MULTILAYER BARRIER PACKAGING FILM

Abstract: A multilayer barrier packaging film suitable for use in fabricating bags for packaging primal and sub-primal meat cuts and processed meats. The multilayer film has a first outer layer of a thermoplastic polymer or copolymer, a first core layer of a barrier film comprising vinylidene chloride-methyl acrylate copolymer, a second core layer of a thermoplastic polymer or copolymer, a third core layer of a barrier film including vinylidene chloride-methyl acrylate copolymer, and a second outer layer of a thermoplastic polymer or copolymer. More specifically, EVA, LLDPE, VLDPE and blends of these materials may be used in the first and second outer layer and in the second core layer. The multilayer film is preferably made by co-extrusion of the layers, and then it is biaxially stretched. After biaxial stretching, the multilayer film may be irradiated to a dosage level of between 1 megarad and 10 megarads and heat-sealed in the form of a bag. The bag has improved storage stability characteristics.

Description: MULTILAYER BARRIER PACKAGING FILM FIELD OF THE INVENTION This invention relates to multilayer barrier packaging films. This invention has particular application to such films for use in fabricating bags for packaging primal and sub- primal meat cuts and processed meats. However, it is envisaged that films in accordance with the present invention may find use in other packaging applications such as packaging air or moisture sensitive compositions generally such as curable putties and sealants, other foodstuffs such as tofu, or the like. BACKGROUND OF THE INVENTION The meat packaging industry may be commonly divided into three segments, being fresh meats, frozen meats and processed meats. Fresh and processed meats are of inherently short shelf life compared to frozen storage. Processed meats and red meats are routinely packed in airtight plastic film packs to aid cold storage. In the fresh red meat industry, the cattle and swine are slaughtered and broken down into primal and sub-primal meat cuts. The primal and sub-primal meat cuts are large cuts of meat. They are smaller than a side of beef, for example, but larger than the ultimate cut which is sold at retail to the consumer. A primal cut comprises the entire section of a side beef, such as the rib section or the rump roast section, while a sub-primal cut comprises only a portion of such a section. Primal and sub-primal cuts are prepared at the slaughter house and are then shipped to a retail meat store, or to an institution such as a hospital, hotel or restaurant, where they are butchered into small cuts of meat suitable for the individual consumer. The processed meat industry takes various portions of the animal carcasses and processes these portions under varying conditions to produce finished meat products which may be used directly by the consumer. Products may include ham, smoked picnics, smoked butts, corned beef, turkey breast and various sausage products such as frankfurters, smoked sausage links, bologna, salami and the like. These products may be packaged in consumer portions or they may be packaged in bulk for shipment to a retail meat store, restaurant or hotel. Bulk shipments may include such items as ham chunks, cooked turkey breasts, bologna chubs, long bologna for delicatessen sale, rings of bologna, corned beef brisket, smoked picnics, smoked butts and linked products such as smoked sausage. When fresh red meat cuts, such as roast or rib sections, and bulk processed meats are prepared for shipment or storage, they are usually packaged in such a way that air ( ie oxygen) is prevented from contacting the meat and moisture is prevented from leaving the meat. This is done in order to minimize spoilage and discoloration during shipping and handling. One desirable way to package fresh red meats and processed meats so as to protect them from contact with air and from moisture loss is to shrink package them with a packaging material that has good oxygen and moisture vapour barrier properties. One such shrink packaging material that has good oxygen and moisture vapour barrier properties is polyvinylidene chloride film. Vinylidene chloride-vinyl chloride copolymers are commonly referred to as PVDC. While vinylidene chloride-vinyl chloride copolymer film has excellent barrier properties, in actual practice, when PVDC is used as a monolayer film, it must be plasticized in order for the film to have adequate abrasion resistance and flexibility at storage temperature of, for example, 30°F to 50°F. Unfortunately, the addition of plasticizer sufficient to provide the requisite low temperature properties to the PVDC monolayer film has a significant adverse effect on the barrier properties of the film. While increasing the thickness of the film from the conventional thickness of 30 to 50 microns to 125 microns or more, for instance, would improve the barrier properties of the film, it would be economically undesirable to use a monolayer film of PVDC having a thickness of 125 or more microns. Also, if such thick films were employed, bags made from the film would be difficult to gather and clip closed at the open end. One approach to the provision of a film having barrier properties which are better than those of the 38 to 50 micron monolayer PVDC film previously used for shrink packaging meat, is to employ a multilayer film, one layer of which is vinylidene chloride-vinyl chloride copolymer having a minimum amount of plasticizer. The other layer or layers of such multilayer films are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in the vinylidene chloride-vinyl chloride layer containing little or no plasticizer. While multilayer films containing one layer of PVDC have proven superior to a mono-layer film of PVDC it has been proposed