Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- ' -- 2092968
MULTI-LAYERED THERMOPLASTIC PACKAGING FILM
WITH IMPROVED OXYGEN PERMEABILITY
Background of the Invention
The present invention relates to a multi-layer film
which is particularly useful as packaging material. More '
specifically, this invention relates to an oxygen permea-
ble, thermoformable, multilayer film especially useful for
packaging products that require oxygen such as fresh poul-
try and frozen red meat.
Description of the Prior Art
Numerous film products are employed for packaging and
for delivery of food products. These films were developed
to have particular properties and often employ multiple
layers to obtain the desired properties. For example, it
is well known to use polyolefin based films which are char-
acterized by high strength, excellent moisture and water
vapor resistance, fair chemical resistance and variable
processability. These polymers are often used in combina-
tion with other polymers. It has been found that no single
polymer or copolymer however can possess all the desired
properties and thus the proper combination of different
polymers and multilayered structures have been found to
provide a good balance depending on the end use of the film.
Many fiLas designed for packaging applications in the
food industry incorporate "barrier" polymers to prevent the
5/920821.5/SPECFLDR
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2092J68
passage of oxygen. The present invention however is direct-
ed to films to be used for packaging certain food products
such as fresh poultry which must necessarily possess high
oxygen permeability.
For many such packaging applications it is also desir-
able that the oxygen permeable film is capable of
thermoforming. Most typically, a non-thermoforming film or
web will be used in combination with a thermoforming film
or web to produce a final package. In a typical operation
a forming web is formed into a mold to provide a film cavi-
ty in Which a food product is placed. A non-forming web
can then be placed over the cavity and vacuum sealed by
means well known in the art to the periphery of the forming
web. Many meat products are packaged in this manner.
A prior art non-thermoformable web is available which
employs biaxially oriented polypropylene as an outermost
heat resistant layer. However, heretofore there has been
only one commercially available thermoformable film having
good oxygen permeability properties. That is, it is gener-
ally known that thermoformable mono-layer ionomers provide
high oxygen permeability. Generally speaking, ionomers are
metal neutralized salts of ethylene acrylic acid or
methacrylic acid copolymers and are most typically sold
under the trade name Surlyn(TM) by DuPont. Although mono-
layer Surlyn produced by a blown or cast process.provides a
heat sealable thermoformable oxygen permeable film, product
failure rate .is high because the periphery of the web must
be heated to its softening point in order for sealing to
occur. Thus, burn through is common.
Thus there is a need in the art for a heat-sealable
thermoformable oxygen permeable film or web which does not
degrade upon sealing.
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Summary of the Invention
It is thus an object of the present invention to
provide heat-sealable thermoplastic multilayer film having good
oxygen permeability.
It is a more particular object of the present
invention to provide a suitable thermoplastic material for
packaging fresh poultry or frozen meat.
It is yet another object of the present invention to
provide a multilayer thermoplastic film having an oxygen
l0 transmission rate greater than 2000 cc. mil/m2-24 hr. atm. at
73°F.
Such objects are generally achieved by providing a
multilayer, thermoformable film which includes a sealing layer
and an outer heat resistant layer, preferably of a propylene
1S based polymer, wherein the polymeric composition of the outer
heat resistant layer has a melting point greater than that of
the sealing layer.
Such objects are more particularly achieved by
providing such a film wherein the outer heat resistant layer
20 has a melting point at least about 10°F greater than that of
the sealing layer.
Thus, there is provided a multilayer film comprising:
an outer heat-sealable layer comprising an ethylene-based
polymer; and a polymeric outer heat-resistant layer having a
25 melting point that is at least 10°F greater than the melting
point of the heat-sealable layer, wherein the multilayer film
is unperforated, thermoformable and has an oxygen transmission
rate greater than 2000 cm3 mil/m2-24 hrs atmosphere at 73°F,
and wherein all the layers of the film are co-extruded.
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CA 02092968 2000-11-02
64'536-807
Description of Preferred Embodiments
The present invention provides a multilayer packaging
film characterized by having excellent oxygen permeability and
thermoformability properties which includes at least a sealing
layer and a heat resistant layer. The sealing layer most
preferably includes an ethylene based polymer such as low
density polyethylene or a copolymer of ethylene
3a
.. 2os~~s~
and one or more comonomers. Preferred ethylene copolymers
include ethylene/alpha-olefins, ethylene vinyl acetates,
ethylene alkyl acrylates, ethylene acrylic acid copolymers
as well as the metal neutralized salts of ethylene acrylic
or methacrylic acid copolymers commonly referred to as
ionomers.
