U.S. patent application number 10/751756 was filed with the patent office on 2005-07-07 for easily openable closure for a retortable container having a metal end to which a membrane is sealed.
This patent application is currently assigned to Sonoco Development, Inc.. Invention is credited to Morin, Jeremy E., Nelson, Brian E., Williams, Alan.
Application Number | 20050145630 10/751756 |
Document ID | / |
Family ID | 34711495 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050145630 |
Kind Code |
A1 |
Williams, Alan ; et
al. |
July 7, 2005 |
Easily openable closure for a retortable container having a metal
end to which a membrane is sealed
Abstract
A retortable, easily openable closure of the type having a metal
end and a membrane closure heat-sealed thereto, wherein the metal
end has a coating of a heat-seal composition comprising a
food-compatible metal-coating composition blended with
polypropylene. The polypropylene disperses within the
food-compatible metal-coating composition during curing of the
coating on the metal end such that an outer surface of the coating
defines a multitude of separate, spaced islands of polypropylene
dispersed within the food-compatible metal-coating composition so
as to form bonding sites. The membrane closure's lower surface has
a heat-seal layer of polypropylene for sealing to the coating on
the metal end.
Inventors: |
Williams, Alan; (Camden,
SC) ; Nelson, Brian E.; (Camden, SC) ; Morin,
Jeremy E.; (Florence, SC) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
Sonoco Development, Inc.
|
Family ID: |
34711495 |
Appl. No.: |
10/751756 |
Filed: |
January 5, 2004 |
Current U.S.
Class: |
220/359.3 ;
220/359.4 |
Current CPC
Class: |
B65D 2517/5032 20130101;
B65D 2517/5048 20130101; B65D 17/502 20130101; B65D 2517/5013
20130101; B65D 2517/5056 20130101; B65D 2517/5008 20130101; B65D
2517/0013 20130101 |
Class at
Publication: |
220/359.3 ;
220/359.4 |
International
Class: |
B65D 041/00 |
Claims
What is claimed is:
1. A retort container assembly for producing a retort container,
the retort container assembly comprising: a container body having a
top end; a metal end joined to the top end of the container body,
the metal end defining an opening through which contents of the
container are dispensed, an upper surface of the metal end that
surrounds the opening having a coating of a heat seal composition
comprising a food-compatible metal-coating composition blended with
polypropylene, the polypropylene dispersing within the
food-compatible metal-coating composition during curing of the
coating on the metal end such that an outer surface of said coating
defines a multitude of separate, spaced islands of polypropylene
dispersed within the food-compatible metal-coating composition so
as to form bonding sites; and a membrane for attachment to the
metal end to seal closed the opening thereof, a lower surface of
the membrane having a heat-seal layer thereon for heat-sealing to
the polypropylene bonding sites on the coating on the metal end,
the heat-seal layer comprising polypropylene and bonding to the
membrane with a bond strength exceeding that between the heat-seal
layer and the coating on the metal end.
2. The retort container assembly of claim 1, wherein the coating on
the metal end comprises about 1 to 20 weight percent
polypropylene.
3. The retort container assembly of claim 2, wherein the coating
comprises about 10 weight percent polypropylene.
4. The retort container assembly of claim 1, wherein the coating
comprises epoxy and polypropylene.
5. The retort container assembly of claim 1, wherein the membrane
includes a foil layer, and the heat-seal layer of polypropylene is
bonded to the foil layer.
6. The retort container assembly of claim 5, wherein the membrane
further includes a layer of polyethylene terephthalate, the foil
layer being disposed between the layers of polypropylene and
polyethylene terephthalate.
7. A process for forming a seal between a metal end of a container
and a closure membrane such that a peel strength of the seal is
substantially unaffected by a retort process and such that the seal
has a controlled low peel strength rendering the seal readily
openable, the process comprising the steps of: providing the
membrane to have a heat-seal layer exposed on a side of the
membrane to be sealed to the metal end, the heat-seal layer
comprising a layer of polypropylene; providing the metal end to
have a coating of a heat seal composition comprising a
food-compatible metal-coating composition blended with
polypropylene, the polypropylene dispersing within the
food-compatible metal-coating composition during curing of the
coating on the metal end such that an outer surface of said coating
defines a multitude of separate, spaced islands of polypropylene
dispersed within the food-compatible metal-coating composition so
as to form bonding sites for bonding to the polypropylene heat-seal
layer on the membrane; and attaching the membrane to the metal end
by heat-sealing the heat-seal layer on the membrane to the coating
on the metal end via said bonding sites, wherein the coating on the
metal end is formulated such that the heat-seal layer on the
membrane bonds to the coating with a bond strength less than that
between the heat-seal layer and the membrane, whereby failure
occurs between the heat-seal layer and the coating when the
membrane is pulled to open the container.
