Packaging with peelable lid
FIELD OF THE INVENTION 
     This invention relates to improved packaging with peelable lid and relates particularly, but not exclusively, to packaging of the plastics material type wherein there is a generally rigid base, and a potentially rupturable, flexible web of plastics material over the base and sealed to the base to define a space therebetween and wherein there is a relatively rigid lid sealed over the flexible web. 
     DESCRIPTION OF PRIOR ART 
     Hitherto, packaging of the above type has been produced. An example of such packaging is disclosed in U.S. Pat. No. 4,685,274 where A. J. M. Garwood is listed as the inventor. The packaging disclosed in the aforementioned U.S. patent specification relates to packaging whereby foodstuffs can be kept for a substantial period of time with enhanced keeping properties. Typically such packaging comprises a plastics material base, foodstuffs such as red meat placed on the base, a thin flexible web skin packaged to the base over the goods and a lid provided over the base to define a space between the lid and the plastics web. The space contains a gas for enhancing the keeping properties of the meat by permeating through the flexible web. The aforementioned specification also makes mention of the fact that the lid can be peelable from the base whereby to allow gases such as oxygen to then permeate through the flexible web to enhance the restoration of a bright red colour to the meat which may have turned dark brown/purple during the storage life of the meat in the package. 
     Thus, whilst the present invention has particular application to packaging of the above type it is not intended to be limited to such packaging and has application generally to packaging where goods are packaged to a base by a thin flexible web and wherein a lid is required to be placed over the thin flexible web and subsequently stripped therefrom without rupturing the thin flexible web. 
     In the embodiments of the packaging described above in relation to the U.S. patent specification, we have experienced rupturing of the thin flexible web when the lid is peeled from the package. This rupturing occurs at the edges of the seal which are innermost relative to the package during the peeling process. 
     OBJECT AND STATEMENT OF THE INVENTION 
     It is an object of the present invention to provide an improved method and product whereby rupturing of the thin flexible web around the seal is inhibited during the peeling process. 
     This desired end result can be achieved by providing the seal with a peelable adhesion gradient across the width of the seal and wherein the greatest adhesion is outermost of the package. 
     Therefore in accordance with the first broad aspect of the present invention there may be provided a method of sealing a peelable lid to a package which contains a base and a potentially rupturable flexible web of plastics material covering and held to the base which together define a space therebetween, said method comprising the steps of: 
     1. Placing said lid over said thin flexible web; 
     2. Sealing said lid to said thin flexible web over said base with a strip like seal which extends around the perimeter of said space so said lid will be sealed at the same region as the thin flexible web is held to said base, said sealing being by a thermosealing process which provides a temperature gradient across the width of the strip like seal during the sealing process, 
     whereby to provide said seal with a peelable adhesion gradient across the width of said strip like seal there being a lower adhesion adjacent the space so that when said lid is stripped from said package, the stripping of said lid will be unlikely to rupture said thin flexible web. 
     In accordance with a further broad aspect of the present invention there may be provided a package comprising a base, a potentially rupturable flexible web of plastics material covering said base and adhered to said base to define a space therebetween, and a lid sealed to said flexible web at the same region as said flexible web is adhered to said base, the sealing of said lid to said flexible web being by way of a strip like seal which extends around said space, and wherein there is a peelable adhesion gradient across the width of the strip, there being a lower adhesion adjacent the space, whereby to inhibit against rupturing of said flexible web when said lid is peeled from said package. 
     Most preferably said strip like seal is formed by two sealing strips which run generally parallel to each other around the perimeter of said space and wherein the outermost strip has a greater peeling adhesion than the innermost strip. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In order that the invention can be more clearly ascertained examples of preferred embodiments will now be described with reference to the accompanying drawings wherein: 
     FIG 1 is a cross-sectional view taken through a prior art package of the type disclosed in U.S. Pat. No. 4,685,274. 
     FIG. 2 is a view similar to FIG. 1 showing a different form of prior art package, shown for example in PCT patent specification no. PCT/AU87/00297. 
     FIG. 3 is a close-up view showing an edge portion of the packaging in FIG. 2. 
