BARRIER FILMS

Abstract: There is provided an oxygen barrier film which is a biaxially oriented, multi-layer film having an oxygen barrier layer sandwiched between two layers of polyolefin. The oxygen barrier layer is composed of a blend of an ethylene vinyl alcohol copolymer and a compatible different polymer. The blend is capable of being coextruded and biaxially oriented along with the polyolefin, whereas the ethylene vinyl alcohol copolymer alone is not.

Description: BARRIER fILMS This invention provides a biaxially oriented multi-layer barrier film having an oxygen barrier layer sandwiched between two layers of polyolefin. Polypropylene films are common packaging materials in view of their relative low cost and moisture resistance. However, polypropylene has a fairly high permeability to gases, including oxygen. Therefore, films composed entirely of polypropylene are not adequate for packaging oxygen sensitive foods and other materials that degrade in the presence of oxygen or other atmospheric gases. U.S. Patent No. 4,254,169 describes the use of an ethylene vinyl alcohol layer, as an oxygen barrier layer, sandwiched between two layers of polyolefin in a coextruded multi-layer film. However, the multi-layer film disclosed in this patent is not biaxially oriented. In order to achieve optimal oxygen impermeability in a multi-layer film such as described in the above-mentiond reference, it is necessary to use an ethylene vinyl alcohol copolymer having a relatively high vinyl alcohol to ethylene monomeric ratio. However, such films have extremely poor rheology characteristics as compared with polypropylene films. Consequently, composite films containing polypropylene and such ethylene vinyl alcohol copolymer are not readily biaxially orientable. According to one aspect of the invention, there is provided a biaxially oriented, polyolefin containing five-layer film structure comprising: (i) two outermost layers composed of oxygen,permeable polyolefin; (ii) one innermost oxygen barrier layer comprising a blend of: : (a) an ethylene vinyl alcohol copolymer which has no more than about 70 mole % of ethylene, the copolymer having melting and softening temperatures higher than the polyolefin of outermost layers (i); and (b) a different polymer compatible with the ethylene vinyl alcohol copolymer (a) and having melting and softening temperatures lower than the polyolefin of outermost layers (i); ; and (iii) two adhesion promoting layers interposed between the outermost polyolefin layers and the innermost oxygen barrier layer, wherein the different polymer (b) is blended with the ethylene vinyl alcohol copolymer in an amount sufficient to achieve melting and softening characteristics of the blend sufficient to render the five-layer film structure capable of being c coextruded and biaxially oriented, the film structure being formed by coextruding the layers and biaxially orienting the coextruded layers. According to another aspect of the invention, there is provided a biaxially oriented, polypropylene containing five-layer film structure consisting essentially of: (i) two outermost layers composed of polypropylene; (ii) one innermost oxygen barrier layer comprising a blend of: (a) an ethylene vinyl alcohol copolymer which has from 28 to 36 mole % of ethylene, the remainder being essentially vinyl alcohol; and (b) poly(ethyloxazoline); and (iii) two adhesion promoting layers interposed between the outermost polypropylene layers and the innermost oxygen barrier layer, the adhesion promoting layers being composed of a polyolefin polymer or copolymer blended with'a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride, wherein the oxygen barrier layer (ii) constitutes from 5% to 20% by weight of the film structure, the adhesion promoting layers (iii) each constitute from 2% to 10% by weight of the film structure, and wherein the poly(ethyloxazoline) (b) constitutes from 5% to 20% by weight of the blend of ethylene vinyl alcohol and poly(ethyloxazoline). According to another aspect of the invention, there is provided a method for imparting oxygen impermeability to a film composed predominantly of biaxially oriented polyolefin, the method comprising: (i) coextruding an innermost oxygen barrier layer between two outermost polyolefin layers to form a coextruded multi-layer film; and (ii) biaxially orienting the coextruded film of step (i), wherein the oxygen barrier layer comprises a blend of: (a) an ethylene vinyl alcohol copolymer which has no more than about 70 mole X of ethylene, the copolymer having melting and softening temperatures higher than the polyolefin of the outermost layers; and (b) a different polymer compatible with the ethylene vinyl alcohol copolymer (a) and having melting and softening temperatures lower than the polyolefin of the outermost layers, wherein the different polymer (b) is blended with the ethylene vinyl alcohol copolymer in an amount sufficient to achieve melting and softening characteristics of the blend sufficient to render the multi-layer film structure capable of being coextruded and biaxially oriented. According to another aspect of the invention, there is provided a food container formed from a biaxially oriented, polyolefin containing five-layer film structure comprising: (i) two outermost layers composed of polyolefin; (ii) one innermost oxygen barrier layer comprising a blend of: (a) an ethylene vinyl alcohol copolymer which has no more than about 70 mole % of ethylene, the copolymer having melting and softening temperatures higher than the polyolefin of outermost layers (i); and (b) a different polymer compatible with the ethylene vinyl alcohol