8. The process of claim 7, wherein the entire outer surface of the
coating is provided to have a substantially uniform distribution of
said islands of polypropylene.
9. The process of claim 7, wherein the coating on the metal end is
provided to have about 1 to 20 percent polypropylene by weight.
10. The process of claim 7, wherein the coating is formulated of
epoxy and polypropylene.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a container having a
closure formed by a metal end joined to a top end of the container
and defining an opening through which contents of the container are
dispensed, and a peelable membrane sealed to the metal end so as to
hermetically close the opening. The invention relates more
particularly to such a container closure that is capable of
withstanding a retort sterilization process without failing, but
that has a controlled peel strength that is sufficiently low to be
readily openable by hand.
BACKGROUND OF THE INVENTION
[0002] A variety of food products are packaged in containers that,
after filling and sealing, are subjected to a retort sterilization
process to render the filled containers shelf-stable such that they
do not require refrigeration until opened (referred to herein as
"retort containers"). The retort process typically involves
elevating the temperature of the filled and sealed container to
about 250-260.degree. F. for about one hour. During the retort
process, pressure builds up in the container. It is critical that
the container closure not fail or leak during or after the retort
process.
[0003] Various types of closures have been developed for retort
containers. One type of closure includes a metal end that 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 a
flexible membrane that is sealed to the metal end so as to cover an
opening defined therein. The membrane generally includes a metal
foil layer and may include one or more additional layers such as
polymer and/or paper. One side of the foil layer typically is
sealed to the metal end on the container using suitable sealing
materials. Heat sealing is generally employed to seal the membrane
to the metal end.
[0004] The sealing material used for attaching the membrane to the
metal end must have a sufficiently high melting temperature so that
the strength of the seal during the retort process is not impaired
to such an extent that the seal fails. Polypropylene has commonly
been used as the sealing material for retort containers because it
is heat-sealable and has a melting point exceeding the temperature
of the retort process. A heat-seal layer of polypropylene is
disposed on the lower side of the membrane. Similarly, a coating of
polypropylene is applied to the metal end, for example by applying
a composition comprising polypropylene suspended in an organic
solution and then evaporating the solvent. After filling of the
container, the membrane is sealed to the metal end by heat-sealing
the polypropylene heat-seal layer on the membrane to the
polypropylene coating on the metal end.
[0005] The polypropylene-to-polypropylene bond is strong enough to
withstand the retort process. However, the bond tends to be so
strong that the peel force required to peel the membrane from the
metal end is unacceptably high.
[0006] Retortable easy-peel closures have been developed to try to
overcome the above problem. The easy-peel closures generally
include some type of failure mechanism in a heat-seal layer on the
membrane so that the heat-seal layer fails internally. In some of
these closures, a controlled defect is designed into the heat-seal
layer by adding incompatible polymers, fillers, or both, to
polypropylene or other heat-sealable polymer such as polyethylene.
For example, low-density polyethylene and/or high-density
polyethylene have been added to polypropylene to make a heat-seal.
Talc has been used as a filler in polypropylene as well as in
polyethylene. The incompatible polymers and fillers work on the
principle of creating a weakness within the heat-seal layer to
initiate a crack in the layer when the membrane is peeled from the
metal end. In other closures, the membrane has included a
coextruded heat-seal layer comprising a layer of polypropylene
coextruded with a layer of polypropylene to which fillers and/or
incompatible polymers have been added, and the intended failure
mechanism is a delamination between the coextruded layers as a
result of a weakening of their bond caused by the added fillers
and/or polymers. Such membranes having controlled defects are
relatively complex and thus tend to be relatively expensive to
produce.
