Note: Descriptions are shown in the official language in which they were submitted.
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HIGH MODULUS OXYGEN-PERMEABLE MULTILAYER
FILM, PACKAGING PROCESS USING SAME, AND
PACKAGED PRODUCT COMPRISING SAME
10
Field of the Invention
The present invention relates to multilayer films, particularly multilayer
films suitable
for use as packaging films. The present invention is particularly directed to
multilayer films
and packaged products using same, wherein the film has a relatively high
modulus and a
relatively high Ortransn-~ission rate. The present invention is also directed
to the use of such
films in the packaging of OZ-sensitive products, such as lettuce, etc.
Background of the Invention
Multilayer films have been utilized for the packaging of "oxygen-sensitive
products",
2 0 such as lettuce, l. e., products which exhibit lower shelf life in the
presence of either too much
oxygen in the package, or too little oxygen in the package. In such multilayer
films, the 02-
transmission rate, and even the COZ-transmission rate, are of primary
importance, especially
in the packaging of such Oz-sensitive products as vegetables, fiwits, and
cheese. For
example, in the packaging of precut lettuce, the presence of too much 02 in
the package
2 5 results in an enzymatic browning of cut surfaces, known as pink ribbing.
On the other hand,
if the concentration of OZ in the package is too low, the lettuce tends to
spoil due to
anaerobiosis.
One of the commercially-available multilayer films which has been used in the
packaging of oxygen-sensitive products has an outer heat-resistant layer of an
elastomer,
3 0 such a styrene-butadiene copolymer, and an outei sealant layer of a
metallocene-catalyzed
ethylenelalpha-olefin copolymer. Although this multilayer film exhibits
desired OZ and COz
transmission rates, as well as a desirable stiffness, this multilayer film has
been found to
exhibit an undesirable level of "curl," thereby exhibiting less-than-desired
machinability
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performance. In form-fill-and-seal machinery ("FFS" machinery), curl causes
film threadup
problems as well as film tracking problems. Furthermore, in FFS packaging, the
presence of
the outer heat-resistant elastomer layer restricts package formation to a fin-
type backseal, as
the metallocene-catalyzed outer film layer does not seal well to the heat-
resistant elastomer
layer.
It has also been found that there is a pervasive belief among many skilled in
the
packaging of a variety of products that a heat seal cannot be made using a
film having both
outer layers of a low melt point polyethylene-based polymer, i.e., including
both polyethylene
homopolymer as well as ethylene/alpha-olefin copolymers, as it is believed
that the hot bar
will tend to stick to the film during sealing.
it would be desirable to provide a film which exhibits desirable 02 and CO~
transmission rates and a desirable sti$ness, while reducing or eliminating the
curl of the film,
as well as providing a film which is suitable to a lap-type backseal for FFS
packaging, using
constant heat as opposed to impulse heating. Since fogging of the package is
also a common
problem in the packaging of produce and other food products, it would also be
desirable that
the film resist fogging, in order to provide the consumer with a clear view of
the contents of
the package, and in order to provide a more aesthetically appealing package,
especially in
retail applications where product presentation is important. However, those
surface active
agents which are effective antifog agents tend to interfere with ink adhesion
to the film. This
2 0 detrimental effect occurs because the antifog agent blooms to the outside
surface of the
package and interferes with the adhesion of the ink to the film. This
detriment is significant
for packages designed for consumer end use, as the consumer does not find such
a package
to be appealing if the ink is smeared or if the ink comes off onto other
articles or the
consumer. Thus, it would be also desirable to provide the filin with an
antifog agent on an
2 5 outer film surface which forms the inside surface of the package, while
also providing
adequate adhesion for printing on an outer sur.Face of the film, which outer
surface serves as
the outside surface of the package.
Summary of the Invention
3 0 It has been discovered that curl can be eliminated by providing a film
with outer
layers of ethylene/alpha-olefin copolymer and an inner layer of elastomer,
such as
styreneJbutadiene copolymer. Furthermore, by providing both outer layers of
the film with
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ethylenelalpha-olefin copolymer, a lap-type backseal can be used in FFS type
packaging. It
has also been discovered that the homogeneous ethyIene/alpha-olefin copolymers
(e.g.,
metalIocene-catalyzed ethylene/alpha-olefin copolymers) can be provided on one
or both
outer film layers, in order to provide the film with further enhanced sealing
characteristics.
,, 5 As a first aspect, the present invention pertains to a multilayer film
comprising a first
outer layer, a second outer layer, and an inner layer. The first outer layer
comprises a first
ethylene/alpha olefin copolymer, which, in turn, comprises a homogeneous
copolymer. The
second outer layer comprises a second ethylene/alpha-olefin copolymer. The
inner layer
comprises a thermoplastic elastomer. The inner layer is between the first
outer layer and the
second outer layer, and the inner layer is chemically difrerent from the first
outer layer and
the second outer layer. The multilayer film has an OZ-transmission rate of
from about 500 to
50,000 cc/m2 24hr STP; more preferably, from 1,000 to 20,000 cc/m2 24 hr STP;
still more
preferably, from about 2,000 to 10,000 cc/m2 24 hr STP. The multilayer film
has a modulus
of at least 60,000 psi; more preferably, from about 60,000 to I50,000 psi;
still more
preferably, from about 70,000 to 120,000 psi; and yet still more preferably,
from about
80,000 to 100,000 psi.
Preferably, the first outer layer further comprises a surface-active agent
component
comprising at least one member selected from the group consisting of ester of
aliphatic
alcohol, polyether, polyhydric alcohol, ester of polyhydric aliphatic alcohol,
and
2 0 polyethoxylated aromatic alcohol, wherein the surface-active component is
present over the
entire outside surface of the first outer layer.
Preferably, the first outer layer has a thickness of from about 0.3 to 0.8
mil, the inner
layer has a thickness of from about 0. l to 1 mil, the second outer layer has
a thickness of
from about 0.3 to 0.8 mil, and the multilayer film has a total thickness of
from about 1 to 3
2 5 mils. More preferably, the inner layer has a thickness of from about 0.4
to 0.8 mil.
It has been found that the curl problem can be reduced or eliminated by
providing a
film having a symmetrical cross-section. That is, by providing a multilayer
film having a
cross section which is symmetrical in terms of layer arrangement, layer
thickness, and layer
chemical composition, curl is reduced or eliminated. Of course, some
variation, i.e., lack of
3 0 symmetry, can be present, while still substantially reducing or
eliminating the amount of curl
which would otherwise result without substantial symmetry. For example, one
outer layer
can comprise a homogeneous ethylene/alpha-olefin copolymer while the other
outer layer
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comprises a heterogeneous ethylene/alpha-olefin copolymer. One outer layer may
also
contain an antifog agent, while the other layer does not. Such minor
variations in the cross-
sectional symmetry of the filin have not been found to have a particularly
detrimental erect
on the curl level exhibited by the filin. More severe variations are also
possible, while
obtaining a relatively low curl level. However, the presence of only two
layers, such as a first
layer comprising an ethylenelalpha-olefin copolymer and a second layer
comprising a
styrene/butadiene elastomeric copolymer, has been' found to exhibit
substantial curl, and it is
believed that it is the lack of symmetry of such a film which is the cause of
the undesirable
level of curl.
