Patent Application
20050077298
1) Field of the Invention
This invention relates to an easy-opening closure for hermetic sealing of an open end of a retortable container and an easy-opening container that is hermetically sealed by such a closure.
2) Description of Related Art
A variety of closures are known for the hermetic sealing of a container, such as conventional tin-plated steel cans that are widely used for containing food products. Retortable containers are those that can withstand a pasteurization or retort process comprising heat and pressure for preserving the food contents of the container. During retort, the container can be subjected to temperatures above 212° F., e.g., up to 280° F., under pressures of 15 to 30 psi.
Easy-opening containers are those that can be opened without undue effort and without the use of a special tool such as a rotary can opener. In order for an easy-opening container to be retortable, the closure must be sufficiently strong to resist stresses that develop as a result of the retort heat and pressure but easily overcome during opening. One conventional easy-opening, retortable container includes a closure that is stronger in shear than tension. The closure is strong enough to withstand the shear force that develops during retort, while a relatively small tensile force is required to open the container. For example, U.S. Pat. No. 5,752,614, titled “Easy-Opening Closure for Hermetic Sealing a Retortable Container,” to Nelson describes an easy-opening closure that includes a metal end ring that can be seamed to an open end of a retortable container and defines a central opening that is covered by a membrane patch. The membrane patch is bonded to the end ring such that the bond is unaffected during retort processing but has a predetermined tensile force strength that is preferably less than 5 psi to allow peeling of the membrane patch from the end ring. Thus, the container can be retorted and subsequently easily opened.
Other easy open containers are formed partially or wholly of plastic. For example, a lid that includes a metallic foil can be adhered to a plastic container, which can be used for retort. Attempts have also been made to manufacture retortable containers exclusively of plastic. For example, a plastic membrane can be heat sealed to a plastic container. Alternatively, the plastic membrane can be heat sealed to a plastic end ring that is similar to the metal end ring described by Nelson, and the plastic end ring can be fusion bonded to the container by spin welding. A pull ring or tab can also be provided on the plastic membrane so that a user can peel the plastic membrane from the container, thereby breaking the bond between the plastic membrane and the container or plastic end ring. The bond can be difficult to form by heat sealing due to the poor thermal conductivity of the plastic. Further, the bond must be sufficiently strong to resist the stresses that are applied during manufacture, for example, heat and pressure that occur during retort. However, if the bond is too strong, a user may be unable to open the container.
Thus, there exists a need for an easy-opening closure for hermetically sealing an open end of a retortable container and an easy-opening container that is hermetically sealed by such a closure. The closure should be strong enough to withstand the stresses induced during retort, but easily removed by a user. Additionally, the closure should be compatible with closures formed of plastic, in which a bond between plastic members is to be overcome during opening of the container.
The present invention provides an easy-opening container and a closure with a bond that is weakened with radiation so the container sealed by the closure can be easily opened by a consumer. The bond, which can join a plastic closure and a plastic body portion, can be weakened after a retort process so that the container is strong enough to resist the forces produced by internal pressures and temperatures that are associated with retort processing.
According to one embodiment of the present invention, the container includes a base portion with a bottom and side that define an interior space and an opening thereto. A plastic membrane of sufficient size to cover the opening has an outer peripheral area attached to the base portion by a bond having a predetermined shear force strength sufficient to withstand forces created during retort processing of the container. The bond is configured to be radiation weakened to have a predetermined tensile force strength to allow easy-opening of the container by peeling the membrane from the base portion. The base portion can also be formed of plastic and can include a plastic end ring that is fusion bonded to a body member such as the side of the container. The end ring can define the opening, such that the plastic membrane can be attached to the end ring by the bond to seal the container. The membrane and the base portion can be formed of a high barrier material, such as a co-extruded sheet of polypropylene and ethylene vinyl alcohol, to prevent transmission of moisture and oxygen therethrough. Oxygen scavengers can also be disposed in either the membrane or the base portion.
The bond can be formed of an adhesive such as polyurethane, and can be adapted to be weakened by radiation such as microwave radiation. For example, the shear force strength of the bond before weakening can be at least about 20 psi, the tensile force strength of the bond after weakening can be less than about 14 pounds.
The present invention also provides a method of hermetically sealing an open end of a retortable, easy-opening container. A plastic base portion defining an opening is provided, for example, by fusion welding an end ring to a body member. A plastic membrane is disposed to cover the opening of the base portion. The plastic membrane can be formed by co-extruding a sheet of polypropylene and ethylene vinyl alcohol. In addition, oxygen scavengers can be disposed in the plastic membrane. A bond is formed between the membrane and the base portion, for example, by disposing an adhesive between the plastic membrane and the base portion and urging the plastic membrane and the base portion together. The bond is irradiated with microwave or other radiation, and thereby weakened so that the bond allows easy opening of the container by peeling the membrane from the base portion. According to one aspect of the invention, the bond is formed with a shear force strength of at least about 20 psi and weakened to have a tensile force strength of less than about 14 pounds. Thus, the container can be retorted or otherwise processed after the bond is formed and before the bond is weakened.
