CA1051821A

CA1051821A - Cross-linked amide/olefin polymeric laminates

Info

Publication number: CA1051821A
Application number: CA237,971A
Authority: CA
Inventors: Anne C. Bieler; Henry G. Schirmer
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co Conn
Priority date: 1975-03-10
Filing date: 1975-10-20
Publication date: 1979-04-03
Also published as: GB1480204A; SE7603086L; FR2309333B3; JPS5918223B2; JPS51112888A; AU496808B2; FR2309333A1; US4104404A; IT1056950B; US3997383A; DE2609352A1; AU1176876A

Abstract

Abstract of the Disclosure An amide/olefin polymeric film laminate suitable for packaging, water cooking, and storing food within the package is prepared by coextruding polyamide and polyolefin layers to form a laminate and then cross-linking the laminate by irradiation. Prior art polyolefin/polyamide laminates delaminate readily when subjected to agitation in hot water, but this tendency is significantly reduced by laminates according to the present invention.

Description

~05~8Z~
FIELD OF THE INYENTION
This înventîon relates to la~înated fil~s in tubular and sheet configurations for packaging, cooking, and storing food.
Particularly, the invention relates to laminates having polyamide and polyolefin layers, specifically nylon and polyethylene layers.
BACKGROUND OF THE INVENTION
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 is an oxygen barrier, has a high melting point, and is strong and clear. Polyethylene is used as an inner surface for such la~inates because it is easily heat lS sealable, is moiature impermeable, and is relatively chemically inert to many food stuffs.
The disadvantages of nylon are principally its high cost, moisture permeability, and poor heat sealability; but, these disadvantages are largely offset by polyethylene's low cost, low moisture permeability, and good heat sealability. While the complimentary characteristics of nylon and polyethylene make them suitable for use as laminates, especially for packages containing food, the materials are somewhat incompatible because ~ they are considerably difficult to initially bond together. This difficulty is thought to be attributable to their differences in physlcal and chemical structures. 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 upwardly.

-2-~05~8Zl A number of techniques have been developed in an attempt to oyercome the bonding difficulties of nylon to polyethylene.
These ~echniques include chemically and electrically treating the surface of the polyethylene and the incorporation of a layer of adhesiYe between the nylon and polyethylene. Some of the adhesives used in the prior art are ethylene-acrylic acid copolymers and the zinc or magnesium neutralized ionic copolymers . . .
known as ionomers. Even blends of an ionomer and polyethylene have been used in an attempt to find an adhesive that maintains sufficient cohesion in hot water or at boiling water temperatures.
However, in an elevated temperature medium such as hot or boiling water, pouches, bags, and clipped casings of bulk packaged food, particularly packages containing more than ten pounds of food, all fail because the amide/olefin polymeric laminates tend lS to delaminate and shred within a short period of time. Accordingly, it i9 a principal object of the present invention to provide an amide/olefin polymeric laminate which will not delaminate under physical deformation at elevated temperatures.
It is another obiect of this invention to provide a laminated film in tubular or casing form which is suitable for packaging foodstuffs.
It is another object of the subject invention to provide a package in which food may be both cooked and stored without opening the package between the cooking and storing phases.
These and other ohjects are accomplished by the invention which is described herein below.

-3-105~82~
SUMMARY OF THE INVENTION
It has been surprisingly discovered that delamination of a poly-olefin/polyamide laminate may be prevented and the strength of the laminate increased by irradiating the laminate to a dosage level of at least 6 MR.
Accordingly, in one aspect, this invention is a method of making a laminate which will resist delamination and shredding in an elevated temperature medium.
First, the laminate is formed by coextruding an amide polymer and an olefin polymer with a polymeric adhesive composed predominately of olefin units dis-posed therebetween. Next, the coextruded laminate is cross-linked by ionizing radiation, preferably in the dosage range of 6 to 10 MR. Higher dosages could be used but are not necessary and are uneconomical.
In another aspect, the invention is a method of producing an olefin/
amide laminate for prolonged use in elet~ated temperature media in which the method comprises the steps of: coextruding a tubular laminate having poly-amide and irradiatively cross-linkable polyolefin layers with an adhesive disposed between the polyolefin and polyamide layers, the major component of the adhevive being olefin units which are cross-linkable by radiation; cooling and flattening the tubular laminate; and, irradiating the laminate by ionizing radiation to a dosage level of at least 6 MR and conveniently in the range of 6 to 10 MR.
In still another aspect, the present invention is a laminated film which comprises a polyamide layer; a polyolefin layer; an adhesive disposed between said polyamide and polyolefin layers, the major component of the ad-hesive being olefin units; and, the laminate being cross-linked to the equi-valent of an irradiation dosage of at least 6 MR and conveniently in the range of 6 to 10 MR.