Preferably, the homogeneous ethylene/alpha-olefin copolymer in the first outer
layer
is a first homogeneous ethylene/aipha-olefin copolymer, and the second outer
layer
comprises a second homogeneous ethylenelalpha-olefin copolymer. More
preferably, the
first homogeneous ethylene/alpha-olefin copolymer has a density of less than
about 0.915
grams per cubic centimeter, and the second homogeneous ethylene/alpha-olefin
copolymer
has a density of less than about 0.915 grams per cubic centimeter. Still more
preferably,
the first homogeneous ethylene/alpha-olefin copolymer comprises ethylene mer
in an
amount of from about 99 to 80 weight percent ethylene, based on copolymer
weight, and a
first alpha-olefin mer in an amount of from about 1 to 20 weight percent,
based on
copolymer weight, wherein the first alpha-olefin mer comprises at least one
member selected
2 0 from the group consisting of Ca, Cs, and Cs; and, the second homogeneous
ethylene/alpha-
olefin copolymer comprises ethylene mer in an amount of from about 99 to 80
weight
percent ethylene, based on copolymer weight, and a second alpha-olefin mer in
an amount of
from about 1 to 20 weight percent, based on copolymer weight, and wherein the
second
alpha-olefin mer comprises at least one member selected from the group
consisting of G4, C~,
2 5 and Cx. Still more preferably,
the first homogeneous ethylene/alpha-olefin copolymer comprises ethylene mer
in an amount
of from about 95 to 85 weight percent, based on copolymer weight, and a first
alpha-olefin
mer in an amount of from about 5 to 15 weight percent, based on copolymer
weight,
wherein the first alpha-olefin mer comprises at least one member selected from
the group
3 0 consisting of Cs, and a blend of C~ and Ca; and the second homogeneous
ethylene alpha-
olefin copolymer comprises ethylene mer in an amount of from about 95 to 85
weight
percent, based on copolymer weight, and a second alpha-olefin mer in an amount
of from
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about 5 to 15 weight percent, based on copolymer weight, wherein the second
alpha-olefin
mer comprises at least one member selected from the group consisting of Cs and
a blend of
Cs and C4; and the multilayer film has an o2-transmission rate of from about
2,000 to 10,000
cc/mz/ 24hr STP.
. 5 Preferably, the inner layer comprises styrene-containing polymer. The
inner layer can
comprise a blend of styrene homopolymer and styrenelbutadiene block copolymer.
Preferably, styrene homopolymer is present in an amount of from about 5 to 50
weight
percent, based on layer weight; more preferably, from about 10 to 30 weight
percent; still
more preferably, from about 10 to 20 weight percent; preferably, the balance
of the inner
layer is styrene/butadiene block copolymer.
The inner layer can be a first inner layer, with the film further comprising a
second
inner layer and a third inner layer. The second inner layer comprises a first
polyoIefin and the
third inner layer comprises a second polyolefin, with the first inner layer
being between the
second inner layer and the third inner layer. Preferably, the first polyolefin
comprises at least
one member selected fi-om the group consisting of propylene homopolymer, and
propyleneJethylene copolymer containing ethylene mer in an amount of from
about 0.1 to 6
weight percent; preferably, the second polyolefin comprises at least one
member selected
from the group consisting of propylene homopolymer, and propylenelethylene
copolymer
containing ethylene mer in an amount of from about 0.1 to 6 weight percent.
2 0 Preferably, the inner layer is a first inner layer comprising a first
styrene-containing
polymer, with the multilayer film further comprising a second inner layer
comprising
polyolefin, and a third inner layer comprising a second styrene-containing
polymer.
Preferably, the second inner layer of polyolefin is between the first inner
layer and the third
inner layer; preferably, the polyolefin comprises at least one member of the
group consisting
2 5 of propylene homopolymer, and propylene/ethylene copolymer containing
ethylene mer in an
amount of from about 0.1 to 6 weight.
Preferably, the styrene-containing polymer comprises styrene/butadiene
copolymer.
' Preferably, the styrene/butadiene copolymer comprises styrene/butadiene
block copolymer
comprising butadiene mer in an amount of from about 20 to 40 weight percent,
based on
3 0 block copolymer weight.
As a second aspect, the present invention pertains to a packaged product
comprising
a product packaged in, i.e., surrounded by, a package comprising a multilayer
film according
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to the present invention. The product comprises at least one member selected
from the
group consisting of lettuce, cabbage, broccoli, green beans, cauliflower,
spinach, kale, carrot,
onion, pepper, corn, radish, endive, chard, chicory, radicchio, greens, peas,
squash, escarole,
brussel sprout, mushroom, melon and berry. Preferably, the muitilayer filin is
a preferred
multiiayer film according to the present invention. In the packaged product
according to the
present invention, preferably the multilayer film has a tear notch, so that
the package is an
easy-open package. Preferably, the package is a sealed pouch having two end
seals and a
backseal connecting the two end seals. The backseal can be a fin seal or a lap
seal.
The 02-sensitive product preferably comprises a cut vegetable or fiuit
comprising at
least one member selected from the group consisting of lettuce, cabbage,
broccoli, green
beans, cauliflower, spinach, kale, carrot, onion, pepper, corn, radish,
endive, chard, chicory,
radicchio, greens, peas, squash, escarole, brussel sprout, mushroom, melon and
berry.
Preferably, the Oz-sensitive product comprises a cut vegetable comprising at
least one
member selected from the group consisting of lettuce, cabbage, broccoli,
cauliflower, kale,
carrot, onion, radish, endive, chard, chicory, radicchio, and escarole, and
the film has an 02-
transmission rate of from about 3,000 to 8,000 ccJm2/ 24hr STP.
As a third aspect, the present invention is directed to a packaging process
for
packaging an 02-sensitive product. The process comprises the steps of (A)
forwarding a
supply of a multilayer film into a vertical form fill and seal apparatus; (B)
passing the film
2 0 over a collar member of the vertical form fill and seal apparatus, so that
substantially
vertically-oriented edge portions of the film are adjacent one another; (C)
forming a
longitudinal seal along at least a segment of the adjacent edge portions of
the film, to form a
sealed tube segment; (D) collapsing a lower end portion of the sealed tube
segment; (E)
forming a bottom package seal across the collapsed lower end portion of the
sealed tube
2 5 segment, to form a pouch; (F) adding an appropriate quantity of the 02-
sensitive product to
the pouch; (G) collapsing an upper end portion of the pouch; and (H) forming a
top package
seal across the collapsed upper end portion to form a sealed pouch containing
the oxygen-
sensitive product, whereby a package is formed. The multilayer film is a film
according to
the multilayer filin of the present invention, preferably, a preferred
multilayer film according
3 0 to the present invention. The product is an 02-sensitive product as in the
packaged product
according to the present invention. Preferably, the vertical form fill and
seal machine forms,
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fills, and seals at least 15 packages per minute, without substantial burn
through of the film at
the seals. Preferably, the film is sealed at a temperature of from about
70°C to 150°C.
Brief Descri,~tion of the Drawings
Figure 1 illustrates an enlarged cross-sectional view of a three-layer
multilayer film of
the present invention.
Figure 2 illustrates an enlarged cross-sectional view of a five-layer
multilayer film
according to the present invention.
Figure 3 illustrates a schematic view of a process according to the present
invention.
2 0 Figure 4 illustrates a vertical form fill and seal apparatus to be used in
packaging
process according to the present invention.
