WO2012152608A1

WO2012152608A1 - Peelback

Info

Publication number: WO2012152608A1
Application number: PCT/EP2012/057895
Authority: WO
Inventors: Gerry Patrick DOYLE; Ian Maxwell; Mark Jonathan Prestidge
Original assignee: Crown Packaging Technology, Inc.
Priority date: 2011-05-06
Filing date: 2012-04-30
Publication date: 2012-11-15

Abstract

An improved closure for a metal container body (1) in which a lid (3) of peelable lidding material is sealed directly to an upwardly and outwardly inclined sealing surface (2), the sealing surface being an inner surface of the container body. Another aspect of the invention relates to a method of making such a closure. The sealing surface at and outwardly adjacent to the inside edge of an annular bond region (4) is inclined such that a non-peel loading is applied to the annular bond region.

Description

PEELBACK

Technical Field

[0001] This invention relates to an improved closure for a metal container body in which a lid of peelable lidding material is sealed directly to an upwardly and outwardly inclined sealing surface, the sealing surface being an inner surface of the container body. Another aspect of the invention relates to a method of making such a closure.

Background Art

[0002] EP 1855822 B (CROWN PACKAGING TECHNOLOGY, INC) 30/12/2009 discloses a metal container having a lid of peelable lidding material sealed directly to a sealing surface, the sealing surface being an inner surface of a container body delimiting the access opening of the container body. The peelable lidding material is a multilayer structure with an aluminium layer of from 6 to 90 microns thickness and a bond layer. Having the lidding material sealed directlyio the inner surface of the can body avoids having to bond the lidding material to an intermediate metal ring, which in turn would have to be fastened to the container body. To provide the advantages of stackability of one container upon another, the sealing surface is upwardly and outwardly inclined (as shown in figures 3 to 5 of EP 1855822 B1 ). The stackability results from the upward and outward inclination of the sealing surface, which defines a recess within which the base of another container can nestle. The inclined sealing surface also has the benefit of increasing the rigidity of the container and its ability to sustain being dropped without damage being caused to the container.

[0003] However, despite these advantages, there is a problem with containers of the type disclosed in EP 1855822 B1. Generally, at some point during the container's lifecycle, the filled and sealed container will be subject to negative differential pressure. By "negative differential pressure" is meant where the pressure outside the enclosed volume of the container is greater than that within the enclosed volume of the container. For example, the container may be vacuum packed, in which case the negative differential pressure would remain until the container is opened. Alternatively, where the container is used for storing food product requiring sterilisation (for example, as performed in a retort), the negative differential pressure may arise during the sterilisation process. Regardless of precisely how the negative differential pressure occurs, it causes the lid to be stretched across the access opening into an inward facing dome. This is shown clearly in figure 1 which shows a container body 1 of the type disclosed in EP 1855822 B1 having an inclined sealing surface 2, to which is sealed a lid 3 of peelable lidding material to define an annular bond region 4. The figure shows the action of negative differential pressure P resulting from vacuum packing or retort processing in stretching the lidding material across the access opening into an inward facing dome having a radius of curvature R. The change in curvature in the lid where the dome meets the annular bond region 4 induces a peel force F_p component acting at the inside edge of the bond region. This peel force component F_p can result in progressive peeling of the lid 3 from the inclined sealing surface 2 (this is also shown on figure 1) - a phenomenon known as "peelback". Most worryingly, as peelback initiates from the inside of the container, it is not visible to either the filler or a consumer. In serious cases, peelback can result in rupture of the seal between the lid and sealing surface. Even where rupture does not occur, it can result in a reduction of the seal width and therefore an undesirable weakening of the seal.

[0004] There is therefore a need for an improved closure suitable for direct

sealing to a container body in which the risk of peelback is reduced or eliminated, whilst maintaining the advantages of having an upwardly and outwardly inclined sealing surface. The invention provides both a closure and a method of making a closure having these advantages.