     FIG. 4 is an example of a preferred embodiment of the present invention showing a view similar to that in FIG. 3. 
     FIG. 5 is a view similar to FIG. 4 showing another example of a preferred embodiment of seal. 
     FIG. 6 is a view similar to that of FIGS. 4 and 5 showing apparatus for effecting such a seal. 
     FIGS. 7 and 8 show different examples of apparatus which can be used in the same manner as that shown in FIG. 6. 
    
    
     Referring now to FIG. 1 there is shown a packaging having a base 1, goods 3 on the base, a flexible web of potentially rupturable plastics material 5 covering the goods 3, and a lid 7 over the base 1. The lid 7 closes a space 9 between the lid 7 and the flexible web 5. The goods 3 are received in a space 11 which is a depression in the base. Typically the base 1 and the lid 7 are of rigid plastics material such as PVC and the flexible web 5 is of a thin flexible plastics material which will permit wrapping onto the goods 3. The technique for effecting such wrapping is fully disclosed in the aforementioned U.S. patent specification. The flexible web 5 is gas permeable and the space 9 is filled with a gas which can pass through the flexible web 5 by permeation to contact the surface of the goods 3 so as to maintain the keeping qualities of the packaged good 3 which may conveniently be red meat. In the embodiment disclosed, the base 1 and lid 7 are substantially gas impervious. 
     The embodiment of FIG. 2 is slightly different to that in FIG. 1 as here the height of the goods 3 extends above the upper surface of the base 1. In this embodiment, the flexible web 5 is stretched over the goods 3 and onto the upper surface of the base 1 and then adhered thereto. In this way, the flexible web 5 compresses the goods 3 onto the base 1 and inhibits against flopping around of the goods 3 on the base 1. The lid 7 is shaped so as to provide the space 9 similar to that in FIG. 1. 
     FIG. 3 shows a close-up view of a corner edge of the packaging shown in FIG. 2. It should be appreciated that the same points explained in relation to this embodiment apply in relation to the embodiment shown in FIG. 1. The flexible web 5 is held to the base 1 by a seal 13 which comprises a strip like seal which extends completely around the perimeter of the base 1 over a lip 15 of the base 1. The seal 13 is typically effected by a thermo sealing process. Thus, a heated platen (not shown) is brought down to clamp together the edge surfaces of the lid 7, the flexible web 5 and the lip 15, and the plastic materials are melted thereby resulting in sealing of the components one to another. The aforementioned specification discloses that the sealing may be such as to permit the lid 7 to be peeled from the base 1 leaving the flexible web 5 intact and sealed to the base 1. This is required so as to permit oxygen which is in atmospheric air or in another gas to permeate the flexible web 5 and contact the surface of the goods 3, such as red meat, to attempt to restore a bright red colour to the meat. It has been found that when meat is packaged in this way, it deteriorates in colour over a period of time and turns a dark brown or purple deoxymyoglobin. By removing the lid 7, oxygen in the air can then contact the surface of the meat to turn it a bright red oxymyoglobin colour. This, in turn, permits the meat to be perceived to be totally fresh and of the normal colour associated with red meats. 
     The peelability of the lid 7 has been obtained by suitable choice of plastics material for the lid 7 and the flexible web 5. Thus, whilst there is a substantially gastight seal effected by such seal 13, the seal between the flexible web 5 and the lid 7 is such that the lid 7 can be peeled therefrom leaving the flexible web 5 intact and sealed on the base 1 without substantially affecting the seal between the flexible web 5 and the lip 5. It has been discovered however, that when the lid 7 is peeled in this way, the flexible web 5, being potentially rupturable, because of its relatively thin nature, can tear on the innermost side of the seal 13 relative to the packaging during the peeling process. This has been perceived to occur as a result of the sharp transition of adhesion gradient at the inner edge of the seal 13. 
