Abstract:
A multilayer plastic film has a series of component layers and bonding layers bonding said component layers together, the component layers including an inner hydrolysed ethylene vinyl acetate copolymer (EVOH) layer, an adjacent polyvinylidene chloride (PVDC) layer, and outer layers comprising a polyolefin homopolymer, copolymer or ionomer.

Description:
FIELD OF THE INVENTION 
     This invention relates to multilayer plastic films suitable for packaging food products such as cheese, processed meat and raw meat. 
     BACKGROUND OF THE INVENTION 
     There is a need in the food packaging industry for packing film with improved structural integrity and which can be made into bags. Food products such as cheese, processed meat and raw meat are preferably packed by using film with high barrier resistance to moisture and oxygen. A typical way of protecting such food products from moisture and oxygen is by shrink packaging in high barrier film. A useful film with good moisture and oxygen barrier properties for this purposes comprises a copolymer of vinylidene chloride with a monomer such as methyl acrylate, vinyl chloride or other unsaturated monomer or hydrolysed ethylene vinyl acetate (EVOH) or any other barrier polymer which can be fabricated in the form of film and has good oxygen and moisture barrier properties. 
     It is known to provide a multilayer shrink film with the following structure, namely a first outer layer, a bonding layer, a barrier layer, a further bonding layer and a further outer layer to achieve the desired properties. A known multilayer film of this kind having good moisture and oxygen barrier properties for use in packaging food products has a core layer of a barrier polymer such as a copolymer of vinylidene chloride or hydrolysed ethylene vinyl acetate (EVOH) or other barrier polymer. A multilayer plastic film from which bags are to be manufactured must meet a number of criteria, such as being heat sealable, delamination resistant, and have good moisture and oxygen barrier properties. For heat shrinkable uses, the film must also be capable of shrinking from about 30 to about 50 percent at 90 degrees C. in both longitudinal and transverse directions. The film should also be capable of withstanding physical abuse during transportation. 
     A significant property of EVOH and polyvinylidene chloride (PVDC) based films is their barrier resistance to oxygen and other gasses. Compared to PVDC based films, EVOH based films have better barrier properties at low relative humidity, i.e. 30 to 40 percent, than commonly used multilayer films. 
     U.S. Pat. No. 4,469,742 (Oberle et al) describes a multilayer cook-in film with six layers. The middle barrier layer is of EVOH, selected because of its superior oxygen barrier properties compared to PVDC. 
     However, EVOH is hydrophillic, and therefore EVOH based films are moisture sensitive. As the moisture content of an EVOH based film increases, the permeation rate increases, i.e. barrier properties decrease, which is in contrast to PVDC based films because the permeability of PVDC does not vary with humidity, i.e. PVDC is not moisture sensitive. Accordingly therefore, EVOH based films are often produced in a multilayer structure or buried under moisture barrier materials such as polyolefins or sometimes with polyamides in order to protect an EVOH layer from moisture. 
     If a material with a much higher barrier layer for moisture compared to polyolefins or polyamide is used to protect an EVOH layer in a multilayer film from both the environmental and product sides thereof, the barrier properties and consequently the shelf life of the product can be increased significantly. 
     However, U.S. Pat. No. 4,161,562 (Yoshikawa et al) teaches that, in multilayer barrier films, a barrier layer of crystalline polymers such PVDC or EVOH has a tendency to crack if the thickness exceeds 15 microns, due to low temperature brittleness of these polymers. 
     It is therefore an object of the present invention to provide a multilayer plastic film which is at least substantially free from the problems referred to above. 
     SUMMARY OF THE INVENTION 
     According to the invention, a multilayer plastic film has a series of component layers and bonding layers bonding said component layers together, the component layers including an inner hydrolysed ethylene vinyl acetate copolymer (EVOH) layer, an adjacent polyvinylidene chloride (PVDC) layer, and outer layers comprising a polyolefin homopolymer, copolymer or ionomer. 
     The inner EVOH layer may have a thickness in the range of from about 2 to about 20 microns, preferably from about 5 to about 15 microns. 
     The adjacent PVDC layer may comprise from about 85 to about 98 percent by weight of vinylidene chloride, and may be selected from the group consisting of vinylidene chloride—methyl acrylate copolymer, vinylidene chloride-vinyl chloride copolymer and a copolymer of vinylidene chloride with an unsaturated monomer. The adjacent PVDC layer may have a thickness in the range of from about 2 to about 30 microns preferably from about 5 to about 15 microns. 