SUMMARY OF THE INVENTION
[0007] The present invention provides a retortable, easily openable
closure of the type having a metal end and a membrane, wherein the
failure mechanism during peeling of the membrane from the metal end
is not a cohesive failure within the heat-seal layer on the
membrane. The heat-seal layer on the membrane in accordance with
the invention does not require incompatible polymers or fillers to
be added, nor does it require a coextruded structure, because the
failure mechanism involves a failure between the heat-seal layer on
the membrane and a corresponding heat-seal coating on the metal
end. Accordingly, the heat-seal layer on the membrane is simplified
and therefore less costly.
[0008] In accordance with the invention, the metal end has a
coating of a heat-seal composition comprising a food-compatible
metal-coating composition with polypropylene dispersed therein. The
polypropylene disperses within the food-compatible metal-coating
composition during curing of the coating on the metal end such that
an outer surface of the coating defines a multitude of separate,
spaced islands of polypropylene dispersed within the
food-compatible metal-coating composition so as to form bonding
sites. The polypropylene islands are microscopic in scale. The
membrane's lower surface has a heat-seal layer thereon, comprising
a layer of polypropylene. The polypropylene heat-seal layer on the
membrane is heat-sealed to the coating on the metal end, whereby
the polypropylene heat-seal layer bonds to the bonding sites on the
coating. The peel strength of the resulting bond depends on the
fraction of the coating's surface area that is represented by the
bonding sites. As the collective area of the bonding sites becomes
a greater fraction of the coating's total surface area, the peel
strength increases. In accordance with the invention, the heat-seal
layer of polypropylene on the membrane bonds to the membrane with a
strength exceeding that between the heat-seal layer and the coating
on the metal end, such that when the membrane is pulled to open the
container, the failure occurs between the heat-seal layer on the
membrane and the coating on the metal end. A controlled low peel
strength can be achieved by suitably formulating the
polypropylene-filled metal-coating composition.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0009] 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:
[0010] FIG. 1 is a cross-sectional view through a top end portion
of a container in accordance with one embodiment of the
invention;
[0011] FIG. 2 is a cross-sectional view taken on line 2-2 in FIG.
1, with the membrane and metal end shown separated for clarity;
and
[0012] FIG. 3 is a top view of the coating on the metal end,
greatly enlarged in scale to show the microscopic islands of
polypropylene.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present inventions now will be described more fully
hereinafter with reference to the accompanying drawings, in which
some but not all embodiments 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.
[0014] FIGS. 1 through 3 illustrate one embodiment of the invention
relating to a metal end and membrane closure for a retortable
container. The container 10 includes a flange 12 at its top end,
the flange projecting radially outwardly in an initial state (not
shown) of the container prior to incorporation of a closure. The
closure for the container includes a metal end 14 attached to the
top end of the container, such as by double-seaming the metal end
with the flange as shown. The metal end 14 defines an opening 16
through which contents of the container can be dispensed. The metal
end includes a generally horizontal upper surface 18 that encircles
the opening 16. The closure also includes a flexible membrane
closure 20 that is sealed to the upper surface 18 of the metal end
to hermetically seal the container. The membrane closure
advantageously includes a pull tab 22 that is not sealed to the
metal end so that it can be readily grasped and pulled to remove
the membrane from the metal end.
[0015] In accordance with the invention, the membrane closure 20 is
heat-sealed to the metal end 14 by cooperative heat-seal layers
applied to the closure and metal end. More particularly, a
heat-seal layer 24 (FIG. 2) is applied to the upper surface 18 of
the metal end. The heat-seal layer 24 comprises a food-compatible
metal-coating composition into which polypropylene is blended. The
food-compatible metal-coating composition should bond well to metal
and should have good corrosion resistance, and should be capable of
withstanding a retort process without substantially degrading.
Various food-compatible metal-coating compositions can be used in
the practice of the invention, including but not limited to
epoxies, plastisols, polyesters, food-grade lacquers, and
others.
[0016] When the polypropylene is blended into the food-compatible
metal-coating composition and the composition is coated onto the
metal end and cured, the surface of the resulting heat-seal layer
24 is characterized by a multitude of microscopic spaced "islands"
26 of polypropylene that are dispersed within the matrix of the
epoxy 28 (see FIG. 3). These polypropylene islands 26 comprise
bonding sites to which the membrane closure 20 can be sealed as
further described below.