-4-,* ii ~051821 Preferably, the olefln is ethylene 80 that the preferred polyolefin layer is polyethylene and the preferred adhesive is an ionomer in which ethylene is the predominate monomeric unit.
Also, within the scope of the invention the polyolefin layer may be an e~hylene vinyl acetate copolymer and the adhesive may comprise an acid copolymer resin.
DEFINITIONS
. . .
As used herein the terms set forth below will be understood to have the following meanings:

"Polymer" includes homopolymers, polymers, copolymers, terpolymers, and block, graft, random, or alternating polymers.
"Ionomer" means a polymer which has ethylene as its major component but has both covalent and ionic bonds. The anions hang from the hydrocarbon chain and the cations are metallic, e.g. magnesium or zinc so that ionomers may sometimes be referred to as a zinc neutralized ionic copolymer.
"Adhesive" means a polymeric substance capable of bonding two polymeric film layers together and for this application specifically refers to resins comprising ionomers and blends thereof with polyolefins and to acid copolymer resins and blends thereof with polyolefin resins.
"Amide" or "amide polymer" means a nylon selected from the group consisting of polycaproamide, polyhexamethylene, -~-- adipamide, polyhexamethylene sebacamide, polycaprylamide, polyundecanoamide, and polydodecanamide. These nylons are respectively commonly known as nylon-6; nylon-6, 6; nylon 6, 10;
nylon 8; nylon 11; and nylon 12 with the preferred nylon being nylon-6 such as type 8207 which is sold ~y Allied Chemical Corporation.

-5-~05~821 "Olefin" means the group of unsaturated hydrocarbons of the general formula CnH2 and includes ethylene, propylene, butene-l, etc., and blends thereof. In the present application the olefins of interest are the mono-alpha olefins having 2 to 8 carbon atoms and which cross-link when exposed to ionizing radiation.
"Irradiation" means exposure to high energy radiation such as electrons, X-rays, gamma rays, beta rays, etc. which induce cross-linking between the molecules of the irradiated material. Preferably, irradiation are carried out by an electron accelerator and the dosage levels is determined by the insoluble gel in the irradiated material. The dosage is measured in "rads"
wherein one rad is the absorbed dose of ioni~ing radiation equal to an energy of 100 ergs per gram of irradiated material.
A megarad (MR~ is one million rads.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred embodiment of the present invention a tubular casing which is 10 inches wide in the flattened condition and which has a 4.5 mil thickness was manufactured by a coextrusion process. A coextrusion die fed by four e~truders divided the melt streams into ten separate layers. Beginning from the inside of the tube, the first two layers were polyethylene having a density of 0.92 gms./cc. These two layers were fed frolll a ~ - 3 1/2" Hartig extruder and the two layers had a combined extruded thickness of 1.2 mils. Next to the inner polyethylene layers was the first adhesive layer with the adhesive being an ionomer resin fed from a 1 1/2" MPM extruder and the thickness of the first adhesive layer was 0.3 mils. Following the adhesive layer