copolymer (a) and having melting and softening temperatures lower than the polyolefin of outermost layers (i); and (iii) two adhesion promoting layers interposed between the outermost polyolefin layers and the innermost oxygen barrier layer, wherein the different polymer (b) is blended with the ethylene vinyl alcohol copolymer in an amount sufficient to achieve melting and softening characteristics of the blend sufficient to render the five-layer film structure capable of being coextruded and biaxially oriented, the film structure being formed by c coextruding the layers and biaxially orienting the coextruded layers. A multi-layer film according to an aspect of the present invention contains an oxygen barrier layer composed of a blend of ethylene vinyl alcohol and a different polymer. This oxygen barrier layer may be characterized as a core layer. Due to the excellent oxygen barrier properties associated with the ethylene vinyl alcohol component of this core layer, this layer may be quite thin, in the range of 0.1 mil or less, while still obtaining adequate oxygen barrier properties. Ethylene vinyl alcohol copolymers suitable for use according to the present invention have no more than about 70 mole % of ethylene, the remainder being essentially vinyl alcohol. A preferred ethylene vinyl alcohol copolymer has from 28 to 36 mole % of ethylene. Kuraray of Japan produces an ethylene vinyl alcohol copolymer under the designation "EP-F" (trademark) which has 32 to 36 mole % ethylene, a molecular weight of 29,500 and a melting point of 1800C (3560F). Other manufacturers produce suitable ethylene vinyl alconol copolymers. One suitable copolymer has about 29 mole X ethylene, a molecular weight of 22,000, and a melting point of 1850C (3650F). The polymer (b) which is compatible with ethylene vinyl alcohol may be a polyamide. As used herein the term "polyamide" is intended to connote a homopolymer or copolymer having repeating units of the formula -N-C=O. Examples of such polyamides include nylons such as nylon 6', nylon 66 and nylon 610 and polymers of 2-oxazolines, of the formula EMI5.1 where R is C1-C6 alkyl and n is at least 100. Polymerization of 2-oxazolines is discussed in Frump, J.A., Chemical Reviews, 71, Oct. 1971, pp. 483-505. Dow produces a commercially available poly(ethyloxazoline) where R in the above formula is ethyl. The amount of polymer (b) which may be blended with ethylene vinyl alcohol is that amount which improves the rheology of the resulting film, such that this film is biaxially orientable with the other layers of the coextruded film. This amount of poly(ethyloxazoline) may be, for example, from 5% to 20% by weight of the ethylene vinyl alcohol/poly(ethyloxazoline) blend. The oxygen barrier layer is preferably composed entirely of a blend of ethylene vinyl alcohol and polymer (b) and is preferably bonded to the outermost layers of polyolefin by means of adhesion promoting layers. However, less preferably, an adhesion promoting component may be blended along with the ethylene vinyl alcohol and polymer (b) and the core layer may be bonded directly to the outermost polyolefin layers. The blending of an adhesion promoting component into an ethylene vinyl alcohol core layer is described in the U.S. Patent No. 3,882,259. ,The adhesion promoting layer which may be used in the present invention is preferably a modified polyolefin as described in the U.S. Patent No. 4,254,169. More particularly, such modified polyolefins which form layers adjacent the core layer are blends of polyolefin and a graft copolymer of high density polyethylene (HDPE) with an unsaturated fused ring carboxylic acid anhydride. ,The polyolefin component of the blend may include a number of resins such as high, medium and low density polyethylene (DUPE, MOPE, LDPE), and polyolefin copolymers such as ethylene vinyl acetate copolymer (EVA) and ethylene acrylic acid (EAA). Modified polyolefin blends are disclosed in U.S. Patent Nos. 4,087,587 and 4,087,588. Suitable modified polyolefin blends are available from the Chemplex Company of Rolling Meadows, Illinois under the name Plexar (trademark). Commercially available grades of such modified polyolefins are blends of the graft copolymer with different polyolefins. These include Plexar-I (trademark), a blend with ethylene vinyl acetate copolymer; Plexar-II (trademark), a blend with high density polyethylene; Plexar-II further modified with an elastomer such as polyisobutylene; and Plexar-Ili (trademark), a blend with ethylene vinyl acetate polymer adapted for cast film coextrusion. It has been found that these materials coextrude well with extnJsion grades of ethylene vinyl alcohol without need for adding materials to either the polyolefin or the core layers in order to obtain the desired level of adhesion between layers. The melting and softening temperatures of the ethylene vinyl alcohol are generally higher than the melting and softening temperatures of the polyolefin of the outermost layers which are, in turn, higher than the melting and softening temperatures of the polymer (b). Without wishing to be strictly limited to any particular temperatures, the melting point of the ethylene vinyl alcohol may be, e.g., from 170 to 19O0c, the melting point of the polyolefin may be from 150 to 170do and the melting point of the polymer (b) may be from 110 to 1500C. However, when higher melting polyolefins and ethylene vinyl alcohols are used, the melting point of the polymer (b) may be as high as 190at. In a preferred process for the production of the multi-layer film, resins of the polyolefin, modified polyolefin and the core barrier material are melted