Ethylene alpha-olefins are, generally speaking,
copolymers of ethylene with one or more comonomers selected
from C3 to about C1o alpha olefins but especially com-
prises ethylene copolymers with C, to about C1o alpha
olefins such as butene-1, pentane-1, hexene-1, octene-1,
and the like, in which the polymer molecules comprise long '
chains with few side chains or branches and sometimes are
referred to as linear polymers. These polymers are ob-
tained by low pressure polymerization processes and the
side branching which is present will be short compared to
non-linear ethylenes. Ethylene/alpha-olefin copolymers
have a density in the range of from about 0.860 g/cc to
about 0.940 g/cc. The term linear low density~polyethylene
is generally understood to include that group of
ethylene/alpha-olefin copolymers which fall into the densi-
ty range of about 0.915 to about 0.940 g/cc. Sometimes
linear polyethylene in the density range from about 0.926
to about 0.940 is referred to a linear medium density poly-
ethylene (L2~PE). Lower density ethylene alpha olefins may
be referred to as very low density polyethylene (VLDPE,
typically used to refer to the ethylene butene copolymers
-~ supplied by Union Carbide) and ultra-low density polyethyl-
ene (ULDPE, typically used to refer to the ethylene octene
copolymers supplied by Dow). It should be noted although
specific density ranges is for vLDPE, ULDPE, LLDPE, and
LI4aPE have been set forth herein, that no bright line can
be drawn for density classification and such will vary by
supplier.
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2092J68
Recently a new type of ethylene based linear polymers
have been introduced. These new resins are produced by
metallocene catalyst polymerization and are characterized
by narrow or more homogenous compositional properties,
such as molecular weight distribution, than resins produced
by conventional Ziegler-Natta polymerization processes.
Conventional Ziegler-Natta polymerization systems have
discreet catalyst composition differences which are mani-
fested as different catalyst reaction sites with each site
having different reaction rates and selectivities.
Metallocene catalyst systems are characterized as a single
identifiable chemical type which has a singular rate in
selectivity. Thus the conventional systems produce resins '
that reflect the differential character of the different
catalyst sites while resins produced by metallocene systems
reflect the single catalytic site. However, it should be
noted that at least some previously available, ethylene
based linear polymers approximated the physical and composi-
tional properties achieved by the present metallocene cata-
lyzed polyolefins. That is traditional metallic catalyzed
polymerization processes operating at low reaction rates
can produce relatively homogenous resins that compare favor-
ably with .the homogeneity of metallocene catalyzed resins.
An example of such are the resins sold under the trade name
Tafmer(TM) by Mitsui. Both metallocene catalyzed ethylene
alpha olefins and the Tafmer-type of resins are appropriate
for use in the heat seal layer of the present invention.
Another .resin which may be used in the present heat
seal layer is a butadiene styrene copolymer (BDS) such as
DR10, one of the R resin series available from Phillips
Chemical Company. Also, within the scope of the present
heat resistant layer are modified polyvinyl chlorides (PVC)
such as supplied by B.F. Goodrich. Inclusion of such res-
ins provides stiffness to the overall film structure as
well as imparting excellent oxygen permeability qualities.
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As noted above, ionomers are also within the scope of
the present seal layer. As has been seen in the prior art
mcnolayer Surlyri films, such resi~ provides excellent heat
sealability as well as high oxyge.~. permeability. However,
unlike the mono-layer films of the prior art, the present
film structure will also include a heat resistant layer
such that upon sealing the entire thickness of the film
structure is not heated to its sof~,.~ning point.
Accordingly, the present fiL~~ structure includes a
heat resistant layer having a melting point greater than
that of the sealing layer. Thus, when a non-forming web in
accordance with the present invention is positioned above a
thermoformed web containing product and sealed about the
periphery thereof, the outermost surf aces of each of the
two webs need not be heated to ti:e ~ point of softening by
the sealing mechanism in order for adequate sealing to
occur between the two respective seal layers.
Furthermore, it should be noted that although the
present invention is generally directed to thermoformable
webs, non-forming webs are also within the scope of the
invention. That is, thermoformable webs which are not
thermoformed in the end-use application are considered
non-forming and are covered by the present invention.