     Referring now to FIG. 4, there is shown one example of a preferred embodiment wherein the strip-like seal 13 which extends completely around the perimeter of the packaging is formed by two sealing strips 17 and 19 which extend side by side. Thus, two separate platens (not shown) can be used at different temperatures so that strips 17 and 19 can be produced with different adhesive strengths. Strip 17 will have a higher peelability adhesive gradient than strip 19, in other words, there will be greater adhesive strength in strip 17 than in strip 19. Thus, strip 17 can provide high adhesion of lid 7 relative to the flexible web 5 while strip 19 will provide lower adhesion. The adhesion may be chosen to be just sufficient to permit a gas tight adhesive seal in strip 19 but in strip 17 it will be sufficient to provide both a gas tight adhesive seal and positive but peeling adhesion of lid 7 to flexible web 5. 
     FIG. 5 shows an example similar in concept to that shown in FIG. 4. Here, the strip seal 13 comprises a single strip which extends around the package and around the space 11 but in this case, the strip 13 has an adhesion gradient across its width as shown diagrammatically by the graph in FIG. 5. Thus, the outermost perimeter of the strip has higher adhesion than the innermost perimeter of the strip. In this embodiment it is achieved by a single strip 13 as distinct from two separate strips 17 and 19 in FIG. 4. 
     FIG. 6 shows an example of how the two strips 17 and 19 of FIG. 4 can be produced during manufacture. In this case, a lower chamber part 21 is provided to receive bases 1 with goods 3 on the bases 1 and with the thin flexible 5 over the goods 3. The lip 15 of the base locates on the upper surface of the lower chamber part 21. A cooled platen 23 can be provided to push the lid 7 and the flexible web 5 down onto the lip 15 without effecting sealing. Two further platens 25 and 27 can then be lowered either together or at different times to effect sealing strips 17 and 19 shown in FIG. 4. In this connection, platen 27 will be at a higher temperature than platen 25. Thus, platen 27 will effect greater adhesive strength in the seal strip 17 and greater peelability removal forces will be required across the seal strip 17 than across seal strip 19. Platen 23 can be cooled by known techniques such as by using water passing through suitable hollow cores thereof. Platens 25 and 27 can be heated by known means such as by hot water or steam or by an electric heating element which all can be suitably thermostatically controlled and which passes through suitable hollow cores thereof. 
     FIG. 7 shows an example of an edge portion of a further platen 29 which can be used to produce two different temperature surfaces 31 and 33 and thereby simulate the effect of platens 25 and 27 in FIG. 6. Here, platen 29 can be made of suitable material such as machine steel and contain a core passageway 35 through which hot water or steam can pass. The passageway 35 is closer to face 31 than to face 33 and hence face 31 will be elevated to a higher temperature than face 33. 
     FIG. 8 shows a further embodiment to produce a sealing strip 13 shown in FIG. 5. Here, the platen 37 has a main bulky portion 39 with an extending foot 41 and a thin interconnecting portion 43. A core passageway 45 extends through the foot 41 and hot water or steam can be arranged to pass through the passageway 45 thereby heating the outermost edge (left hand side) as shown in FIG. 8 to a higher temperature than the right hand edge of FIG. 8. Thus, the surface 47 will have a temperature gradient being highest at the left hand side and lowest at the right hand side. 
     The plastics materials are preferably chosen such that the base 1 adheres to the flexible web 5 at a lower temperature than the lid 7. In this way, the flexible material 5 will be adhered to the base 1 on the innermost edge of strip 19 whilst the lid 7 will be only partially adhered or may not even be adhered at strip 19. Thus, as the lid 7 is peeled from the packaging, it will be unlikely to rupture the flexible web 5 in the region of the strip 19. 
     Typical examples of plastics material for lid 7 comprise APET (Amorphous Polyethylene Terephthalate) which has an overall thickness of approximately 0.013 inches. The face or undersurface of lid 7 relative to the package may comprise a blend of 90% APET plus 10% PETG 6763. The thickness of the blend coating or layer may be 0.0015 inches. Thus, the lid 7 may be a multi-layered web which is formed either by bonding of two separate webs or by a co-extrusion or by a coating of one web to the other. 
     The flexible web 5 may comprise a plasticised polyvinyl chloride of 0.001 inches thickness. 