     A first outer layer may have a thickness in the range of from about 10 to about 40 microns, preferably from about 15 to about 25 microns. The first outer layer may comprise at least one material selected from the group consisting of low density polyethylene, linear low density polyethylene, metallocene-based linear low density polyethylene, polypropylene, copolymers of polypropylene, high-density polyethylene, ionomers, and mixtures thereof. 
     A second outer layer may have a thickness in the range of from about 10 to about 40 microns preferably from about 15 to about 25 microns. 
     The second outer layer may comprise at least one material selected from the group consisting of low density polyethylene, linear low density polyethylene, metallocene-based linear low density polyethylene, polypropylene, copolymers of polypropylene, high-density polyethylene, ionomers, and mixtures thereof. 
     Each bonding layer may have a thickness in the range of from about 1 to about 10 microns, and may be selected from the group consisting of ethylene vinyl acetate, ethylene methyl acrylate, and ethylene acrylic acid copolymer and ethylene vinyl acetate—maleic anhydride copolymer. 
     The thickness of the film may be in the range of from about 25 to about 300 microns, preferably from about 50 to about 100 microns. 
     An adjacent PVDC layer may be provided on each side of the EVOH layer. 
     Thus, in a multilayer plastic film in accordance with the invention, the EVOH is “locked up” in an adjacent PVDC layer. PVDC has better barrier properties not only for moisture and oxygen but also for aroma compared to polyolefins and polyamides which have conventionally been us ed to protect an EVOH layer in known multilayer plastic films. 
     A multilayer plastic film in accordance with the invention can be produced by methods such as blown film, cast film, lamination, extrusion coating and extrusion coating lamination. A multilayer plastic film in accordance with the invention can also be used to produce bottles and other containers by blow molding. 
     The present invention also provides a method of producing a multilayer plastic film including extruding multilayer plastic material in tubular bubble form to provide a multilayer plastic film having a series of component layers and bonding layers bonding said component layers together, said component layers including an inner hydrolysed ethylene vinyl acetate copolymer (EVOH) layer, an adjacent polyvinylidene chloride (PVDC) layer, and outer layers comprising a polyolefin homopolymer, copolymer or ionomer, and cooling and collapsing the tubular film to sheet form. 
     The above method may also include subsequently returning the collapsed film to a second tubular bubble form, stretching the second tubular bubble radially and longitudinally to produce a biaxially oriented heat shrinkable film, and collapsing the second tubular bubble to a further sheet form. The further sheet form may be partially cross-linked by exposure to high energy electrical radiation to improve the sealing properties of a sealing layer. 
     The present invention further provides a method of producing a multilayer plastic film including extruding multilayer plastic film in cast form to provide a multilayer plastic film having a series of component layers and bonding layers bonding said component layers together, said component layers including an inner hydrolysed ethylene vinylacetate copolymer (EVOH) layer, an adjacent polyvinylidene chloride (PVDC) layer, and outer layers comprising a polyolefin homopolymer, copolymer or ionomer, and cooling the film. The multilayer plastic film may be laminated to a web before cooling. 
     The present invention also provides a blow molded container formed from multilayer plastic film having a series of component layers and bonding layers bonding said component layers together, said component layers including an inner hydrolysed ethylene vinyl acetate copolymer (EVOH) layer, an adjacent polyvinylidene chloride (PVDC) layer, and outer layers comprising a polyolefin homopolymer, copolymer or ionomer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
     FIG. 1 is a cross-sectional view of a nine layer plastic film in accordance with one embodiment of the invention, 
     FIG. 2 is a schematic drawing showing the production of a biaxially oriented heat shrinkable multilayer plastic film in accordance with the invention by means of a double bubble blown film process, and 
     FIG. 3 is a schematic drawing showing the production of multilayer plastic film in accordance with the invention by means of a single bubble blown film process, 
     FIG. 4 is a schematic drawing showing the production of multilayer plastic film in accordance with the invention by means of cast film process, and 
     FIG. 5 is a schematic drawing showing the production of multilayer plastic film in accordance with the invention by means of an extrusion coating or lamination process. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In one embodiment of the invention, a nine layer plastic film as (see FIG. 1) a series of components layers and bonding layers bonding the component layers together. One component layer is a central layer five of hydrolysed ethylene vinyl acetate copolymer (EVOH) with a melt index of from about 0.8 to about 8, preferably from about 1.6 to about 6, with ethylene content from about 27 to about 48 percent, preferably about 44 percent. 
