GB2024713A

GB2024713A - A nylon/olefin polymer laminate film and method of preparing it

Info

Publication number: GB2024713A
Application number: GB7923194A
Authority: GB
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co
Priority date: 1978-07-03
Filing date: 1979-07-03
Publication date: 1980-01-16
Also published as: FR2434696A1; CA1111372A; BE877423A; DK279179A; SE7905774L; CH643185A5; IT7924040A0; IT1192652B; NL7905133A; GB2024713B; FR2434696B1; DE2926530A1; FI792087A

Abstract

A nylon/olefin polymer especially nylon/polyethylene laminate film is useful for packaging food to be cooked in hot water. However, special steps are required to bond the nylon and olefin polymer firmly enough to prevent delamination in these severe conditions. Various adhesives have been tried and in particular combinations of use of an ionomer as adhesive and irradiation of the laminate film. However, a high dosage of radiation is required. The solution to this problem provided by the present invention involves use as an adhesive of an ethylene-vinyl acetate copolymer and irradiation of the laminate film. A dosage as low as 2.0 megarads is possible using this adhesive.

Description

SPECIFICATION A nylonlolefin polymer laminate film and method of preparing it This invention relates to a nylon/olefin polymer laminate film, whether in tubular or sheet configuration, for use in elevated temperature media. One important use is for packaging food which is to be cooked in hot water and subsequently stored.

Laminates of nylon and polyethylene are widely used for packaging various products and have been found especially suitable for foodstuff containing packages such as bags or pouches subjected to widely varying temperatures and conditions. Nylon is used for such packages because it has low oxygen permeability, has a high melting point, and is strong and clear. Polyethylene is used as an inner surface for such laminates because it is easily heat sealable, is moisture-impermeable, and is relatively chemically inert to many foodstuffs.

The disadvantages of nylon are principally its high cost, moisture permeability, and poor heat sealability. These disadvantages are largely offset by polyethylene's low cost, low moisture permeability and good heat sealability. While the complementary characteristics of nylon and polyethylene make them suitable for use as laminates, especially for packages containing food, the materials are somewhat incom patible because they are very difficult initially to bond together. Also, once joined, the two materials will often separate upon physical deformation, particularly when agitated at water cooking temperatures which generally range from 140"F (60"C) upwardly.

As boiling water temperatures are approached the occurrence of delamination in prior art laminates becomes a serious problem.

A number of techniques have been developed in an attempt to overcome the bonding difficulties of nylon to polyethylene. These techniques include chemically and electrically treating the surface of the polyethylene and the incorporation of a layer of adhesive between the nylon and polyethylene.

Some of the adhesives used in the prior art are ethylene-acrylic acid copolymers and the zinc or magnesium neutralised ionic copolymers known as ionomers, see United States Patent No. 3,423,231.

Blends of an ionomer and polyethylene have even been used in an attempt to find an adhesive that maintains sufficient cohesion in hot water or at boiling water temperatures, see United States Patent No. 3,697,368. However, in an elevated temperature medium such as hot or boiling water, pouches, bags and clipped casings of bulk packaged food, particu larly packages containing more than 10 1 b. (4.5 kg) of food, all fail because the nylon/olefin polymer laminates tend to delaminate and shred quickly.

Our United States Patent No. 3,997,383 describes a polyolefin/adhesive/polyamide laminate film in which the adhesive is an ionomer or acid-olefin copolymer or a blend thereof with a polyolefin. This patent describes preventing delamination and in creasing the strength of the laminate by irradiating it to a dosage of at least 6 megarads (MR), to cross-link the olefin-derived polymeric units in the laminate structure.

In the present invention a specific laminate structure has been discovered which resists delamination and shredding in an elevated temperature medium more satisfactorily than prior art structures and which requires lower radiation dosages to achieve satisfactory cross-linking.