to use a multilayer film with two PVDC layers separated by a thermoplastic polymer or copolymer. Such laminates offer significantly superior barrier to a multilayer film with a single layer of PVDC. The use of two layers allows the use of higher plasticizer levels to provide better resistance to cracking of PVDC layers during the abusive handling and also still provides barrier properties in the event that one of the PVDC layers is damaged during handling. In providing such a film, however, it must be recognized that good oxygen and moisture vapour barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging processed meats and primal and sub-primal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient for the film to heat-shrink within a specified range of percentages, eg from about 15 to 60 percent at about 90°C, in both the machine and the transverse directions. (Conventionally, the term "MD" refers to a machine direction and the term "TD" refers to transverse direction. ) The film must be heat sealable in order to be able to fabricate bags from the film and in order to heat seal the open ends of the fabricated bags after insertion of the meat product. The heat sealed seams of the bags must not pull apart during the heat shrinking operation, and the film must resist puncturing by sharp bone edges during the heat shrinking operation. It has been proposed to prepare multilayer films, one layer of which is a vinylidene chloride-vinyl chloride copolymer and at least one other layer of which is an ethylene-vinyl acetate copolymer. For example, such films are proposed in McFedries, Jr, et al. US Pat No 3,600,267, Peterson US Pat No 3,524,795, Titchenal et al US Pat No 3,625,348, Schirmer US Pat Nos 3,567,539 and 3,607,505, and Widiger et al US Pat No 4,247,584. In addition, multilayer films comprising a core layer of a vinylidene chloride copolymer, wherein the vinylidene chloride copolymer is a copolymer of a vinylidene chloride monomer and a vinyl chloride monomer, are known, for example as disclosed in Brax et al, US Pat Nos 3,741,253 and 4,278,738, Baird et al US Pat No 4,112,181 and Lustig et al Canadian Pat No 982,983. Multilayer films comprising a very low density polyethylene which is a linear copolymer of ethylene and higher alpha olefin containing from 3 to 8 carbon atoms, having a density below about 0.91g/cm^ and a secant modulus below about 140,000kPa are also known and disclosed in Lustig et al US Pat No 4,976,898. Kuo US Pat No 5,491,019 discloses multilayer films comprising "ethylene alpha-olefin copolymer", or "ethylene/α- olefin copolymer" which is defined as such heterogeneous materials as linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene catalyzed polymers such as EXACT™ materials supplied by Exxon, and TAFMER™ materials supplied by Mitsui Petrochemical Corporation. These materials generally include copolymers of ethylene with one or more comonomers selected from C4 to CIO alpha-olefins such as butene-1 ( ie 1-butene), hexene-1, octene-1, etc in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylene ' s which are more highly branched than their respective counterparts. LLDPE, as used herein, has a density usually in the range of from about 0.91 grams per cubic centimetre to about 0.94 grams per cubic centimetre. Other ethylene/alpha-olefin copolymers, such as the long chain branched homogeneous ethylene/alpha-olefin copolymers available from the Dow Chemical Company, known as AFFINITY™ resins, are also included as another type of ethylene alpha-olefin copolymer useful in the present invention. Also in the prior art, cross-linking by irradiation has been used to enhance the properties of films containing a single barrier layer employed in packaging operations. For example, US Pat No 3,741,253 to Brax et al teaches a multi-ply laminate having a first ply of ethylene-vinyl acetate which is cross-linked by irradiation. The second ply and the third ply of the laminate are not irradiated. The thus-prepared laminate may then be biaxially stretched. Baird et al US Pat Nos 3,821,182 and 4,112,181 teaches a three layer film combination which has been irradiated before stretching. Further, Bernstein et al US Pat Nos 4,391,862 and 4,352,844 disclose co-extruding first and second polymeric layers, irradiating the co-extruded layers, joining a third layer to the second polymeric layer, and then stretching the multilayer film. Still further, Bieler et al US Pat No 4,318,763 teaches that the seals of the bags made of multilayer film may be strengthened by cross-linking the seal area of the bag by irradiation. The barrier layer aforementioned references describe a single barrier layer of PVDC. US Patent 4542075 discloses a multilayer laminate film including two barrier layers of vinylidene chloride copolymer spaced apart by a non barrier layer, by virtue of the collapse of a melt blown layflat tube, which in its layflat configuration is laminated to an irradiatively crosslinked polyethylene. The extra lamination of the layflat tube onto the heat shkinkable layer, rather than coextrusion and orientation of the whole film, is a disadvantage. A further disadvantage is the need for tacky coatings or layers to promote self-welding of the interior layflat surface of the layflat tubes. The disclosure indicates no exact measure of the barrier layer thickness and so presumes an industry- standard PVDC copolymer thickness for each barrier layer of 0.5-1.0 mil (approx 12.5 - 25 micron). US Patent 5529833 discloses a multilayer laminated film having one oxygen barrier layer and an oxygen scavenging layer in a conventional construction, or two oxygen barrier layers where the inner oxygen barrier layer acts as a spacer between the product and an obligatory oxygen scavenging layer. In this publication, the oxygen barrier layers