Figure S illustrates a packaged product of the present invention, the product
being
packaged in the multilayer film of the present invention.
Detailed Description of the Invention
As used herein, the term "monomer" refers to a relatively simple compound,
usually
containing carbon and of low molecular weight, which can react to form a
polymer by
combination with itself or with other similar molecules or compounds.
As used herein, the term "comonomer" refers to a monomer which is
copolymerized
2 0 with at Least one different monomer in a copolymerization reaction, the
result of which is a
copolymer.
As used herein, the term "polymer" refers to the product of a polymerization
reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc.
As used herein, the term "homopolymer" is used with reference to a polymer
2 5 resulting from the polymerization of a single monomer, i.e., a polymer
consisting essentially
of a single type of repeating unit.
As used herein, the term "copolymer" refers to polymers formed by the
polymerization reaction of at least two different monomers. For example, the
term
"copolymer" includes the eopolymerization reaction product of ethylene and an
alpha-olefin.,
a 3 0 such as 1-hexene. However, the term "copolymer" is also inclusive of,
for example, the
copolymerization of a mixture of ethylene, propylene, I-hexene, and I-octene.
7
CA 02245678 2004-O1-14
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As used herein, the term "copolymerization" refers to the simultaneous
polymerization of two or more monomers.
As used herein, terminology employing a "/" with respect to the chemical
identity of a
copolymer (e.g., "an ethylene/alpha olefin copolymer"), identifies the
comonomers which are
copolymerized to produce the copolymer. This terminology, as used herein,
refers to the
primary comonomer first, followed by the secondary comonomer. The
copolymerization is
preferably carned out in the presence of more (on a weight percent basis) of
the primary
comonomer than the secondary comonomer.
As used herein, the phrase "heterogeneous polymer" refers to polymerization
reaction products of relatively wide variation in molecular weight and
relatively wide
variation in composition distribution, i.e., polymers made, for example, using
conventional
Ziegler-Natta catalysts. Such polymers typically contain a relatively wide
variety of chain
len~,rths and comonomer percentages.
As used herein, the phrase "heterogeneous catalyst" refers to a catalyst
suitable for
use in the polymerization of heterogeneous polymers, as defined above.
Heterogeneous
cataaysts are comprised of several kinds of active sites which differ in Lewis
acidity and steric
emrironment. Ziegler-Natta catalysts are heterogeneous catalysts. Examples of
Ziegler
Natta heterogeneous systems include metal halides activated by an
organometallic co
catalyst, such as titanium chloride, optionally containing magnesium chloride,
complexed to
2 0 trialkyl aluminum and may be found in patents such as U.S. Patent No.
4,302,565, to
GOEKE, et. al., and U.S. Patent No. 4,302,566, to KAROL, et. al.:
As used herein, the phrase "homogeneous polymer" refers to polymerization
reaction
products of relatively narrow molecular weight distribution and relatively
narrow
2 5 composition distribution. Homogeneous polymers are useful in various
layers of the
multilayer film used in the present invention. Homogeneous polymers exhibit a
relatively
even sequencing of comonomers within a chain, the mirroring of sequence
distribution in all
chains, and the similarity of length of all chains, and are typically prepared
using metallocene,
or other single-site type catalysis.
3 0 More particularly, homogeneous ethylene/alpha-olefin copolymers may be
characterized by one or more methods known to those of skill in the art, such
as molecular
weight distribution (M,yiM"), composition distribution breadth index (CDBI),
and narrow
8
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tne;lting point range and single melt point behavior. The molecular weight
distribution
(NI",/M"), also known as polydispersity, may be determined by gel permeation
chromatography. The homogeneous ethylene/alpha-olefin copolymers useful in
this
invention will have a (M,y/M;,) of less than 2.7. Preferably, the (M"!M") will
have a range of
about 1.9 to 2.5. More preferably, the (M,~/M") will have a range of about 1.9
to 2.3. The
composition distribution breadth index (CDBI) of such homogeneous
ethyIene/alpha-olefin
copolymers will generally be greater than about 70 percent. The CDBI is
defined as the
weight percent of the copolymer molecules having a comonomer content within 50
percent
(l. e., plus or minus 50%) of the median total molar comonomer content. The
CDBI of linear
polyethylene, which does not contain a comonomer, is defined to be 100%. The
Composition Distribution Breadth Index (CDBI) is determined via the technique
of
Temperature Rising Elution Fractionation (TREF). CDBI determination clearly
distinguishes
the homogeneous copolymers used in the present invention (narrow composition
distribution
as assessed by CDBI values generally above 70%) from heterogeneous polymers
such as
VLDPEs available commercially which generally have a broad composition
distribution as
assessed by CDBI values generally less than 55%. The CDBI of a copolymer is
readily
calculated from data obtained from techniques known in the art, such as, for
example,
temperature rising elution fractionation as described, for example, in Wild
et. al., J. Poly. Sci.
Poly. Phys. Ed., Vol. 20, p.441 ( 1982). Preferably, the homogeneous
ethylene/alpha-olefin
2 0 copolymers have a CDBI greater than about 70%, i.e., a CDBI of from about
70% to 99%.
In general, the homogeneous ethylene/alpha-olefin copolymers in the multilayer
films of the
present invention also exhibit a relaitively narrow melting point range, in
comparison with
"heterogeneous copolymers", i.e., polymers having a CDBI of less than 55%.
Preferably, the
homogeneous ethylene/alpha-olefin copolymers exhibit an essentially singular
melting point
2 5 characteristic, with a peak melting point (Tm), as determined by
Difi'erential Scantling
Colorimetry (DSC), of;from about 60°C to 110°C. Preferably, the
homogeneous copolymer
has a DSC peak T," of from about 90°C to 110°C. As used herein,
the phrase "essentially
sing;Ie melting point" means that at least about 80%, by weight, of the
material corresponds
to a single Tm peak at a temperature within the range of from about
60°C to 1 I O°C, and
3 0 essentially no substantial fraction of the material has a peak melting
point in excess of about
1 l 5°C., as determined by DSC analysis. DSC measurements are made on a
Perkin Elmer ~'
System 7 Thermal Analysis System. Melting information reported are second
melting data,
*Trade-mark
9
CA 02245678 2004-O1-14
64536-951
i. e., the sample is heated at a programmed rate of 10°C./min. to a
temperature below its
critical range. The sample is then reheated (2nd melting) at a programmed rate
of 10°C/min.
A homogeneous ethylene/alpha-olefin copolymer can, in general, be prepared by
the
copolymerization of ethylene and any one or more alpha-olefin. Preferably, the
alpha-olefin
is a C3-CZO a-monoolefin, more preferably, a Ca-C,2 a-monoolefin, more
preferably, a Ca-Cg
a-monoolefin. Still more preferably, the alpha-olefin comprises at least one
member selected
from the group consisting of butene-1, hexene-1, and octene-1, i.e., l-butene,
1-hexene, and
1-octene, respectively. Yet still most preferably, the alpha-olefin comprises
octene-1, and/or
a blend of hexene-1 and butene-1.
Processes for preparing homogeneous polymers are disclosed in U.S. Patent No.
5,206,075, U.S. Patent No. 5,241,031, and PCT International Application WO
93/03093.
