OXYGEN BARRIER ORIENTED FILM

Abstract: A coextruded multiple layer oriented film comprises a core layer of a blend of ethylene vinyl alcohol copolymer and polyamide, two intermediate adhesive layers, and two outer layers of a blend of linear low density polyethylene, linear medium density polyethylene, and ethylene vinyl acetate copolymer. A preferred embodiment of the multilayer film ex­ hibits excellent optics and shrink properties while providing high oxygen barrier especially useful in food packaging app­ lications.

Description: This invention relates to oriented thermoplastic films for packaging; and more particularly, this invention relates to a coextruded, multilayer, oriented film having high oxygen barrier characteristics. Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse during the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product. Various polymeric materials have been used to provide lower gas permeability in order to reduce the transmission of oxygen through the packaging film and thereby retard the spoilage and extend the shelf life of products such as food items which are sensitive to oxygen. It is also desirable to include in a packaging film a shrink feature, i.e., the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is stretched in either a longitudinal (machine) direction, a transverse direction, or both, in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films. It is sometimes also desirable to orient the packaging film and thereafter heat set the film by bringing the film to a temperature near its orientation temperature. This produces a film with substantially less shrinkability, while retaining much of the advantages of orientation, including improved modulus and optical properties. Ethylene vinyl alcohol copolymer (EVOH) is known as an oxygen barrier material and has been used in the past in conjunction with multilayer packaging films. EVOH also provides a good barrier to odors or fragrances. Orienting EVOH to produce a heat shrinkable film has proven to be difficult. During the stretching or racking step for orienting such a film, the EVOH can sometimes develop voids. This phenomenon can result in some loss of oxygen barrier properties, which can affect, i.e. reduce the effective shelf life of food products packaged in EVOH film. The presence of voids in the EVOH layer can also result in discoloration of a food product, such as processed meat, and therefore reduce the appearance and market value of a packaged food item. U.S. Patent No. 4,064,296 issued to Bornstein et al discloses a film formed by the coextrusion of hydrolyzed ethylene vinyl acetate (HEVA) with outside layers of, for example, ethylene vinyl acetate copolymer (EVA). Also of interest is U.S. Patent No. 4,464,443 issued to Farrell et al showing the use of EVOH in a multilayer polymer structure, and including drying agents or desiccants such as sodium phosphate-di-basic and calcium chloride. EVOH, although a good barrier material, is moisture sensitive, and loses a great deal of its barrier properties at higher levels of relative humidity. Also of interest is US. Patent No. 4,457,960 issued to Newsome which discloses the use of EVOH and EVOH blends in a multiple layer film. The film may be made as shrinkable film, and may be melt extruded. The outside layer of the multiple layer film may be a blend of linear low density polyethylene (LLDPE) and EVA. Also of interest is U.S. Patent No. 4,495,249 issued to Ohya et al and disclosing a multilayer laminate film with a core layer of a saponified copolymer of ethylene and vinyl acetate, and including two outer layers of a mixture of EVA and LLDPE. The multilayer laminate film of this reference can be made heat shrinkable and has gas barrier properties. U.S. Patent No. 4,501,797 issued to Super et al discloses an unbalanced oriented multiple layer film including an intermediate layer of anhydride modified polypropylene and a barrier layer of a blend of ethylene vinyl alcohol and nylon. U.S. Patent No. 4,501,798 issued to Koschak et al also discloses the use of a blend of EVOH and nylon and an unbalanced multiple layer polymeric film also including LLDPE or EVA in a sealant layer. Adhesive layers of materials having carboxy moieties and preferably anhydride derivatives are present. The film of the reference is characterized by having high barrier to gaseous transmission, high gloss, transparency and stiffness. U.S. Patent No. 4,347,332 issued to Odorzynski et al discloses a film having a blend of nylon and ethylene vinyl alcohol copolymer. U. S. Patent No. 4,514,465 issued to Schoenberg discloses a five layered thermoplastic film, oriented and irradiated, having (a) a core layer comprising a three component blend of LLDPE, LMDPE, and EVA, (b) two interior layers comprising LLDPE and (c) two surface layers having a four component blend of LLDPE, LMDPE, and EVA combined with a UV stabilizer. EP-A-0141555 discloses an oriented five layered film having (a) a core layer of a blend of an ethylene-vinyl alcohol copolymer and nylon or nylon copolymer, (b) two adhesive