The present invention provides a laminate film for use in elevated temperature media comprising nylon and olefin polymer layers with an adhesive layer therebetween, said adhesive layer comprising predominantly a copolymer of ethylene and vinyl acetate, at least the olefin polymer and the adhesive being cross-linked. A preferred such laminate has the following sequence of layers: (1) olefin polymer, (2) adhesive, (3) nylon, (4) adhesive and (5) olefin polymer.

The invention also includes a method of preparing a laminate film for use in elevated temperature media, comprising: (a) preparing a laminate of nylon and olefin polymer'layers with an adhesive layer therebetween, said adhesive layer comprising predominantly a copolymer of ethylene and vinyl acetate; and (b) irradiating said laminate with ionizing radiation to cross-link at least the olefin polymer and the ethylene-vinyl acetate copolymer.

The ionizing radiation is preferably a stream of electrons. Preferably the laminate is prepared by co-extrusion of all layers. Alternatively it can be prepared by extruding a layer of olefin polymer and extrusion-coating the adhesive and nylon layers.

A substantial component of the adhesive used in the present invention is of polymeric units of ethylene, thus enabling the adhesive to be crosslinked. Preferably the laminate is radiation crosslinked. However it is cross-linked, the degree of cross-linking is preferably equivalent to that of a radiation dosage of from 2.0 to 10 MR, preferably at least 2.0 MR up to but less than 6.0 MR and less preferably from 6.0 to 10 MR. The adhesive can contain other components, apart from the ethylenevinyl acetate copolymer, provided that a substantial number of the polymeric units in the adhesive is cross-linkable, preferably of ethylene.

The olefin polymer layer is preferably of an ethylene-vinyl acetate copolymer or polyethylene.

The olefin polymer can be a homopolymer or copolymer.

The preferred nylon layer is of nylon-6.

As used herein the terms set forth below will be understood to have the following meanings: "Polymer" includes homopolymers, polymers, copolymers (including bipolymers and terpolymers) and block, graft, random, or alternating polymers.

"Adhesive" means a polymeric substance capable of bonding two polymeric film layers together and for this application specifically refers to resins comprising ethylene-vinyl acetate copolymers and blends thereof with other olefin polymers or adhesive materials. Ethylene-vinyl acetate copolymer is referred to hereinafter as "EVA". The term includes modified EVA. "Nylon" means a polymer selected from polycaproamide, polyhexamethylene, adipa mide, polyhexamethylene sebacamide, polycapryla mide, polyundecanoamide, and polydodecanamide.

These nylons are respectively commonly known as nylon-6; nylon-6,6; nylon-6,10; nylon-8; nylon-11; and nylon 2.

"Olefin" means the group of unsaturated hydrocarbons of the general formula CnH2n and includes ethylene, propylene, butene-1, etc., and blends thereof. In the present application the olefins of primary interest are the mono-aipha olefins having 2 to 8 carbon atoms and which cross-link when exposed to ionizing radiation.

"Irradiation" means exposure to high energy radiations such as electrons, X-rays, gamma rays, beta rays, etc. which induce cross-linking between the molecules of the irradiated material. Preferably, irradiation is carried out by an electron accelerator and the dosage levels are determined by the insoluble gel in the irradiated material. The dosage is measured in "rads" wherein one rad is the absorbed dose of ionizing radiation equal to an energy of 100 ergs per gram of irradiated material. A megarad (MR) is one million rads.

The invention also includes a package of food in a thermoformed pouch of a laminate film of the invention sealed to a covering sheet to close it. The laminate film may alternativeiy take the form of a tubular casing. Thus for example it can be cut into tubular segments and the segments clipped by a metal U-shaped clip to form a casing which is filled with a food product and closed by the application of a second clip. Experience with casings has shown that as the temperature of boiling water is approached, e.g. temperature above 1 80 F (82"C), that delamination in prior art (unirradiated) casings becomes a pronounced problem whereas laminate films of the present invention show no tendency to delaminate.