Summary of invention

[0005] Accordingly, there is provided a closure for a container, the closure

comprising a metal container body having an access opening, and a lid for closing the access opening,

the lid formed of a peelable lidding material and having a seal portion and a central portion, the seal portion enclosing the central portion and fixed directly to a sealing surface to define an annular bond region, the sealing surface being an inner surface of the container body delimiting the access opening, at least part of the sealing surface being upwardly and outwardly inclined from the container body's central axis;

characterised in that:

the sealing surface at and outwardly adjacent to the inside edge of the annular bond region (4) is inclined (2b) such that when a negative differential pressure is applied over the lid's central portion to stretch the central portion across the access opening into an inwardly domed profile, at any given annular location on the inside edge of the annular bond region the relative inclination of the central portion and the seal portion is such that a non-peel loading is applied to the annular bond region; and wherein the sealing surface (2) at and outwardly adjacent to the inside edge of the annular bond region comprises an annular step (2a), the annular step (2a) extending either:

i) generally parallel to a horizontal plane defined by the periphery of the access opening;

ii) or downwardly out from the container body's central axis at an acute angle of less than 20°. The annular bond region represents the seal established between the lid and the container body.

[0006] By generally parallel to the horizontal plane is meant to within a tolerance of ± 5° relative the horizontal plane.

[0007] This preferential inclination of the sealing surface at and outwardly

adjacent to the inside edge of the annular bond region helps to avoid the problem of peelback by ensuring that the annular bond region is

predominantly loaded under shear force(s) when the lid is subject to a negative differential pressure. In simple terms, this inclination ensures that where the domed central portion meets the annular bond region, any mismatch in curvature at the meeting location acts to assist in ensuring that the annular bond region remains predominantly loaded in shear rather than in peel under the action of the negative differential pressure. It therefore enables containers of the type disclosed in EP 1855822 B1 to be manufactured with increased confidence that negative differential pressures will not cause progressive peeling of the lidding material from the inside edge of the annular bond region, i.e. peelback. This is particularly beneficial for containers intended for the packaging of products under vacuum conditions (for example, sweetcorn), because the preferential inclination of the sealing surface at and outwardly adjacent to the inside edge of the annular bond region reduces or eliminates any peel force loading on the inside edge of the bond region. This reduction or elimination of the peel force loading is best understood from looking at figure 3.

[0008] The invention provides a solution to the problem of peelback, whilst

retaining the advantages of EP 1855822 B1 , namely:

the ability to stack one container upon another, with a recess defined by the upwardly and outwardly inclined part of the sealing surface of a first container enabling the base of a second container to nest securely on top of the first container;

the upwardly and outwardly inclined part of the sealing surface providing enhanced rigidity relative to a conventional straight walled container.

[0009] Having the step extending generally parallel to the horizontal plane

provides a flat surface for stacking. However, incorporating a limited downward inclination to the step (of up to 20°) ensures that the annular bond region also has an increased ability to withstand positive differential pressure without peelback occurring, whilst still retaining stackability. For a container body of 73 mm in diameter, the step need only be of the order of 1 to 2 mm in width; therefore, it does not add appreciably to the size and footprint of the container.

[0010] As a variation or addition to the use of an annular step, peelback

avoidance can be further improved by providing the central portion of the lid with a profile comprising one or more annular beads. Alternatively or in addition, the lid is formed so that an annular unbonded region of the central portion of the lid extends down along the inner surface of the container body. Both of these aspects of the invention provide the central portion of the lid with additional surface area relative to having the lid simply being bonded to the sealing surface and pulled taut across the access opening. The additional surface area ensures that on application of a negative differential pressure (whether the result of vacuum packing, processing or otherwise), the central portion is able to deform into a deeper inwardly domed profile having a smaller and tighter radius of curvature. This helps to ensure that at the meeting location between the domed central portion and the annular bond region, any mismatch in curvature acts to ensure that the bond region remains predominantly loaded in shear rather than in peel. In summary, the likelihood of the inside edge of the annular bond region being subjected to peel forces is further reduced.

