Abstract:
A sealable and sterilizable package having a bottom of metal foil or light-gauge sheet with a thermoplastic lining and a lid of metal foil or light-gauge sheet with a thermoplastic lining sealingly adhered around its edge portion to the bottom lining to provide a fully sealed, sterilizable and shelf-stable container for food products or the like. The lid is manually peelable from the bottom, with defoliation of the lid lining along the locality of the seal, providing a neat and attractive appearance for the opened container. The lid lining is constituted of a material that effectively and reliably adheres to the metal surface of the lid at sterilization temperatures while forming with the bottom lining a complete but peelable seal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to sealable and sterilizable packages made of metal foil or light-gauge sheet suitable for containing products such as foodstuffs and the like. More particularly, it relates to packages of the type described including a bottom and a lid readily separable by manual peeling of the lid for access to the contents. In an important specific sense, the invention is directed to aluminum foil and/or light gauge sheet packages having a bottom member and lid member each internally lined with a coating of thermoplastic material. The term &#34;aluminum,&#34; as herein used, embraces aluminum metal and alloys thereof. 
     For simplicity of discussion, detailed reference will be made herein to aluminum foil containers as an illustrative and indeed especially advantageous example of the types of containers with which the present invention is concerned; but it is to be understood that in a broad sense the invention generally embraces containers made of metal foil and/or light-gauge metal sheet. 
     Aluminum foil and light-gauge sheet are widely employed in the packaging of foodstuffs, e.g. in containers for retail sale. By way of example, an aluminum foil package for such purposes may comprise a relatively stiff, shaped foil bottom and a lid of foil lighter in gauge than the bottom, with an internal thermoplastic lining or coating on both the lid and the bottom to separate the contained food from the metal of the foil. These linings must be inert with respect to the foodstuff; i.e. they must not exhibit water staining or blushing or affect the taste of the food. 
     Aluminum foil containers and the like have heretofore commonly been used for packaging frozen foods. There are various disadvantages associated with frozen foods, however, including the necessity of providing refrigerated storage as well as the energy consumption and sometimes inconvenient delays involved in heating the food from frozen condition before or during cooking. It has, of course, long been known to package foods for preservation at room temperature under sterile conditions by canning, but metal containers for that purpose have conventionally been relatively heavy cans adapted to be opened only by means of a tool. 
     The provision of a manually openable aluminum foil or like container, which can be sealed and sterilized to provide shelf-stable packaging of foods at room temperature (i.e. packaging that preserves the contained food in unimpaired and safely usable condition for extended periods of room-temperature shelf storage) constitutes a particularly important aspect of the present invention. Such packaging is desirable as obviating both the disadvantages of frozen foods and the inconvenience and other drawbacks of conventional cans. In this aspect, the invention is concerned with provision of a foil or like container that is capable of replacing conventional &#34;tin cans&#34; for shelf-stable room-temperature food storage, as distinguished from frozen food packages, and which provides at the same time the advantages of light-weight and easy manual access to the contents without use of a can opener. 
     For these and other uses of aluminum foil containers, preservation of the contents in satisfactory condition for safe use requires that the container be sealed and sterilized. It has heretofore been proposed to seal the lid and bottom thermoplastic linings of an aluminum foil package together, utilizing as the lining materials thermoplastic substances which are stable at sterilization temperatures. A typical sterilization temperature for many food products is about 120° C. 
     At the same time, it is desirable that such a package be readily manually openable for convenience of access to the contents by a consumer. Prior foil packages adapted for sealing and sterilization have been difficult to open, either tearing unevenly when an attempt is made to separate the lid from the bottom, or requiring the use of an implement such as a knife for cutting through the lid and its associated lining. Efforts previously made to achieve a sealable and sterilizable shelf-stable foil package in which the lid and bottom are easily separable by parting of the seal between them, so as to enable manual peeling of the lid from the bottom, have been unsatisfactory owing to failure to achieve a reliably sealed and sterile package. This failure is presently believed attributable to the fact that the lid lining material selected to provide a seal separable by peeling has failed to maintain satisfactory adherence to the foil surface of the lid and/or to the lining material of the bottom with which it is supposed to form a seal. 
