tooling. With a
pressure sensitive adhesive, it would be more difficult
and would require the adhesive to be printed onto the
closure film in the desired pattern.
FIG. 16 is a graph showing a typical variation of
peel force (90° peel test) as the closure is peeled
open. As the peel is initiated, the force rapidly
increases as the foil peels away from the region of the
flange on the pull tab side 128b. As the foil is peeled
from the remainder of the flange and opens the aperture,
the peel force remains fairly constant, rising to a
second maximum at the end of the aperture. At this
point, the foil is not sealed to the lid, and the peel
force falls quickly to a low value. At the start of the
"stay-on" extension region, the peel force rises to a
high value to discourage the consumer from completely
removing the closure foil.
Furthex control of the peel force can be obtained
by varying the heat sealing conditions in the different
regions of the closure. For example, if the temperature
of the heat seal in the stay-on extension region were
increased, a high peel strength would result. It is
also possible to use a different heat seal lacquer, with
a higher inherent peel strength, in the "stay-on"
extension region. Yet another method of increasing the
peel force requirement in the "stay-on" tab region is by
the use of one or more ridges or other profiled features
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(not shown). Such features would serve to increase the
effective area of the seal and to provide a degree of
mechanical keying for the closure.
As~discussed above with reference to FIG. 15, the
peel force varies as the closure is peeled back. The
detailed variation of the peel force required can be
adjusted and controlled by the various methods
described. The variation shown in FIG. 16 corresponds
to a desirable behavior for the consumer, in that the
uniform peel force after an initial higher start force
provides ease of opening for the container; the
subsequent drop in peel force gives the consumer an
indication (by feel) that the aperture is completely
opened; and, finally, the rapid rise of the force due to
the "stay-on" extension signals the consumer that the
closure is intended to stay on and be folded back for
drinking.
With an aluminum foil closure material, employing a
"stay-on" arrangement as described, the closure can be
easily folded down so that it does not significantly
interfere with the drinking experience of the consumer.
Furthermore, since the foil has good dead-fold charac-
teristics (i.e. it does not exhibit any noticeable
spring back), the closure can be folded back over the
aperture if desired. Although this does not reseal the
can, it would prevent the undesired ingress of dirt or
insects into the beverage between drinks, and may also
reduce the spillage if a can is accidentally tipped.
Yet another advantageous feature of the invention,
in particular embodiments as illustrated in FIGS. 17-20,
is the incorporation of a source of a fragrance or aroma
in the can lid, so that peeling of the closure member to
open the can also acts to expose a small quantity of an
oil or wax based aroma concentrate, located on the lid
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in a position which is in close proximity to the
nostrils of a person drinking from the can aperture.
The aroma is selected to enhance or complement the taste
of the beverage.
It is well known that the senses of smell and taste
are closely related, and in particular that the sense of
smell can significantly enhance the taste experience.
Preservation or enhancement of a smell associated with a
particular beverage, thereby improving the aroma of the
product, may serve to increase the overall enjoyment of
the product. Fragrances which may be thus provided may
include (by way of nonlimiting illustration) lemon,
orange, lime, mint, etc.
The aroma-enhancing feature may, for example,
advantageously be incorporated in a can lid 116 having a
"stay-on" foil closure member 128 as described above
with reference to FIGS. 15-16. A small part of the lid
area, initially covered by the foil closure member (FIG.
17A) but exposed upon peeling of the closure member
(FIG. 17B), is modified so as to receive a small
quantity 156 of an oil- or wax-based fragrance. This
can be achieved by forming a small upwardly opening
depression or reservoir 158 in the lid 116 (FIG. 18)
and/or by forming a similar receptacle indentation
(facing the lid; not shown) in the foil closure member
itself.
The reservoir, and hence the supply of fragrance,
are disposed on the side of the aperture 124 away from
the edge of the lid so as to be close to the nostrils of
a person drinking from the can. This location is
between the aperture 124 and the stay-on heat seal
portion 146c and is thus covered by the closure
extension 128a when the closure member is sealed on the
lid.
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A wide variety of concentrated fragrances are
readily available and, for the described use, the volume
required is about one drop (less than 0.1 ml). Since
the fragrance is sealed between the lid 116 and the
5 closure member 128, there is little if any loss of
fragrance during storage, owing to the excellent barrier
properties of aluminum.
When the foil closure member is peeled back (FIG.
17B) to open the can it exposes the fragrant oil 156,
10 releasing the aroma. As will be apparent from the
drawings, the fragrance reservoir 158 is positioned on
the can lid in close proximity to the nose of a person
drinking straight from the can, to maximize the
effectiveness of the aroma.
