Abstract:
Disclosed is a closure liner composition comprising a mixture (or blend) of a thermoplastic polymer, an oxygen barrier polymer, and a particulate oxygen scavenging material. The oxygen barrier polymer preferably comprises polyisobutylene. The particulate oxygen scavenging material preferably is selected from the group consisting of an alkali metal salt of sulfite, ascorbate, isoascorbate, and a mixture of two or more of these materials. The closure liner composition may be used to fabricate a liner for a closure (e.g., a metal crown used to cap a bottle) that provides superior oxygen reduction properties and improved shelf life.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a closure liner composition with improved oxygen reduction. 
         [0002]    Closures (or caps) for use in food, beverage and pharmaceutical containers are typically made of plastic or metal or both metal and plastic. Such closures generally include a liner (or gasket) on the inner surface to provide a hermetic seal at the container opening. Since many products that are packaged in a container can be adversely affected by the presence of oxygen, it is particularly desirable if the closure liner is formulated to prevent oxygen ingress. Various products are oxygen sensitive and must be protected from oxygen ingress, the most typical one being beer. Such oxygen sensitive products, particularly beer, are typically packaged in a bottle capped with a metal closure (or crown). 
         [0003]    Various techniques have been employed to formulate liner compositions to prevent oxygen ingress. In one technique, as described, for example, in U.S. Pat. No. 5, 075,362 and EP 0 328 336, the liner composition includes an oxygen scavenger, such as an ascorbate or sulfite salt, blended into the base polymer composition that can be molded into the form of a closure liner. With this type of closure liner, the oxygen scavenger will reduce the amount of residual oxygen in the headspace of the container upon filling, as well as consume any external oxygen that permeates through the closure. Since the oxygen scavenger is a reducing agent that reacts with oxygen in the presence of water vapor (but is substantially inert in the absence of moisture), one reasonably might conclude that the base polymer composition should have some permeability to both oxygen and water vapor in order to permit these materials to reach the oxygen scavenger incorporated into the polymer. 
         [0004]    Another approach to prevent oxygen ingress is to utilize a liner composition that is an oxygen barrier—i.e., the liner is a physical barrier that prevents ingress of oxygen. Such a liner composition is described, for example, in U.S. Pat. No. 6,677,397 and includes polyisobutylene as an oxygen barrier material in the base polymer composition that can be molded into the form of a closure liner. While this type of liner may reduce ingress of external oxygen into the container, it will not reduce the amount of oxygen in the headspace of the container. 
         [0005]    A further approach to prevent oxygen ingress is to utilize a closure liner that has both an oxygen scavenger and an oxygen barrier. Such an approach is described, for example, in U.S. Pat. No. 6,194,042. However, the liner described in this patent consists of multiple layers, where one layer is a foamed layer that includes an oxygen scavenging material and another, separate layer serves as a gas barrier layer. The foamed layer permits the free flow of oxygen (and moisture) to come in contact with the oxygen scavenger. A multilayer device such as this is expensive and complicated to manufacture because it requires multiple different manufacturing process steps: e.g., a multilayer liner sheet is coextruded, then formed into individual liners and then placed into the cap. 
         [0006]    It would be highly desirable to provide a closure liner composition with improved oxygen reduction properties and, thereby, improved shelf life. In addition, it would be highly desirable to provide a closure liner composition that is both cost effective and capable of being processed (e.g., molded into closures such as bottle crowns) in a single manufacturing step. It would be further desirable to provide a closure liner composition that prevents contamination from other undesirable gases or solvents, such as trichloroanisole (TCA). 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention embraces a closure liner composition comprising a mixture (or blend) of a thermoplastic polymer, an oxygen barrier polymer, and a particulate oxygen scavenging material. The oxygen barrier polymer preferably comprises polyisobutylene. The particulate oxygen scavenging material preferably is selected from the group consisting of the alkali metal salts of sulfite, ascorbate, isoascorbate, and a mixture of two or more of these materials. In other words, the oxygen scavenging material may be one or more of an alkali metal sulfite, an alkali metal ascorbate or an alkali metal isoascorbate. The closure liner composition of the present invention provides superior oxygen reduction properties and improved shelf life. 
         [0008]    The above-described closure liner composition may be used to fabricate a liner for a closure. The liner is preferably fabricated as a single layer. If the liner includes more than one layer, then at least one layer must comprise the above-described closure liner composition. 
