Abstract:
A retortable, easily openable closure of the type having a metal end and a membrane closure heat-sealed thereto, wherein the metal end has a coating of a heat-seal composition comprising a food-compatible metal-coating composition blended with polypropylene. The polypropylene disperses within the food-compatible metal-coating composition during curing of the coating on the metal end such that an outer surface of the coating defines a multitude of separate, spaced islands of polypropylene dispersed within the food-compatible metal-coating composition so as to form bonding sites. The membrane closure&#39;s lower surface has a heat-seal layer of polypropylene for sealing to the coating on the metal end.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a container having a closure formed by a metal end joined to a top end of the container and defining an opening through which contents of the container are dispensed, and a peelable membrane sealed to the metal end so as to hermetically close the opening. The invention relates more particularly to such a container closure that is capable of withstanding a retort sterilization process without failing, but that has a controlled peel strength that is sufficiently low to be readily openable by hand.  
       BACKGROUND OF THE INVENTION  
       [0002]     A variety of food products are packaged in containers that, after filling and sealing, are subjected to a retort sterilization process to render the filled containers shelf-stable such that they do not require refrigeration until opened (referred to herein as “retort containers”). The retort process typically involves elevating the temperature of the filled and sealed container to about 250-260° F. for about one hour. During the retort process, pressure builds up in the container. It is critical that the container closure not fail or leak during or after the retort process.  
         [0003]     Various types of closures have been developed for retort containers. One type of closure includes a metal end that is attached to the top of the container, typically by double-seaming the metal end to a flange at the top of the container, and a flexible membrane that is sealed to the metal end so as to cover an opening defined therein. The membrane generally includes a metal foil layer and may include one or more additional layers such as polymer and/or paper. One side of the foil layer typically is sealed to the metal end on the container using suitable sealing materials. Heat sealing is generally employed to seal the membrane to the metal end.  
         [0004]     The sealing material used for attaching the membrane to the metal end must have a sufficiently high melting temperature so that the strength of the seal during the retort process is not impaired to such an extent that the seal fails. Polypropylene has commonly been used as the sealing material for retort containers because it is heat-sealable and has a melting point exceeding the temperature of the retort process. A heat-seal layer of polypropylene is disposed on the lower side of the membrane. Similarly, a coating of polypropylene is applied to the metal end, for example by applying a composition comprising polypropylene suspended in an organic solution and then evaporating the solvent. After filling of the container, the membrane is sealed to the metal end by heat-sealing the polypropylene heat-seal layer on the membrane to the polypropylene coating on the metal end.  
         [0005]     The polypropylene-to-polypropylene bond is strong enough to withstand the retort process. However, the bond tends to be so strong that the peel force required to peel the membrane from the metal end is unacceptably high.  
         [0006]     Retortable easy-peel closures have been developed to try to overcome the above problem. The easy-peel closures generally include some type of failure mechanism in a heat-seal layer on the membrane so that the heat-seal layer fails internally. In some of these closures, a controlled defect is designed into the heat-seal layer by adding incompatible polymers, fillers, or both, to polypropylene or other heat-sealable polymer such as polyethylene. For example, low-density polyethylene and/or high-density polyethylene have been added to polypropylene to make a heat-seal. Talc has been used as a filler in polypropylene as well as in polyethylene. The incompatible polymers and fillers work on the principle of creating a weakness within the heat-seal layer to initiate a crack in the layer when the membrane is peeled from the metal end. In other closures, the membrane has included a coextruded heat-seal layer comprising a layer of polypropylene coextruded with a layer of polypropylene to which fillers and/or incompatible polymers have been added, and the intended failure mechanism is a delamination between the coextruded layers as a result of a weakening of their bond caused by the added fillers and/or polymers. Such membranes having controlled defects are relatively complex and thus tend to be relatively expensive to produce.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a retortable, easily openable closure of the type having a metal end and a membrane, wherein the failure mechanism during peeling of the membrane from the metal end is not a cohesive failure within the heat-seal layer on the membrane. The heat-seal layer on the membrane in accordance with the invention does not require incompatible polymers or fillers to be added, nor does it require a coextruded structure, because the failure mechanism involves a failure between the heat-seal layer on the membrane and a corresponding heat-seal coating on the metal end. Accordingly, the heat-seal layer on the membrane is simplified and therefore less costly.  