Further details regarding the production and use of one
species of homogeneous ethylene/alpha-olefin copolymers is
disclosed in U.S. Patent No. 5,206,075, to HODGSON, Jr.;
U.S. Patent No. 5,241,031, to MEHTA; PCT International
Publication Number WO 93/03093, in the name of Exxon
Chemical Company; PCT International Publication Number WO
90,/03414, in the name of Exxon Chemical Patents, Inc.
Still another species of homogeneous ethylene/alpha-olefin
copolymers, generally referred to as long-chain-branched
0 homogeneous ethylene/alpha-olefin copolymers, is disclosed
in U.S. Patent No. 5,272,236, to LAI, et al., and U.S.
Patent No. 5,278,272, to LAI, et al.
As used herein, the term "polyolefin" refers to any polymerized olefin, which
can be
linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted.
2 5 As used herein, the phrases "inner layer" and "internal layer" refer to
any film having
its two principal surfaces with other layers of the multilayer film.
As used herein, the phrase "outer layer" refers to any film layer, of a
multilayer film,
having only one of its principal surfaces directly adhered to another layer of
the film.
As used herein, the phrase "directly adhered", as applied to film layers, is
defined as
3 0 adhesion of the subject film layer to the object film layer, without a tie
layer, adhesive, or
other layer therebetween. In contrast, as used herein, the word "between", as
applied to a
film layer expressed as being between two other specified layers, includes
both direct
CA 02245678 1998-08-06
WO 97/28964 PC~'IUS97/00694
adherence of the subject layer between to the two other layers it is between,
as well as
including a lack of direct adherence to either or both of the two other layers
the subject layer
is between, i.e., one or more additional layers can be imposed between the
subject Layer and
one or more of the layers the subject layer is between.
As used herein, the term "core", and the phrase "core layer", as applied to
multilayer
films, refer to any internal film layer which has a primary fixnction other
than serving as an
adhesive or compatibiIizer for adhering two layers to one another. Usually,
the core layer or
layers provide the multiIayer film with a desired level of strength, i.e.,
modulus.
As used herein, the phrase "sealant layer", with respect to multilayer films,
refers to
an outer film layer which is involved in the sealing of the film to itself or
another layer.
Although the phrase "sealant layer" as herein used refers only to outer film
layers, no matter
how thin, it should also be recognized that in general, the outer 0.5 mil to
1.0 mil of a film is
involved in the leafing of the film to itself or another layer. With respect
to packages having
only fin-type seals, as opposed to lap seals, the phrase "sealant layer"
generally refers to the
inside film layer of a package, as well as supporting layers adjacent this
sealant layer often
being sealed to itself, and frequently serving as a food contact layer in the
packaging of
foods.
As used herein, the phrase "tie layer" refers to any internal layer having the
primary
purpose of adhering two layers to one another.
2 0 As used herein, the term "lamination", and the phrase "laminated film",
refer to the
process, and resulting product, made by bonding together two or more layers of
film or other
materials. Lamination can be accomplished by joining layers with adhesives,
joining with
heat and pressure, and even spread coating and extrusion coating. Multilayer
films can be
made via coextrusion and/or lamination.
2 5 As used herein, the term "extrusion" is used with reference to the process
of forming
continuous shapes by forcing a molten plastic material through a die, followed
by cooling or
chemical hardening. Immediately prior to extrusion through the die, the
relatively high-
viscosity polymeric material is fed into a rotating screw of variable pitch,
which forces it
through the die.
3 0 As used herein, the term "coextrusion" refers to the process of eluding
two or
mare materials through a single die with two or more orifices arranged so that
the extrudates
merge and weld together into a laminar structure before chilling, i.e.,
quenching.
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Coextrusion can be employed in film blowing, free film extrusion, and
extrusion coating
processes.
As used herein, the phrase "machine direction", herein abbreviated "1V>D",
refers to a
direction "along the length" of the film, i.e., in the direction of the film
as the filin is formed
during extrusion andlor coating.
As used herein, the phrase "transverse direction", herein abbreviated "TD",
refers to
a direction across the film, perpendicular to the machine or longitudinal
direction.
As used herein, "02-transmission rate", also referred to as "OTR" and "oxygen
permeability", is measured according to ASTM D 3985, a test known to those of
skill in the
film art.
As used herein, the "melt index" of a polymer is the amount, in grams, of a
thermoplastic resin which can be forced through an orifice of 0.0825 inch
diameter when
subjected to a force of 2160 grams in ten minutes at a specified temperature,
e.g., 190°C for
many polymers. The test is performed by an extrusion rheometer described in
ASTM D
1238.
Figure 1 illustrates a cross-sectional view of a preferred embodiment of
multiIayer
film 10 of the present invention. The film comprises first layer 11 (an outer
layer), second
layer 12 (a core layer), and third layer 13 (also an outer layer). Preferably,
the first and third
layers 11 and 13 are designed to serve as sealing layers, i.e, comprise a
polymer suitable for
2 0 farming a seal via the application of heat or radiation, as is known to
those of skill in the art.
The film of the present invention comprises at least 3 layers. The two outer
layers
fiznction as sealing layers, while the sole core layer, or at least one of a
plurality of inner
layers, provides the multilayer film with a desired tensile properties, while
permitting a
desired level of transmission of oxygen and carbon dioxide therethrough.
Preferably, the film
2 5 comprises from 3 to I 5 layers, and more preferably, from 3 to 7 layers,
and still more
preferably, from 3 to 5 layers. Preferably, the outer layers, i.e., the first
and third layers, are
of substantially identical chemical composition and are of substantially
identical thickness. In
general, the care Iayer should be at least as thick as each of the outer
layers, and preferably
the core layer is thicker than either of the outer layers.
3 0 Although the multilayer film of the present invention can have any total
thickness
which provides a desired rate of oxygen and carbon dioxide transmission, abuse
resistance,
tensile strength, etc., preferably, the multilayer film of the present
invention has a total
12
CA 02245678 1998-08-06
WO 97!28964 PCT/US97/00694
thickness {i.e., a combined thickness of all layers), of from about 0.5 to 10
mils { 1 mil equals
0.001 inch); more preferably, from about 1 to 5 mils; still more preferably,
from 1 to 3 mils;
yet still more preferably, from about 1 to 2.5 mils. Most preferably, the
multilayer film has a
thickness of from about 1.5 to 2 mils.
Preferably, in the multilayer film of the present invention, the two outer
layers each
make up from about 10 to 80 weight percent of the total weight of the
multilayer film.
Furthermore, the second, or core layer, can also make up from about 10 to 80
weight
percent of the total weight of the film. More preferably, the two outer layers
each make up
from about 10 to 40 weight percent of the total weight of the multilayer film,
and preferably
1 o the core layer makes up from about 20 to 80 weight percent of the total
weight of the
multilayer film.
Preferably, the outer film layers each have a thickness of from about 0.05 to
4 mils;
more preferably, from about 0.1 mil to 2 mils; still more preferably, from
about 0.1 mil to 1.2
mils; yet still more preferably, from about 0.3 mil to 0.8 mil; even yet still
more preferably,
from about 0.4 to 0.5 mil.
Preferably, the inner layer (or each of a plurality of inner layers) has a
thickness of
from about 0.1 mil to 4 mils; more preferably, from about 0.2 mil to 2 mils;
still more
preferably, from about 0.5 mil to 1 mil.