are specified in accordance with the usual parameters for such layers in laminated films, in terms of the thickness of PVDC copolymer used as each barrier layer or layers, of 0.5 mil (12.5 micron approx ) . It has surprisingly been determined by the present applicant that there is a closer correlation between the number of barrier layer interfaces and oxygen barrier performance than there is between the thickness of the film and the barrier performance, that is, that for a given total thickness of barrier material, oxygen barrier performance increases with an increase in the number of barrier layers in the laminate. This phenomenon is observed by the present applicant to hold true to at least a degree when the barrier layers are formed up adjacently in the laminate, leading the applicant to speculate that the barrier characteristics of the layer is as much a surface property of the material as a bulk property of the material. The relationship also appears to hold true for all of the known organic polymer barrier materials in current usage, such as homopolymers and copolymers of vinylidene dichloride (PVDC), certain polyamides, ethylene-vinyl alcohol copolymers ( EVOH ) , polyethylene terephthalate (PET), polyvinyl chloride (PVC), and especially EVOH copolymers, and copolymers of vinylidene dichloride with minor proportions of vinyl, acrylic, or other unsaturated monomers. In each of the prior art constructs using one or more oxygen barrier layers, the thickness of the barrier layers required to provide adequate barrier performance adds to the bulk thickness of the laminate without contributing significantly to strength and moreover requires the use of significant quantities of the expensive barrier copolymers. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide multilayer barrier packaging films which substantially overcome at least one of the disadvantages of the prior art films and to provide multilayer barrier packaging films that are effective in use. Other objects and advantages of the invention will hereinafter become apparent. With the foregoing and other advantages in view, this invention in one aspect resides broadly in a multilayer barrier packaging film including an irradiated biaxially orientated coextrusion of at least four layers including at least two layers of a thermoplastic polymer or copolymer and at least two relatively thin thermoplastic polymer oxygen barrier layers. By "relatively thin" it is meant that the thickness of barrier layer is less than industry-standard. For example, the industry-standard thickness for a PVDC copolymer barrier layer is in the order 0.5-1.0 mil (approx 12.5 - 25 micron). Preferably, the total thickness of the barrier layers being selected is less than the thickness of a single barrier layer of the barrier layer material required to give a selected rate of oxygen transmission. The layers may be formed up in the laminate with adhesive, tie layers or further polymer layers in between, or may be formed up adjacent in the laminate. Adhesive and tie layer composites include those that are well known in the art. Preferably, the packaging film includes outer layers of a thermoplastic polymer or copolymer and at least two core thermoplastic polymer oxygen barrier layers. More preferably, the core barrier layers are separated by at least one layer of a thermoplastic polymer or copolymer. Additional polymer layers may be included by coextrusion, coating, lamination, or a combination thereof. Preferably, the barrier layers are selected from copolymers of vinylidene dichloride (PVDC), certain polyamides, ethylene-vinyl alcohol copolymers (EVOH), polyethylene terephthalate ( PET ) , polyvinyl chloride ( PVC ) , and especially EVOH copolymers, polyamides and copolymers of vinylidene dichloride with minor proportions of vinyl, acrylic, or other unsaturated monomers. Preferably, each said layer having a finished thickness of at least 2.5 microns to provide physical integrity. In one embodiment the multilayer film has a first outer layer of a thermoplastic polymer or copolymer, a first core layer of a barrier film including vinylidene chloride-methyl acrylate copolymer, a second core layer of a thermoplastic polymer or copolymer, a third core layer of a barrier film including vinylidene chloride-methyl acrylate copolymer, and a second outer layer of a thermoplastic polymer or copolymer. More specifically, EVA, LLDPE, VLDPE and blends of these materials may be used in the first and second outer layer and in the second core layer. The multilayer film is preferably made by coextrusion of the layers, and then it is biaxially stretched. After biaxial stretching, the multilayer film may be irradiated to a dosage level of between 1 megarad and 10 megarads and heat-sealed in the form of a bag. The bag has improved storage stability characteristics. In a further aspect there is provided a biaxially oriented laminate having outer heat shrinkable layers and at least 2 PVDC barrier layers each having a thickness of at least 2.5μ and having a total thickness of PVDC of less than 12.5μ, and having an oxygen transmission of less than 25. lcc/sq . m/24hr/atm. In a further aspect there is provided a heat-shrinkable multilayer film having a first outer layer of a polymer or copolymer, a first core layer of a barrier film including vinylidene chloride-methyl acrylate copolymer, a second core layer of a thermoplastic polymer or copolymer, a third core layer of a barrier film including vinylidene chloride-methyl acrylate copolymer, and a second outer layer of a polymer or copolymer wherein the multilayer film has been biaxially stretched and then irradiated to a dosage level of between about 1 megarad and about 10 megarads, when employed to make bags for packaging primal and sub-primal meat cuts and processed