         [0008]     In accordance with the invention, the metal end has a coating of a heat-seal composition comprising a food-compatible metal-coating composition with polypropylene dispersed therein. The polypropylene disperses within the food-compatible metal-coating composition during curing of the coating on the metal end such that an outer surface of the coating defines a multitude of separate, spaced islands of polypropylene dispersed within the food-compatible metal-coating composition so as to form bonding sites. The polypropylene islands are microscopic in scale. The membrane&#39;s lower surface has a heat-seal layer thereon, comprising a layer of polypropylene. The polypropylene heat-seal layer on the membrane is heat-sealed to the coating on the metal end, whereby the polypropylene heat-seal layer bonds to the bonding sites on the coating. The peel strength of the resulting bond depends on the fraction of the coating&#39;s surface area that is represented by the bonding sites. As the collective area of the bonding sites becomes a greater fraction of the coating&#39;s total surface area, the peel strength increases. In accordance with the invention, the heat-seal layer of polypropylene on the membrane bonds to the membrane with a strength exceeding that between the heat-seal layer and the coating on the metal end, such that when the membrane is pulled to open the container, the failure occurs between the heat-seal layer on the membrane and the coating on the metal end. A controlled low peel strength can be achieved by suitably formulating the polypropylene-filled metal-coating composition.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0009]     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:  
         [0010]      FIG. 1  is a cross-sectional view through a top end portion of a container in accordance with one embodiment of the invention;  
         [0011]      FIG. 2  is a cross-sectional view taken on line  2 - 2  in  FIG. 1 , with the membrane and metal end shown separated for clarity; and  
         [0012]      FIG. 3  is a top view of the coating on the metal end, greatly enlarged in scale to show the microscopic islands of polypropylene. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     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.  
         [0014]      FIGS. 1 through 3  illustrate one embodiment of the invention relating to a metal end and membrane closure for a retortable container. The container  10  includes a flange  12  at its top end, the flange projecting radially outwardly in an initial state (not shown) of the container prior to incorporation of a closure. The closure for the container includes a metal end  14  attached to the top end of the container, such as by double-seaming the metal end with the flange as shown. The metal end  14  defines an opening  16  through which contents of the container can be dispensed. The metal end includes a generally horizontal upper surface  18  that encircles the opening  16 . The closure also includes a flexible membrane closure  20  that is sealed to the upper surface  18  of the metal end to hermetically seal the container. The membrane closure advantageously includes a pull tab  22  that is not sealed to the metal end so that it can be readily grasped and pulled to remove the membrane from the metal end.  
         [0015]     In accordance with the invention, the membrane closure  20  is heat-sealed to the metal end  14  by cooperative heat-seal layers applied to the closure and metal end. More particularly, a heat-seal layer  24  ( FIG. 2 ) is applied to the upper surface  18  of the metal end. The heat-seal layer  24  comprises a food-compatible metal-coating composition into which polypropylene is blended. The food-compatible metal-coating composition should bond well to metal and should have good corrosion resistance, and should be capable of withstanding a retort process without substantially degrading. Various food-compatible metal-coating compositions can be used in the practice of the invention, including but not limited to epoxies, plastisols, polyesters, food-grade lacquers, and others.  
         [0016]     When the polypropylene is blended into the food-compatible metal-coating composition and the composition is coated onto the metal end and cured, the surface of the resulting heat-seal layer  24  is characterized by a multitude of microscopic spaced “islands”  26  of polypropylene that are dispersed within the matrix of the epoxy  28  (see  FIG. 3 ). These polypropylene islands  26  comprise bonding sites to which the membrane closure  20  can be sealed as further described below.  