and coextruded as a cast film. Blown film coextrusion and water quench coextrusion may also be utilized. All such coextrusion processes yield a multi-layer film which has excellent adhesion between the layers and high resistance to passage of gas and moisture. Seat sealing of polyolefin outer layers is readily obtained. Thus, the multi-layer film product is well adapted to use in packaging food products as well as many non-food products which require moisture and oxygen barrier packaging. Biaxial orientation of the coextruded, composite film may take place by any suitable means. In this regard, reference is made to U.S. Patent No. 4,294,935 which describes the biaxial orientation of ethylene vinyl alcohol films. The multi-layer film of the present invention may have a wide range of thicknesses,such as from 0.5 to 5 mils. The oxygen barrier layer may constitute from 5% to 20%, preferably 10% by weight, of the multi-layer film. The adhesion promoting layers, when present, may each constitute from 2% to 10%, preferably 5%, by weight of the multi-layer film in equal portions. The remainder of the multi-layer film is preferably constituted by the outermost polyolefin layers in equal portions. Although the various layers described herein have often been characterized as being "composed of" certain resins, it will be understood that these resins may include minor portions of various additives associated with these resins such as fillers, pigments, stabilizers. Containers may be made from the multi-layer films of the present invention by heat sealing to form a pouch or by thermoforming to make containers in which foods are packaged and stored. EXAMPLE PEOX 425 (trademark), a poly(ethyloxazoline) from Dow with a molecular weight of 425,0(3(3, was blended with EP-F (trademark), an ethylene vinyl alcohol copolymer from Kuraray containing from 32 to 36 mole % of ethylene and having a molecular weight of 29,500 and a melting point of 180 C (356 F). The amount of PEOX 425 in this blend was about 10S by weight of the blend. This blend was coextruded with a polypropylene having a melting point of about 1600C, sold under the trademark "Arco 8670't, a modified polyolefin adhesive available from Chemplex under the trademark Plexar 2109, to form a composite film of the structure: PP/Plexar 2109/EP-F + PEOX 425/Plexar 2109/PP in a weight ratio of approximately 40:5:10:5:40, where PP represents the polypropylene. This coextruded composite film was biaxially oriented.

Claims: CLAIMS: 1. A biaxially oriented, polyolefin containing film structure comprising: (i) two outermost layers composed of oxygen-permeable polyolefin; (ii) one innermost oxygen barrier layer comprising a blend of: (a) an ethylene vinyl alcohol copolymer which has no more than about 70 mole X of ethylene, the copolymer having melting and softening temperatures higher than that of the polyolefin layers (i); and (b) a different polymer compatible with the ethylene vinyl alcohol copolymer (a) and having melting and softening temperatures lower than that of the polyolefin layers (i); and (iii) two adhesion promoting layers interposed between the polyolefin layers and the oxygen barrier layer. 2. A film structure according to claim 1, wherein the different polymer (b) is a polyamide. 3. A film structure according to claim 1 or 2, wherein the adhesion promoting layers are composed of a polyolefin polymer or copolymer blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride. 4. A film structure according to claims 1 2 or 3, wherein the ethylene vinyl alcohol copolymer contains 28 to 36 mole % of ethylene. 5. A film structure according to claims 2, 3 or 4, wherein the polyolefin of the outermost layers (i) has a melting point of from 150 to 1700c, the ethylene vinyl alcohol copolymer (a) has a melting point of from 170 to 1900C and polyamide (b) has a melting point of from 110 to 150 C. 6. A biaxially oriented, polypropylene containing five-layer film structure consisting essentially of: (i) two outermost layers composed of polypropylene; (ii) one innermost oxygen barrier layer comprising a blend of: (a) an ethylene vinyl alcohol copolymer which has from 28 to 36 mole % of ethylene, the remainder being essentially vinyl alcohol; and (b) poly(ethyloxazoline); and (iii) two adhesion promoting layers interposed between the polypropylene layers and the innermost oxygen barrier layer, the adhesion promoting layers being composed of a polyolefin polymer or copolymer blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride, wherein the oxygen barrier layer (i) constitutes from 5% to 20% by weight of the film structure, the adhesion promoting layers (ii) each constitute from to 10% by weight of the film structure, and wherein the poly(ethyloxazoline) (b) constitutes from 5% to 20% by weight of the blend of ethylene vinyl alcohol and poly(ethyloxazoline). 7. A film structure according to claim 6, wherein the outermost polypropylene layers each constitute about 40% by weight of the film structure, the adhesion promoting layers each constitute about 5 by weight of the film structure and the oxygen barrier layer constitutes about 10% by weight of the film structure. 8. A film structure according to claim 7, wherein the poly(ethyloxazoline) (b) constitutes about 10% by weight of the oxygen barrier layer (ii). 9. .A film structure according to claim 8, having a thickness from 0.0127 to 0.127mm (0.5 to 5 mils). 10. A food container made from the film structure described in any preceeding claim.

Inventors: LU, PANG-CHIA
Applicants: MOBIL OIL CORPORATION
Application year: 1983
Main classification: B32B27
IPC codes: B65D1, B32B27, B65D65, B32B7