meats, such a film provides bags having improved physical characteristics, whereby the bags when stored provide better shelf life, are more abuse resistant, have the ability to withstand high sealing temperatures, and greater seal strength than those of the prior art. In a further aspect this invention relates to an irradiated multilayer film suitable for use in the manufacture of bags for packaging primal and sub-primal meat cuts and processed meats. This invention also relates to such film including an irradiated five-layer film wherein the outer layers of the film comprise ethylene-vinyl acetate copolymers, and the first and third core layer comprises copolymers of vinylidene chloride and methyl acrylate and the second core layer comprises a thermoplastic polymer or copolymer, and to the process for manufacturing such film. In another embodiment blends of polymers and copolymers may be substituted into the first and second outer layer and the second core layer. DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "comonomer" refers to a monomer which is copolymerized with at least one different monomer in a copolymerisation reaction, the result of which is a copolymer. As used herein, the term "polymer" refers to the product of a polymerization reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc. As used herein, the term "copolymer" refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the term "copolymer" includes the copolymerisation reaction product of ethylene and an alpha-olefin, such as 1-hexene. However, the term "copolymer" is also inclusive of, for example, the copolymerisation of a mixture of ethylene, propylene, 1- hexene, and 1-octene. As used herein, a copolymer identified in terms of a plurality of monomers, eg "propylene/ethylene copolymer", refers to a copolymer in which the first listed monomer copolymerizes in a higher weight percent than the second listed monomer, and, for copolymers which are terpolymers, the first monomer copolymerizes in a higher weight percent than the second monomer, and the second monomer copolymerizes in a higher weight percent than the third monomer, etc. As used herein, terminology employing a "/" with respect to the chemical identity of a copolymer (eg "an ethylene/alpha-olefin copolymer" ) identifies the comonomers which are copolymerized to produce the copolymer. This terminology, as used herein, refers to the primary comonomer first, followed by the secondary comonomer. The copolymerisation is carried out in the presence of more (on a weight percent basis) of the primary comonomer than the secondary comonomer . As used herein, the phrase "heterogeneous polymer" refers to polymerization reaction products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, ie polymers made, for example, using conventional Ziegler-Natta catalysts. Such polymers typically contain a relatively wide variety of chain lengths and comonomer percentages. As used herein, the phrase "ethylene alpha-olefin copolymer" and "ethylene/alpha-olefin copolymer" refer to such heterogeneous materials as linear low density polyethylene ( LLDPE ) , and very low and ultra low density polyethylene (VLDPE and ULDPE ) ; and homogeneous polymers such as metallocene catalyzed polymers such as EXACT™ materials supplied by Exxon, and TAFMER™ materials supplied by Mitsui Petrochemical Corporation. These materials generally include copolymers of ethylene with one or more comonomers selected from C4 to CIO alpha-olefins such as butene-1 ( ie 1-butene), hexene-1, octene-1, etc in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylenes which are more highly branched than their respective counterparts. LLDPE, as used herein, has a density usually in the range of from about 0.91 grams per cubic centimetre to about 0.94 grams per cubic centimetre. Other ethylene/alpha-olefin copolymers, such as the long chain branched homogeneous ethylene/alpha-olefin copolymers available from the Dow Chemical Company, known as AFFINITY™ resins, are also included as another type of ethylene alpha- olefin copolymer useful in the present invention. As used herein, the term "polyolefin" refers to any polymerized olefin, which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted. In accordance with one embodiment of this invention, there is provided heat-shrinkable multilayer film having: a first outer layer of, (a) an ethylene-vinyl acetate copolymer, said first ethylene- vinyl acetate copolymer having a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 9 to about 25 weight percent, based on the weight of said first ethylene-vinyl acetate; (b) including very low density polyethylene of density not greater than about 0.915 grams per cubic centimetre, a second outer layer of, (a) an ethylene-vinyl acetate copolymer, said first ethylene- vinyl acetate copolymer having a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 9 to about 25 weight percent, based on the weight of said first ethylene-vinyl acetate copolymer; (b) a blend of two ethylene-vinyl acetate copolymers, wherein one of said ethylene-vinyl acetate copolymers has a melt index of from about 0.1 to 1.0 decigram per minute and a vinyl acetate content of from about 10 to 18 weight percent, based on the weight of said copolymer, and the other ethylene-vinyl acetate copolymer has a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 2 to about 10 weight percent, based on the weight of said copolymer. T h e blend (b) of said two ethylene-vinyl acetate copolymers has a vinyl acetate content of from about 3 to about 18 weight percent, and preferably from about 10 to about 15 weight percent, based on the weight of said copolymers. The first ethylene-vinyl acetate copolymer can be a single ethylene-vinyl acetate copolymer or a blend of at least two ethylene-vinyl acetate copolymers