Abstract:
A co-extruded microlayered polymer barrier film is provided. The barrier film can be applied to paperboard to form a container having reduced oxygen gas permeability. Co-extrusion of the barrier film allows for the simultaneous fabrication of tens or hundreds of alternating microlayers of two different polymers that combine to form a film having reduced oxygen gas permeability compared to macro layer thickness films of the same two polymers.

Description:
BACKGROUND 
     In paperboard containers, a flat paperboard blank is folded over on itself to form a container that is square or rectangular in cross-section. The side ends of the container are sealed together to form the final structure. The paperboard is coated on its exterior and interior surfaces with a heat-sealable material that will bond to form the container. Typically, this heat sealable material is a low density polyethylene (LDPE) or other material having a melting point low enough to seal without damaging the paperboard. 
     There can be additional layers of material on the product side of the paperboard, between the paperboard and the interior sealing layer. One of these layers is usually a gas barrier layer. The gas barrier layer is either laminated, extruded, or co-extruded onto the paperboard to provide a board with gas barrier properties. Extrusion or co-extrusion is a faster, lower cost process than lamination. The cost of a laminate film and associated handling is also avoided with co-extrusion. 
     The need for a gas barrier in paperboard packages is well known and there have been many solutions to reduce gas transfer through the paperboard assembly. The amount and rate of gas transfer will depend on the type of barrier layer or layers that is used. 
     Particularly, the need for an oxygen barrier is well known. Oxygen contacting the product reduces the shelf life of the product. The amount and rate of oxygen transfer will depend on the type of barrier layer or layers that is used. The oxygen barrier layer is laminated, extruded or co-extruded onto the paperboard to provide a paperboard assembly with oxygen barrier properties. Oxygen is the gas for which barriers are tested but the barriers are useful for other gases so the barriers are commonly termed “gas barriers.” 
     There are other factors than oxygen permeability that enter into the choice of a barrier material. Cost is a factor, both in the type and cost of the barrier material being used and also in the amount required to obtain the necessary barrier properties. The number of layers or laminations of material are a factor in the cost. 
     The most effective, and most costly, oxygen barrier has been an aluminum foil barrier. Food, such as chips, has been placed in foil or plastic pouches. Aluminum foil has also been used in juice containers. Plastic materials, such as nylon, have also been used as barrier materials. Many other materials have been proposed and used in order to reduce the cost of the barrier material. Each of these materials has attributes and shortcomings. Usually, a number of layers of different materials are provided, with each layer having a special function. These functions might be the ease of attachment, a barrier for certain gases, a barrier for certain chemicals in the contained liquid, the ability to adhere two layers together, and the ability to seal the container. Each of these layers adds cost to the container, both in material cost and production cost. 
     An effective oxygen barrier will have an oxygen transfer rate (OTR) of 40 or less cubic centimeters of oxygen per square meter per day (cc/m.sup.2/day) at standard temperature and pressure (STP) and 50% relative humidity (% RH). Water soluble or water dispersible oxygen barrier materials such as polyvinyl alcohol require large quantities of material per square meter to be effective. This is usually more material than can be applied in a coating application such as a size press without manufacturing difficulty. 
     A paperboard packaging material minimizing oxygen permeability is desired. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one aspect, a paperboard assembly with oxygen barrier properties is provided. In one embodiment, the paperboard assembly includes: 
     a paperboard having a first side and a second side opposite the first side; and 
     a composite barrier film adjacent the second side of the paperboard, the composite barrier film comprising: 
     a first containment layer, disposed closest to the paperboard; 
     a second containment layer, disposed farthest from the paperboard; and 
     a microlayered composite layer intermediate the first containment layer and the second containment layer and comprising a plurality of unit layers, each unit layer comprising one layer of a first microlayer polymer and one layer of a second microlayer polymer, without a tie layer intermediate, wherein the first microlayer polymer is a polyolefin or a polyolefin modified for improved adhesion, and wherein the second microlayer polymer is selected from the group consisting of a polyamide (PA), polypropylene (PP), polyethylene terephthalate (PET), and blends thereof. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional diagram of a paperboard assembly with gas barrier properties in accordance with the disclosed embodiments; 
         FIG. 2  is an enlarged view of a portion of the paperboard assembly of  FIG. 1 ; 