15 For use with a lid having a fragrance reservoir,
the heat seal 146 securing the closure member 128 to the
lid 116 is configured to fully surround the reservoir
158 containing the supply of fragrance. Two specific
heat seal designs for this purpose are respectively
20 shown in FIGS. l9 and 20. In FIG. 19, the heat seal
area 146a around the aperture 124 is contiguous with the
heat seal area 146b surrounding the fragrance reservoir
or well 158 and the heat seal portion 146c that secures
the "stay on" extension 128a of the closure member to
25 the lid; the design is such that as the lid is peeled
back from the aperture, there is a high probability that
the fragrance-containing depression 158 in the lid will
be partially or fully exposed and the fragrance will
start to be released. In FIG. 20, the heat seal area
30 146d surrounding the fragrance containing reservoir is
isolated from the heat seal portions 146a (around the
aperture) and 146c (bonding the stay-on closure member
extension to the lid), but again, the action of peeling
back the closure member results in partial or complete
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opening of the reservoir to release the fragrance. In
the case of FIG. ~0, by isolating the fragrance
reservoir 158 from the main heat seal areas 146a and
146c, the probability of premature evaporation of the
fragrance owing to heat input from the heat sealing
tools is significantly reduced.
In brief summary, the present invention provides a
novel can end with a safe and convenient aperture and a
heat sealable foil closure, suitable for use with
carbonated beverages or similar products. Among the
benefits and advantages that may be achieved with the
pans of the invention are the following:
-- improved sanitary characteristics, because no
external exposed surface is introduced into the
beverage, as occurs when present-day scored lids are
opened with a riveted pull-tab system;
-- enhanced aesthetics, in that the peelable foil
closure can be embossed and printed (inside and/or
outside);
-- increased selection of aperture size and shape
since, while there will be some limitations, a wider
range of aperture sizes and shapes will be possible than
is the case with present-day scored lids;
-- greater safety, in particular because the
reverse curl of the aperture-defining bead eliminates
sharp edges;
-- ease of opening, and concomitant consumer
satisfaction, since marketing studies in the food
industry indicate that consumers prefer easy-peel
closures to the scored ends of present-day carbonated
beverage cans as well as to the use of can openers;
-- ease of use, since a can with this end design
has better pouring characteristics and may be easier to
drink from directly.
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Especially preferred embodiments of the invention
are carbonated beverage cans with readily peelable
closure members characterized by a burst resistance of
at least about 620 kPa or higher, e.g. 689 kPa or above,
and a shelf life of at least six months or more. The
creep resistance and barrier properties of foil
closures, together with the shear strength of heat
seals, enable attainment of the desired extended shelf
life.
By way of further illustration of the invention,
reference may be made to the following specific
examples, in which Example 1 is a hypothetical example
and Examples 2 and 3 describe burst resistance tests
performed on actual samples of can lids with heat-sealed
l5 closures embodying features of the invention, while
Example 4 describes actual tests related to shelf life.
In these Examples, identifications of aluminum alloys by
four-digit numbers with the prefix "AA" refer to
designations of aluminum alloy compositions registered
with the Aluminum Association, as will be understood by
persons skilled in the art.
EXAMPLE 1
An illustrative can end (lid) embodying the present
invention with a heat sealed foil/polymer laminate
closure might be constructed with the following
specification:
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Aperture diameter (A) . 25.4 mm
Flange angle . 20-25
Laminate . 100 a foil (AA 3104) + 25
polymer (e. g., polyethylene,
polypropylene, polyester)
Heat seal width . 2.5 mm
Cand id sheet . 228 ~ (AA 5182 alloy) with a heat
sealable coating
It will be understood that a range of values for each
parameter should be possible. The target burst
resistance for such a lid would be > 620 kPa and the
target peel force (at 90° to the plane of the aperture)
would be < 1.8 kg,
EXAMPLE 2
Tests were performed to determine peel strength and
burst resistance for can ends (lids) of "202" can end
size (a standard can size designation) in accordance
with the invention, having an annular frustoeonical
flange with an 18° angle of slope defining an aperture
19 mm in diameter, covered by a foil closure heat sealed
to the flange around the aperture. The lids were formed
from can end sheet of AA5182 aluminum alloy at a gauge
of 22 ~, and their outer surfaces were coated with
"Valspar" unicoat at a coating weight of 1.5 mg/in2
(approximately 1.5 a thick). The closures were made
from heat sealable stock of 50 a foil of AA3105 aluminum
alloy, coated on its inner surface (the surface in
contact with the aperture-defining frustoconical flange)
with Rorschach TH388~ vinyl heat seal lacquer at a
coating weight of 6 g/m~ (about 6 ~ thick). Heat
sealing was performed at various selected tool
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temperatures (on the side of the foil closure) of from
230° to 280°C, with a pressure of 975 N and a time of 0.3
sec.