having melt indices and vinyl acetate contents within the aforementioned ranges; (c) very low density polyethylene of density not greater than about 0.915 grams per cubic centimetre, a second core layer of a thermoplastic polymer or copolymer which could be a blend of the first and second outer layers, and a first and third core layer including a vinylidene chloride methyl acrylate copolymer containing from about 5 weight percent to about 15 weight percent methyl acrylate, based on the weight of said copolymer. The heat shrinkable multilayer film of this invention can be produced by known techniques. The film may be produced by a four or more layer die with at least two of the inner layers designed for the extrusion of PVDC. Each layer of the die is connected to an extruder. The extruders feeding the PVDC are specifically designed to melt and pump PVDC. The preferred method is by coextruding the multiple layers into a primary tube, followed by biaxially stretching the tube by known techniques to form a heat shrinkable film. The "double bubble" technique disclosed in Pahlke US Pat No 3456044 is suitable for use in producing the film of this invention. In this Pahlke-type double bubble process a primary tube is prepared, cooled, reheated and the tube is simultaneously stretched in the machine direction (MD) by operating longitudinally spaced nip rolls at different speeds, and in the transverse direction (TD) by inflating air inside the tube. Suitable stretch ratios are from about 2 to about 6, of which about 3 to 5 is preferred. The heat shrinkable multilayer film of this invention may also be formed into a primary tube by known techniques such as by co-extruding at least the core layer and the first and second outer layer on each side of the core layer to form a primary tube. For example, this process is described in Canadian Patent No. 982923. Alternatively, coating lamination may be used, wherein a first outer tubular layer is extruded and thereafter the core and second outer tubular layers are sequentially coated onto the outer surface of the first tubular layer and the core layer to form the composite primary tube. As another alternative, the first outer and core outer layer may themselves be coextruded, and the second outer layer thereafter coated onto the outside surface of the core layer. Coating lamination procedures are described in Brax et al. US Patent 3741253. After biaxial stretching, the multilayer film may then irradiated to a dosage level of between about 1 megarad and about 10 megarads, such as by passing it through an electron beam irradiation unit. The multilayer film may then be employed to manufacture heat-shrinkable bags useful in packaging primal and sub-primal meat cuts and processed meats . In accordance with a preferred embodiment of this invention, the first outer layer of the multilayer film is an ethylene-vinyl acetate copolymer containing from about 9 to about 15 weight percent of vinyl acetate, based on the weight of the copolymer, said copolymer having a melt index of between about 0.1 and about 1.0 decigram per minute, and it may be selected from the group consisting of (a) a single ethylene-vinyl acetate copolymer; and (b) a blend of ethylene-vinyl acetate copolymers having melt indices and vinyl acetate contents within the aforementioned ranges of values; or (c) a very low density polyethylene of density not greater than about 0.915 grams per cubic centimetre; or (d) a blend of the aforementioned ethylene-vinyl acetate copolymers and the very low density polyethylene. Further, in a preferred embodiment of this invention the first and third core layers of the multilayer film of this invention comprises a vinylidene chloride-methyl acrylate copolymer containing at least 85 weight percent of vinylidene chloride, based upon the weight of the vinylidene chloride copolymer. The remainder of the vinylidene chloride copolymer is methyl acrylate. More preferably, the vinylidene chloride-methyl acrylate copolymer will contain at least about 85 weight percent, and not more than about 95 weight percent, of polymerized vinylidene chloride because when the vinylidene chloride copolymer contains less than about 85 weight percent vinylidene chloride, the methyl acrylate content would be greater than the maximum amount approved by the Food and Drug Administration for food contact uses, which is 15 percent by weight of the copolymer. If the vinylidene chloride content is more than 95 weight percent, the vinylidene chloride copolymer is generally not extrudable. The vinylidene chloride copolymer may contain but is not limited to less than 5 weight percent plasticizer, the percentage being based on the total weight of the blend of copolymer and all additives including plasticizer, in order to maximize the barrier properties of the thin film. The levels may be higher than 5% for equivalent or better barrier properties than a multilayer film containing only one layer of vinylidene chloride copolymer. Conventional plasticizers such as dibutyl sebacate and epoxidized soybean oil can be used. The second outer layer of the multilayer film of this invention comprises an ethylene-vinyl acetate copolymer selected from the group consisting of ( a ) an ethylene-vinyl acetate copolymer having a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 3 to about 18 weight percent, and preferably from about 10 to about 15 weight percent, based on the weight of said second ethylene- vinyl acetate copolymer; and (b) a blend of two ethylene-vinyl acetate copolymers, wherein one of said ethylene-vinyl acetate copolymers has a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 10 to about 18 weight percent, based on the weight of said copolymer, and the other ethylene-vinyl acetate copolymer has a melt index of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 