         FIG. 3  is a cross-sectional diagram of a paperboard assembly with gas barrier properties in accordance with the disclosed embodiments; 
         FIG. 4  is a cross-sectional diagram of a paperboard assembly with gas barrier properties in accordance with the disclosed embodiments; and 
         FIG. 5  is a micrograph of a cross-section of an exemplary composite barrier film in accordance with the disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, a paperboard assembly with oxygen barrier properties is provided. In one embodiment, the paperboard assembly includes: 
     a paperboard having a first side and a second side opposite the first side; and 
     a composite barrier film adjacent the second side of the paperboard, the composite barrier film comprising: 
     a first containment layer, disposed closest to the paperboard; 
     a second containment layer, disposed farthest from the paperboard; and 
     a microlayered composite layer intermediate the first containment layer and the second containment layer and comprising a plurality of unit layers, each unit layer comprising one layer of a first microlayer polymer and one layer of a second microlayer polymer, without a tie layer intermediate, wherein the first microlayer polymer is a polyolefin or a polyolefin modified for improved adhesion, and wherein the second microlayer polymer is selected from the group consisting of a polyamide (PA), polypropylene (PP), polyethylene terephthalate(PET), and blends thereof. 
       FIG. 1  is a cross-sectional diagram of the paperboard assembly  100  with oxygen barrier properties. As used herein, “oxygen” typically refers to molecular oxygen gas (O 2 )—which is the most critical gas that must be prevented from diffusing through a paperboard container—although it will be appreciated that other forms of oxygen, such as ozone are also encompassed. 
     In certain embodiments, the paperboard assembly  100  may also have water vapor barrier properties. 
     Referring to  FIG. 1 , the paperboard assembly  100  includes a paperboard  105  having a first side and a second side opposite the first side. A barrier film  110  is adjacent the second side of the paperboard  105 . The barrier film  110  is comprised of at least three different regions: a first containment layer  115 , disposed closest to the paperboard  105 ; a second containment layer  125 , disposed farthest from the paperboard  105 ; and a microlayered composite layer  120  intermediate the first containment layer  115  and the second containment layer  125 . 
     An enlarged view of the circular region  2  of  FIG. 1  is shown in  FIG. 2 . Referring to  FIG. 2 , the microlayered film  120  is intermediate the first containment layer  115  and the second containment layer  125 . In the microlayered film  120 , a plurality of unit layers  123  are comprised of a first microlayer polymer  121  and a second microlayer polymer  122 . The unit layer  123  repeats throughout the thickness of the microlayered film  120 . 
     Each of the components of the paperboard assembly  100  of  FIGS. 1 and 2  will now be discussed in further detail. 
     The paperboard assembly  100  provided herein has specific applications in food and beverage packaging or any applications where it is desirable to exclude oxygen from the interior of a container. The paperboard assembly  100  may be formed into food packages or into containers and cups or any other use known to those of skill in the art. 
     The foundation of the paperboard assembly  100  is the paperboard  105  itself. The paperboard  105  can be any type of paperboard known to those of skill in the art. By applying the barrier film  110  to the paperboard  105 , the oxygen permeability of the paperboard assembly  100  is reduced compared to the oxygen permeability of the paperboard  105  itself. For example, in one embodiment the oxygen transmission rate OTR through the paperboard assembly is 30 g/m 2 ·day or less at 23° C. and 50% relative humidity (RH). In another embodiment, the OTR is 40 g/m 2 ·day or less at 23° C. and 50% RH. 
     In the embodiment illustrated in  FIG. 1 , the barrier film  110  is abutting the paperboard  105  to form the paperboard assembly  100 . The barrier film  110  is comprised of a first containment layer  115 , a second containment layer  125 , and a microlayered composite layer  120  intermediate the containment layers. The barrier film is formed from two or more polymers that are co-extruded to form a barrier film  110  that is then applied to the paperboard  105  to form the paperboard assembly  100 . 
     The barrier film  110  is of unitary construction, meaning that when the barrier film  110  is formed it is a single piece of polymer composite. This can be distinguished from attempting to recreate the barrier film  110  by sequentially depositing the dozens of polymer layers that comprise the barrier film  110  (i.e., depositing a first containment layer  115 , depositing the many layers of the microlayered film  120 , and then depositing the second containment layer  125 ). The barrier film  110  can be unitarily fabricated using a co-extrusion die with a layer multiplier, such as those sold by Extrusion Dies Industries, LLC. and Cloeren, Inc. 