     Component layers  3  and  7  on each side of the central EVOH layer  5  are of poly vinylidene chloride (PVDC) comprising vinylidene chloride-methyl acrylate copolymer with from about 70 to about 98 percent by weight of vinylidene chloride and from about 2 to about 30 percent by weight of methyl acrylate (or vinyl chloride or a suitable unsaturated monomer). The VDC resin contains small amounts of plastizer and stabilizers such as dioctil adipate and epoxyidized soybean oil as known in the art. The component layers one and nine, namely first and second outer layers, are of low density polyethylene with a melt index of from about 0.5 to about 6.0 decigram per minute. 
     Bonding layers  4  and  6  are of ethylene-vinyl acetate-malaic anhydride copolymer with a melt index of from about 0.1 to about 6.0 decigram per minute and a vinyl acetate content from about 9 to about 28 percent by weight, used on the weight of the ethylene vinyl acetate copolymer. Bonding layers  2  and  8  are of ethylene-vinyl acetate copolymer with a melt index of from about 0.1 to about 6 decigram per minute and a vinylacetate content of from about 9 to about 28 percent by weight. 
     In other embodiments, the outer layers may be of linear low density polyethylene, polypropylene, polystyrene, high density polyethylene and linear low-density polyethylene, or blends of polyolefins, polyamide or polyesters. Such polymers can also provide a sealant layer which may also include metallocene based plastomer, ethylene vinyl acetate, ionomers or blends of these materials. The bonding layers may alternatively be anhydride modified ethylene vinyl acetate, anhydride modified linear low density polyethylene, ethylene vinyl acetate, ethylene acrylic acid, ethylene methacrylic acid or salts of ethylene acrylic acid or methacrylic acid. 
     The PVDC layer or layers made the copolymer of vinylidene chloride may have from about 70 to about 98 percent by weight of this monomer and from about 2 to about 30 percent by weight of at least one unsaturated monomer which is copolymerizable with vinylidene chloride, such as methyl acrylate or vinyl chloride. The PVDC layer or layers may also contain additives such as heat stabilizers and plasticizing compounds such as epoxysidized soybean oil and steramide as known in the art. A certain amount of comonomer with vinylidene chloride is required for easy processability of PVDC copolymer but higher amounts of the comonomer in the copolymer lower the barrier properties of the PVDC. 
     The EVOH layer comprises hydrolysed ethylene-vinyl acetate copolymer, with an ethylene content from about 27 to about 48 mole percent. One of the PVDC layers and an adjacent bonding layer of the embodiment shown in FIG. 1 may be omitted. In this case, the resulting seven layer of film should be used in such a way that the PVDC side faces the higher moisture content side, which could be either the environment side or the packaged product side. 
     The abuse resistance, toughness and puncture resistance can be increased by forming an outside layer of co-extruded polyamide such as Nylon 6-12, Nylon 11, Nylon 12, Nylon 6, Nylon 6/66 or other polyesters. Another advantage to using co-extruded polyamide or polyester as an outside layer is when this layer is used as a seal through layer, because polyamides and polyesters usually have a higher melting point and therefore multilayer films containing polyamides as an outside layer can be sealed quicker with stronger seals. Materials such as Nylon 6 or Nylon 6/66 or certain polyesters have much higher melting points and can be coextruded as an outside layer with a multilayer film by an extrusion method such as described in U.S. patent application Ser. No. 08/947936 filed Oct. 9, 1997, the contents of which are hereby incorporated herein by reference. 
     A biaxially oriented heat shrinkable multilayer film in accordance with the present invention may be produced by the known double bubble method. Referring now to FIG. 2, a tubular nine layer film is produced by using nine extruders  12  (only one of which is shown). The polymers extruded by the extruders  12  are fed to an annular die  14 , and the tubular film with nine layers is extruded downwardly therefrom. The tubular nine layer film is cooled in a cold water tank  16  located under the die  14  and containing water at a temperature of about 25 degrees C. or lower. The bubble  10  formed by the multilayer film is squeezed out by nip rolls  20  in the cold water tank  16  which collapse the film from bubble form to sheet form. 
     The cold water in the tank  16  quenches the tubular film to maintain the amorphous state of the plastic material and to lower the temperature thereof so that substantially no crystalline growth occurs in the PVDC layers which inhibit the subsequent process of orientation. 