2 to about 10 weight percent, based on the weight of said copolymer. The blend (b) of said two ethylene-vinyl acetate copolymers has a vinyl acetate content of from about 3 to about 18 weight percent, and preferably from about 10 to about 15 weight percent, based on the weight of said copolymers; (c) a very low density polyethylene of density not greater than about 0.915 grams per cubic centimetre; or (d) a blend of the aforementioned ethylene-vinyl acetate copolymers and the very low density polyethylene. The multilayer film of this invention will generally have a total thickness of from about 38 microns to about 90 microns, and preferably of from about 44 microns to about 75 microns, because when the thickness of the multilayer film is more than 75 microns, no improvement in performance is gained except for extreme applications. When the thickness of the multilayer film is less than 44 microns, the bag will have diminished puncture resistance. The first outer layer will normally have a thickness of from about 20 microns to about 33 microns, the first and third core layers will normally have a thickness of from about 2.5 microns to about 5.0 microns; the second core layer will normally have a thickness of from about 9.0 microns to about 33 microns and the second outer layer will normally have a thickness of from about 10 microns to about 20 microns. The thickness of the first outer layer, which is the inner layer of the bag, should be within the aforementioned range because the sealing and processability properties of the film layer would otherwise be diminished. The thickness of the first and third core layers should be within the above- indicated range because the film would provide inadequate barrier properties if the individual core layer thickness is less than about 2.5 microns. The upper limit of 5.0 microns for the individual core layers is primarily due to economic considerations. The thickness of the second outer layer, which is the outer layer of the bag, is selected within the aforementioned range to provide an abuse cover over the barrier layer. The thickness of the second core layer is selected in order to provide a total thickness of the multilayer film in the range of from about 44 microns to about 75 microns. After biaxial stretching by any suitable method well known in the art, the multilayer film of this invention is irradiated to a dosage level of between about 1 megarad and about 10 megarads, and preferably between about 2 megarads and about 5 megarads, by any suitable method such as by employing an electron beam. It has been found that the irradiation energy applied to the multilayer film herein is important. That is, when the energy level is below the indicated range, sufficient cross-linking is not obtained so as to improve the heat sealing characteristics of the multilayer film or to have any enhanced effect upon the toughness properties of the film. When the energy level is above the aforementioned range, film discoloration due to degradation of the polyvinylidene chloride copolymer core layer is accelerated, the degree of the film shrinkage is significantly reduced, and further improvements in the heat sealing characteristics and toughness properties of the film are not achieved. In another aspect of this invention, bags suitable for the shrink packaging of primal and sub-primal meat cuts and processed meats are provided from the aforedescribed multilayer film. The bags may be produced from the five-layer film of this invention by heat sealing. For instance, if the film of this invention is produced in the form of tubular film, bags can be produced therefrom by heat sealing one end of a length of the tubular film or by sealing both ends of the tube; then slitting one edge to form the bag mouth. If the film of this invention is made in the form of flat sheets, bags can be formed therefrom by heat sealing three edges of two superimposed sheets of film. When carrying out the heat sealing operation, the surfaces which are heat sealed to each other to form seams are the said first outer layers of the films of the invention. Thus, for example, when forming a bag by heat sealing one edge of a length of tubular film, the inner surface of the tube, ie the surface which will be heat sealed to itself, will be the said first outer layer of the film. The invention is further illustrated by the examples which appear below. In the examples, parts and percentages are by weight, unless otherwise specified. The following test methods were used in determining the properties of the resins and films used in the examples: Melt index values were obtained pursuant to ASTM Method D-1238, condition E. Tensile strength values were obtained following ASTM Method D-882, procedure A. Oxygen transmission rate is measured according to ASTM D3985. Non-ASTM test methods employed are described in the following discussion. Shrinkage values were obtained by measuring unrestrained shrink at 90 °C for five seconds. To demonstrate the significant reduction in oxygen transmission rate obtained when a three layer film with a single core layer of vinylidene chloride vinyl chloride copolymer containing about 85 weight percent vinylidene chloride and about 15 weight percent vinyl chloride (VDC-VC Copolymer); is compared to a five layer film with first and third core layer of vinylidene chloride vinyl chloride copolymer containing about 85 weight percent vinylidene chloride and about 15 weight percent vinyl chloride (VDC-VC Copolymer), the first and third layers being separated by a second core layer of a polymer or copolymer, with the combined thickness of the first and third layers approximately equal to the thickness of the single core layer of the three layer film the following samples were prepared. EXAMPLE 1 Biaxially