     The first containment layer  115  is comprised of a polymer that is capable of binding (“tying”) the paperboard  105  to the first microlayer polymer  121  of the microlayered film  120 . Therefore, the composition of the first containment layer  115  must be selected to provide a bond between the paperboard  105  and the first microlayer polymer  121 , and so the composition of the first containment layer  115  may vary depending on the characteristics of the paperboard  105  and the first microlayer polymer  121 . In a representative example, the first containment layer  115  is a polyolefin, such as a polyethylene (e.g., polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), or linear LDPE), a polyamide (such as nylon), or a polyamide/PET blend. Additional representative first containment layer  115  polymers include “modified” versions of the above-listed polymers, which are modified to include additional moieties within the polymer, such as maleic anhydride (which improves adhesion properties). 
     The first containment layer  115  may be a water vapor barrier, depending on its composition. For example, if a blend of PET and amorphous nylon is used, the first containment layer  115  could be a water vapor barrier depending on the amount of PET. 
     The second containment layer  125  is disposed furthest from the paperboard  105  and provides structural containment of the microlayered film  120  in a similar manner to the first containment layer  115 . The composition of the second containment layer  125  is such that it will bond, without a tie layer, to whichever of the first microlayer polymer  121  and second microlayer polymer  122  is disposed at the surface of the microlayered film  120  closest to the second containment layer  125 . 
     The composition of the second containment layer  125  may be the same or different from the first containment layer  115 . In the configuration of the paperboard assembly  100  illustrated in  FIG. 1 , the second containment layer  125  is the last layer of the barrier film  110  applied to the paperboard  105 , such that the second containment layer  125  will be the material of the paperboard assembly  100  exposed to the product contained within a container formed from the paperboard assembly  100 . With this in mind, a representative second containment layer is LDPE, which is known to those of skill in the art as a polymer layer acceptable for contacting typical products stored within paperboard containers. LDPE also provides water vapor barrier properties. However, it will be appreciated by those of skill in the art that other polymers for the second containment layer  125  can be used as long as the requirements of the paperboard assembly  100  and the product contained therein are met. 
     The first containment layer  115  and second containment layer  125  may have thicknesses that are the same or different. Each layer must be at least thick enough to support the microlayered film  120 , which typically requires a minimum thickness for the containment layers  115  and  125  of 1 micron or greater. However, it will be appreciated that the properties of the polymers forming the containment layers  115  and  125  will dictate the minimum thickness required for the layers. 
     The microlayered film  120  has oxygen barrier properties and may also have water vapor barrier properties, depending on its composition. The microlayered film  120  includes a plurality of unit layers  123 , each comprised of a first microlayer polymer  121  and a second microlayer polymer  122 , without a tie layer in between. That is, the first microlayer polymer  121  and the second microlayer polymer  122  abut each other in the unit layer  123 . Similarly, there are no tie layers in between abutting unit layers  123  in the microlayered film  120 . 
     The number of unit layers  123  can be adjusted according to the mechanical and oxygen transport characteristics desired from the paperboard assembly  100 . In certain embodiments, the microlayered film  120  comprises between 10 and 100 unit layers  123 . The microlayered film  120  can be configured such that either the first microlayer polymer  121  or the second microlayer polymer  122  can contact either or both of the first containment layer  115  and second containment layer  125 . It will be appreciated that in the configuration illustrated in  FIG. 2 , the first microlayer polymer  121  contacts the first containment layer  115 , and the second microlayer polymer  122  contacts the second containment layer  125 . While this is a particular embodiment, the aspects provided herein are not limited to such an embodiment. 
     The thicknesses of the first microlayer polymer  121  and the second microlayer polymer  122  can be varied according to the co-extrusion process. In certain embodiments, the first microlayer polymer  121  and the second microlayer polymer  122  each have a thickness of from 0.1 to 0.8 microns. 
     In certain embodiments, thickness of the barrier film  110  is from 0.4 mil to 2.0 mil. 
     The first microlayer polymer  121  is a polyolefin. Polyolefins are known to those of skill in the art to include the various forms of LDPE and HDPE. In one embodiment, the polyolefin is selected from the group consisting of a polyethylene, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, and combinations thereof. The polyolefin  121  may be modified to improve adhesion to polymer  122 . The addition of maleic anhydride to the polyolefin is one way of providing improved adhesion. 
