CA1144728A - Multi-layer barrier film - Google Patents

Abstract

ABSTRACT

A delamination resistant multi-layer film is comprised of a core layer of vinyl alcohol polymers or copolymers such as polyvinyl alcohol or ethylene vinyl alcohol between outer layers of a polyolefin blended with a chemically modified polyolefin having functional groups added to the basic polymer such that strong adhesion to the core layer is obtained. The core layer provides a good oxygen barrier, while the outer layers protect the core layer from degradation by exposure to water vapor.
Additional layers of various polymer materials may be provided over the layers of modified polyolefin. The film can be prepared by coextrusion techniques.

Description

'7Z~ ( BACKGROUND OF THE Il`lVENTION

1. Field of the Invention This invention per~ains generally ~o multi-layer polymer films and to methods for producing the same.

2. Description of the Prior ~rt Polyolefin film such as polyethylene and polypropylene are common packaging materials because of their relative low cost, heat sealability, and moisture resistance. ~owever, these polyolefins have a fairly high permeability to gases including oxygen, so that, used alone, they are not adequate for packaging oxygen sensitive Eoods and other materials that degrade in the presence of oxygen or other atmospheric gases.
It is known that polymers and copolymers of vinyl alco-hol, hereafter called vinyl alcohol films, have excellent resis-tance to pérmeatian by gases, particularly to oxygen. Vinyl al-coh~l films include polyvinyl alcohol and ethylene vinyl alcohol copolymer. The desirable proper~ies of the vinyl alcohol films degrade in the presence of moisture. Vinyl alcohol films also tend to be structurally brittle, par~icularly when formed in re-latively thin layers, i.e. in the range of 1 mil or less. Thus, efforts have been made to laminate a vinyl alcohol web between two outer webs of polyolefin which provide a moisture barrier and structural support for the vinyl alcohol core. An example of such a structure, a core layer of ethylene vinyl alcohol blended wi~h another polymer and sandwiched between ~wo ou~er layers of '~

_ 1 _ polyolefin, is shown in the U.S. Patent to Nohara! et. al., No.

3,882,259. Since the oxygen ba~rier property of substantially pure vinyl alcohol is superior to that of vinyl alcohol blends, it is desirable to produce a film having a subs~antially pure vinyl alcohol layer within polyolefin layers. However, substan-tially pure ethylene vinyl alcohol and polyvinyl alcohol do not bond well to many polymer films, particularly polyolefin films.

SUMMARY OF THE INVENTION

This invention provides a film structure in which a substantially pure polyvinyl alcohol or ethylene vinyl alcohol layer is joined with one or more polyolefin layers with adequate adhesion to prevent delamination.
A multi-layer fllm in accordance with the invention in-cludes a core layer of substantially pure vinyl alcohol polymer or copolymer such as polyvinyl alcohol (PVOH) or ethylene vinyl alcohol copolymer (EVOHj adhered to at least one layer of chemically modified polyolefin having surprisingly strong adhesion charac~eristics to the vinyl alcohol core layer. The multi-layer film can be made by coextrusion. Because the core layer is formed of substantially pure polyvinyl alcohol or ethylene vinyl alcohol, the entire film may be made very thin while still retaining high oxygen impermeability. The modified polyolefin layer and the core may be coextruded with additional layers of other polymers to provide an in~egrated film structure having a composite of ~he characteristics of the various films in the structure and which has sufficient adhesion between layers to 2 ~ _ __ _ resist delamination.
The modified polyolefins that have been found to have suitable adhesion to the vinyl alcohol core layer are blends of a polyolefin and a graft copolymer of high density polye~hylene having an unsaturated fused ring carboxylic acid anhydride grafted thereto. The polyolefin component of the blend may be polyethylene or ethytlene copolymers such as ethylene vinyl ace-tate. Such modified polyolefins have shown a marked ability to adhere to vinyl alcohol materials, particularly tv ethylene vinyl alcohol when coextruded therewith.
The atmospheric gas barrier layer is preferably ethylene vinyl alcohol which is readily coextrusible with the modified polyolefin layers and with additional layers. Ethylene vinyl alcohol is approximately fifty times less permeable to oxygen than extrusion grade saran (vinylidine chloride). Heretofor, saran has been d~minant in the field v oxygen barrier materials for films. The lower oxy~en permeability of EVOH allvws it tv be used as a very thin layer. Polyvinyl alcohol also displays superior barrier properties, but is difficult to coextrude without the addition of water to the resin thereby at least temporarily impairing its barrier properties. Polyvinyl alcohol is better suited to laminated film structures wherein a polyvinyl alcohol core web is ex~rusion coated or otherwise plied with the adherent modified polyolefin layers.
- In a preferred process for the production of the multi-layer film, resins of the modified polyolefin and the core bar-rier material are melted and coextruded a$ joined concentric tubes which are blown to the desired thickness. Addi~ional layers of polymer material similarly may be c~extruded over the modified polyolefin layers. Cast 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. Heat sealing of polyolefin outer layers is readily ob-tained. 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.
The coextruded multi-layer film can be processed further in hydraulically lubrica~ed compression rolls to reduce the thickness of the film, to improve its clarity, and to provide improved physical properties by orientation.
Fur~her objects, features, and advantages of the inven-tion will be apparent from the following detailed description illustratiny preferred embodiments of the invention~