stretched three-layer films had been prepared by a "double bubble" process similar to that disclosed in US Pat No 3,456,044 by co-extruding the following compositions through a multilayer die, biaxially stretching the co-extruded primary tube, and then irradiating the biaxially stretched tube were used in the tests. These films are shown as A, B, and C on Table 1. The composition of the films tested were three layers having an inner and an outer layer of ethylene-vinyl acetate copolymer containing and a four layer of comprised of vinylidene chloride-methyl acrylate copolymer. To demonstrate that a reduction in oxygen transmission results when two or more barrier layers are used versus one barrier layer the film was separated into the individual layers and then recombined into multiple layers. Where the films had been separated and recombined glycerin was introduced to eliminate air that would confuse the oxygen transmissions tests. The test data demonstrates that glycerin has no resistance to the flow of oxygen at the levels measured. Therefore, the test film combinations are a true indication of the oxygen transmission rate of the same structures which had been made by coextrusion or extrusion coating or laminating techniques well known in the art. This is shown on Table 1. EXAMPLE 2 To demonstrate that the benefit of two or more barrier layers is not dependent on being combined into a multilayer film with outer layers consisting of other polymers, biaxially stretched monolayer PVDC films were prepared by the well known double bubble process. These films are labeled as C, D, E, F, G, & H on Table 1. Surprisingly, when these single layer films were combined into multiple layers separated only by glycerin the same benefits of more than one barrier layer were realized as with the multilayer structures A, B, & C. These tests also indicate that this benefit is greater than would be anticipated simply by an increase in gauge alone as is shown in Table 1. TABLE 1 Table 1 indicates the oxygen transmission rates for eight different films labelled A, B, C, D, E, F, G, H. Test 1 is the oxygen transmission rate with the base film A. Tests 2 through 6 were with the film separated and re combined as described earlier. Tests 2, 3, 5 & 6 are with two barrier layers and indicate about a 14% to 19% reduction in oxygen transmission rate per micron of PVDC thickness. Test 4 indicates that a single layer of PVDC with the outer layer removed still has the same barrier properties as the base film in test 1. Surprisingly test 5 has the same reduction in oxygen transmission rate as compared to tests 2 & 3 even though the two PVDC layers are separated only by glycerin whereas test 2 & 3 have a total of 80u and 40u of EVA separating the two layers of PVDC respectively. This indicates that the EVA has essentially no resistance to the flow of oxygen. Test 7 & 12 is the oxygen transmission rate with the base film B. Tests 8, 9, 10, 11, 13 & 14 were with the film separated and re combined as described earlier. Tests 9, 10 & 14 are with two barrier layers and indicate about a 16% to 20% reduction in oxygen transmission rate per micron of PVDC thickness. Test 8, 11 & 13 indicates that a single layer of PVDC with the outer layer removed still has about the same barrier properties as the base in film tests 7 & 12. Comparison of test 13 to tests 7 & 12 indicates that the glycerin has no resistance to the flow of oxygen within the experimental accuracy of the test. Tests 15 through 18 were 1, 2, 3 & 4 layers of the same film C separated only by glycerin. The oxygen transmission was reduced when an additional layer of PVDC was added however the decrease was significantly less when the 4th layer was added as compared to the 3rd layer. This suggests that there is some limit beyond which the additional layers will provide minimum benefit. Tests 19, 20 & 21 were 1, 2 & 3 layers of the same film D separated only by glycerin as was done with film C. Again the benefit of multiple layers of PVDC is shown. Tests 22 through 30 were with 4 different films E, F, G & H each with a single layer at a different measured thickness. The purpose of these tests was to demonstrate that the oxygen transmission rate is constant for different thickness of PVDC. The change in rate with respect to an increase in film thickness ranged from a reduction in rate of 5.49% for film E to an increase in rate of 12.45% for film H. Films F & G had a slight reduction in rate. These results support the contention that for a given PVDC formulation the oxygen transmission rate is constant and not a function of the thickness over the range of thicknesses tested. In summary, the novel film compositions of this invention have been shown to possess physical properties required for use in packaging primal and sub-primal meat cuts and processed meats, while additionally having a significantly reduced oxygen transmission rate which will result in improved shelf life of the product being packaged. In general, various conventional additives such as slip agents, antiblock agents, and pigments may be incorporated in the films of the present invention in accordance with conventional practice. Although preferred embodiments of this invention have been described in detail, it is contemplated that modifications thereof may be made and some preferred features may be employed without others, all within the spirit and scope of the invention. Additionally, although up to 6 layer films are illustrated in the examples, multilayer films having more than 6 layers are contemplated within the scope of this invention provided that at least one of the plurality of layers comprises two layers of vinylidene chloride-methyl acrylate copolymer. It will of course be realised that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth.