     The base 1 can comprise a material of APET of thickness of 0.020 inches which can have a layer on the innermost face to abut with flexible web 5 of a blend of 85% PETG 6763 an amorphous polymer based on poly(ethylene terephthalate) having a number average molecular weight of about 26,000 and 15% PET 5116 a terephthalate copolyester having a low postextrusion crystallinity and low glass-transition temperature of about 58° C. which enables heat-seal bonds to be achieved over a temperature range of 95° to 205° C. This layer, or web, can have a thickness of about 0.0015 inches. Thus, the base material may be a web coated or bonded with a further web or it may be a co-extruded web. The above materials and thickness are preferred when using Eastman Chemical Products, Inc. Plastics. 
     In an alternative material combination the lid 7 may have a total thickness of 0.015 inches and be formed of a co-extruded laminate of an outer layer relative to the packaging of Eastman PET 9921 a condensation polymer of a poly(ethylene terephthalate) thermoplastic copolyester produced by a continuous meltphase polymerization process followed by a solid-state polymerization process and an inner layer relative to the packaging comprising a blend of Eastman PET 9921 of 84% and PETG of 6763 of 16%. The percentages are by weight. The outer layer can be 0.013 inches thick and the inner layer 0.002 inches thick. 
     The flexible web 5 may have a total thickness of 0.001 inches and be formed of pPVC. 
     The base 1 may have a total thickness of 0.018 inches and be formed of a co-extruded laminate of an inner layer relative to the packaging comprising a blend of Eastman PETG 6763 of 50% and Kodabond PET 5116 of 50%. The percentages are by weight. The outer layer may be Eastman 9921. The inner layer can be 0.002 inches thick and the outer layer can be 0.016 inches thick. 
     In the above material combination the platen 27 can be heated to 400° F. and platen 25 can be heated to 385° F. 
     Thus, in these examples the lid 7 and the base 1 have a PETG component in their surfaces facing the flexible web 5 of PPVC at the seal. Accordingly good sealing can be effected and subsequent peel of the lid is possible. 
     The above materials have been found particularly suitable in a packaging of the type used for storing perishable goods such as red meats wherein the package contains a desired gas in the space 9 which can permeate the flexible web 5 to contact the surface of the goods 3 to enhance keeping properties of the goods 3. After a period of time, should the red meat discolour by deoxymyoglobin, the lid 7 can be peeled from the package whilst leaving the flexible web 5 held to the base 1 without rupturing around the seal 13. In this way, oxygen or other gas can pass through the web 5 and contact the surface of the goods 3 to provide for further treatment of the goods 3 such as restoration of a desired colour. 
     It is particularly required that the flexible web 5 not rupture when the lid 7 is stripped therefrom, as a ruptured package does not have the same aesthetic qualities to an intending purchaser as a totally unruptured package. 
     It has been observed that when the lid 7 is removed oxygen in atmospheric air or in other gases takes some time to permeate the flexible web 5. Thus, when the oxygen concentration under the membrane is in the approximate percentage ration to the volume of gas under the membrane of 0%-5% metmyoglobin has a tendency to form on the meat. This can result in a permanent dark brown colouring to the meat and this cannot be changed to the required bright red oxymyoglobin colour even after being subjected to large amounts of oxygen. Thus, it is desirable to permit quick transfer of oxygen through the flexible web 5 once the lid 7 is removed. This can be achieved by providing apertures 49 in the flexible web 5, preferably in that portion of the flexible web 5 that spans the space between the sides of the meat and the base 1. The apertures 49 may be pre-provided in the flexible web 5 before the meat is packaged or provided after removal of the lid 7. The apertures 49 can be provided by a suitable punch which will produce apertures 49 in a known or predetermined array in the flexible web 5. By having apertures 49 in a predetermined array they will not detract from the aesthetic appearance of the package. If desired, the apertures 49 may be covered by a sales label after the meat has turned to the bright red oxymyoglobin colour. 
     Modifications may be made to the present invention as would be apparent to persons skilled in the art of producing packaging. These and other modifications may be made without departing from the amhit of the invention, the nature of which is to be determined from the foregoing description.