layers and (c) two surface layers of a blend of LLDPE and EVA. Copending U. S. application serial No. 694,362, assigned to a common assignee with the present application, corresponding to European Specification 169640, discloses a five-layer thermoplastic film having two cross-linked surface layers of LLDPE/LMDPE/EVA. The present invention provides a coextruded thermo-plastic multilayer film having one or more of the following properties: good oxygen barrier properties over a wide range of moisture conditions; substantial freedom from voids in the barrier material of the film; an aesthetic appearance with good clarity, and other desirable optical properties; superior toughness and abrasion resistance. The multilayer film may totally coextruded, and oriented to provide a film with good shrink properties as well as good barrier properties. It may also be oriented yet substantially shrink free. The present invention provides an oriented multilayer film comprising a cross-linked core layer comprising a blend of an ethylene vinyl alcohol copolymer and a polyamide resin; two cross-linked interior layers each comprising an adhesive resin; and two cross-linked surface layers each comprising a three component blend of a linear low density polyethylene, a linear medium density polyethylene, and an ethylene vinyl acetate copolymer. In another aspect of the invention, a method of making an oriented multilayer film comprises the steps of simultaneously coextruding a core layer of a blend of an ethylene vinyl alcohol copolymer and a polyamide resin, two intermediate layers of an adhesive material, and two outer layers of a blend of a linear low density polyethylene, a linear medium density polyethylene, and an ethylene vinyl acetate copolymer; rapidly cooling the coextruded film; crosslinking the cooled film; heating the crosslinked film to a temperature between 105°C and 115°C; and stretching and orienting the heated film. "Intermediate layer", and "interior layer" is used herein to define a layer in a multilayer film enclosed on both sides by other layers. The term "oriented" is used herein to define a polymeric material in which the molecules have been aligned by a process such as racking or blown bubble process. The term "ethylene vinyl alcohol copolymer" and "EVOH" is used herein to include saponified or hydrolyzed ethylene vinyl acetate copolymers. The term "racking" is used herein to define a well-known process for stretching coextruded and reheated multilayer film by means of tenter framing or blown bubble processes. The term "linear low density polyethylene", "LLDPE", and the like are used herein to refer to copolymers of ethylene with one or more comonomers selected from C₄ to C₁₀ alpha oleofins such as butene-1 or octene, in which the molecules of the copolymers comprise long chains with 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 defined herein has a density usually in the range of from 0.916 grams per cubic centimeter to 0.925 grams per cubic centimeter. The terms "linear medium density polyethylene", "LMDPE" and the like as used herein refers to copolymers as described above and having a density usually in a range of from 0.926 grams per cubic centimeter to 0.941 grams per cubic centimeter. The terms "ethylene vinyl acetate copolymer", and "EVA" is used herein to refer to a copolymer formed from ethylene and vinyl acetate monomers wherein the ethylene derived units in the copolymer are present in major amounts, preferably between 60% and 98% by weight, and the vinyl acetate derived units in the copolymer are present in minor amounts, preferably between 2% and 40% by weight. The term "oriented" is used herein to define a material which, when heated to an appropriate temperature above room temperature (for example 96°C), will have free shrink of 5% or greater in at least one linear direction. All compositional percentages used herein are calculated on a "by weight" basis. The term "polyamide" and the like refers to high molecular weight polymers having amide linkages along the molecular chain, and refers more specifically to synthetic polyamide such as various nylons. Further details are given below with reference to the sole drawing figure where figure 1 is a schematic cross-section of a preferred embodiment of a multilayer film of the invention. Referring specifically to the drawings, in figure 1, a schematic cross-section of the preferred embodiment of the coextruded multilayer oriented film of the invention is shown. Film structure is directed to a multilayer film having the generalized structure of A/B/C/B/A where A is an outer layer, B is an intermediate adhesive layer, and C is a core layer containing a blend of a barrier material and a polyamide. Preferably, the outer layers A each comprise about 35% of the total multilayer film thickness; the intermediate layers B each comprise about 10% of the film thickness, and the barrier/polyamide layer C about 10% of the total film thickness. The total thickness of the multilayer film is preferably between 0.013 to 0.05 mm (0.5 and 2.0 mils), and are