[0017] The membrane 20 can include various layers to provide
desired gas and moisture barrier properties and to have sufficient
tear-resistance; such layers can include one or more of a metal
foil, a polymer layer (e.g., polyethylene terephthalate), a kraft
paper layer, a lacquer layer, etc. In the illustrated embodiment,
the membrane 20 includes a metal foil layer 30 and a polymer layer
32 laminated to the upper side of the foil layer. The foil layer
provides gas and moisture barrier properties, but would be easily
tearable if not reinforced. The polymer layer imparts
tear-resistance to the membrane and can also impart additional
barrier properties.
[0018] A heat-seal layer 36 is applied to the lower surface of the
membrane 20. In accordance with the invention, the heat-seal layer
36 does not require fillers or incompatible polymers and need not
be a complex coextruded laminate as in the prior art, because the
heat-seal layer 36 is not intended to fail within itself when the
membrane is peeled from the metal end. Instead, the intended
failure mode is an adhesive failure between the heat-seal layer 36
on the membrane and the heat-seal layer 24 on the metal end. In
accordance with the invention, the heat-seal layer 36 comprises a
layer of polypropylene without the addition of any other components
such as fillers or incompatible polymers that would substantially
affect the ability of the layer 36 to remain substantially intact
during removal of the membrane from the metal end.
[0019] The polypropylene of the heat-seal layer 36 bonds to the
polypropylene bonding sites 26 on the metal end when the membrane
is heat-sealed to the metal end. By virtue of the structure of the
surface of the heat-seal layer 24, the bond between the membrane
and the metal end is not as strong as the bond that would exist if
the heat-seal layer 24 were pure polypropylene. As noted, such a
bond between two pure polypropylene layers would be too strong to
allow the membrane to be readily peeled from the metal end. In
contrast, the bond between the polypropylene layer 36 and the
epoxy/polypropylene layer 24 is weaker because less than the full
surface area of the layer 24 is bonded to the polypropylene layer
36 of the membrane.
[0020] The peel strength of the bond between the membrane 20 and
the metal end 14 can be controlled by varying the fraction of the
total surface area of the heat-seal layer 24 that is made up of the
polypropylene bonding sites or islands 26. This fraction is a
function primarily of the relative proportions of the metal-coating
composition and polypropylene making up the heat-seal material of
the layer 24. In accordance with the invention, the polypropylene
advantageously comprises from about one percent to about 20 percent
by weight of the composition, and suitably can comprise about 10
percent by weight of the composition.
[0021] The closure system in accordance with the invention is
suitable for retort containers. Polypropylene has a melting
temperature of about 160.degree. C. (about 280.degree. F.), and
thus can withstand a retort process, which typically involves
heating a sealed container to about 250 to 260.degree. F. for about
one hour. The closure system in accordance with the invention also
can provide a relatively low peel strength between the membrane and
the metal end.
[0022] A series of peel strength tests were conducted to assess the
peel strength of a closure system in accordance with the invention
as well as a number of prior-art closure systems. The tests also
sought to determine whether the peel strength is affected by
subjecting the closure to an elevated temperature similar to a
retort process. Five different closure configurations were tested.
In each configuration, a sheet metal precursor or blank from which
a metal end is made had a coating of an identical Watson Rhenania
epoxy/polypropylene blend applied to it and cured, such that the
sheet metal precursors in all cases had substantially identical
heat-seal layers of the epoxy/polypropylene composition. The
surface of the heat-seal layer included polypropylene islands
dispersed in the epoxy matrix generally as shown in FIG. 3. The
membranes were heat-sealed to the sheet metal precursors with 40
psi pressure for one second at 400.degree. F. with a rubber bar
under the precursor to help ensure that a uniform pressure was
applied over the surface of the membrane.
[0023] The membrane structures used in the five test configurations
are shown in Table I below:
1TABLE I Membrane Structures Tested Membrane Configuration Layer 1
Layer 2 Heat-Seal Layer A 1 .mu.m lacquer 62 .mu.m foil Coex PP/PP
mixed with LDPE and filled with talc and B 1 .mu.m lacquer 72 .mu.m
foil Coex PP/PP mixed with HDPE C 16 .mu.m PET 21 .mu.m foil PP
mixed with LDPE D 3 .mu.m lacquer 66 .mu.m foil LDPE mixed with
HDPE and filled with talc E 12 .mu.m PET 9 .mu.m foil PP
[0024] Configuration A was a Lawson Mardon Gold Foil membrane,
configuration B was a Lawson Mardon Silver Foil membrane,
configuration C was a Lawson Mardon PET film membrane,
configuration D was an Alcoa membrane, and configuration E was a
membrane in accordance with one embodiment of the present
invention.
