Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTAINER SEALING
This invention relates to the sealing of
containers, and in particular to the material that is
used to provide a seal between a container body and an
end closure, as well as to a process for forming a
seal.
Much research has gone into the problem of
providing a secure seal between containers and
corresponding end closures. A seal must be formed
between these two components after the container is
~0 filled with, say, a beverage or microwavable food. It
is of course important to ensure that the contents of
the sealed container do not leak out, and also to
prevent bacteriological or other contamination of the
contents from the ambient atmosphere.
The container may be of metal or plastic
material. The closure member is usually of metal. It
will often include a ring-pull or other frangible
portion, in order to allow ready access to the
contents.
~0 It is conventional to crimp or otherwise form the
end closure onto a flange at the open end of the
container, and to provide a sealant composition
between the two components. Metal cans formed in this
way are described in, for example, US-A-3403813,
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US-A-3774560, US-A-3882763 and US-A-4089283; reference
to these specifications will show various forms, and
procedures for forming, container-closure seals.
In US-A-3403813, the sealant is atactic amorphous
polypropylene; it states that this material has
viscosity characteristics such that it provides a
permanently tacky sealant between the can end and can
body. In US-A-3774560, an expandable foam
seam-sealing compound is used, which expands, on
10 heating, to fill voids that may be present in the seam
area.
Despite these various proposals, sealed
containers of the type described suffer from various
disadvantages. Firstly, it is difficult to fill all
15 the voids that may be formed during sealing,
especially when the two components are respectively of
metal and plastics. Secondly, the compressive force
that is applied when bringing the two components into
contact leads to the possibility of buckling of
20 relatively weak containers, e.g. of plastics.
Thirdly, the stress imposed by the forming of the
respective components at the area of sealing means
that multi-layer plastics materials may delaminate
during seaming, when the curl overlap between
25 container and closure takes place.
According to a first aspect of the present
invention, in a sealed container comprising a
container body, an end closure member and a sealant
therebetween, the sealant is bonded to the body and to
30 the closure me~ber by a bond having a peel/bond
strength of at least 30, e.g. up to 50, 75, 100 or
more, Newtons, by a flat seal peel test which involves
flat-sealing a 15 mm wide strip of coated aluminium to
a 15 mm wide strip of polypropylene, as used in a
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container; the samples are then peeled apart at an
angle of 180 and at a speed of -100 mm/min.
According to a second aspect of the present
invention, an end closure member, of the type suitable
for sealing to a container body, carries a composition
that can be activated indirectly to form a sealant
bonded to the closure member and also to a container
body.
According to a third aspect of the present
invention, a method for producing a sealed container
comprises seaming a container body and an end closure
member with an indirectly-activatable composition
therebetween, and activating the composition so that
it forms bonds to the closure member and to the
container body.
The present invention is based on the utility of
a strong bond between the container body and the
sealant, and between the sealant and the end closure.
This is in contrast to the previous use of materials
20 which have various properties but are designed
primarily to prevent the passage of contaminants in
and/or contents out. Further, by contrast with the
known system of treating a sealant composition to foam
it in situ, the present invention can involve the use
Of an activatable material to form the desired bond.
The effect of the strong bond is to provide
several important advantages to products and processes
of the present invention. Firstly, the burst strength
of the sealed container is enhanced. Secondly, the
fact that a strong bond can be formed with plastics
material indirectly means that little compressive
force is needed when bringing the components together;
the system is therefore as useful for plastics as
metal containers. Thirdly, the sealant fixes the
relative positions of the components in the sealed
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container and resists the forces that are built in
during crimping; this is a particular advantage when
using plastics laminates that are liable to
delaminate. By creating a weld between the materials,
the strength of the finished seam will be increased
even if delamination has taken place during seaming.
Unlike unsealed containers, further delamination will
be restricted and therefore a much stronger and higher
integrity container will result.
In general terms, the nature of the container
body and closure member may be conventional. The
container will usually be provided with a flange at
its open end, to facilitate the contact between the
components. The closure member may be formed with a
ring-pull or other frangible portion.