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were four layers of nylon coextruded from a 2" NRM extruder and the total finished thickneas of these four layers was 1.5 mils.
Adjacent to the nylon layers was a second adhesive layer fed from the same extruder as the first layer and haYing the same thickness of 0.3 mils. The outer two layers were the same polyethylene as the first two layers except that the die for the outer two layers was fed from a 1 3/4" Prodex extruder and the total thickness of .. . .
these two layers was 1.2 mils.
The polyethylene and the ionomer were products of E. I.
du Pont de Nemours& Co. of ~ilmington, Delaware, the polyethylene being PE-2650 and the ionomer being Surlyn 1650. The nylon resin was nylon 6, type 8207 from Allied Chemical Corp. of Morristown, New Jersey.
The complete laminate ha6 a structure as follows:
polyethylene/polyethylene/ionomer/nylon/nylon/nylon/nylon/ionomer/
polyethylene/polyethylene. Thus, considering the four central nylon layers as sublayers forming one unit layer, the unit layer has adhesive on either side and to the respective adhesive layers areapplied the polyethylene unit layers which consist of sublayers.
In this instance, one polyethylene layer forms the inner surface of the tubing and one of the other polyethylene layers forms the outer surface of the tubing.
EXAMPLE
- Multilayer tubings as described immediately above were irradiated to different dosage levels and then each tubing was cut into 5 tubular sections or casings and one end of each casing was closed with a U-shaped metal clip. Each casing was then filled with 1 cup vegatable oil and 5 quarts of water, closed

-7-lOS18Zl with a clip, and tumbled in water at 190 F. The tumbling was accomplished hy placing the filled casings in a modified, front loading, commercial laundry machine which was filled with the heated water. Placing the casings in the water initially cooled the water to about 172 F. After about 20 minutes the water again reached 190 F and agitation began with the following results:
TABLE

Tlme st_0 F ln Nlnutes Irrndlntlon 0 Dossge (~R) 10 IDln. 20 ~oln. 30 Qln. 40 min. 50 mln.
0 511ght dclan. PE lnyer PE lsyer PE l~yer PE l~yer Co~cp. delnm~ Co~p. d~lan. Co~p. delnn. COIDP. deln:l.
2 Very nll&ht Sllght deln~. PE layer PE lnyer PE layer deln~. Coa~p. deln~. Conp. delnn. Col;p. delnrs.
4 V~ry nlight Very sllght Very nll&ht 511ght delasl. PE lnyer delna. delnGI. del~m. Co~p. delnQ.
6 llo dela~. No delnn. No deln~. No delnDI. ~lo delaQ.
9 No dela~. No. delnrd. No deln~l. No dtlnr~. No delam.

A portion of the preferred tubing was irradiated to a dosage of approximately 8 MR and then both irradiated and nonirradiated tubing was cut in 30" lengths and closed at one end with a U-shaped metal clip to form a casing. The casings were then filled with threerepresentative products, these being ~1) a mixture of 20%
vegetable oil with water as it was known that the ionomer/polyethylene bond is weakened by fatty substances; (2) creamed carrots;
(3) macaroni and cheese. All of the casings were hand filled with -fiye quarts of the test product and sealed with a clip after expressîng excess air by hand. A modified laundry machine of the horizontal rotating drum, front loading type with a capaclty of 125 lbs. was equipped with a steam injector and thermostat 105182~

and was filled ~ith 50 gallons of tap ~ater. The casings were then subjected to agitated tumbling at 210F for 2~ minutes. Afterwards the interior layers of the cas;ngs were examined for delamination and shredding and then microscopically examined. The results were 5as follows:
TABLE II
SAMPLE NU~BER OF CASINGS PRODUCT EVALUATION

Irradiated 3 Oil & Water Intact Unirradiated 3 Oil & Water Delamination & Shredding Irradiated 2 Creamed Carrots Intact Unirradiated 2 Creamed Carrots Delamination & Shredding Irradiated 1 Mac. & Cheese Intact Unirradiated 1 Mac. & Cheese Delamination & Shredding In both Examples 1 and 2 when the dosage level was greater than 6 MR, none of the irradiated casings failed under these abusive test conditions whereas all of the unirradiated casings either delaminated or shredded or both and even the lower dosage level casings delaminated somewhat.
The practical significance of a test like the foregoing is that in certain institutional food cooking processes it has been discovered that food products may be cooked in casings and, after cooling, the food may be stored unfrozen at a temperature in the range of 28F to 32F. In these institutional cooking processes, it has been found quite advantageous to tumble the filled casings .
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 and agitation in cooled water.