Further the invention includes a method of preparing food for large scale catering which comprises packaging the food by sealing it in a pouch or casing comprising a film laminate of the invention, tumbling the pouch or casing in heated water to cook the food, after cooking agitating the pouch or casing in cool water to reduce the temperature of the food rapidly, and storing the packaged food at a temperature of from 28 to 320F (-2.2 to 0 C). In these institutional cooking processes, it has been found quite advantageous to tumble the filled casings or pouches in heated water thus increasing the transfer of heat into the food product within the casing and thereby reducing the cooking time. In like manner, after cooking, the food may be rapidly cooled to its storage temperature by tumbling or other agitation in cooled water.By making it possible for the temperature of the food to be reduced quickly from its cooking temperature to its storage temperature the growth of microorganisms can be effectively restricted thus increasing the storage lifetime of the food and enhancing its quality. The irradiated casings and pouches according to the present invention are some of the very few packages made from thermoplastic materials which have been found to have satisfactory abuse resistance for the above described cooking and storage process.

The following Examples illustrate the invention.

Example I In a preferred embodiment of the present invention a tubular casing which is 10 inches (25 cm) wide in the flattened condition and which has a 4.5 mil (115 micron) thickness was manufactured by a coextrusion process. Coextrusion dies fed by extruders provide the melt streams which comprise five basic layers. Beginning from the inside of the tube the first layer was polyethylene having a density of 0.92 g./cm3. This layer was fed from a 3 1/2(89 mm) Hartig extruder and the layer has an extruded thickness of 1.2 mils (30 microns). Next to the inner polyethylene layer was the first adhesive layer with the adhesive being a modified ethylene-vinyl acetate copolymer resin fed from a 11/2" (38 mm) MPM extruder and the thickness of the first adhesive layer was 0.3 mils (7.6 microns).Following the adhesive layer was a nylon layer coextruded from a 2" (51 mm) NRM extruder and the total finished thickness of this layer was 1.5 mils (38 microns). Adjacent to the nylon layer was a second adhesive layer fed from the same extruder as the first layer and having the same thickness of 0.3 mils (7.6 microns). The outer layer was the same polyethylene as the first layer except that the die for the outer layer was fed from a 1 3/4" (44 mm) Prodex extruder and the total thickness of this layer was 1.2 mils (30 microns).

As an alternative to coextrusion, the laminate can be prepared by first extruding a substrate layer and extrusion-coating the additional layers on to it. the olefin polymer layer is the preferred substrate.

The polyethylene was PE-2650 of E.l. du Pont de Nemours & Co. of Wilmington, Delaware, U.S.A. The nylon resin was nylon-6, type 8207 Allied Chemical Corporation of Morristown, New Jersey, U.S.A. The modified EVA adhesive was "Plexar" adhesive from Chemplex Company of Rolling Meadows, Illinois, U.S.A.

The complete laminate has a structure as follows: polyethylene/EVAtnylon/EVAtpolyethylene. The laminate, as flattened tubing, was next passed through a beam of electrons from a commercial electron accelerator, using a radiation dosage of more than 2.0 and less than 6.0 MR. Tests on laminates prepared by the present invention show that no delamination occurred in laminates irradiated to dosage levels as low as 2.0 MR. Dosage levels of 6 MR or above are necessary for known laminates. Such dosages could be used in the present invention but would be uneconomical as no increase in resistance to delamination and shredding was observed for these higher dosage levels. The increased abuse-resistance and resistance to delamination is thought to be due to one or more of the following factors: irradiation induces cross-linking at and across the polyethylene/adhesive interface and the adhesivetnylon interface as there is some commingling of the melts in a co-extruded laminate; the cross-linked adhesive no longer melt-flows; and the strength of the polyethylene layers increases, particularly at high temperatures, because of crosslinking. However, this invention is not limited by any particular theory explaining the increased resistance to delamination.

Example 2 Tubing was prepared as described above and then slit to produce film which was 6" (150 mm) wide.

Using a Hooper model 503 vacuum packaging machine, cavities 1" to 2" (25 to 51 mm) deep were thermoformed in the film and filled with water and vegetable oil. The filled cavities were then moved to a vacuum chamber and a thermoplastic sheet was heat sealed to the film around the cavity perimeter to form a sealed package or pouch. Next, the packages were placed in 1 80 F (60 C) water for 72 hours and no delamination occurred.