Most preferred for use in the heat resistant layer of
the present film structure are propylene based resins such
as propylene homopolymers and propylene copolymers.
Ethylene propylene copolymers which have a major portion of
prop~rlene and a minor portion of a ~'.'.~.ylene are desirable for
use in the present heat resistant layer as such do not
become brittle at freezing temperatures. Conventional
polypropylenes are desirable because of their high melting
point, approximately 160°C, but may become brittle at freez-
ing temperatures. Thus, the end-use application as well as
the melting point of the polymeric composition of the heat
6
2092968
seal layer must be considered in choosing the polymeric
composition of the heat resistant layer. Specifically, the
heat resistant layer must have a melting point greater than
that of the heat seal layer and most preferably at least
10°F greater than that of the heat seal layer.
Other resins appropriate for use in the heat resistant
layer include polystyrene and styrene butadiene copolymer.
Although such resins have melting points less than that of
polypropylene, they can be employed in accordance with the
present invention so long as the polymeric components of
the heat seal layer have an even lower melting point.
It has further been found that advantages of different
heat resistant polymers may be incorporated into a single
structure by incorporating two or more of such resins into
a single film, either in separate layers or by blending.
For example, in a preferred embodiment, the outermost layer
is a polypropylene but an internal layer is included of an
ethylene propylene copolymer (EPC). The internal EPC layer
is, of course, more heat resistant than the seal layer but
adds a pliancy to the structure not afforded by the more
heat resistant but less pliant polypropylene. Thus, crack-
ing at freezing temperatures is reduced. Other polymers
appropriate for use in the outer heat resistant layer may
also be used internally in the structure although it would
generally be preferred to provide the mast heat resistant
polymeric composition in the outermost layer.
Also within the scope of the present invention are
internal tie layers which add bulk and prevent delamination
without decreasing the oxygen transmitability of the entire
structure. Preferred tie layers include ethylene vinyl
acetates, ethylene methyl acrylates, ethylene butyl acry-
lates, very low density polyethylenes, ultra low density
polyethylenes, Tafmers, as well as metallocene catalyzed
ethylene alpha-olefins of lower densities. Generally speak-
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ing, most resins suitable f or use in the seal layer will
also serve appropriately as tie layer resins. A preferred
tie resin is a high vinyl acetate BVA which promotes adhe-
sion between the various layers of the film structure.
The following examples are intended to illustrate the
pref erred embodiments of the invention.
a v T rm r c~ ~
Sample films were prepared by coextrusion, i.e., all
of the layers are extruded at once. The polymer melt from
the extrusion dies was cooled and cast into solid sheets
having a thickness of 7.46 mils.
P.P. EVA P.P. EVA P.P. EVA LLDPE
22% 12% 7% 6% 7% 8% 38%
P.P. - Quantum Petrothene Polypropylene Homopolymer
PP 2004-MR
EVA - Exxon Ethylene Vinyl Acetate LD 720.92 19% V.A.
LLDPE= Dow DowleX Linear Low Density Polyethylene
2044A
One of the sheets was tested for oxygen permeability on an
OX-TRANS Ten-,Fifty Oxygen Permeability Tester (Test Method
E-160) and the results for three c~~ts were 1061.8; 1077.4;
and 1037.0 cc/m2/24 hr.-atm, respectively.
8
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TABLE I
LONGITUDINAL
Specimen Stress at Strain at Modulus
Number ~ Max. Load Max. Load
( pSl ) ( >o ) ( PSIXI000
)
1 4017.5 942.0 34.128
2 405.9 941.5 29.417
3 4020.3 942.0 42.016
Mean 4031.2 941.8 35.187
TRANSVERSE
Specimen Stress at Strain at Modulus
Number Max. Load Max. Load
(psi) (%) (PSIX1000).
1 3236.9 942.0 41.791
2 3407.0 942.0 43.214
3 3029.8 942.0 42.251
Mean 3224.6 942.0 42.418
'L~ V T M'i~T L~ 7
Another sample film was prepared by blending 93% of
LLDPE (DowleXM2044A) and 15% ULDPE (AttaneM4201) with about
2% of a master batch concentrate containing slip and
antiblcck additives. The antiblock master batch included
AmpacetTM (10853). This heat sealing layer was coextruded
into a film structure containing alternating layers of
ethylene vinyl acetate and ethylene-~olypropylene copolymer.
Films having the following formulation were cast into
sheets having'a thickness of 3.0, 3.5, 5.0 and 11 mils.