     A further important consideration, in a practical sense, is that opening of the package should not result in visible and unsightly irregularities along the rim of the bottom. Such irregularities are aesthetically undesirable, especially where the bottom may be used as a serving vessel for the contents, and may seriously detract from consumer acceptance of the package. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a new and improved sealed, sterilizable light-gauge metal sheet and/or foil package characterized both by high reliability of the sterile seal and ease and neatness of manual opening of the package. Another object is to provide such a package which is shelf-stable, i.e. capable of preserving contained foodstuffs for extended periods of room-temperature shelf storage. Still another object is to provide such a package wherein the lid separates cleanly from the bottom when manually opened, leaving no unsightly marring or visible irregularity along the rim of the bottom. A further object is to provide a sealed sterilizable aluminum light-gauge sheet and/or foil package which can be opened by peeling of the lid from the bottom. 
     To these and other ends, the present invention broadly contemplates the provision of a package including a metal foil or light-gauge sheet bottom, a first thermoplastic lining adhering to and covering the inner surface of the bottom, a metal foil or light-gauge sheet lid, and a second lining of thermoplastic material adhering to and covering the inner surface of the lid, the bottom having a lip over which the first lining extends, and the lid having an edge portion overlying the lip with the second and first linings sealed together along the lip. Both thermoplastic linings are formed of material or materials which are inert with respect to the contents of the package (that is, materials which are not attacked by foods or by acids or salt present in or with the contained food) and which are stable at sterilization temperatures of at least about 120°-125° C. In accordance with the present invention and as a particular feature thereof, the thermoplastic material of the second (i.e. lid) lining is a material which effectively and reliably adheres to the metal surface of the lid and to the first lining material at temperatures up to and including the aforementioned sterilization temperatures and which forms with the first lining material a fully effective sterilizable seal providing a shelf-stable package that opens evenly and neatly when the lid is peeled manually from the bottom without tearing of the lid, unsightly disfigurement of the bottom rim, or separation of the lid lining from the metal of the lid except along the locality of the seal. 
     More particularly, the lid lining in accordance with the invention is of such character that when the lid is peeled from the bottom of the container, that portion of the lid lining which is bonded to the bottom lining (i.e. along the rim of the bottom) is selectively or partly stripped from the metal surface of the lid. Stated in other words, the seal formed by the mutually bonded portions of the lid and bottom linings parts from the lid metal as the lid is peeled off but remains adherent to the rim of the bottom, and also parts from the remainder of the lid lining, which thus remains adherent to the lid metal. This selective stripping or parting of the seal-forming portion of the lid lining, both from the lid metal and from the remainder of the lid lining, is herein termed defoliation. By virtue of such defoliation, visible marring or disfigurement of the bottom rim is avoided, because the rim remains fully coated with material of the linings (so that no bare metal is exposed at the rim), yet no unsightly loose fragments or tatters or lid lining material project therefrom. 
     The metal of both the lid and the bottom may be aluminum, e.g. aluminum foil. In such case, the foil of the bottom may for example be of a heavier gauge than the lid foil so as to retain a desired container shape when formed into such shape. 
     In a specific embodiment of the invention, the material of the bottom lining may be polypropylene while the lid lining is constituted of a graft copolymer as hereafter further defined. The graft copolymers herein contemplated are found to form fully effective yet readily peelable seals with polypropylene, characterized by the property of defoliation described above, and to provide completely satisfactory adherence to the lid foil surface throughout the range of temperatures to which the container is subjected during and after sterilization. 
    
    
     Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawing. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a foil container embodying the present invention in a particular form; 
     FIG. 2 is an enlarged fragmentary elevational sectional view of one side portion of the container, with the lid sealed to the bottom; 
     FIG. 3 is a view similar to FIG. 2 but showing the lid being peeled away; and 
     FIG. 4 is an enlarged perspective view of a marginal portion of the lid undersurface after peeling, further illustrating the property of defoliation. 
