Patent Application
20050109784
1. Technical Field
The invention relates generally to easy-open containers for consumer products, wherein the container is sealed closed by heat-sealing a flexible membrane closure to a metal end, flange, or rim that encircles the top opening of the container. The invention more particularly relates to such a container employing a heat-seal material composed of an acidic ionomer.
2. Description of Related Art
There is a class of easy-open product containers that are sealed closed by heat-sealing a membrane closure to a metal end or to a flange or rim that encircles the open top of the container. Such containers, for example, are marketed under the ULTRASEAL® trademark for containing various food products. In the case of containers having a metal end, the metal end generally comprises tin-plate steel, and its upper surface is coated with a heat-seal material The metal end is double-seamed to a flange formed on the container. The membrane closure can have various constructions, often including a foil layer as a barrier, and/or one or more polymer layers. The lower surface of the membrane closure has a heat-seal material on it for heat-sealing to the heat-seal material coated on the metal end.
In prior seals of this type, the heat-seal material that has been used is an acidic ionomer, in particular an ethylene methacrylic acid ionomer (i.e., a copolymer of ethylene and methacrylic acid wherein the methacrylic acid groups are partially neutralized with a cation, typically zinc or sodium ions), such as the composition marketed under the SURLYN® trademark by DuPont. Current ionomer heat-seal material used in this sealing application has an acid content of about 15%. This relatively high acid content provides grease-resistance to the bond between the membrane closure and the metal end. However, the bond tends to become stronger with aging, to the point that it can become very difficult for the consumer to remove the membrane closure from the metal end.
It has been discovered that a higher acid content of the heat-seal material promotes such bond-strengthening with age (referred to as the “age-up” phenomenon). The invention addresses this problem by providing a heat-seal material comprising a blend of two different acidic ionomers of respectively higher and lower acid content such that the resulting blend has an intermediate acid content. In one embodiment, the more-acidic ionomer has an acid content of about 15% and the less-acidic ionomer has an acid content of about 7%, and the blend contains approximately equal proportions of the two ionomers, such that the acid content of the blend is about 11%. The reduced acid content of the resulting blend leads to a reduction in the age-up problem, while still retaining sufficient oil- and grease-resistance for the seal.
In a preferred embodiment, the more-acidic ionomer comprises an ethylene methacrylic acid ionomer (i.e., a copolymer of ethylene and methacrylic acid wherein the methacrylic acid groups are partially neutralized with a cation, typically zinc or sodium ions), such as the composition marketed under the SURLYN® trademark by DuPont. The less-acidic ionomer comprises an ethylene acrylic acid ionomer (i.e., a copolymer of ethylene and acrylic acid wherein the acrylic acid groups are partially neutralized with a cation, typically zinc or sodium ions), such as the composition marketed under the TRANCEND® trademark by DuPont.
A container in accordance with one embodiment of the invention comprises a container body defining a top opening, a sealing surface being defined on the container body encircling the top opening, and a closure sealed to the sealing surface by a heat-seal material comprising a blend of two different acidic ionomers having respectively higher and lower acid content such that the heat-seal material has an acid content intermediate between that of the two different ionomers, whereby the resulting heat seal strikes a balance between oil-resistance and age-up.
A container in accordance with another embodiment of the invention comprises a container body defining a top opening, a metal end attached to the container body encircling the top opening, and a membrane closure sealed to the metal end by a heat-seal material comprising a blend of two different acidic ionomers having respectively higher and lower acid content such that the heat-seal material has an acid content intermediate between that of the two different ionomers.
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:
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.
A container 10 in accordance with one embodiment of the invention is depicted in
An end member 20 is attached to the container body 100 by double-seaming with the flange 101. With reference to
The end member 20 can be formed of various materials, as long as it is plastically deformable so as to accommodate the double-seaming operation. For example, the end member can be formed of a suitable metal such as aluminum or tin-plate steel of relatively large thickness, e.g., about 0.005 to about 0.025 inch. Metal end members on food containers typically are coated with a suitable coating (not shown) to prevent direct contact between the metal and the container contents. An alternative end member material is composite material comprising a relatively thin metal foil of aluminum or steel laminated with one or more polymer layers, such as disclosed in U.S. Pat. No. 5,069,335, incorporated herein by reference. The metal foil layer in such a composite end member is substantially thinner than a conventional metal end member, e.g., about 0.00015 to 0.003 inch for aluminum and about 0.0005 to 0.0025 inch for steel. The particular construction of the end member is not critical to the present invention, and various other constructions can be used.
The container 10 also includes a flexible membrane closure 30 that is attached to the end member 20 by heat-sealing, in such a manner that the closure is readily peelable from the end member to open the container. The closure preferably includes a tab portion 33 that remains unattached to the end member after heat-sealing (see
To attach the membrane closure 30 to the end member 20, each of these components has a heat-seal material disposed thereon. As shown in
The end member 20 likewise has a heat-seal material 25 disposed on at least the ledge portion 23 that is contacted by the closure 30 (i.e., disposed on at least the sealing surface to which the closure is to be attached); the heat-seal material 25 can, if desired, cover the entire upper surface of the end member. There may be one or more intervening layers (not shown), such as adhesive layers, between the heat-seal material 25 and the main substrate of the end member to facilitate adhering the heat-seal material to the substrate and/or for other purposes. The presence or absence of such additional layer(s) is not critical to the present invention.
In accordance with the invention, the heat-seal material 25, 31 comprises a blend of two different acidic ionomers having different levels of acid content. More particularly, the heat-seal material in a preferred embodiment comprises a blend of a first ionomer having an acid content of about 15% by weight, and a second ionomer having an acid content of about 7% by weight. The first and second ionomers are present in approximately equal proportions in the preferred embodiment. Preferably, the first ionomer comprises an ethylene methacrylic acid ionomer (i.e., a copolymer of ethylene and methacrylic acid wherein the methacrylic acid groups are partially neutralized with a cation, typically zinc or sodium ions), such as the composition marketed under the SURLYN® trademark by DuPont. The second ionomer preferably comprises an ethylene acrylic acid ionomer (i.e., a copolymer of ethylene and acrylic acid wherein the acrylic acid groups are partially neutralized with a cation, typically zinc or sodium ions), such as the composition marketed under the TRANCEND® trademark by DuPont. When equal proportions of these two ionomers are blended, the resulting heat-seal material has an acid content of about 11% by weight.
The reduced acid content of the heat-seal material (i.e., reduced relative to the conventional SURLYN® material that has typically been used in the past, which has a 15% acid content) has been found to lead to a reduction in the age-up phenomenon. At the same time, heat seals produced using the material still have adequate oil- and grease-resistance.
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. For example, although the sealing surface to which the closure 30 is heat-sealed in the disclosed embodiment is defined on a ring-shaped end member 20 that is double-seamed to the container body, it is within the scope of the invention to provide the sealing surface in other ways, such as by integrally forming the sealing surface on the container body. Additionally, while the sealing surface is shown as being planar and normal to a longitudinal axis of the container body, alternatively the sealing surface could be shaped and/or oriented in other ways. It will also be appreciated that although a container of circular cross-section has been described, the invention is not limited to circular containers; terms such as “ring-shaped”, “encircling”, and the like, as used in the specification and claims, are intended to apply to both circular and non-circular objects. 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.