         [0017]     The membrane  20  can include various layers to provide desired gas and moisture barrier properties and to have sufficient tear-resistance; such layers can include one or more of a metal foil, a polymer layer (e.g., polyethylene terephthalate), a kraft paper layer, a lacquer layer, etc. In the illustrated embodiment, the membrane  20  includes a metal foil layer  30  and a polymer layer  32  laminated to the upper side of the foil layer. The foil layer provides gas and moisture barrier properties, but would be easily tearable if not reinforced. The polymer layer imparts tear-resistance to the membrane and can also impart additional barrier properties.  
         [0018]     A heat-seal layer  36  is applied to the lower surface of the membrane  20 . In accordance with the invention, the heat-seal layer  36  does not require fillers or incompatible polymers and need not be a complex coextruded laminate as in the prior art, because the heat-seal layer  36  is not intended to fail within itself when the membrane is peeled from the metal end. Instead, the intended failure mode is an adhesive failure between the heat-seal layer  36  on the membrane and the heat-seal layer  24  on the metal end. In accordance with the invention, the heat-seal layer  36  comprises a layer of polypropylene without the addition of any other components such as fillers or incompatible polymers that would substantially affect the ability of the layer  36  to remain substantially intact during removal of the membrane from the metal end.  
         [0019]     The polypropylene of the heat-seal layer  36  bonds to the polypropylene bonding sites  26  on the metal end when the membrane is heat-sealed to the metal end. By virtue of the structure of the surface of the heat-seal layer  24 , the bond between the membrane and the metal end is not as strong as the bond that would exist if the heat-seal layer  24  were pure polypropylene. As noted, such a bond between two pure polypropylene layers would be too strong to allow the membrane to be readily peeled from the metal end. In contrast, the bond between the polypropylene layer  36  and the epoxy/polypropylene layer  24  is weaker because less than the full surface area of the layer  24  is bonded to the polypropylene layer  36  of the membrane.  
         [0020]     The peel strength of the bond between the membrane  20  and the metal end  14  can be controlled by varying the fraction of the total surface area of the heat-seal layer  24  that is made up of the polypropylene bonding sites or islands  26 . This fraction is a function primarily of the relative proportions of the metal-coating composition and polypropylene making up the heat-seal material of the layer  24 . In accordance with the invention, the polypropylene advantageously comprises from about one percent to about  20  percent by weight of the composition, and suitably can comprise about 10 percent by weight of the composition.  
         [0021]     The closure system in accordance with the invention is suitable for retort containers. Polypropylene has a melting temperature of about 160° C. (about 280° F.), and thus can withstand a retort process, which typically involves heating a sealed container to about 250 to 260° F. for about one hour. The closure system in accordance with the invention also can provide a relatively low peel strength between the membrane and the metal end.  
         [0022]     A series of peel strength tests were conducted to assess the peel strength of a closure system in accordance with the invention as well as a number of prior-art closure systems. The tests also sought to determine whether the peel strength is affected by subjecting the closure to an elevated temperature similar to a retort process. Five different closure configurations were tested. In each configuration, a sheet metal precursor or blank from which a metal end is made had a coating of an identical Watson Rhenania epoxy/polypropylene blend applied to it and cured, such that the sheet metal precursors in all cases had substantially identical heat-seal layers of the epoxy/polypropylene composition. The surface of the heat-seal layer included polypropylene islands dispersed in the epoxy matrix generally as shown in  FIG. 3 . The membranes were heat-sealed to the sheet metal precursors with 40 psi pressure for one second at 400° F. with a rubber bar under the precursor to help ensure that a uniform pressure was applied over the surface of the membrane.  
         [0023]     The membrane structures used in the five test configurations are shown in Table I below:  
                                 TABLE I                           Membrane Structures Tested            Membrane                   Configuration   Layer 1   Layer 2   Heat-Seal Layer               A    1 μm lacquer   62 μm foil   Coex PP/PP mixed with                   LDPE and filled with                   talc and       B    1 μm lacquer   72 μm foil   Coex PP/PP mixed with                   HDPE       C   16 μm PET   21 μm foil   PP mixed with LDPE       D    3 μm lacquer   66 μm foil   LDPE mixed with HDPE                   and filled with talc       E   12 μm PET    9 μm foil   PP                  
 