     The composition of the second microlayer polymer  122  is selected from the group consisting of polyamide (such as a nylon), polypropylene, polyethylene terephthalate (PET), and blends thereof. As disclosed in the experimental evidence set forth below in Table 1, microlayers incorporating polyamide or PET as the second microlayer polymer  122  both dramatically reduce the oxygen transmission across a representative barrier film  110 . 
     In one embodiment, the polyamide is selected from the group consisting of a nylon, PA6, PA66, amorphous nylons such as DuPont Selar 3426 or EMS Grivory G21, aromatic ring containing nylons such as MXD6 or MXD6/MXDI, and combinations thereof. 
     With regard to the composition and thickness of the microlayer polymers  121  and  122 , as well as the thickness of the microlayered film  120 , it will be appreciated that the design trade offs involved include variations of mechanical flexibility, conservation of materials, and resistance to oxygen transmission. In this regard, the greater the number of unit layers  123 , the greater the reduction in oxygen permeability for the barrier film  110 . However, increasing the number of unit layers  123  also requires more polymer material to be used (increasing cost). Therefore, by balancing these characteristics, one of skill in the art can form an appropriate barrier film  110  according to the design requirements of a particular packaging requirement. 
     Referring now to  FIG. 3 , another embodiment of a paperboard assembly  300  is illustrated, wherein an additional tie layer  130  is inserted intermediate the paperboard  105  and the barrier film  110  when compared to the paperboard assembly  100  illustrated in  FIGS. 1 and 2 . The tie layer  130  may serve several purposes, and may also be comprised of one or more layers of one or more different polymers. While the purpose of the tie layer  130  may be to physically tie the barrier film  110  to the paperboard  105 , the tie layer  130  may also, or instead, serve the purpose of providing further water vapor barrier properties and/or durability. For example, a water vapor barrier polymer may be used for the tie layer  130 , which would provide an additional layer of protection for the paperboard assembly  300  compared to the paperboard assembly  100 , which does not include the tie layer  130 . In the representative case where the tie layer  130  is a water vapor barrier polymer, both HDPE and LDPE are representative materials that could be used for such an application. In the representative case where the tie layer  130  provides improved durability, both PP and linear-LDPE are representative polymers that can be used for the tie layer  130 . 
     Referring now to  FIG. 4 , another embodiment of a paperboard assembly  400  is illustrated. The paperboard assembly  400  is similar to the paperboard assembly  300  of  FIG. 3 , although it includes a printing layer  140  abutting the first side of the paperboard  105 . The printing layer  140  is a layer known to those of skill in the art as being the “outside” face of a container made from paperboard, and therefore is configured to receive and support a printed image, such as packaging logos, colors, words, etc. The printing layer  140  also acts as a barrier against the migration of moisture (e.g., vapor or condensation) into the paperboard  105 . 
       FIG. 4  also illustrates an optional product layer  145  that can be included in any embodiment disclosed herein (i.e., this layer is not limited to the configuration shown in  FIG. 4  but is also applicable to the configurations illustrated in  FIGS. 1-3 ). The product layer  145  has a composition compatible with the product which the paperboard assembly  400  will come into contact with when formed into a container. As discussed above, LDPE is a common polymer that is used in paperboard packaging, which is why it is an exemplary material useful as the second containment layer  125  in the embodiments disclosed herein. However, if the second containment layer  125  is a material that is not compatible with the product, a product layer  145  can be applied over the barrier film  110  so as to improve the compatibility of the paperboard assembly  400  with the product. While LDPE is a representative material useful as a product layer  145 , although any material known to those of skill in the art will be useful in the described embodiments. Product layer  145  may also act as a barrier to reduce water vapor from migrating into the paperboard  105 . 
     Experimental testing was performed on several exemplary paperboard assemblies of the type described in the embodiments herein. Table 1 summarizes these results. Both a polyamide (PA) and a PET control sample were used, which were compared to microlayered PA and microlayered PET. The samples included a barrier film  110  having a first containment layer  115  consisting of LDPE, a second containment layer  125  consisting of LDPE, and a polymer layer varied between PA, PET, microlayered LDPE/PA, and microlayered LDPE/PET. Two independent testing facilities were used to test the barrier films for both oxygen transmission rate (OTR) and water vapor transmission rate (WVTR). As can be seen from the data in Table 1, oxygen transmission is dramatically reduced in both the microlayer PA films and the microlayer PET films compared to the respective control films. Accordingly, the barrier films disclosed herein are particularly useful as oxygen barriers. Some combinations of microlayered films may also reduce water vapor transmission. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Water vapor and oxygen permeability test results. 
               