Claims: CLAIMS 1. A multilayer barrier packaging film including an irradiated biaxially orientated coextrusion of at least four layers including at least two layers of a thermoplastic polymer or copolymer and at least two relatively thin thermoplastic polymer oxygen barrier layers. 2. A multilayer barrier packaging film according to Claim 1, wherein the barrier layers may be formed up in the laminate with tie layers or polymer layers in between, or may be formed up adjacent in the laminate. 3. A multilayer barrier packaging film according to any one of Claims 1 or 2, wherein additional polymer layers are included by coextrusion, coating, lamination, or combination thereof . 4. A multilayer barrier packaging film according to Claim 1, wherein the film includes outer layers of said thermoplastic polymer or copolymer. 5. A multilayer barrier packaging film according to Claim 1, wherein said barrier layers are at least two core layers separated by at least one layer of a polymer or copolymer. 6. A multilayer barrier packaging film according to Claim 1, wherein the barrier layers are selected from copolymers of vinylidene dichloride (PVDC), certain polyamides, ethylene- vinyl alcohol copolymers ( EVOH ) , polyethylene terephthalate (PET), polyvinyl chloride (PVC), and EVOH copolymers, polyamides and copolymers of vinylidene dichloride with minor proportions of vinyl, acrylic, or other unsaturated monomers. 7. A multilayer barrier packaging film in accordance with any one of the preceding Claims, wherein each said barrier layer has a thickness of at least 2.5 microns. 8. A multilayer barrier packaging film according to any one of the preceding Claims, wherein said multilayer film includes : a first outer layer of a thermoplastic polymer or copolymer; a first core layer of a thermoplastic polymer oxygen barrier film; a second core layer of a thermoplastic polymer or copolymer; a third core layer of a thermoplastic polymer oxygen barrier film, and a second outer layer of a thermoplastic polymer or copolymer. 9. A multilayer barrier packaging film according to Claim 8, wherein the first and third core layers includes vinylidene chloride-methyl acrylate copolymer. 10. A multilayer barrier packaging film according to Claim 8, wherein EVA, LLDPE, VLDPE and blends of these materials are included in the first and second outer layer and in the second core layer. 11. A multilayer barrier packaging film according to any one of Claims 8 to 10, wherein said multilayer film is made by coextrusion of the layers, followed by biaxial stretching and irradiation. 12. A multilayer barrier packaging film according to any one of Claims 1 or 11, wherein said multilayer film is irradiated to a dosage level of between 1 megarad and 10 megarads. 13. A multilayer barrier packaging film multilayer film according to Claim 9, wherein said vinylidene chloride-methyl acrylate copolymer contains at least about 85 weight percent of vinylidene chloride, based on the weight of said vinylidene chloride copolymer. 14. A multilayer barrier packaging film according to Claim 13, wherein said vinylidene chloride copolymer contains a maximum of 15 weight percent plasticizer, based on the total blend weight of additives and said vinylidene chloride copolymer. 15. A multilayer barrier packaging film according to any of Claims 8 to 14, wherein said first outer layer has a thickness from about 20 microns to about 33 microns. 16. A multilayer barrier packaging film according to any one of Claims 8 to 15, wherein said second and third core layers have a thickness from about 2.5 microns to about 5.0 microns. 17. A multilayer barrier packaging film according to any one of Claims 8 to 16, wherein said second core layer has a thickness from about 9.0 microns to about 12.5 microns. 18. A multilayer barrier packaging film according to claims any one of Claims 8 to 17, wherein said second outer layer has a thickness from about 10 microns to about 20 microns. 19. A multilayer barrier packaging film according to any one of Claims 5 to 18, wherein said film has a total thickness from about 44 microns to about 75 microns. 20. A multilayer barrier packaging film according to any one of the preceding Claims, wherein the film is fabricated into the form of a bag. 21. A multilayer barrier packaging film including outer heat shrinkable layers and at least 2 PVDC barrier layers each having a thickness of at least 2.5μ and having a total thickness of PVDC of less than 12.5μ, and having an oxygen transmission of less than 25.1cc/sq.m/24hr/atm.

Inventors: APREA JOHN J, GINOSSATIS STAMATIS, APREA, JOHN, J., GINOSSATIS, STAMATIS
Applicants: APREA JOHN J, GINOSSATIS STAMATIS, SPOROS S A, APREA, JOHN, J., GINOSSATIS, STAMATIS, SPOROS S.A.
Application year: 1998
Main classification: B32B27
IPC codes: B32B27, B65D65