         [0009]    The present invention also includes a closure for a container, wherein the closure includes a closure liner fabricated from the afore-described closure liner composition. In particular, the present invention is directed to a metal crown for a beverage container, wherein the metal crown includes a closure liner fabricated from the afore-described closure liner composition. In addition, the present invention embraces a container filled with a product, wherein the container is capped by a closure that includes a liner fabricated from the afore-described closure liner composition. In particular, the present invention embraces a bottle filled with a beverage, wherein the bottle is capped by a metal crown that includes a liner fabricated from the afore-described closure liner composition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a graph that depicts the oxygen reducing (i.e., absorbing/blocking) properties over time of a closure liner of the present invention (C) that includes both an oxygen scavenger material and an oxygen barrier polymer compared to closure liners that include either an oxygen scavenger material (A) or an oxygen barrier polymer (B). 
           [0011]      FIG. 2  is a graph of the data contained in  FIG. 1 , but without the control in order to permit expansion of the graph and more clearly depict the oxygen reducing properties of a closure liner of the present invention (C) over time compared to closure liners A and B. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    In particular, the closure liner composition of the present invention advantageously comprises, by weight of the composition, 40% to 80%, preferably 50% to 70%, of a thermoplastic polymer, 5% to 50%, preferably 10% to 30%, of an oxygen barrier polymer, and 1% to 30%, preferably 3% to 20%, more preferably 5% to 15%, of a particulate oxygen scavenging material. 
         [0013]    The oxygen barrier polymer preferably comprises polyisobutylene, and more preferably consists essentially of polyisobutylene. Typically, polyisobutylene is a copolymer of isobutylene with a small amount (e.g., about 2% by wt.) of isoprene. The particulate oxygen scavenging material preferably is selected from the group consisting of an alkali metal sulfite, an alkali metal ascorbate, an alkali metal isoascorbate, and a mixture of two or more of these materials. The preferred alkali metal is sodium or potassium. Sodium sulfite is especially preferred as the oxygen scavenging material. It is preferred that the particulate oxygen scavenging material have an average particle size in the range of about 0.1 μm to about 50 μm. 
         [0014]    The thermoplastic polymer generally should be permeable, or at least partially permeable, to oxygen and water vapor. The thermoplastic polymer typically will comprise one or more polymers selected from the group consisting of polyethylene, polypropylene, ethylene propylene copolymer, styrene butadiene rubber, carboxylated styrene butadiene, polyisoprene, styrene isoprene styrene block copolymer, styrene butadiene styrene block copolymer, styrene ethylene butylene styrene block copolymer, ethylene vinyl acetate copolymer, ethylene (meth) acrylate copolymer (for instance ethylene butyl acrylate copolymer), ethylene vinyl alcohol copolymer, vinyl chloride homopolymer or copolymer, styrene acrylic polymer, polyamide, and vinyl acetate polymer and blends of one or more of these. Particularly preferred thermoplastic polymers include ethylene vinyl acetate, polyethylene, particularly high density polyethylene, and blends thereof. 
         [0015]    The closure liner composition may contain other optional components such as filler, slip aids, process aids, pigments, lubricants, stabilizers, antioxidants, UV stabilizers, plasticizers, mineral oil, tackifying resins and other conventional additives in conventional amounts, depending upon the nature of the composition and the properties desired. Typically, the total amount of such additives is generally below 20%, preferably below 10%, by weight of the composition. 
         [0016]    The above-described components for the closure liner composition typically are subjected to a twin screw compounding process (e.g., using step melt temperatures of about 150° C. to 220° C.) to form pellets, which pellets may then be molded into a closure liner, typically via a high speed cold molding process. A closure liner fabricated of the above-described closure liner composition preferably will have a Shore hardness between 70 Shore A and 100 Shore A, and a melt flow index (MFI) between 0 and 20 g/min (2.16 kg, 190° C.). 
         [0017]    The closure liner may be pre-molded, then inserted into a closure, or preferably it will be molded directly (i.e., in situ) into the inner portion of the closure, particularly where the closure is a metal crown for use in capping a bottle. The closure with liner may then be fitted onto a container that has been filled with a product, e.g., a bottle filled with beer. 
       EXAMPLE 
       [0018]    Closure liners are prepared from the closure liner compositions shown in Table 1 below. Compositions A and B are prior art compositions for comparison, while composition C is a composition of the present invention. Composition A includes an oxygen scavenging material (sodium sulfite), but does not include an oxygen barrier polymer. Composition B includes an oxygen barrier polymer (polyisobutylene), but does not include an oxygen scavenging material. Composition C includes both an oxygen scavenging material (sodium sulfite) and an oxygen barrier polymer (polyisobutylene). 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Amount (parts by weight) 
               