Preferably, the multilayer film of the present invention has an O2-
transmission rate of
2 0 from about 500 to 50,000 cc/m2/ 24hr STP; more preferably, from about
1,000 to 20,000
cc,/m2124hr STP; still more preferably, fi-om about 2,000 to 10,000 cc/m2/
24hr STP; even
yet still more preferably, from about 3,000 to 8,000 cclm2/ 24hr STP.
Preferably, at least one of the outer film layers comprises a homogeneous
ethylenelalpha-olefin copolymer which permits the multilayer film to have an
02-transmission
rate of from about 500 to 50,000 cc/m2/ 24hr STP. Although the outer film
layers, i.e.,
layers 10 and 13 in the preferred embodiment illustrated in Figure 1, can have
the same or
differing chemical composition, preferably the outer layers comprise
substantially identical
ethylenelalpha olefin copolymer, more preferably, each comprises a homogeneous
ethylenelalpha-olefin copolymer substantially identical to the other.
Preferably, the
3 0 ethylene/alpha-olefin copolymer in the outer layers has a density of less
than or equal to
about 0.915 glee, i.e., up to and including about 0.915 g/cc. Preferably, an
antifog agent is
present on the outer surface of an outer film layer which later becomes the
inside layer of the
13
CA 02245678 1998-08-06
WO 97/28964 PCT/US97/00694
package. If the package is printed, preferably the printing is on the outer
surface of the other
outer film layer which forms the outside layer of the package.
Homogeneous ethylene/alpha-olefin copolymers may be characterized by one or _
more methods known to those of skill in the art, such as molecular weight
distribution
(M~,JM"), composition distribution breadth index {CDBI), and narrow melting
point range ,
and single melt point behavior. The molecular weight distribution (MwINI"),
also known as
polydispersity, may be determined by gel permeation chromatography. The
homogeneous
ethylenelalpha-olefin copolymers useful in this invention will have a (M,uJM")
of less than 2.7.
Preferably, the (M"JM") will have a range of about 1.9 to 2.5. More
preferably, the (MW/M")
will have a range of about 1.9 to 2.3.
The composition distribution breadth index (CDBI) of such homogeneous
ethylenelalpha-olefin copolymers will generally be greater than about 70
percent. The CDBI
is defined as the weight percent of the copolymer molecules having a comonomer
content
within SO percent (i.e., plus or minus SO%) of the median total molar
comonomer content.
The CDBI of linear polyethylene, which does not contain a comonomer, is
defined to be
100%.
The Composition Distribution Breadth Index (CDBI) is determined via the
technique
of Temperature Rising Elution Fractionation {TREE). CDBI determination clearly
distinguishes the homogeneous copolymers used in the present invention {narrow
2 0 composition distribution as assessed by CDBI values generally above 70%)
from
commercially available heterogeneous polymers commonly referred to as VLDPEs,
which
differ from homogeneous polymer in having a broad composition distribution as
assessed by
CDBI values generally less than 55%. The CDBI of a copolymer is readily
calculated from
data obtained from techniques known in the art, such as, for example,
temperature rising
2 5 elution fractionation as described, for example, in Wild et. al., J. Poly.
Sci. Poly. Phys Ed
Voi. 20, p.44-1 ( 1982). Preferably, the homogeneous ethylene/alpha-olefin
copolymers to be
used in the film according to the present invention have a CDBI greater than
about 70%, i.e.,
a CDBI of from about 70% to 99%.
In general, the homogeneous ethylenelalpha-olefin copolymers in the multilayer
films
3 0 of the present invention also exhibit a relatively narrow melting point
range, in comparison
with "heterogeneous copolymers", i.e., polymers having a CDBI of less than
55%.
Preferably, the homogeneous ethylene/alpha-olefin copolymers exhibit an
essentially singular
14
CA 02245678 1998-08-06
WO 97fZ8964 PCT/US97/~Off694
melting point characteristic, with a peak melting point (Tm), as determined by
Differential
Scanning Colorimetry (DSC), of from about 60°C to 110°C.
Preferably the homogeneous
copolymer has a DSC peak Tm of from about 85°C to 105°C. As used
herein, the phrase
"essentially single melting point" means that at least about 80%, by weight,
of the material
corresponds to a single Tm peak at a temperature within the range of from
about 60°C to
110°C, and essentially no substantial fraction of the material has a
peak melting point in
excess of about 115°C., as determined by DSC analysis. The presence of
higher melting
peaks is detrimental to film properties such as haze, and compromises the
chances for
meaningful reduction in the seal initiation temperature of the final film. DSC
measurements
l0 are made on a Perkin Elmer System 7 Thermal Analysis System. Melting
information
reported are second melting data, i.e., the sample is heated at a programmed
rate of
10°C.lmin. to a temperature below its critical range. The sample is
then reheated (2nd
melting} at a programmed rate of 10°C/min.
The homogeneous ethyIene/alpha-olefin copolymer in the outer layers can, in
general,
be prepared by the copolymerization of ethylene and any one or more alpha-
olefin.
Preferably, the alpha-olefin is a C3-C~ a-monoolefin, more preferably, a C4-
C12 a-monoolefin,
still more preferably, a Cø-Cg a-monoolefin. Still more preferably, the alpha-
olefin comprises
at least one member selected from the group consisting of butene-I, hexene-1,
and octene-1,
i.e., 1-butene, 1-hexene, and 1-octene, respectively. Most preferably, the
alpha-olefin
2 o comprises octene-1, andlor a blend of hexene-l and butene-1.
In general, the ethylene/alpha-olefin copolymer comprises a copolymer
resulting from
the copolymerization of from about 80 to 99 weight percent ethylene and from 1
to 20
weight percent alpha-olefin . Preferably, the ethylene/alpha-olefin copolymer
comprises a
copolymer resulting from the copolymerization of from about 85 to 95 weight
percent
2 5 ethylene and from 5 to 15 weight percent alpha-olefin.
The outer layers can consist essentially of the homogeneous ethylene/alpha-
olefin
copolymer (or copolymers), or can have still additional polymers blended
therewith.
However, in each of the outer layers, the homogeneous ethylenelalpha-olefin is
preferably
present in an amount of at least about 50 weight percent, based on the weight
of the
3 0 respective outer layer. More preferably, the homogeneous ethylene/alpha-
olefin is present in
an amount of at least about 75 weight percent, based on the weight of the
respective outer
layer. Still more preferably, the homogeneous ethylene/alpha-olefin is present
in an amount
6 4 5 3 6 - 9 51 ~ 02245678 2004-O1-14
of about 100 weight percent, based on the weight of the respective outer
layer. If another
polymer, i.e., a "secondary polymer", is present in admixture with the
homogeneous
ethylenelalpha-olefin copolymer, preferably this secondary polymer comprises
at least one
member selected from the group consisting of polyethylene, ethylene vinyl
acetate, ethylene
methyl acrylate, ethylene butyl acrylate, ethylene methyl acrylic acid,
ionomer, and
ethylene/aipha-olefin.
Preferably, the first film Iayer is directly adhered to a first side of the
second film
layer, and the third film layer is directly adhered to a second side of the
second film layer.
The homogeneous ethylene/alpha-olefin copolymers can be prepared through the
use
of a metallocene catalyst, and/or any additional single site catalyst.
Furthermore, the
homogeneous ethylenelalpha-olefin copolymers can be prepared in accordance
with any
suitable polymerization process, including slurry polymerization, gas phase
polymerization,
and high pressure polymerization processes. U. S. Patent No. 5,206,075, U. S.