preferably between 0.02 to 0.04 mm (.75 and 1.5 mils). Even are preferably, the multilayer film or the present invention is about 0.025 mm (1 mil) thick. Preferably, core layer 10 is a blend of ethylene vinyl alcohol copolymer and a polyamide or copolymer comprising polyamide comonomers. Core layer 10 preferably forms between 5% and 25% of the total film thickness. Thicknesses less than about 5% result in a very thin film with possible voids in the barrier material. Thicknesses greater than about 25% make the film difficult to stretch or rack, and also result in increased cost due to the expensive barrier component. Intermediate layers 12 and 14 are preferably acid or acid anhydride-modified polymeric material which can bond the core layer 10 to the outer layers 16 and 18. This material preferably includes a graft copolymer of a polyolefin, such as polyethylene, or ethylene-ester copolymer substrate and an unsaturated carboxylic acid or acid anhydride, blended with a polyolefin, such as polyethylene, or ethylene-ester copolymer. Outer layers 16 and 18 are preferably a three component blend of LLDPE, LMDPE, and EVA. These outer layers preferably include from 40% to 60% by weight of LLDPE, from 20% to 30% by weight of LMDPE, and from 20% to 30% by weight of EVA. More preferably, the outer layers 16 and 18 include 50%, by weight, of a LLDPE, 25%, by weight, of LMDPE, and 25%, by weight, of EVA. The EVA has a vinyl acetate (VA) content of preferably between 3.5 and 9% by weight and more preferably between 3.5 and 5% by weight. At VA contents greater than about 9%, the multilayer film becomes too sticky or tacky for many applications, or requires the use of relatively large amounts of slip and anti-block additives. The film is preferably irradiated with between 3 and 13 megarads (M.R.) of irradiation, even more preferably between 5 and 10 M.R., prior to orientation of the film. Orientation is done by racking or stretching the film at a racking ratio of from between 3.0 and 5.0 times the original dimensions of the film in the longitudinal (machine) and transverse directions. EXAMPLE 1 A sample film was prepared by blending 50% of LLDPE (Dowlex 2045), 25% LMDPE (Dowlex 2037) and 15% EVA having a vinyl acetate of about 3.6%, blended with about 10% of a masterbatch concentrate containing slip and antiblock additives compounded with EVA of about 3½% vinyl acetate content by weight. This outside blend layer was coextruded with a core layer containing a blend of 90% EVOH (EVAL H) and 10% of a nylon 6/nylon 12 copolymer (Grillon CA-6), and an intermediate adhesive (Norchem Plexar 3150). The Dowlex 2045 may be obtained from Dow Chemical Company. This is an especially preferred LLDPE for use in this invention, and is a copolymer of ethylene and octene and has a density at 23°C of about 0.920 grams per cubic centimeter and a melt flow index of from 0.7 to 1.2 grams per ten minutes (as measured by ASTM-D-1238, E-28). LLDPE adds toughness to the film. A preferred LMDPE is Dowlex 2037, also obtainable from Dow Chemical Company. This resin is a copolymer of ethylene and octene and has a density at 23°C of about 0.935 grams per cubic centimeter and a melt flow index of about 2.55 grams per ten minutes (ASTM-D-1238, E-28). The LMDPE imparts stiffness, i.e. high modulus, to the film without significantly sacrificing toughness. The high modulus characteristic of the film is especially desirable in form-fill-seal applications where the film is fed as a lay-flat film and then formed on a forming shoe into a tube. The EVA of the outside blend layer is commercially available from El Paso Polyolefins Company under the trade designation El Paso PE 204-CS95. This material has a density at 23°C of from 0.9232 to 0.9250 grams per cubic centimeter and a melt flow (ASTM-D-1238, E-28) of 2.0 ± 0.5 grams per ten (10) minutes. The vinyl acetate content of this EVA is about 3.6% by weight. EVA improves the processability of the film, and also provides a sealing layer which can be adequately sealed at relatively low temperatures, or else provides a stronger seal at a given temperature, than many other polymeric resins. The EVOH of the core blend layer was EVAL H, available from EVAL Company of America and having an ethylene content of about 38% by weight and a melt index of about 1.5 grams/10 minutes. Other suitable EVOH resins include EVAL E, EVAL F, and EVAL K, as well as blends of the above, and preferably such resins or blends having a melt index of between 1 to 4 grams per ten minutes (ASTM 1238). Grillon CA-6, available from Emser Industries, was blended with the EVOH. The Grillon CA-6 is a nylon copolymer having about 60% nylon 6 and about 40% nylon 12 by weight. Although nylon 12 would be effective alone as a blending material in the core layer, this is a relatively expensive material. Nylon 6 alone would be effective as a blending material, but with some difficulty in processing. The particular blend employed proved to be very advantageous in providing an economical yet effective means for providing a core blend having the good barrier properties