Example 3 Bologna was packaged in thermoformed cavities made as described in Example 1. After being boiled in water for 30 minutes no delamination occurred.

For comparison, similar packages were prepared in which the film had not been irradiated. In these packages with non-cross-linked film delamination occurred after the packages had been in boiling water for approximately one minute.

Example 4 Packages containing beef, corned beef, chicken parts, and turkey parts were prepared as described in Example 1. Two thicknesses of film were used, 4.5 mils and 6 mils (115 and 150 microns) thick before thermoforming. These packages were boiled for 45 minutes and then cooked for 5 hours at 180"F (82"C) and then for 1 minute in a microwave oven. No delamination nor seal failures occurred.

As our alternative to the low density polyethylene (0.92 g./cm3) used in the laminate described in Examples 1 and 2, high density (0.96 g./cm3) polyethylene may be used as one or both of the polyethylene layers or an ethylene-vinyl acetate copolymer may be used for one or both of the polyethylene layers.

Claims

1. A laminate film for use in elevated temperature media comprising nylon and olefin polymer layers with an adhesive layer therebetween, said adhesive layer comprising predominantly a copoly mer of ethylene and vinyl acetate, at least the olefin polymer and the adhesive being cross-linked.

2. A laminate film according to claim 1 wherein the olefin polymer and adhesive are radiation cross linked.

3. A laminate film according to claim 1 or 2, wherein the degree of cross-linking is equivalent to that which would be induced by absorption of at least 2.0 MR up to but less than 6.0 MR of ionising radiation.

4. A laminate film according to claim 1,2 or 3 having the following sequence of layers: (1) olefin polymer, (2) adhesive, (3) nylon, (4) adhesive and (5) olefin polymer.

5. A laminate film according to claim 1,2,3 or 4 wherein said olefin polymer layer is of polyethylene.

6. A laminate film according to claim 1,2,3 or 4, wherein said olefin polymer layer is of a copolymer of ethylene and vinyl acetate.

7. A laminate film according to any preceding claim in tubular form.

8. A method of preparing a laminate film for use in elevated temperature media, comprising: (a) preparing a laminate of nylon and olefin polymer layers with an adhesive layer therebetween, said adhesive layer comprising predominantly a copolymer of ethylene and vinyl acetate; and (b) irradiating said laminate with ionizing radiation to cross-link at least the olefin polymer and the ethylene-vinyl acetate copolymer.

9. A method according to claim 8 wherein the laminate is irradiated to a dosage of at least 2.0 MR, up to but less than 6.0 MR.

10. A method according to claim 8 or 9 wherein the laminate is prepared by co-extruding all layers.

11. A method according to claim 8 or 9 wherein the laminate is prepared by extruding a layer of olefin polymer and extrusion-coating the adhesive and nylon layers.

12. A method according to any one of claims 8 to 11 wherein a tubular laminate is prepared and is irradiated in a iaid-flatform.

13. A method according to any one of claims 8 to 12 wherein the olefin polymer is polyethylene or a copolymer of ethylene and vinyl acetate.

14. A method according to claim 8 substantially as described in Example 1.

15. A laminate film when prepared by a method claimed in any one of claims 8 to 14.

16. A laminate film according to claim 1 substantally as described in Example 1.

17. A laminate film according to any one of claims 1 to 7, 15 and 16 in the form of a thermoformed pouch or a tubular casing.

18. A package of food in a thermoformed pouch claimed in claim 17 sealed to a covering sheet to close it.

19. A method of preparing food for large scale catering which comprises packaging the food by sealing it in a pouch or casing comprising a film laminate claimed in any one of claims 1 to 7, 15 and 16, tumbling the pouch or casing in heated water to cook the food, after cooking agitating the pouch or casing in cool water to reduce the temperature of the food rapidly, and storing the packaged food at a temperature of from 28 to 32"F (-2.2 to 0 C).