Patem No.
5,241,031, and PCT International Application WO 93/03093, di~ose homogeneous
. polymers and methods for making same.
Slurry polymerization processes generally use superatmospheric pressures and
temperatures in the range of 40°-100°C. In a slurry
polymerization, a suspension of solid,
particulate polymer is formed in a liquid polymerization medium to which
ethylene and
2 0 comonomers and often hydrogen along with catalyst are added. The liquid
employed in the
polymerization medium can be an alkane, cycloaIkane; or an aromatic
hydrocarbon such as
toluene, ethylbenzene or xylene. The medium employed should be liquid under
the
conditions of polymerization, and relatively inert. Preferably, hexane or
toluene is employed.
Alternatively, the homogeneous ethylene/alpha-olefin copolymer is prepared by
gas-
2 5 phase polymerization. A gas-phase polymerization process utilizes super-
atmospheric
pressure and temperature in the range of about 50°-120°C. Gas
phase polymerization can be
performed in a stir ed or fluidized bed of catalyst and product particles in a
pressure vessel
adapted to permit the separation of product particles from unreaeted gases.
Ethyiene,
comonomer, hydrogen and an inert diluent gas such as nitrogen can be
introduced or
30 recirculated so as to maintain the particles at temperatures of 50°-
120°C. Triethylaluminum
may be added as needed as a scavenger of water, oxygen, and other impurities.
Polymer
product can be withdrawn continuously or semicontinuously, at a rate such as
to maintain a
1H
CA 02245678 2004-O1-14
64536-951
constant product inventory in the reactor. After polymerization and
deactivation of the
catalyst, the product polymer can be recovered by any suitable means. In
commercial
practice, the polymer product can be recovered directly from the gas phase
reactor, freed of
residual monomer with a nitrogen purge, and used without fiuther deactivation
or catalyst
removal.
The homogeneous ethylene/alpha-olefin copolymer can also be produced by a high
pressure process, in the presence of a catalyst system comprising a
cyclopentadienyl-
transition metal compound and an alumoxane compound. It is important, in the
high-
preasure process, that the polymerization temperature be above about
120°C., but below the
decomposition temperature of the polymer product. It is also important that
the
polymerization pressure be above about 500 bar (kg/cm2). In those situations
wherein the
molecular weight of the polymer product that would be produced at a given set
of operating
conditions is higher than desired, any of the techniques known in the art for
control of
molecular weight, such as the use of hydrogen or reactor temperature, may be
used in the
process of this invention.
Further details regarding the production and use of linear homogeneous
ethylenelalpha-olefin copolymer are disclosed in U.S. Patent No. 5,206,075, to
HODGSON,
Jr.; U.S. Patent No. 5,241,031, to MEHTA; PCT International Publication Number
WO
93103093, in the name of Exxon Chemical Company; PCT Inte7national Publication
Number
2 0 WO 90103414, in the name of Exxon Chemical Patents, Inc. Details of the
production of
substantially linear long chain branched homogeneous ethylene/alpha-olefin
copolymers is
disclosed in U.S. Patent No. 5,272,236, to LAI, et. al., and U.S. Patent No.
5,278,272,
to LAI, et. al.
2 5 Although the multilayer film of the present invention can have a plurality
of inner
layers, including a plurality of tie layers as well as a plurality of core
layers, in one preferred
embodiment, the multilayer film of the present invention has no tie layers,
and only one core
layer as the sole inner layer of the film.
In Figure I, core layer 12 comprises a thermoplastic elastomer. As used
herein, the
30 phrase''thermoplastic elastomer" refers to a family of polymers which
resemble elastomers in
that they are highly resilient and can be repeatedly stretched to at least
twice their initial
lengths with full, rapid recovery, but are true thermoplastics and thus do not
require curing or
17
CA 02245678 2004-O1-14
64536-951
vulcanization as do most robbers. Preferred thermoplastic elastomers for use
in the present
invention include styrene-containing polymers, more preferably
styrenelbutadiene copolymer.
Preferably, the core layer comprises a styrene-containing elastomeric polymer.
Styrene/butadiene block copolymer is a preferred styrene-containing
elastomeric polymer. A
blend of styrene homopolymer and styrene/butadiene block copolymer is a
preferred blend
for use in the core layer. Preferably, the blend comprises 5-50% styrene
homopolymer, with
the balance being styrene-butadiene block copolymer; more preferably, 10-30%
styrene
homopolymer; still more preferably, IO-20% styrene homopolymer. Preferably,
the
styc~enelbutadiene copolymer comprises styrene/butadiene block copolymer
comprising
butadiene mer in an amount of from about 20 to 40 weight percent, based on
block
copolymer weight.
As with the homogeneous ethylene/alpha-olefin copolymer polymer of the outer
layers of the multilayer, film. of the present invention, the polymer or
polymers in the inner
layer, including both core layers as well as tie layers, can be produced in
accordance with any
suitable polymerization process, including slurry polymerization, gas phase
polymerization,
and high pressure poiyrnerization processes, as discussed in above, in detail.
Furthermore, in
addition other catalysts, the polymers) in core and/or tie layer or layers can
be prepared
through the use of a single site catalyst, such as a metallocene catalyst, as
discussed above.
Figure 2 illustrates a cross-sectional view of a five-layer film I4 according
to the
2 0 present invention, which is an alternative of the preferred rnultilayer
film illustrated in Figure
1. In Figure 2, multilayer film 14 is composed of five layers, including first
layer 15, second
layer 16, third layer 17, fourth layer I8, and fifth layer 19. First layer 15,
and fifth layer 19,
which are outer layers, are both designed to serve as sealing layers. Inner
second layer I6
and inner fourth layer 18 can be polyethylene-based layers, e.g., can comprise
an
2 5 ethylenelaipha-olefin copolymer or propylene/ethylene copolymer, or can be
designed to
serve as tie layers, or can be high modulus elastomer-containing layers. Third
layer 1 ~, also
an inner layer, can be a thigh modulus eIastomer containing layer, or a
polyethylene-based
layer, or even a tie layer. If an antifog agent is used in the film, i.e., a
surface active agent, it
should be present on an outer surface of an outer film layer which later
becomes the inside
3 0 layer of the package. If the package is printed, the printing is on the
outer surface of the
other outer film layer, which forms the outside layer of the package.
18
CA 02245678 1998-08-06
WO 97/28964 PCT/US97100694
Although most preferably the multilayer film of the present invention does not
comprise a tie layer, in general, the multilayer film of the present invention
can comprise one
or more tie layers. In general, the tie layer or layers may comprise any
polymer which
adheres to both the layers which the tie layer is tying together. The
composition, number,
and thickness of the tie layer or layers are as known to those of skill in the
art of films. Tie
layers need be only thick enough to effectuate the desired tying function.
Preferably, the
thickness of the tie layers is from about 0.001 to 0.5 mil., more preferably
from about 0.01 to
0.4 mil., and most preferably from about 0.1 to 0.3 mil.
The polymer components used to fabricate multilayer films according to the
present
invention may also contain appropriate amounts of other additives normally
included in such
compositions. These include slip agents such as talc, antioxidants, fillers,
dyes, pigments and
dyes, radiation stabilizers, antistatic agents, elastomers, and the like
additives known to those
of skill in the art of packaging films. The presence of an antifog agent is a
particularly
preferred additive for at least one of the outer layers.