    
    
     DETAILED DESCRIPTION 
     For purposes of specific example, the invention will be described as embodied in a container 10 (FIG. 1) having a bottom 11 and lid 12 both fabricated of aluminum. In the illustrated embodiment of the invention, the container 10 is generally rectangular in plan, with rounded corners. It will be understood that the rectangular container shape shown is merely exemplary and that the invention may be embodied in containers of circular or other configuration as well. 
     The bottom 10 is formed from light-gauge sheet, or from foil sheet of sufficiently heavy gauge to sustain its illustrated shape when filled with a food product or the like. A suitable range of gauges for the bottom, in this embodiment, is about 0.001-0.012 inch; conventionally, the term &#34;foil&#34; is used for sheet up to 0.008 inch in thickness. The bottom has a base 14 and side walls 15 integral therewith, cooperatively defining an upwardly open but otherwise fully liquid-and gas-tight receptacle. The upper edges of the side walls are bent outwardly to provide a continuous annular lip or rim 16 extending entirely around the circumference of the container with the outer margin of the lip shaped to form a bead 18. 
     The lid 12 is a single continuous sheet of aluminum foil, typically somewhat lighter in gauge (e.g. about 0.001-0.005 inch) than the foil of the bottom for desired flexibility and ease of peeling. As shown, the lid is generally rectangular in configuration with three rounded corners corresponding to three of the corners of the bottom, and is so dimensioned that the peripheral edge 20 of the lid uniformly overlies the lip 76 of the bottom (terminating just inwardly of the bead 18), thereby to provide full closure of the package. The fourth corner 22 of the lid is not rounded, but instead projects outwardly beyond the underlying rounded corner 23 of the lip, to constitute a free tab that can readily be grasped for manual peeling of the lid from the bottom. 
     As best seen in FIGS. 2 and 3, the inner surface of the bottom 10 is entirely covered with a first lining 24 of thermoplastic material, i.e. a layer or coating of the thermoplastic material which adheres fully and uniformly to the inner foil surface of the bottom. This coating extends over the entire upwardly facing surface of the lip 16. Similarly, the inner or downwardly facing surface of the lid 12 is entirely covered with a second lining 26 of thermoplastic material which completely coats and uniformly adheres to the lid foil surface and extends outwardly to the edges thereof, so that when the lid is placed on the bottom with its peripheral edge overlying the lip 16, the peripheral portion of the lid lining 26 is in facing contact with the portion of the bottom lining 24 that coats the lip. The linings 24 and 26 are bonded together to form a seal 28 extending entirely around the circumference of the package and holding the lid securely on the bottom, thus providing fully sealed enclosure of the package contents. Stated in general, the thermoplastic material or materials of the linings 24 and 26 are selected to be inert with respect to the contents of the package, so as not to be attacked e.g. by acids or salt in the contained food, and are selected to have a melting range above the temperature of sterilization (e.g. about 120°-125°C) so that sterilization procedures will not disrupt the integrity of the linings and the seal they cooperatively provide. 
     In accordance with the present invention, and as a particular feature thereof, the thermoplastic material constituting the lid lining 26 (in addition to satisfying the foregoing general criteria) is a material that effectively and reliably adheres to the aluminum foil surface of the lid and to the first lining material at temperatures ranging upwardly through sterilization temperatures of e.g. about 120°C, and which forms with the first lining material a sterilizable seal that provides a shelf-stable package and parts by defoliation from the lid when the lid is peeled manually from the bottom. 
     More particularly, the package of the invention advantageously has lid and bottom linings characterized by such bonding and strength properties that peeling of the lid selectively strips the material of seal 28 (including the lid lining portion previously incorporated in the seal by bonding) from the metal surface of the lid, leaving the seal material fully adherent to the bottom ring 16, and also leaving the remainder of the lid lining 26 full adherent to the lid. This selective defoliation of the seal 28 (or more precisely the seal-forming portion of lid lining 26) exposes bare metal of the lid as indicated at 30 in FIGS. 3 and 4, but this is of no consequence; since the metal of the bottom rim remains fully covered with lining material, there is no unsightly marring or disfigurement of the rim as would occur if lining material were stripped therefrom. 
