Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE IN~ENTION
This invention relates to laminated films 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 or ethylene-vinyl acetate
copolymer 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 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 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. These attributes of polyethylene are also
shared by ethylene-vinyl acetate copolymer. 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. Th;s difficulty is thought to be attributable to their
differences in physical and chemical structures. Also, once joined,
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the two materials will often separate upon physical deformation,
particularly when agitated at water cooking temperatures which generally
range from 140F 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 neutralized
ionic copolymers known as ionomers as described in U.S. Patent No.
3,423,231 which issued to H. H. Lutzmann on January 21, 1969. 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. Such blends are set forth in U.S. Patent
No. 3,697,368 which issued to M. Bhuta et al on October 10, 1972.
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 to delaminate and shred within a
short period of time. Accordingly, it is a principal object of the
present invention to provide an amide/olefin polymeric laminate which
will not delaminate under physical deformation at elevated te~peratures.
It is another object of this invention to provide a laminated
film in tubular or casing form which is suitable for packaging food-
stuffs.
Yet anGther object of this invention is to provide a laminate
which requires a lower radiation dosage to become resistant to delamination.
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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 objects are accomplished by the invention
which is described herein below.
SUMMARY OF THE INVENTION
In U.S. Patent No. 3,997,383 ~hich issued on December 14, 1976
to Anne C. Bieler et al it was discovered that delamination of a poly-
olefin/adhesive/polyamide laminate may be prevented and the strength of
the laminate increased by irradiating the laminate to a dosage of at
least 6 MR to cross-link the olefin monomers in the laminate structure.
In this 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
laminate is preferably formed by coextruding an amide polymer and an
olefin polymer with an adhesive comprising predominately cross-linkable
olefin homopolymers or copolymers disposed therebetween, or, the adhesive
and amide polymer can be extrusion coated onto a polyolefin substrate.
Accordingly, in one aspect, the subject invention is a method of forming
a laminate wherein the adhesive contains predominately an olefin copolymer
such as ethylene-vinyl acetate copolymer. After formation, the laminate
is cross-linked by ionizing radiation, preferably by electrons. U.S.
Patent No. 3,734,843 which issued on May 22, 1973 to R. K. Tubbs discloses
irradiation of ethylene-vinyl acetate pellets in order to increase the
molecular weight and lower the melt index. However, in the Tubbs patent
no use is shown of ethylene-vinyl acetate as a film or as an adhesive.
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In another aspect, the present invention is a laminated film
which comprises a polyamide layer; a polyolefin layer, an ethylene-vinyl
acetate copolymer adhesive disposed between said polyamide and poly-
olefin layers, a substantial component of the adhesive being ethylene
monomeric units, and, the laminate including the adhesive being cross-
linked to the equivalent of an irradiation dosage in the range of 6 to
10 MR.
The polyolefin layer is preferably an ethylene-vinyl acetate
copolymer or polyethylene. The preferred ethylene-vinyl acetate copolymer
component of the adhesive may be blended with other adhesive components,
the significant requirement being that a substantial number of the
monomeric units in the adhesive be cross-linkable monomeric units,
preferably ethylene monomers so that the adhesive will be substantially
cross-linked when irradiated.
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.
"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 polyolefins or adhesive materials. Ethylene-vinyl
acetate copolymer is referred to hereinafter as "EVA."
"Amide," "amide polymer," or "nylon" means a polymer selected
from the group consisting of polycaproamide, polyhexamethylene, adipamide,
polyhexamethylene sebacamide, polycaprylamide, polyundecanoamide, and
polydodecanamide. These nylons are respectively commonly known as
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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 by
Allied Chemical Corporation.
"Olefin" means the group of unsaturated hydrocarbons of the
general formula CnH2n and includes ethylene, propylene, butene-l, etc.,
and blends there-of. 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 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.
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. Coextrusion f
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 gms./cc. This layer was fed from
a 3 1/2" Hartig extruder and the layer has an extruded thickness of 1.2
mils. 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 1 1/2" MPM extruder and the thickness of the first
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adhesive layer was 0.3 mils. Following the adhesive layer was a nylon
layer coextruded from a 2" NRM extruder and the total finished thickness
of this layer was l.5 mils. 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. The outer layer was the same poly-
ethylene as the-first layer except that the die for the outer layer was
fed from a l 3/4" Prodex extruder and the total thickness of this layer
was l.2 mils.
In addition to coextrusion, the laminate can be prepared by
first extruding a substrate layer and extrusion coating the additional
layers on to it. The inner polyethylene ,layer would be the substrate.
The polyethylene was the product of E. I. du Pont de Nemours &
Co. of Wilmington, Delaware, the polyethylene being PE-2650. The nylon
resin was nylon 6, type 8207 from Allied Chemical Corp. of Morristown,
New Jersey. The modified ethylene-vinyl acetate adhesive was "Plexar"
adhesive from Chemplex Company of Rolling Meadows, Illinois.
The complete laminate has a structure as follows: poly-
ethylene/EVAjnylon/EVA/polyethylene. The laminate, as flattened tubing
was next passed through a beam of electrons from an accelerator to
cross-link the cross-linkable polymers in the laminate structure.
Example
Tubing was prepared as described above and then slit to
produce film which was 6" wide. Using a Hooper model 503 vacuum packaging
machine, cavities l" to 2" 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 180F water for 72 hours and no delamination
occurred.
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Example 2
Bologna was packaged in thermoformed cavities made as described
in Example 1. After being boiled in water for 30 minutes no delami-
nation occurred. 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 3
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 thick before thermoforming.
These packages were boiled 45 minutes and then cooked 5 hours at 180F
and then 1 minute in a microwave oven. No delamination nor seal failures
occurred.
In addition to the above described thermoformed pouches, the
tubular laminate may 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. temperatures above 180F, that delamination in prior
art (unirradiated~ casings becomes a pronounced problem whereas laminates
according to the present invention show no tendency to delaminate.
The practical significance of improved resistance to delami-
nation is that in certain institutional food cooking processes it has
been discovered that food products may be cooked in pouches, casings
and, after cool;ng, 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 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
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storage temperature by tumbling and 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 descrihed cooking and
storage process.
A commercial electron accelerator was used to irradiate the
polyethylene/adhesive/nylon/adhesive/polyethylene laminate described
hereinabove. In prior art structures it was found that for the most
satisfactory results it was necessary to irradiate the laminate to a
dosage of about 6 MR. However, using the ethylene-vinyl acetate copolymer --
adhesive of the present invention field tests show that no delamination
occurred in laminates irradiated to dosage levels as low as 2.0 MR.
Dosage levels above 6 MR could be used 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 poly-
ethylene/adhesive interface and the adhesive/nylon interface as there is
some commingling of the melts in the coextruded laminate; the cross-
linked adhesive no longer melt flows; and, the strength of the poly-
ethylene layers increases, particularly at high temperatures, because of
cross-linking. However, this invention is not limited to any particular
theory explaining the increased resistance to delamination.
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372
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.
Having described my invention,
,
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