Multilayer films in accordance with the present invention can be manufactured
using
film fabrication technologies well-known in the art. For example, the base
film may be
extruded into a film using a flat die, or extruded into a film using an
annular die, and the heat
seal layer formed thereon by solvent deposition, lamination or coextrusion
techniques.
However, the preferred method of manufacture of the multilayer film of the
present invention
2 0 is via simultaneous coextrusion, in an annular die, of all the layers of
the multilayer film,
including the outer (sealing) layers, the core layer, and optionally, the one
or more tie layers.
Figure 3 illustrates a schematic view of a process according for making the
film
according to the present invention, i.e., either multilayer film 10 or
multilayer film 14, both of
which are in accordance with the present invention. Although for the sake of
simplicity only
2 5 one extruder 20 is illustrated in Figure 3, there are preferably at least
2 extruders, and more
preferably, at least three extruders. That is, preferably at least one
extruder, and more
preferably two extruders, supply molten polymer to coextrusion die 21 for the
formation of,
for example, outer layers 11 and 13 as illustrated in Figure 1, and at least
one additional
extruder supplied molten polymer to coextrusion die 21 for the formation of,
for example,
3 0 core layer 12 as illustrated in Figure 1. Each of the extruders is
supplied with polymer pellets
suitable for the formation of the respective layer it is extruding. The
extruders subject the
19
CA 02245678 2004-O1-14
64536-951
polymer pellets to sufficient pressure and heat to melt the polymer and
thereby prepare it for
extrusion through a die.
Taking extruder 20 as an example, each of the extruders is preferably equipped
with
a screen pack 22, a breaker plate 23, and a plurality of heaters 24. Each of
the coexttuded
film layers is extruded between mandrel 25 and die 21, and the extrudate is
cooled by cool air
flowing from air ring 26. The resulting blown bubble 2 ~ i s t he re a f t a r
gu i de d
into a collapsed configuration by nip rolls 29, via guide
rolls 28. The collapsed tube is optionally passed over
treater bar 30, and is thereafter passed over idler rolls
31, and around dancer roll 32 which imparts tension control
to collapsed tube 33, after which the collapsed tube is
wound into roll 34 via winding mechanism 35.
Although the multilayer film of the present invention is preferably not
irradiated,
optionally the film may be irradiated. In the irradiation process, the film is
subjected the film
to an energetic radiation treatment, such as corona discharge, plasma, flame,
ultraviolet, X-
ray, gamma ray, beta ray, and high energy electron treatment, which induce
cross-linking
between molecules of the irradiated material.
Radiation dosages are refen-ed to herein in terms of the radiation unit "RAD",
with
one million RADS, also known as a megarad, being designated as "MR". A
suitable
2 0 radiation .dosage of high energy electrons is in the range of up to about
12 MR, more
preferably about 2 to about 9 MR~ and still more preferably, about 3 MR.
Preferably,
irradiation is carried out by an electron accelerator and the dosage level is
determined by
standard dosimetry methods.
Figure 4 illustrates a vertical form fill and seat apparatus to be used in
packaging
2 5 process according to the present invention. Vertical form fill and seal
equipment is well
known to those of skill in the packaging arts. The following documents
disclose a variety of
equipment suitable for vertical form fill and seal: U.S. Patent No. 2,956,383;
U.S. Patent No.
3,340,129 to J.J. GREVTCH; U.S. Patent No. 3,611,657, to KIYOSHI INOUE, et.
a1_; U.S_
Patent No. 3,703,396, to INOUE, et. al.; U.S. Patent No. 4,103,473, to BAST,
et. al.; U.S.
30 Patent No. 4,506,494, to SHIMOYAMA, et. al.; U.S. Patent No. 4,589,247, to
; U.S. Patent
No. 4,532,752, to TAYLOR; U.S. Patent No. 4,532,753, to KOVACS; U.S. Patent
No.
4,5TI,926, to SCULL'; and Great Britain Patent Specification No. 1 334 616, to
de
GROOT, et. at .
CA 02245678 1998-08-06
WO 97/28964 PCT/US97/OQ694
In Figure 4, a vertical form fill and seal apparatus 40 is schematically
illustrated.
Apparatus 40 utilizes multilayer film 41 according to the present invention.
Product 42, to
be packaged, is supplied to apparatus 40 from a source (not illustrated), from
which a
predetermined quantity of product 42 reaches upper end portion of forming tube
44 via
funnel 43, or other conventional means. The packages are formed in a lower
portion of
apparatus 40, and flexible sheet material 41 from which the bags or packages
are formed is
fed from roll 51 over certain forming bars (not illustrated), is wrapped about
forming tube
44, and is provided with longitudinal seal 47 by longitudinal heat sealing
device 46, resulting
in the formation of vertically-oriented tube 48. End seal bars 45 operate to
close and seal
horizontally across the lower end of vertically-sealed tube 48, to form pouch
50 which is
thereafter immediately packed with product 42. Film drive belts 52, powered
and directed by
rollers, as illustrated, advance tube 48 and pouch 50 a predetermined
distance, after which
end seal bars 45 close and simultaneously seal horizontally across the lower
end of vertically-
sealed tube 48 as well as simultaneously sealing horizontally across upper end
of sealed
pouch 49, to form a product packaged in sealed pouch 49. The next pouch 50,
thereabove,
is then filled with a metered quantity of product 42, forwarded, and so on. It
is also
conventional to incorporate with the end seat bars a cut-oil' knife (not
shown) which operates
to sever a lower sealed pouch 49 from the bottom of upstream pouch 50.
in carrying out the packaging process of the present invention, preferably the
vertical
2 0 form fill and seal machine forms, fills, and seals at least I 5 packages
per minute, preferably
from about I 5 to 45 packages per minute, without substantial burn through of
the film at the
seals.
Although the packaging process may be carried out with any film according to
the
present invention, the packaging process is preferably carried out using a
preferred film
2 5 according to the present invention. Preferably, the film is sealed at the
lowest possible
temperature at which relatively strong seals are produced. In general, the
film is sealed at a
temperature of from about 70°C to I 50°C.; more preferably, from
about 80°C to 140°C, and
still more preferably, from about 90°C to 130°C.
In general, the packaging process is carried out with the packaging of an
oxygen-
3 0 sensitive product. Preferably, the oxygen-sensitive product comprises at
least one cut
vegetable or fruit selected from the group consisting of lettuce, cabbage,
broccoli, green
beans, cauliflower, spinach, kale, carrot, onion, pepper, corn, radish,
endive, chard, chicory,
21
CA 02245678 1998-08-06
WO 97/28964 PCT/ITS97/00694
radicchio, greens, peas, squash, escarole, brussel sprout, mushroom, melons
and berries;
more preferably, at least one member selected from the group consisting of
lettuce, cabbage,
broccoli, cauliflower, kale, carrot, onion, radish, endive, chard, chicory,
radicchio, and
escarole, where the film has an oxygen permeability of from about 2000 to
10,000 cc/mz/
24hr STP.
Figure 5 illustrates one embodiment of a packaged product 49 of the present
invention, the product being packaged in sealed pouch 56 having vertical seal
47 and end
seals 57. Package 56 is a multilayer film of the present invention as produced
in a vertical
form fill and seal apparatus, in accordance with the present invention as
described above.