The components may be combined in conventional
manner, e.g. using chucks and rolls to press the
components together. A hermetic joint is formed by
interlocking the edges to both the closure and
container components. The joint is generally produced
in two operations, a sealing compound having already
been introduced between the other components, e.g. by
forming a layer thereof on the closure member. The
first operation forms a metal/plastic or metal/metal
curl/overlap, while the second operation flattens them
to produce the required seam. This operation may be
followed, if necessary or desired, by activation of
the sealant, e.g. by induction heating, to a
temperature of, ay, 150 to 200C.
Examples of sealant materials than can be used in
the invention include the following:
Polymers that can be extruded onto aluminium or
steel sheet: these include polypropylene,
polyethylene and polybutylene;
Morprime-based systems;
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Polyolefinic resins: these are mainly modified
ethylene acrylates (trade names include Admer, Modic
and Bynel);
PE-CTFE;
PE-TFE;
PTFE;
EVA;
MAA:
EAA;
Ionomers: and
Modylene P/A (carboxylic acid-modified PP).
By way of example, the invention will be
described below in connection with a plastics or
polypropylene container and a metal or aluminium end
closure, although steel is an alternative metal, and
of course the invention is applicable to metal
container-metal end systems.
As already indicated, a Morprime/polypropylene or
other material can be applied to one of the metal or
plastics components, and most suitably to a metal
closure member. It can be applied as a lacquer, by
way of replacement for a conventional epoxy lacquer.
A primary aim of the lacquer i8 to provide a, say,
polypropylene/aluminium weld in the area of the double
seam, and to provide protection of aluminium from
attack by acids, e.g. in the container contents.
A suitable lacquer may comprise a dispersion of
modified polypropylene or other polymer in a blend of
high boiling solvents. The polypropylene can be
30 blended with epoxy resins to provide excellent
adhesion to aluminium and steel and can be used to
make high strength laminations to polypropylene.
The sealant may be applied in an overall coating
process. Alternatively, the sealant may be applied
selectively. For example, a heat-sealing material may
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be applied only to the flange overlap area on the
aluminium or other closure member.
In either case, bonding between a polypropylene
flange and an aluminium end can be formed indireGtly.
For example, fusion is crea~ed by passing the seamed
container through an induction field. The energy can
be focused on the area of overlap by coil design,
causing the aluminium to heat, and resulting in the
lacquer melting and fusing to the polypropylene.
Conventional contact heating systems or
ultrasonics could create the same effect as induction
sealing. An important advantage of induction sealing
is that the time taken to create the bond is very
short, and will therefore not significantly affect
15 existing canning line speeds. Other suitable heating
systems include sonic welding, induction welding,
radio-freguency welding, conduction welding,
spin-welding and impulse sealing.
In addition to the advantages described above,
20 the present invention may allow greater potential for
the reduction of problems associated with reverse seam
wrinkles. It may also provide wider seaming operating
windows. All these advantages and effects can be
achieved without any significant reduction of
25 conventional double-seaming line speeds with a
heat-sealing system.
The following Example illustrates the invention.
ExamPle
Aluminium sheet was hand-coated with a Morprime
lacquer. The Morprime lacquer basically consists of
an epoxy lacquer containing a polypropylene
suspension. The lacquer was applied as a thin film
and cured to the surface in an oven.
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The sheets were converted to ends and seamed onto
30 cm. diameter containers, and induction-sealed using
a system supplied by Stanelco.
For testing, the containers were split into two
groups. One group was tested without retorting, and
the other group was retorted at 121C and then tested.
The containers were tested by assessing their burst
strength. The burst test involves introducing
compressed air into the container in a controlled
manner, and recording the pressure needed to rupture
the pack's seal.
The results of the burst tests are given below:
Unretorted Containers
_
Conventional Induction-
double-seamed sealed
-
1st OP seam only very low 193 kPa
.
2nd OP seam131 kPa 234 kPa
Retorted Containers
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Conventional Induction-
double-seamed sealed
1st OP seam only very low 213 kPa
2nd OP seam124 kPa 207 kPa
_
These results show that the induction-sealed
containers not only possess a burst strength twice
that of conventionally-seamed containers, but also
possess equivalent bond strengths when only seamed
with the first operation (OP) seaming roll.
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In the flat seal peel test described above,
results of 30-40 Newtons have been obtained by
operation in accordance with the invention.
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