DESCRIPTION OF THE PREFERRED EMBOD-tMENTS

The multi-layer film of the invention includes a core of substantially pure polyvinyl alcohol (PVOH), or ethylene vinyl alcohol (EVOH). Excellent adhesion of the layers in the film is obtained without the need to introduce bond promoting materials into the core layer, and it is highly preferred that they ~e eli~
mina~ed from the core layer since introduction of such extraneous materials may reduce the oxygen barrier properties of the core.

As a result, the core layer may be made quite thin, in the range of 0.2 mil or less, while still obtaining adequa~e ox~gen barrier properties.

Vinyl alcohol polymers and copolymers suitable for ex-trusion are available commercially. Kuraray of Japan produces an ethylene vinyl alcohol copolymer under the trade mark "EP-F" which has 32 to 36% ethylene, a molecular weight of 29,500 and a melt-ing point of 356~F. Nipon Gohsei of Japan produces ethylene vinyl alcohol copolymers under the trade marks "GL-D" and "GL-E". GL-D
has about 29% ethylene, a molecular weight of ~2,000, and a melt-ing point of 365F. GL-E has about 40% ethylene, a molecular wieght of 26,000, and a melting point of 327F. The film struc-tures reported in the tables which follow were made with these commercial EVOH resins.

The modified polyolefins which form the layers adjacen~
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 (HDPE, MDPE, LDPE), and polyolefin copoly-mers such as e-thylene vinyl acetate copolymer (EVA) and ethylene acrylic acid (EAA). Modified polyolefin blends are disclosed in U.S. Patents 4,087,5~7 and ~,087,588. Suitable modified polyole-fin blends are available from the Chemplex Company of Rolling Meadows, Illinois under the trade mar~ Plexar. Commercially available grades of such modified polyole~ins are blends of the graft copolymer with different polyolefins. These include Plexar-I, a trade mark for a blend with ethylene vinyl acetate copoly-mer; Plexar-II, a trade mark for a blend with high density poly-ethylene; Plexar-II (a trade mark) further modified with an elas-tomer such as polyisobutylene; and Plexar-III, a trade mark for a blend with ethylene vinyl acetate polymer adapted for cast film coextrusion. It has been found that these materials coex-trude well with extrusion grades of ethvlene vinyl alcohol '7~3 without need for adding m~terials to either the polyoleEin or the core layers in order to obtain the desired level o~ adhesion between layers.
.

Other materials used for layers overlyiny the modified polyolefin layers, include by way of illustration, high, medium and low density polyethylene, polypropylene, modified polyolefins, polyolefin copolymers such as ethylene vinyl acetate copolymer and ethylene acrylic acid copolymer blends two or more of the foregoing polymers, nylon, and ionomers such as those sold under t~e trade mark "Surlyn" by duPont. Five or more layers can be coextruded to produce a film having a plurality oEdesired pro-perties in a single pass.

Substantial improvements of a coextruded multi-layer structure may be obtained by lubricated compression rolling of the film. Such compression rolling of polymeric films is des-cribed in U.S. Patents 3,504,075, 3~194,863 and Re. 27,~04. The film resulting from the fluid compression roll technique is uni-axially oriented and reduced in thickness, with improved clarity or transparency and improved moisture barrier properties.

Examples of coextruded film structures in accordance with the present invention which have been made and tested are '' ~ t728 reported in the following tables. Table I reports oxygen per-meabi]ity ~or the various st~uctures. The permeability is ex-pressed as milliIiters of oxygen which pass through a square meter of the structure during a 24 hour period. The permea-bilities are reported at different humidities. Table II reports the adhesion between layers of various structures. The data was obtained using an Ins~ron test device which reveals the force in grams required to separate a 10 inch wide fil~ structure.

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In the tables "slip" means that a slip mat~rial such as ~iO2 (Union Carbide 0291) has been added to the outer sur-face layers to make the surface more slippery. Pigments such as supplied under the trademark Ampacet 11078 have been added to some layers. "DNS" means that -the sample did not separate in the adhesion test.

Containers were made from certain of -the films of the examples by heat sealing to form a pouch or by thermoforming to make containers in which foods were packased and stored to eva-luate the performance of the films. The containers were success-ful -~ . `7

Claims (36)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A delamination resistant multi-layer structure com-prising a barrier layer of a substantially pure polymer or co-polymer of vinyl alcohol adhered directly to a layer of a modi-fied polyolefin which consists essentially of a polyolefin polymer or copolymer blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride.

2. The multi-layer structure of claim 1 wherein the barrier layer is located between layers of the modified polyolefin.

3. The multi-layer structure of claim 1 wherein a further layer of a polymer is adhered to the modified polyolefin.

4. The multi-layer structure of claim 2 wherein a further layer of polymer is adhered to at least one of the modi-fied polyolefin layers.