[001 1] Conveniently, the sealing surface is profiled to define a concavely-profiled segment when viewed from above the access opening of the container body. Preferably, the concavely profiled sealing surface is profiled such that its radius of curvature at and outwardly adjacent to the inside edge of the annular bond region corresponds with that of the central portion of the lid when subject to the negative differential pressure. This feature provides the lid with an aesthetically pleasing appearance as it minimises or eliminates the presence of a ridge line being formed in the lid at the junction between the seal and central portions due to the negative differential pressure pulling the lid down into the container.

[0012] The lid is preferably a laminate structure. A preferred form of laminate structure is one having a metal substrate to provide a gas-tight layer (aluminium is particularly preferred), the substrate coated on one surface with a heat sealable coating to enable easy fixing to the container body. Conveniently, the lid comprises an aluminium layer of from 6 to 90 microns thickness. The lid may also include a tab (preferably being an integral part of the lid structure). The tab may be folded back onto the lid and, optionally, at least partly fixed to the lid (for example, by heat sealing) so as to keep the tab folded back onto the lid.

[0013] As for the container body, aluminium or steel are particularly preferred materials. The container body is preferably formed from a metal sheet section deformed into a cylinder, the adjoining sides of the metal sheet welded together along the height of the cylinder. Conveniently, a heat sealable lacquer is applied to the interior of the container body to enable the lid to be heat sealed to the container body. As is known in the field of peelable lids for containers, an especially good heat seal is provided by ensuring that the lacquer system on the container body and the coating on the interior facing surface of the lid each contain polypropylene.

[0014] However, the invention is not linnited to use of particular materials, other than the requirement for the container body to be made of metal. Rather, the lid and container body each have any structure (including any coatings or lacquers) that enable formation of a peelable bond between the lid and the container body.

[0015] In common with the invention of EP 1855822 B1 , the upwardly and

outwardly inclined part of the sealing surface is preferably inclined from the container body's central axis at an acute angle of from 20° to 60°.

[0016] In another aspect of the invention directed towards reducing the likelihood of peelback occurring, there is provided a method of forming a closure for a container, comprising:

i. taking a metal container body;

ii. locally applying a heat sealable lacquer to an annular region of the inner surface of the container body adjacent an end of the container body to define a localised annular coated region of the container body's inner surface, the annular region of the container body's inner surface being free of geometric discontinuities;

iii. outwardly inclining an annular part of the inner surface of the container body at the end of the container body relative to the remainder of the container body's inner surface, such that the localised annular coated region covers a majority of the outwardly inclined part and extends over the transition between the outwardly inclined part and the remainder of the container body's inner surface, thereby ensuring an consistent thickness of lacquer;

iv. heat sealing a lid of peelable lidding material to the outwardly inclined part through bonding to the heat sealable lacquer.

[0017] Previously, containers such as those disclosed in EP 1855822 B1 having a lid sealed directly to an upwardly and outwardly inclined inner surface of the container body would have the heat sealable lacquer applied after formation of the outwardly inclined surface. However, this has been found to result in undesired thinning of the heat sealable lacquer at the transition between the inclined surface and the remainder of the container body's inner surface due to "run-off' of the lacquer material at the transition region. This run-off occurs at the location corresponding to the inside edge of the seal between the lid and the container body and therefore can exacerbate the risk of peelback occurring because there is less material to bond the lid to. In any event, the run-off can result in reduction of the seal area between the lid and the container body and therefore an undesired reduction in the strength of the seal between the lid and container body.

[0018] The method of the invention avoids the above-mentioned problems of the background art by ensuring that the lacquer is applied before formation of the outwardly inclined part to which the lid is later bonded. This thereby allows the coating to dry without undesired "run-off' and therefore avoids undesired localised thinning of the lacquer, especially at the location corresponding to the inside edge of the bond between the lid and the container body.

[0019] To provide additional reassurance against the occurrence of peelback, the method may be adapted to provide the container body and the lid with the features defined for the closure of the invention.