     The collapsed film  21  from the cold water tank  16  is passed over the idler rolls  24  and through a pair of nip rolls  26 . The collapsed film  21  is passed from the nip rolls  26  through an infrared heating section  28  and blown to form a second bubble  30 , which is subsequently collapsed by a collapsing frame  32 . The collapsed film  31  then passes through a pair of nip rolls  34  which are rotated three times faster than the nip rolls  26 , with the air in the bubble  30  being entrapped therein by the rolls  26 ,  34 . This results in biaxial orientation of the film lengthwise and breadthwise. The collapsed film  31  is then passed over further idler rolls  34  and wound in the form of a roll  36 . 
     The collapsed film may travel past an annealing station  38  which stabilized the film to prevent shrinkage on the roll  36 . The film may be slit to remove trim if desired. 
     Multilayer film in accordance with the invention may also be produced by a single bubble blown film process. Referring now to FIG. 3, a tubular nine layer film is produced by using nine extruders  42  (only one of which is shown). The polymers extruded by the extruders  42  are fed to an annular die  44 , and the tubular film is extruded upwardly therefrom through an air ring  46  which cools the film. Tubular film passes upwardly in the form of a bubble  48  to a collapsing frame  50  where the tubular film is collapsed into sheet form and passes through nip rolls  52 . The flattened film  54  subsequently passes over idler rolls  56 ,  58  and is wound onto roll  60 . 
     Use of this invention enables multilayer films to be produced in which each PVDC layer has a thickness less than about 15 microns and therefore has minimal susceptibility to cracking. Also because of the presence of a plurality of PVDC layers, the possibility of problems arising from pinholes therein is minimized since a pinhole in one layer is most unlikely to be aligned with a pinhole in another layer. 
     The dies  14  or  44  may be of the kind described in U.S. Pat. No. 5,788,902 (Planeta) issued Aug. 4, 1998 or in U.S. Pat. No. 6,116,885 (Planeta et al) issued Sep. 12, 2000 which describe and claim inventions relating to the extrusion of plastic materials in multilayer tubular film which is especially useful when one of the plastic materials (such as PVDC) is readily degradable. The contents of these two patent applications are hereby incorporated herein by reference. 
     Multilayer plastic film in accordance with the invention may also be produced by a cast film process. As shown in FIG. 4, a nine layer cast film is produced by using nine extruders  64  (only one of which is shown). The polymers extruded by the extruder  62  are fed to a flat die  64 , and the resultant cast film  65  is passed downwardly therefrom around a pair of chill rollers  66  and an idler roller  68  before being wound onto a roll  70 . 
     As shown in FIG. 5, the multilayer cast plastic film produced as in FIG. 4 may also be coated on or laminated with a substrate web. The substrate web  72  is unwound from a supply roll  74  and passes around a rubber covered roller  76  and, with the multilayer cast plastic film  65 , then passes between the nip formed by a chill roller  78  and the rubber covered roller  76  to produce a laminate  80  which passes around idler rollers  82 ,  84  before being wound onto a roll  86 . 
     A tubular film produced in accordance with the invention may be used to form blow molded containers, which consequently have improved properties as described above, for example, improved oxygen and aroma barrier properties. 
     A specific example of the present invention will now be described. 
     EXAMPLE 
     Nine layer film was prepared by a conventional blown film process by coextruding the composition described below through a multilayer die, biaxially stretching the coextruded tubular film and winding the tube. 
     The outer layer  1  was of low density polyethylene with a melt index of 2.0 decigram per minute, bonding layers  4  and  6  were of ethylene vinylacetate-maleic anhydride having a melt index of 2.5 decigrams per minute and containing about 28 percent by weight of vinylacetate, and bonding layers  2  and  8  were of ethylene vinylacetate having a melt index of 2 to 6 decigram per minute and a vinyl acetate content of 28 percent by weight. PVDC layers  3  and  7  were of vinylidene chloridemethyl acrylate copolymer containing 94 percent by weight of vinylidene chloride and 6 percent by weight of methyl acrylate. EVOH layer  5  was of hydrolysed ethylene vinyl acetate having a melt index of 1.6 and an ethylene content of 44 percent. 
     The resulting biaxially oriented film had a thickness of 50 microns and showed good physical properties, especially those required for packaging cheese, meat and other food products. 
     Other embodiments and examples of the invention will be readily apparent to a person skilled in the art, the scope of the invention being defined in the appended claims.