             
          
           
               
                   
                 Sample Description 
                 Testing Facility A 
                 Testing Facility B 
               
             
          
           
               
                   
                 (0.5 mil LDPE containment layer/ 
                 WVTR 
                 OTR 
                 WVTR 
                 OTR 
               
               
                   
                 Polymer layer listed below/ 
                 gm-mil/ 
                 cc-mil/ 
                 gm-mil/ 
                 cc-mil/ 
               
               
                 Sample 
                 0.5 mil LDPE containment layer) 
                 (100 in 2  · day) 
                 (100 in 2  · day) 
                 (100 in 2  · day) 
                 (100 in 2  · day) 
               
               
                   
               
             
          
           
               
                 Control PA 
                 0.28 mil PA 
                 1.13 
                 22.7 
                 0.34 
                 20.4 
               
               
                 Control PET 
                 0.33 mil PET 
                 1.00 
                 138.5 
                 0.32 
                 49.6 
               
               
                 Microlayer PA 
                 0.28 mil of 48 microlayers of 
                 1.23 
                 12.9 
                 0.34 
                 14.7 
               
               
                   
                 LDPE/PA 
               
               
                 Microlayer PET 
                 0.33 mil of 48 microlayers of 
                 0.80 
                 70.9 
                 0.36 
                 35.9 
               
               
                   
                 LDPE/PET 
               
               
                   
               
             
          
         
       
     
       FIG. 5  is a micrograph of a Microlayer PA film similar to those that were tested to provide the results in Table 1. 
     Methods for forming the paperboard assemblies disclosed herein are also provided. In one aspect, a method for forming a paperboard assembly  100  is provided that includes the steps of providing a paperboard  105 ; co-extruding a barrier film  110  including a first containment layer  115 , a second containment layer  125 , and a microlayered composite layer  120  intermediate; and applying the barrier film  110  to the paperboard  105  to form the paperboard assembly  100 . In order to facilitate bonding of the barrier film  110  to the paperboard  105 , the paperboard  105  is typically pretreated to generate favorable surface chemistry for bonding. For example, either flame or corona pretreatment of the paperboard  105  are representative pretreatment methods. 
     Methods for making the paperboard assembly  100  may also include steps of providing a tie layer  130  intermediate the barrier film  110  and the paperboard  105 . A product layer  145  can be applied to the barrier film  110 . A printing layer  140  can be applied to the paperboard  105 . The tie layer  130  and product layer  145  can be co-extruded with the barrier film  110  or laminated onto the paperboard  105  or barrier film  110  independent of the co-extrusion of the barrier film  110 . 
     The disclosed embodiments are useful for forming gable-top containers, and therefore, in certain embodiments, a gable-top container is provided that includes a paperboard assembly as disclosed herein. Other types of containers include rectangular containers and any other container shapes known to those of skill in the art that can be manufactured using paperboard. Methods for forming containers using the paperboard assembly are also provided. When forming a container from one of the disclosed paperboard assemblies, it is typical for the second containment layer  125  (or the product layer  145 ) to form a seal with itself or the printing layer  140 , depending on the type of container (e.g., rectangular or gable-top). 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.