         [0024]     Configuration A was a Lawson Mardon Gold Foil membrane, configuration B was a Lawson Mardon Silver Foil membrane, configuration C was a Lawson Mardon PET film membrane, configuration D was an Alcoa membrane, and configuration E was a membrane in accordance with one embodiment of the present invention.  
         [0025]     Samples of each test configuration were subjected to a peel test with an Instron tester with a T-peel geometry, and the maximum peel load per inch width of the membrane was recorded. Some samples of each configuration were subjected to simulated retort conditions by heating the samples to 250° F. for one hour, with a 15-minute heat-up period and a 15-minute cool-down period. The peel test was done for each configuration both before the retort process and after the retort process. The results are shown in Table II below:  
                             TABLE II                           Peel Strength Test Results with Epoxy/       Polypropylene Heat Seal Layer on Metal                Peel Strength   Peel Strength       Membrane   Before Retort   After Retort       Configuration   (lb/in)   (lb/in)               A   4.8   4.2       B   4.9   4.6       C   4.8   4.0       D   5.3   3.5       E   6.4   5.0                  
 
 The standard deviation in all cases was 0.2 to 0.3. 
 
         [0026]     Two additional series of tests as described above were conducted on the same five membrane structures described in Table I, but wherein the heat-seal layer applied to the metal precursors was pure polypropylene. Specifically, in one series a coating of Morprime 10B (polypropylene suspended in an organic solution) was applied to the metal precursors using a number 28 metering rod (correlates to a coating weight of about five pounds per ream) and then dried in an oven at 220° C. for 15 to 20 minutes to evaporate the solvent. In the other series, a Morprime 14B coating was applied and dried in the same manner. The results of the first series of tests are shown in Table III and the results of the second series of tests are shown in Table IV below:  
                                                           TABLE III                           Peel Strength Results with Morprime 10B       Polypropylene Heat-Seal Layer on Metal                Peel Strength       Peel Strength           Membrane   Before Retort       After Retort       Configuration   (lb/in)   Std. Dev.   (lb/in)   Std. Dev.                    A   5.5   0.8   5.0   0.4       B   5.2   0.1   4.9   0.2       C   5.6   0.4   4.9   0.7       D   4.3   0.3   3.0   0.4       E   26   2   22   3                  
 
         [0027]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                   
               
               
                 Peel Strength Results with Morprime 14B 
               
               
                 Polypropylene Heat-Seal Layer on Metal 
               
             
          
           
               
                   
                 Peel Strength 
                   
                 Peel Strength 
                   
               
               
                 Membrane 
                 Before Retort 
                   
                 After Retort 
               
               
                 Configuration 
                 (lb/in) 
                 Std. Dev. 
                 (lb/in) 
                 Std. Dev. 
               
               
                   
               
             
          
           
               
                 A 
                 4.9 
                 0.4 
                 4.2 
                 0.2 
               
               
                 B 
                 5.2 
                 0.3 
                 4.3 
                 0.3 
               
               
                 C 
                 4.4 
                 0.4 
                 5.2 
                 0.9 
               
               
                 D 
                 3.6 
                 0.2 
                 2.4 
                 0.2 
               
               
                 E 
                 23 
                 1 
                 14 
                 2.5 
               
               
                   
               
             
          
         
       
     
         [0028]     The results in Table II above indicate that a membrane and metal closure system made in accordance with the invention (configuration E membrane sealed to a metal having an epoxy/polypropylene heat-seal layer) was able to achieve a relatively low peel strength generally of the same magnitude as the other configurations tested. The peel strength declined only moderately after retort.  
         [0029]     Comparison of the results in Tables III and IV with those in Table II further reveals that the membrane configurations A through D did not have substantially different peel strengths whether sealed to the epoxy/polypropylene heat-seal layer or the pure polypropylene heat-seal layer. This is consistent with the failure mechanism in these closure configurations being a failure of the heat-seal layer on the membrane rather than an adhesive failure between the membrane and the metal. In contrast, the configuration E membrane had a much higher peel strength when bonded to the pure polypropylene heat-seal layer than when bonded to the epoxy/polypropylene heat-seal layer. This is consistent with the failure mechanism being an adhesive failure between the heat-seal layer on the membrane and the heat-seal layer on the metal.  
         [0030]     From the foregoing, it will be appreciated that the invention allows a membrane and metal end closure to be constructed with a relatively simple membrane structure. The closure is able to withstand a retort process without substantial degradation of the bond strength between the membrane and the metal end. At the same time, the peel force required to remove the membrane can be controlled to be relatively low so that the membrane is readily removable. The invention allows the peel force to be controlled in a simple fashion by suitably varying the proportions of epoxy and polypropylene in the heat-seal composition applied to the metal end.  
         [0031]     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.