[0020] To provide increased rigidity to the container body structure a peripheral curl is formed at the end of the container body. This is formed before, during or after step iii of the method of the invention.

Brief description of drawings

[0021 ] Figure 1 shows a cross-section through a closure of the prior art, indicating the problems of peelback (see earlier discussion of the Background Art).

[0022] Various embodiments of the invention are described with reference to the following drawings:

[0023] Figure 2 shows a cross-section through a first embodiment of a closure according to the invention, before and after application of a negative differential pressure. [0024] Figure 3 shows a cross-section through a second embodiment of a closure according to the invention, before and after application of a negative differential pressure.

[0025] Figure 4 shows a cross-section through the embodiments of figures 1 and 2 after application of the negative differential pressure.

[0026] Figures 5 to 8 show cross-section views illustrating the method of the

invention.

Description of embodiments

[0027] Figure 2 shows a metal container body 1 having an annular sealing

surface 2. A lid 3 of peelable lidding material is sealed to the sealing surface 2 to define an annular bond region 4. As stated in the general description of the invention, the annular bond region 4 represents the seal established between the lid 3 and the container body 1.

[0028] The sealing surface 2 has a horizontal step portion 2a and an inclined portion 2b. As shown in the figures, the sealing surface 2 is an inner surface of the container body 1 , with the inclined portion 2b being inclined upwardly and outwardly at an angle of around 60° from the central axis 5 of the container body 1. The end of the container body outwards of the sealing surface terminates in a peripheral curl 6.

[0029] The lid 3 is shown divided into two distinct portions:

seal portion 3a, representing that portion of the lid 3 which is bonded to the sealing surface 2 to form the annular bond region 4;

and central portion 3b, which is that part of the lid 3 enclosed by the seal portion 3a.

[0030] For the first embodiment, the lid 3 is profiled with two annular beads 7 (see figure 2) before application of any negative differential pressure.

[0031] The second embodiment differs from that of figure 2 in having an annular unbonded region 8 of the central portion 3b of the lid 3 extending down along the inside surface of the container body 1 before application of any negative differential pressure (see figure 3).

[0032] The dashed line in figures 2 & 3 shows the action of a negative differential pressure P acting over the area of the lid 3. The negative differential pressure P may arise from the filled container undergoing a sterilisation process in a retort, or a filled container being vacuum sealed. As shown by the dashed line in figures 2 & 3, the negative differential pressure P has the effect of deforming the central portion 3b into an inwardly-protruding dome having a radius of curvature R. The annular beads 7 and the annular unbonded region 8 of the first and second embodiments

respectively provide additional surface area to the lid's central portion 3b to enable the dome to extend deeper into the container body 1 than merely having a lid with a planar central portion.

[0033] Figure 4 is applicable to the embodiments of figures 1 & 2 and shows a detail view of the container body 1 and lid 3 in the region of the annular bond region 4, showing the forces imposed on the annular bond region as a result of the central portion 3b being subjected to the negative differential pressure P. As will be understood by comparison of figure 4 (the invention) and figure 1 (the background art), the incorporation of the horizontal step portion 2a at and outwardly adjacent to the inside edge of the annular bond region 4 ensures that the inward doming of the central portion 3b under the action of the negative differential pressure P does not act to impose peel forces on the bond region 4. Rather, the annular bond region 4 is instead loaded in shear τ. Therefore, peelback of the lidding material is avoided. The influence of the horizontal step portion 2a and the doming of the central portion 3b can also be understood by comparison of angles a and β at a given annular location 'A' on the inside edge of the annular bond region 4:

where angle a represents the acute angle between i) a tangent 9 to the central portion 3b of the domed lid and ii) horizontal plane 10; and where angle β represents the acute angle between i) a tangent 12 to the seal portion 3a of the domed lid 3 and ii) the horizontal plane 10. In the embodiment of figure 4 the horizontal step portion 2a is parallel to the horizontal plane 10, meaning that the tangent 12 lies parallel to the plane 9 and therefore angle β is zero.