Initially, to determine peel strength, T-peel test
pieces were prepared from the can end sheet and heat
sealable foil stock described above by heat sealing 15
mm wide strips of the foil stock to 15 mm wide can end
sheet samples for different heat seal temperatures (as
listed in FIG. 10). Results, summarized in FIG. 10,
show that the peel strength can be adjusted for this
combination of materials by modifying the heat seal
temperature. As mentioned above, a peel force of
between about 10 N and about 15 N is generally regarded
as acceptable for an easy opening container. Since the
anticipated width of the heat seal for closures
embodying the present invention may be typically or
conveniently approximately 15 mm, the peel forces will
fall within this acceptable range.
To test burst resistance, a number of formed and
heat sealed can ends as described were subjected to a
standard burst test in which the rim of the can end is
clamped to a rubber gasket seal and a gradually
increasing air pressure is applied to the inner lid
surface. The deformation of the lid and seal can be
observed during the test and the maximum pressure at
failure is recorded. After testing the lids are
examined to determine the mode of failure.
The results of these burst~tests are shown in FIG.
11. For these tests, burst pressures of approximately
424 kPa were recorded. During the tests it was noted
that the foil closure 28 stretched and "domed" to a
point where the tension in the foil had developed a
significant peel component, i.e., the tangent (in a
vertical plane) to the bulged foil closure 28 at the
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edge of the aperture 24 exceeded the 18° slope angle of
the flange 30, as illustrated diagrammatically in
FIG. 9. Failure of the seal occurred by a peel
initiated at the inner edge of the aperture.
5 A 414 kPa burst resistance is sufficient for low
levels of carbonation or for normally carbonated
beverages under standard conditions of use. However,
since carbonated products must be capable of tolerating
varying degrees of extreme conditions (elevated
10 temperature, agitation, etc.), the normal targeted burst
resistance is generally 620 kPa or higher. In the case
of the materials employed in this Example, higher burst
resistance should be achieved with this gauge of foil if
a higher flange angle (e. g. 25°) were to be used.
15 EXAMPLE 3
A further series of can ends in accordance with the
invention were prepared and. tested. The lid members
were the same (dimensions, gauge, alloy, coating, flange
slope angle and aperture diameter) as in EXAMPLE 2, but
20 the closures were made of heat sealable foil stock of 70
a foil of AA9802 aluminum alloy with an inner surface
coated with a vinyl heat seal lacquer of unknown
formulation. Heat sealing was performed with a tool
temperature (on the foil closure side) of 280°C, under
25 the same pressure and time conditions as in EXAMPLE 2.
These materials (can end sheet and foil closure)
were subjected to peel strength testing. Peel strengths
of greater than 20 N /15 mm were recorded for these
samples. This is too high for convenient peeling and
30 indicates that the vinyl lacquer was not a suitable
formulation.
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Samples of the lids and closures were formed,
subjected to heat sealing, and tested for burst
resistance. Burst resistance was found to be > 620 kPa.
During the burst tests, the foil closures bulged to form
a shallow dome, but the distortion was not sufficient to
create a significant peel component to the resultant
tension force.
Failure of the lids eventually occurred by
distortion of the can end shell metal. The foil and the
heat seal survived the test satisfactorily.
With the thicker foil of this Example, the doming
which occurs at pressures below 620 kPa (for the 19 mm
aperture) was below the level at which a peel component
of force would arise.
EXAMPLE 4
The shelf life of cans in accordance with the
invention was tested by preparing a can having a lid in
accordance with the invention, including an angled
flange having an 18° angle of slope and defining a
circular aperture 19 mm in diameter. The closure was
aluminum foil 100 a thick, with a vinyl/acrylic lacquer
("TH 388") used for the heat seal, which had a width of
2 mm. The internal pressure of the can was345 kPa. The
can was examined weekly for over eight weeks.
Throughout this period, there were no detectable changes
in bulge height of the foil closure and there was no
detectable change in the heat seal joint (i.e., no
sliding) .
In a further test, another can was prepared, having
a lid in accordance with the invention, including an
angled flange having an 18° angle of slope and defining
a circular aperture 22.2 mm in diameter. The closure
was aluminum foil 100 a thick, with a vinyl/acrylic
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lacquer ("TH 388") used for the heat seal, which had a
width of 2 mm. The internal pressure of the can was
4l4 kPa. The can was examined weekly for over six
weeks. Throughout this period, there was no change in
bulge height of the foil closure and no detectable
change in the heat seal joint (i.e., no sliding).
It is to be understood that the invention is not
limited to the features and embodiments hereinabove
specifically set forth, but may be carried out in other
ways without departure from its spirit.