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 invention 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, the invention 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.
Referring now to
The openable first end 16 is closed by the closure 30 such that the base portion 12 and the closure 30 define an interior space 22 therein, which can be hermetically sealed. As shown in
The closure 30 can be a generally planar membrane that covers the opening 16 of the base portion 12, e.g., an outer peripheral area 31 can cover an intermediate area 27 of the end ring 24 as shown in
Preferably, both the base portion 12 and the closure 30 are formed of a high barrier material to prevent transmission of moisture, oxygen, and other fluids therethrough, such as where the container 10 is used in food packaging. Either or both of the base portion 12 and the closure 30 can include various types of plastic including, but not limited to, polyolefins such as polypropylene. For example, the base portion 12 and/or the closure 30 can be formed of multilayer structures that include layers of polypropylene, ethylene vinyl alcohol (EVOH), polyethylene, or the like. In particular, a multilayer structure can be formed by co-extruding polypropylene with ethylene vinyl alcohol. The material of the base portion 12 and closure 30 can also include chemical compounds that absorb oxygen, referred to generally as oxygen scavengers, to prevent the transmission of oxygen. For example, a moisture-activated, iron oxide-based oxygen scavenger can be disposed in polyolefin.
A bond 32 connects the closure 30 to the base portion 12, e.g., extending circumferentially around the opening 26 and connecting the closure 30 to the end ring 24. Preferably, the bond 32 is sufficiently strong to maintain the attachment of the closure 30 to the base portion 12 throughout the manufacture, filling, processing, and/or handling of the container 10, but configured so that the closure 30 can be removed from the container 10 by the user. For example, the bond 32 can be formed with a shear force strength of at least about 20 psi so that the container 10 can withstand a retort process in which the sealed container 10 is subjected to relatively high temperatures and corresponding internal pressure, or other processing operations. Subsequently, the strength of the bond 32 can be reduced, for example, to have a tensile force strength of less than about 14 pounds so that a user can open the closure 30 by lifting or peeling the closure 30 from the container 10. According to one embodiment of the present invention, the bond 32 is weakened to have a tensile force strength of between about 6 and 14 pounds so that the user can open the closure 30 by applying a force of no more than about 6 to 14 pounds. In other embodiments, the strength of the bond 32 can be further reduced.
The bond 32 can be formed of an adhesive, such as a polyurethane adhesive or nylon adhesive that is disposed between the closure 30 and the base portion 12. Polyurethane adhesives can be solvent-based or solvent-less. For example, polyurethane adhesives are available from Liofol Company, a division of Henkel Adhesives, Cary, N.C., including solvent-based and solvent-less polyurethane adhesives identified by product numbers 3649/6255 and 7975/7276, respectively.
The closure 30 can be formed separately from the base portion 12 and subsequently attached to the base portion 12. Various methods can be used for disposing the adhesive between the closure 30 and the base portion 12. For example, the closure 30 and/or the base portion 12 can be treated with a corona, which provides an electrostatic charge that forms a primed or roughened surface on which the adhesive can more easily be deposited uniformly. The adhesive is then applied to one or both of the closure 30 and the base portion 12, and the closure 30 is positioned and urged against the base portion 12.
As initially formed, the bond 32 is typically strong enough to withstand retort and other processing, but has a tensile force strength that requires more than the desired 6 to 14 pounds of lifting or peeling by the user. Therefore, the bond 32 can be subjected to radiation for weakening the bond 32. As shown in
A grip portion such as a ring, tab, or other feature can also be provided on the closure 30 to facilitate opening. For example, the closure 30 can include a pull ring 34 that is disposed on an outer surface of the closure 30 as shown in
The closure 30 can provide a hermetic seal to the container 10 such that the container 10 can be used for storing food items and other items requiring a hermetic seal and/or a reduced or enhanced storage pressure. Preferably, the bond 32 is sufficiently strong to withstand shear stresses that result from pressures due to the retort process or other processes associated with the manufacture, filling, processing, and/or handling of the container 10; however, a person can easily open the container 10 by subjecting the bond 32 to a tensile force after the bond 32 has been radiation weakened and thereby peeling or otherwise separating the closure 30 from the container 10. For example, the bond 32 can withstand the stresses applied during retort, but tear when put in tension by the user after radiation weakening.
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.