[0034] From considering figure 4 and angles α, β, it is understood that a positive difference between a and β signifies no peel loading at the inside edge of the annular bond region 4. [0035] Figures 5 to 8 relates to the method of the invention. A straight walled cylindrical container body 10 provided with a peripheral curl 1 1 at one end is shown in figure 5. A heat sealable lacquer, preferably containing polypropylene, is applied by a spraying assembly 12 to a localised annular region of the inner surface of the container body 10 adjacent the end of the container body. This results in a localised annular coated region 13 adjacent the end of the container body 10.

[0036] The lacquer on the annular coated region 13 is then allowed to dry and is cured. The lack of any discontinuity on the surface of the container body 10 to which the lacquer is applied means that the lacquer is able to dry and cure having a consistent uniform thickness. There is no tendency for "run-off' to occur.

[0037] Subsequently, an annular part 14 of the inner surface of the container body 10 is then outwardly inclined relative to the remainder of the inner surface of the container body 10. In one embodiment, this is achieved by use of a spinning tool 15 having an inclined surface 16 and able to rotate about its longitudinal axis 17. Either or both of the spinning tool 15 and the end of the container body 10 are driven towards each other, with the rotating action of the spinning tool 15 causing the annular part 14 to be progressively outwardly inclined. The resulting inclined annular part 14 of the container body's inner surface has an inclination corresponding to that of the inclined surface 16 of the spinning tool 15. Other means, such as die flanging, may be used to incline the annular part 14 of the container body's inner surface. Formation of the inclined annular part 14 results in the annular coated region 13 - being of uniform thickness - extending over a majority of the inclined annular part 14 and just over the transition region 15 with the remainder of the container body's inner surface.

[0038] In a further subsequent step, a lid 18 is sealed to the inclined annular part 14 (as shown in figure 8).

Claims

1. A closure for a container, the container comprising a metal container body (1) having an access opening and the closure comprising a lid (3) for closing the access opening,

the lid (3) formed of a peelable lidding material and having a seal portion (3a) and a central portion (3b), the seal portion enclosing the central portion and fixed directly to a sealing surface (2) to define an annular bond region (4), the sealing surface (2) being an inner surface of the container body (1 ) delimiting the access opening, at least part of the sealing surface (2) being upwardly and outwardly inclined from the container body's central axis;

characterised in that:

ii) or downwardly out from the container body's central axis at an acute angle of less than 20°.

2. A closure as claimed in either of claim 1 , wherein the central portion (3b) of the lid (3) has a profile comprising one or more annular beads (7).

3. A closure as claimed in claim 1 or claim 2, wherein an annular unbonded

region (8) of the central portion of the lid extends down along the inner surface of the container body (1 ).

4. A closure as claimed in claim 1 , wherein the sealing surface (2) is profiled to define a concavely-profiled segment when viewed from above the access opening of the container body.

5. A closure as claimed in claim 4, wherein the radius of curvature (R) of the concavely profiled sealing surface at and outwardly adjacent to the inside edge of the annular bond region corresponds with that of the central portion of the lid when subject to the negative pressure differential.

6. A closure as claimed in any preceding claim, wherein the upwardly and

7. A method of forming a closure for a container, comprising:

i. taking a metal container body (10);

ii. locally applying a heat sealable lacquer to an annular region of the inner surface of the container body adjacent an end of the container body to define a localised annular coated region (13) of the container body's inner surface, the annular region of the container body's inner surface being free of geometric discontinuities;

iii. outwardly inclining an annular part (14) of the inner surface of the container body (10) at the end of the container body relative to the remainder of the container body's inner surface, such that the localised annular coated region covers a majority of the outwardly inclined part and extends over the transition between the outwardly inclined part and the remainder of the container body's inner surface, thereby ensuring an consistent thickness of lacquer;

iv. heat sealing a lid (18) of peelable lidding material to the outwardly inclined part (14) through bonding to the heat sealable lacquer.

8. A method as claimed in claim 7, wherein before, during or after step iii a

peripheral curl (1 1 ) is formed at the end of the container body.