             
          
           
               
                 Ingredient 
                 A 
                 B 
                 C 
                 Control 
               
               
                   
               
             
          
           
               
                 LDPE 1   
                 30 
                   
                   
                 37 
               
               
                 HDPE 2   
                   
                 60 
                 58 
               
               
                 Ethylene Butylene Copolymer 3   
                 18 
                 10 
                 10 
                 18 
               
               
                 Polyisobutylene 4   
                   
                 30 
                 25 
               
               
                 Styrene-Butadiene-Styrene Copolymer 5   
                 30 
                   
                   
                 30 
               
               
                 White Mineral Oil 6   
                 15 
                   
                   
                 15 
               
               
                 Polydimethyl Siloxane 7   
                   
                 0.5 
                 0.5 
               
               
                 Sodium Sulfite (anhydrous; &lt;50 μm) 
                 7 
                   
                 7 
               
               
                   
               
               
                   1 LDPE 1070 MN 18C (Total Petrochemicals) 
               
               
                   2 Stamylan HD7108 (Sabic) 
               
               
                   3 Flexirene CL10 (Polimeri) for A; Clearflex MQB0 (Polymeri) for B&amp;C 
               
               
                   4 Butyl 101-3 (Lanxess) 
               
               
                   5 Kraton D1153 ES (30% styrene; Kraton Polymers) 
               
               
                   6 Finavestan A360B (Total Lubricants) 
               
               
                   7 Siliconol AK1000 (Wacker Chimie) 
               
             
          
         
       
     
         [0019]    Preparation of the closure liner pellets. All the ingredients of the composition are blended in a twin screw extruder at melt temperatures between 150° C. and 220° C. to provide pellets for each of the compositions A, B and C, as well as the Control, which includes no oxygen barrier or scavenger material. 
         [0020]    Preparation of lined crown closures. The above-described extruded pellets are melted, deposited in crown closures and cold molded using conventional crown lining equipment. The closure liner adheres well to the inner surface of the crown (closure) shell. 
         [0021]    Physical barrier test. The lined crown closures are tested for physical barrier performance, namely their ability to block permeation of the contaminant 2,4,6-tricholoranisole (TCA). The test crowns (containing liners A, B and C as described above) are sealed onto bottles containing 5% ethanol solution. The bottles are then placed in a controlled environment containing TCA and stored for 30 days until a detectable level of TCA has permeated the samples and controls. The samples are subjected to liquid/liquid extraction and GC/MS analysis to measure the TCA contaminant permeated into the bottle. The results are shown in Table 2. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Composition 
                 TCA (μg/L) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 3760 
               
               
                   
                 B 
                 40 
               
               
                   
                 C 
                 30 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    As can be seen from the above data, composition C has excellent physical barrier properties to TCA, which means that it also would perform as a physical barrier to oxygen. When compared to the barrier performance of composition B, it can be seen that the presence of oxygen scavenger in composition C does not adversely affect the physical barrier performance. 
         [0023]    Oxygen reduction test. Lined crowns are closed onto 330 ml glass bottles filled with acidified and carbonated water. An oxygen sensor is glued inside the bottles before the filling step. After pasteurization at 62° C. for 20 minutes, the bottles are stored at around 22° C. for oxygen content measurement. Oxygen measurements are performed on shaken bottles using the non-invasive method, Oxysense™ 101. This technique measures the amount of oxygen in the package over time and, thus, evaluates the performance of crown liners to scavenge oxygen within the package and/or block oxygen from permeating into the package. 
         [0024]    The ability of crowns containing closure liners fabricated from compositions A, B and C to reduce oxygen levels in filled bottles versus a Control is depicted in  FIG. 1 . As can be seen, closure liners fabricated of composition C of the present invention show the greatest reduction in oxygen, and the low level of oxygen is maintained for more than 300 days. In contrast, closure liners fabricated of compositions A and B reduce oxygen levels to a much lesser extent, and that level increases over the extended time period. The difference in performance of composition A versus compositions B and C is more clearly depicted in  FIG. 2 , which is a graph based on the same data shown in  FIG. 1 , but without the control, in order to expand the graph. 
         [0025]    The superior performance of closure liners made with composition C is surprising in two respects. First, the oxygen scavenger material requires the presence of moisture to be activated. Since a liner containing the barrier polymer should have a lower permeation to water compared to a liner without the barrier polymer, it would be expected to have a lower oxygen scavenger efficiency by reducing the moisture permeation. Second, since a liner containing the barrier polymer has a reduced permeability to oxygen compared to a liner without the barrier polymer, it would be expected to have a lower oxygen scavenger efficiency by reducing the oxygen permeation. In other words, one would expect that the barrier polymer would inhibit both moisture and oxygen from contacting the oxygen scavenger material. This example indicates that it is not the case. The oxygen scavenger performance of the liner is not adversely impacted by the barrier polymer. In fact, as can be seen, the liner made of composition C reacts faster than the liner made of composition A.