By making it possible for the temperature of the food to be reduced _9_ ~0518Zl quickly from its cooking temperature to its storage temperature the growth of microorganisms can be effecti~ely restricted thus increasing the storage lifetime of the food and enhancîng its quality. The irradiated casings according to the present invention are the only thermoplastic packaging materials which have been found to be satisfactory for the above described cooking and storage process.
A commercial electron accelerator was the radiation unit used to irradiate the polyethylene/nylon/polyethylene laminate described hereinabove, and it was found that it was necessary to irradiate the laminate to a dosage of at least 6 MR. Increased dosage levels could be used but would be uneconomical as no increase in resistance to delamination and shredding was observed for the higher dosage levels. The increased abuse resistance and resistance to delamination is thought to be possibly due to irradiation induced cross-linking at and across the polyethylene/ionomer interface and the ionomer/nylon interface as there is some comingling of the melts inthe coextruded laminate, to increased melting temperature of the cross-linked adhesive, and to increased strength in the polyethylene layers because of cross-linking. However, this invention is not limited to any particular theory explaining the increased resistance to delamination.
Besides the low density polyethylene (0.92 gms/cc) used ~~~- in the laminate described in Examples 1 and 2, high density (0.96 gms/cc) 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.
~aving described our invention,

Claims

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

1. A method of making a polymeric laminate capable of resisting delamination in elevated temperature media comprising the steps of:
(a) coextruding a laminate comprising an amide polymer layer, an olefin polymer layer, and a polymeric adhesive layer therebetween, said adhesive being predominantly comprised of olefin units;
and, (b) cross-linking said coextruded laminate by ionizing radiation, said cross-linking being equivalent to a dosage level of at least 6MR.

2. The method of claim 1 wherein a layer of said adhesive is coextruded onto both sides of said amide layer and a layer of said olefin polymer is coextruded onto the outer sides of each adhesive layer.

3. The method of claim 1 wherein said laminate is coextruded as a tube.

4. A method of producing an olefin/amide laminate for prolonged use in elevated temperature media comprising the steps of:
(1) coextruding a tubular laminate having olefin and amide polymeric layers, said olefin and amide layers including an adhesive disposed therebetween, the major component of said adhesive being olefin units, the olefin in said adhesive and said layer being the same;

(2) cooling and flattening said tubular laminate; and, (3) irradiating said laminate by ionizing radiation to a dosage level of at least 6 MR.

5. The method of claim 4 wherein said olefin polymer is polyethylene.

6. The method of claim 4 wherein said olefin polymer is ethylene vinyl acetate copolymer.

7. The method of claim 5 wherein said said polyethylene is a low density polyethylene.

8. The method of claim 5 wherein said polyethylene is high density polyethylene.

9. The method of claim 4 wherein the tubular laminate is coextruded with inner and outer layers of an olefin polymer and a central layer of an amide polymer.

10. The method of claim 5 wherein said adhesive comprises an ionomer resin.

11. The method of claim 4 wherein said polyamide layer is coextruded between two polyolefin layers with said adhesive being disposed between the polyamide and polyolefin layers.

12. The method of claim 11 wherein said polyamide layer consists of four coextruded sublayers and each polyolefin layer consists of two coextruded sublayers.

13. A laminated polymeric film for use in elevated temperature media comprising:
(a) an amide polymer layer;
(b) an olefin polymer layer;
(c) an adhesive disposed between said amide and olefin layers, said adhesive having olefin units as the major component thereof; and, (d) said laminate being cross-linked to the equivalent of a radiation dosage of at least 6 MR.

14. The laminate of claim 13 wherein there is an olefin layer on each side of said amide layer.

15. The laminate of claim 13 wherein said olefin layer is polyethylene.

16. The laminate of claim 15 wherein the olefin units in said adhesive are ethylene units.

17. The laminate of claim 14 wherein said polyamide layer comprises four sublayers and said polyolefin layers comprise two sublayers.