associated with EVOH, but with the processing and elongation advantages of nylon. Another suitable nylon copolymer is Grillon CR-9, having 20-30% nylon 6 and 70-80% nylon 12 by weight. The intermediate adhesive material, Norchem Plexar 3150, is a low density polyethylene-based anhydride-modified resin produced by Norchem. Other anhydride-modified adhesives such as CXA-162 (duPont) can also be used as the intermediate adhesive. The polymer melt from the coextrusion die was then cooled and cast into a solid tape which was irradiated with about 8 megarads of irradiation. The tape was then heated to about 110°C in an oven and blown into a bubble. The bubble was expanded to about 3.5 times its original dimensions in both the machine (longitudinal) and transverse directions, and then deflated and ply separated into single wound film rolls. The final film had a thickness of about 25.4 µm (one mil), and in addition to the shrink properties imparted by orientation, exhibited excellent toughness, good optics, burn out resistance, resistance to tear propagation, and heat sealability. The film also exhibited good abuse resistance and the necessary stiffness and lower tack required for packaging applications and was substantially free of voids in the EVOH/polyamide blend layer. Test results for the sample film are listed below in Table I. EXAMPLE 2 A sample film was prepared according to the same procedure and formulation described in Example 1, having Plexar 158 as an intermediate adhesive layer, but without a polyamide in the core layer. This film exhibited voids in the EVOH layer. Processed meat (bologna) packaged in the film was found to discolor at regions adjacent the void areas of the EVOH core layer. Plexar 158 is a low density polyethylene-based acid-or acid anhydride-modified polymeric adhesive.

Claims: An oriented multilayer film comprising (a) a cross-linked core layer comprising a blend of an ethylene vinyl alcohol copolymer and a polyamide resin; (b) two cross-linked interior layers each comprising an adhesive resin; and (c) two cross-linked surface layers each comprising a three-component blend of a linear low density polyethylene, a linear medium density polyethylene, and an ethylene vinyl acetate copolymer. A film according to claim 1 in which the cross-linked core layer (a) comprises a blend of (1) from 80%, by weight, to 99%, by weight, of an ethylene vinyl alcohol copolymer and (2) from 1%, by weight, to 20%, by weight of the polyamide resin; and the two cross-linked surface layers (c) each comprise a blend of (1) from 40%, by weight, to 60%, by weight, of a linear low density polyethylene, (2) from 20%, by weight, to 30%, by weight, of a linear medium density polyethylene, and (3) from 20%, by weight, to 30%, by weight, of an ethylene vinyl acetate copolymer. A film according to claim 2 in which the cross-linked core layer (a) comprises a blend of (1) 90%, by weight, of an ethylene vinyl alcohol copolymer, and (2) 10%, by weight, of a polyamide resin; the two cross-linked interior layers (b) each comprise an acid or acid anhydride-modified polymeric material; and the two cross-linked surface layers (c) each comprise a blend of (1) 50%, by weight, of a linear low density polyethylene, (2) 25%, by weight, of a linear medium density polyethylene and (3) 25%, by weight, of an ethylene vinyl acetate copolymer. A film according to claim 1, 2 or 3 wherein said ethylene vinyl acetate copolymer comprises from 3.5%, by weight, to 9%, by weight, of vinyl acetate derived units. A film according to any of claims 1 to 4, which has been cross-linked with from three megarads to thirteen megarads of irradiation. A film according to any of claims 1 to 5 which has been oriented by racking at a racking ratio of from 3.0 to 5.0 in both the longitudinal and transverse directions. A method of making an oriented multilayer film comprising (a) simultaneously coextruding a core layer of a blend of an ethylene vinyl alcohol copolymer and a polyamide resin, two intermediate layers of an adhesive material, and two outer layers of a blend of a linear low density polyethylene, a linear medium density polyethylene, and an ethylene vinyl acetate copolymer; (b) rapidly cooling the coextruded film; (c) crosslinking the cooled film; (d) heating the crosslinked film to a temperature between about 105°C and 115°C; and (e) stretching and orienting the heated film. The method of claim 7 wherein the coextruded film is cooled to room temperature. The method according to claim 7 or 8 wherein the heated film is oriented by racking at a racking ratio of from 3.0 to 5.0 in both the longitudinal and transverse directions. The method according to any of claims 7 to 9 further comprising the step of reheating the oriented film to a temperature near its orientation temperature to provide a substantially non-shrinkable film.

Inventors: SHAH, GAUTAM P.
Applicants: W.R. GRACE & CO.-CONN. (A CONNECTICUT CORP.)
Application year: 1986
Main classification: B32B27
IPC codes: C08L23, B29C47, C08L29, B32B27, C08J3, B29K29, B29K77, B29L9, C08L77, B29C55, B29K23