     As best shown in FIG. 4, when the lid is peeled, that portion of the lid lining which is united with the bottom lining (i.e. to form the seal 28 along the rim) parts from the remainder of the lid lining quite smoothly and uniformly along the lateral margin or margins of the seal. Consequently, the rim 16 of the opened container is essentially completely free of loose scraps or tatters of lid lining material which again would detract from the desired appearance of the rim. In other words, essentially only that portion of the lid lining which is bonded to the bottom lining at the rim is stripped from the lid. 
     This property of selective defoliation of the seal 28 from the lid may be explained as follows: the strength of the bond between the bottom lining 24 and the rim metal, and also the strength of the seal 28 formed by bonding linings 24 and 26, are both greater than the strength of the bond between the lid lining 26 and the lid metal, but the latter bonding strength exceeds the tearing strength of the lid lining material per se. Thus, when the lid is peeled, the lid lining material which is bonded to bottom lining material at seal 28 pulls away from the lid metal but also separates cleanly from the adjacent lid lining material along the margins of the seal. 
     In fabricating a container of the type shown, the linings are first laminated to sheet or foil for the bottom and lid, by preheating the sheet or foil to about 200° C, applying the lining materials and passing the coated sheet through a nip between laminating rollers heated to about 200° C, and thereafter post-heating the laminate at about 200° C. The bottom 11 may then be formed as by deep-drawing. The food to be packaged is placed in the bottom, and covered over by the laminated foil stock for the lid; thereafter any food which may be interposed between the rim 16 and the lid material is expelled by application of high pressure at the rim, and the package is heat-sealed to bond the linings 24 and 26 together entirely around the rim (thereby forming the seal 28) under appropriate conditions of elevated temperature and pressure, and the lid is cut or trimmed to size. The sealed package may be sterilized, e.g. by heating to a temperature in the range of about 120°-125° C; after sterilization, the seal 28 maintains the sterility of the contents providing a shelf-stable package for room temperature storage. 
     The material of the first or bottom lining 24 may, for example, be polypropylene; and in such case, in accordance with the invention, the material of the lid lining 26 may be a so-called graft copolymer. The term &#34;graft copolymer&#34; as herein used includes materials obtained by polymerization of a mono-olefin having 2 or 3 atoms of carbon and grafting a minor proportion (e.g. even less than 1% by weight of the olefin) of an unsaturated monoethylenic carboxylic acid thereon. The acid may, for example, be acrylic or methacrylic acid, and the polymerized mono-olefin may be polyethylene. 
     In specific embodiments of the invention, the graft copolymer is polyethylene having suitable carboxylic acid monomers grafted thereon and constituting a minor proportion by weight of the graft copolymer. Thus, a suitable graft copolymer for the lining 26 is made by treating high density polyethylene granulates with electron beam radiation and subsequently grafting thereon substituted acrylic acid monomers, to produce a polymeric molecule of about 50,000 mol. wt. carrying about 3-7% by weight acrylic groups, such material being termed &#34;PEG&#34; herein. 
     An aluminum foil package having a bottom 11 with a polypropylene lining 24 and a lid 12 with a lining 26 of the PEG graft copolymer just described can be sealed by heat-sealing the lid and bottom linings at the lip 16 around the entire periphery of the package, and subsequently sterilized e.g. at 121° C, with maintained reliability of the seal and continued fully effective bonding of the lining 26 to the lid foil. When the tab 22 is pulled up manually (i.e. in a direction transverse to the major surfaces of the lid), however, the seal parts readily and evenly for assured easy opening of the package without tearing of the lid or separation of the lining therefrom, except for the above-described defoliation. 
     It is to be understood that the invention is not limited to the features and embodiments hereinabove specifically set forth, but may be carried out in other ways without departure from its spirit.