5. The multi-layer structure of claim 1 wherein the barrier layer is ethylene vinyl alcohol copolymer.

6. The multi layer structure of claim 1 wherein the barrier layer is polyvinyl alcohol.

7. The multi-layer structure of claim 1 wherein the modified polyolefin layer consists essentially of polyethylene blended with the graft copolymer of polyethylene.

8. The multi-layer structure of claim 1 wherein the modified polyolefin layer consists essentially of ethylene vinyl acetate blended with the graft copolymer of polyethylene.

9. The multi-layer structure of claim 3 wherein the further layer is a polyolefin.

10. The multi-layer structure of claim 3 wherein the further layer is nylon.

11. The multi-layer structure of claim 3 wherein the further layer is ethylene vinyl acetate copolymer.

12. The multi-layer structure of claim 3 wherein the further layer is an ionomer.

13. A delamination resistant coextruded multi-layer structure comprising a barrier layer of substantially pure ethyl-ene vinyl alcohol copolymer located between adjacent layers con-sisting essentially of a modified polyolefin which consists of a polyolefin polymer or copolymer blended with a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhy-dride.

14 The multi-layer structure of claim 13 wherein the modified polyolefin consists essentially of polyethylene blended with the graft copolymer.

15. The multi-layer structure of claim 13 wherein the modified polyolefin consists essentially of ethylene vinyl acetate blended with the graft copolymer.

16. The multi-layer structure of claim 14 having a further layer over at least one of the modified polyolefin layers comprising a polymer selected from the group consisting of poly-ethylene, polypropylene, ethylene vinyl acetate copolymer, ethyl-ene acrylic acid copolymer, blends of two or more of the foregoing polymers, nylon, and ionomers.

17. The multi-layer structure of claim 15 having a further layer over at least one of the modified polyolefin layers comprising a polymer selected from the group consisting of poly-ethylene, polypropylene, ethylene vinyl acetate copolymer, ethyl-ene acrylic acid copolymer, blends of two or more of the foregoing polymers, nylon, and ionomers.

18. A delamination resistant coextruded multi-layer structure comprising (a) a substantially pure ethylene vinyl alco-hol copolymer core layer; (b) a modified polyolefin layer on each side of and adjacent the core layer, the modified polyolefin consisting essentially of a graft copolymer of polyethylene and an unsaturated fused ring carboxylic acid anhydride blended with a polyolefin selected from the group consisting of polyethylene and ethylene vinyl acetate copolymer; (c) an outer polymer layer over at least one of the modified polyolefin layers, the outer layers being independently selected from the group which consists of polyethylene, polypropylene, ethylene vinyl acetate copolymer, ethylene acrylic acid copolymer, blends of two or more of the foregoing polymers, nylon, and ionomers.

19. A delamination resistant coextruded multi-layer structure comprising a barrier layer of substantially pure ethyl-ene vinyl alcohol copolymer located between adjacent layers of a modified polyolefin which consists essentially of a blend of ethylene vinyl acetate copolymer and a graft copolymer of high density polyethylene and an unsaturated fused ring carboxylic acid anhydride.

20. The multi-layer structure of claim 19 wherein a further layer of a polymer is coextruded over at least one of the modified polyolefin layers.

21. The multi-layer structure of claim 20 wherein the further layer is polyethylene.

22. The multi-layer structure of claim 20 wherein the further layer is ethylene vinyl acetate copolymer.

23. The multi-layer structure of claim 20 wherein the further layer is an ionomer.

24. The multi-layer structure of claim 20 wherein the further layer is nylon.

25. The multi-layer structure of claim 20 wherein one further layer is nylon and another further layer over the other modified polyolefin layer is an ionomer.

26. The multi-layer structure of claim 20 wherein one further layer is nylon and another further layer over the other modified polyolefin layer is ethylene vinyl acetate copolymer.

27. A delamination resistant coextruded multi-layer structure comprising a barrier layer of substantially pure ethyl-ene vinyl alcohol copolymer and an adjacent layer of modified polyolefin which consists essentially of a blend of ethylene vinyl acetate copolymer and a graft copolymer of high density polyethyl-ene and an unsaturated fused ring carboxylic acid anhydride.

28. The multi-layer structure of claim 27 wherein nylon is coextruded on one face of the barrier layer and the modified polyolefin is coextruded on the other face.

29. The multi-layer structure of claim 28 wherein an ionomer is coextruded over the modified polyolefin layer.

30. The multi-layer structure of claim 28 wherein ethylene vinyl acetate copolymer is coextruded over the modified polyolefin layer.

31. A container formed from the structure of claim 1.

32. A container formed from the structure of claim 13.

33. A container formed from the structure of claim 18.

34. A container formed from the structure of claim 29.

35. A container formed from the structure of claim 30.

36. A multi-layer structure according to claim 27 wherein the barrier layer is 0.1 mils in thickness.