[0025] Samples of each test configuration were subjected to a peel
test with an Instron tester with a T-peel geometry, and the maximum
peel load per inch width of the membrane was recorded. Some samples
of each configuration were subjected to simulated retort conditions
by heating the samples to 250.degree. F. for one hour, with a
15-minute heat-up period and a 15-minute cool-down period. The peel
test was done for each configuration both before the retort process
and after the retort process. The results are shown in Table II
below:
2TABLE II Peel Strength Test Results with Epoxy/ Polypropylene Heat
Seal Layer on Metal Peel Strength Peel Strength Membrane Before
Retort After Retort Configuration (lb/in) (lb/in) A 4.8 4.2 B 4.9
4.6 C 4.8 4.0 D 5.3 3.5 E 6.4 5.0
[0026] The standard deviation in all cases was 0.2 to 0.3.
[0027] Two additional series of tests as described above were
conducted on the same five membrane structures described in Table
I, but wherein the heat-seal layer applied to the metal precursors
was pure polypropylene. Specifically, in one series a coating of
Morprime 10B (polypropylene suspended in an organic solution) was
applied to the metal precursors using a number 28 metering rod
(correlates to a coating weight of about five pounds per ream) and
then dried in an oven at 220.degree. C. for 15 to 20 minutes to
evaporate the solvent. In the other series, a Morprime 14B coating
was applied and dried in the same manner. The results of the first
series of tests are shown in Table III and the results of the
second series of tests are shown in Table IV below:
3TABLE III Peel Strength Results with Morprime 10B Polypropylene
Heat-Seal Layer on Metal Peel Strength Peel Strength Membrane
Before Retort After Retort Configuration (lb/in) Std. Dev. (lb/in)
Std. Dev. A 5.5 0.8 5.0 0.4 B 5.2 0.1 4.9 0.2 C 5.6 0.4 4.9 0.7 D
4.3 0.3 3.0 0.4 E 26 2 22 3
[0028]
4TABLE IV Peel Strength Results with Morprime 14B Polypropylene
Heat-Seal Layer on Metal Peel Strength Peel Strength Membrane
Before Retort After Retort Configuration (lb/in) Std. Dev. (lb/in)
Std. Dev. A 4.9 0.4 4.2 0.2 B 5.2 0.3 4.3 0.3 C 4.4 0.4 5.2 0.9 D
3.6 0.2 2.4 0.2 E 23 1 14 2.5
[0029] The results in Table II above indicate that a membrane and
metal closure system made in accordance with the invention
(configuration E membrane sealed to a metal having an
epoxy/polypropylene heat-seal layer) was able to achieve a
relatively low peel strength generally of the same magnitude as the
other configurations tested. The peel strength declined only
moderately after retort.
[0030] Comparison of the results in Tables III and IV with those in
Table II further reveals that the membrane configurations A through
D did not have substantially different peel strengths whether
sealed to the epoxy/polypropylene heat-seal layer or the pure
polypropylene heat-seal layer. This is consistent with the failure
mechanism in these closure configurations being a failure of the
heat-seal layer on the membrane rather than an adhesive failure
between the membrane and the metal. In contrast, the configuration
E membrane had a much higher peel strength when bonded to the pure
polypropylene heat-seal layer than when bonded to the
epoxy/polypropylene heat-seal layer. This is consistent with the
failure mechanism being an adhesive failure between the heat-seal
layer on the membrane and the heat-seal layer on the metal.
[0031] From the foregoing, it will be appreciated that the
invention allows a membrane and metal end closure to be constructed
with a relatively simple membrane structure. The closure is able to
withstand a retort process without substantial degradation of the
bond strength between the membrane and the metal end. At the same
time, the peel force required to remove the membrane can be
controlled to be relatively low so that the membrane is readily
removable. The invention allows the peel force to be controlled in
a simple fashion by suitably varying the proportions of epoxy and
polypropylene in the heat-seal composition applied to the metal
end.
[0032] 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.
* * * * *