9$% P.P. EVA EPC EVA EPC EVA 15o ULDPE
2% Amp.l 83% LLDPE
2% Amp.2
12% 10% 13% 70 13% 7% 33%
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P.P. - Exxon Escorene Polr~ropylene Homopolymer
PD 3345-88
EVA - Exxon Ethylene Vi:~.rl Acetate LD 720.92
19% V.A.
LLDPE - Dow Dowle.~MLinear Low Density Polyethylene
2044A
Amp.l - Ampacet~ PP based slip masterbatch
AmpacetM 40604
Amp.2 - AmpacetTMLLDPE based masterbarch / Ampacet~'
10853
EPC - Exxon Escorene PD9302 3.3% Ethylene 3.8
M.F.
ULDPE - Dow Attane 4201 - 1.0 M.F. 0.912 Density
A sheet of 3.0 and 3.5 mil film was tested for oxygen perme-
ability on an OX-TRANS Ten-Fifty Cxygen Permeability Tester
(Test Method E-160). The resultant transmission rates
shown below in Table II are not as high as would be expect-
ed, especially in view of the data given on Example 1 for a
7.46 mil film. It is believed that the Permeability Tester
employed is somewhat inaccurate at higher permeability.
TABLE II
Oz Transmission
Rate (cc/mZ/24hr-atm)
SAMPLE 1st Cut 2nd Cu'. 3rd Cut
3.0 mil 1776 1876 1965
3.5 mil 1779 1799 1889
Samples of the 3.0 mil, 3.5 m=1 and 5.0 mil films were
tested for tensile strength, eloncation and modulus. The
results are shown in the tables below.
X092968
TABLE III
LONGITUDINAL'-
SpecimenStress Strain at Modulus
at
Number Max. Load Max. Load
(psi) (%) (PSIX1000)
1 5613.0 825.0 31.732
2 6052.8 868.0 28.928
3 6271.0 840. 30.373
Mean 5978.9 844.3 30.345
'- Mean thickness was 2.793 mils.
TRANSVERSE2
SpecimenStress at Strain Modulus
at
Number Max. Load Max.
Load
(psi) (%) (PSIX1000)
1 3643.9 941.5 30.595
2 3548.1 941.5 25.263
3 3917.8 94 1.5 29.452
Mean 3703.3 941 .5 28.437
2Mean thickness was 2.940 mils.
TABLE IV
LONGITUDINAL1
Specimen Stress at Strain Modulus
at
Number Max. Load Max.
Load
(psi) ($) (PSIX1000)
1 6315.6 941.5 33.297
2 6101.6 941.5 32.113
3 6071.0 891.0 39.997
Mean 6162.0 924.7 35.136
1 Mean mils.
thickness
was 3.68
TRANSVERSE=
Specimen Stress at Strain Modulus
at
Number Max. Load Max.
Load
(psi) (%) (PSIX1000)
1 3548.6 941.5 35.332
2 3310.8 941.5 28.852
3 3559.3 941.5 25.209
Mean 350 .3 941.5 29.798
Mean thickness was 5.120 mils.
5/920821.5/SPECFLDR
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2092968
LONGITUDINAL)
TABLE V
Specimen Stress Strain Modulus
at at
Number Max. Max. Load
Load
(psi) (%) (PSIX1000)
1 3204.4 751.5 26.051
2 3303.6 751.5 29.232
3 4573.1 942.0 30.113
_
Mean 3703.7 815.0 28.455
1 Mean thickness
was 5.120
mils.
TRANSVERSE2
Specimen Stress Strain Modulus
at at
Number Max. Max. Load
Load
(psi) (%) (PSIX1000) '
1 3491.8 941.5 35.107
2 3395.6 941.5 33.745
3 3484.1 942.5 29.576
Mean 3457.1 941.8 32.809
1 Mean thickness mils.
was 4.980
Although illustrated embodimentsof the invention have
been described hereinabove, it is to be under-
in detail
stood that is not ited to those precise
the invention lim
embodiments, and modifications may
and that various
changes
be readily rdinary skill without
effected by
persons of
o
department of the invention and
from the spirit
or scope
being set forth followinglaims. For example,
in the c
although it preferred
is generally that
film
structures
in
accordance are coextruded, films
with the present
invention
which are laminated as by sive lamination, heat
such adhe
and pressure also within the scope
or corona
lamination
are
of the present
invention.
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