In general, the product in the package can be any oxygen-sensitive product, as
described above. Preferably, the oxygen-sensitive product comprises at least
one cut
vegetable selected from the group consisting of lettuce, cabbage, broccoli,
green beans,
cauliflower, spinach, kale, carrot, onion, pepper, corn, radish, endive,
chard, chicory,
radicchio, greens, peas, squash, escarole, brussel sprout, mushroom, melons
and bernes;
more preferably, at least one member selected from the group consisting of
lettuce, cabbage,
broccoli, cauliflower, kale, carrot, onion, radish, endive, chard, chicory,
radicchio, and
escarole, where the film has an oxygen permeability of from about 2000 to
10,000 cc/mil/m2/
24hr STP., and, still more preferably, an oxygen permeability of from about
3000 to 8000
cc/m2/ 24hr STP.
2 0 The invention is illustrated by the following examples, which are provided
for the
purpose of representation, and are not to be construed as limiting the scope
of the invention.
Unless stated otherwise, all percentages, parts, etc. are by weight.
EXamples 1 - 6
2 5 A series of coextruded, multilayer films were produced on conventional hot
blown
film equipment using a multilayer annular die, to produce films having an ARIA-
type
structure. The films had average thicknesses of from about 1.5 to 2.0 mils.
Each outer layer
A was designed to serve as a sealing layer. Each outer layer was composed of a
AFFII~1ITY PL 1880 (TM) metallocene-catalyzed ethylene/octene copolymer having
a
3 0 density of about 0.902 gm/cc, and a melt index of about 1.0 gm/10 min
(using Condition E of
ASTM D-1238), obtained from The Dow Chemical Co., 2040 Dow Center, Midland, MI
48674 (hereinafter referred to as "MCPE 1 "). Each outer layer had an average
thickness of
22
CA 02245678 1998-08-06
WO 97/28964 PCTYUS97/00694
about 0.5 mils. For each of the outer film layers A, MCPEl was preblended with
a
"masterbatch," i.e., either: (a) FSU93E {TM) slip/antibiock concentrate,
obtained from A.
Schulman of Akron, Ohio (hereinafter, "MB1"), or (b) FSU255E (TM)
siip/antiblock
concentrate, also obtained from A. Schulman of Akron, Ohio (hereinafter,
"MB2"). The
masterbatch was added to each outer layer, in an amount of about 4 weight
percent, based
on weight of the outer layer. The slip and antiblock agents served to provide
easy separation
of the film plys at the winder, and for good machinability on the VFFS
packaging equipment.
The B-Layer, i.e., the only inner film Layer, contained a styrene/butadiene
copolymer
or a blend of styrene-butadiene copolymer and homogeneous polystyrene. Two
styreneJbutadiene copolymers were evaluated, the first {i.e., "SBC1") being
KK36 (TM)
styrene/butadiene copolymer having 75 percent by weight styrene, a melt index
of 8.0
(Condition G of ASTM D-1238) and a density of 1.01 g/cc, obtained from
Phillips 66
Company, of Pasadena, Texas. The second styrene/butadiene copolymer evaluated
(i.e.,
"SBC2") was STYROLUX 684D ~, obtained from BASF Corporation of 3000
Continental
Drive, N., Mount Olive, N.J. The STYROLUX 684D ~ had a density of 1.01 g/cc
and a
melt index of 8.5 gm/10 min (using Condition G of ASTM method D-1238).
The polymer formulations for the A-layers and B-layer were then fed into the
hoppers of extnaders which fed an annular coextrusion die. The materials were
coextruded
through the die, exited the die, and were blown to a desired width while
simultaneously being
2 0 cooled with an air ring. The cooled film was then collapsed, ply
separated, and wound on
cores for further processing.
Table I, below, provides the structure, total thickness and layer thicknesses,
for each
of the films of Examples 1-6. Table II provides the longitudinal and
transverse modulus,
kinetic coe$'lcient of friction, haze, gloss, and transmission rate data.
Examples 7 - 9
A series of coextruded, multilayer films were produced on conventional hot
blown
film equipment equipped with a multilayer annular die, to produce films having
an
A/B/C/B/A-type structure. The films had average total thicknesses of from
about I .5 to 2.0
A
3 0 mils. For each film, each outer layer A served as a sealing layer and was
composed of
MCPE in an amount of about 96 weight percent, based on weight of the outer
Layer. Each
outer layer A had an average thickness of about 0.5 mils. For each of the
outer film layers,
23
CA 02245678 1998-08-06
WO 97/28964 PCT/US97/00694
the metallocene-catalyzed polyethylene was preblended with MB 1 (in an amount
of about 4
weight percent, based on weight of the outer layer. MB I served to allow easy
separation of
the film plys at the winder, and for good machinability on the VFFS packaging
equipment.
Each of the B-layers contained SBC-1 in an amount of 100 weight percent, based
on weight
of the layer. The C-layer contained ESCORENE PD 9302 ~ propylene/ethylene
copolymer ,
(hereinafter "EPC") , obtained from the Exxon Chemical Company, of Houston,
Texas. This
EPC had an ethylene content of about 3.3 weight percent (based on weight of
EPC), a
density of 0.895, and a melt flow rate of 3.8 g/10 min (Condition L of ASTM D-
1238). The
C layer contained EPC in an amount of about 100 weight percent, based on
weight of the
layer.
The polymer formulations for the A-layers, B-layers and C-layer were then fed
into
the hoppers of extruders which fed the coextrusion die. The materials were
coextruded
through an annular coextrusion die, exited the die, and were blown to a
desired width while
simultaneously being cooled with an air ring. The cooled film was then
collapsed, ply-
separated, and wound onto cores for further processing.
For Examples 7-9, Table I, below, provides the structure, total thickness and
layer
thicknesses; and Table II provides the longitudinal and transverse modulus,
kinetic coefficient
of friction, haze, gloss, and transmission rate data.
2 0 EXdmples 10 & 11 (Comparative)
Two coextruded, multilayer films were produced on conventional hot blown film
equipment equipped with a multilayer annular die, to produce films having an
AB/A-type
structure. The films had average thicknesses of 1.8 mils and 1.3 mils,
respectively. For each
film, the two outer layers A, each of which served as a sealing layer, were
each composed of
2 5 MCPE 1 (96 weight percent) and MB I (4 weight percent). As for the films
of Examples I -6,
MB 1 was added to allow easy separation of the film plys at the winder, and
for good
machinability on the VFFS packaging equipment. The B-layer, i.e., the only
inner layer in the
films of Examples I 0 and 11, was composed of 100 percent EPC.
The polymer formulations for the A-layers and B-layer were then fed into the
3 0 hoppers of extruders which fed an annular coextrusion die. The materials
were coextruded
through the die, exited the die, and were blown to a desired width while
simultaneously being
cooled with an air ring. The cooled film was then collapsed, ply- separated,
and wound on
24
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WO 97/28964 PCT/US97/00694
cores for further processing. For Examples 10 and 11, Table I, below, provides
the
structure, total thickness and various layer thicknesses, and Table II
provides the longitudinal
and transverse modulus, kinectic coei~cient of friction, haze, gloss, and
transmission rate
data.
TABLE I
Film of Film Layer Film Layer
Example Composition & Structure Thickness
No. (mils)
1 96%MCPE 1 + 4%MB 1 / SBC I / 96%MCPE . 5/. 7/.5
1 + 4%MB I
2 95%MCPE 1 + 5%MB2 / SBC 1 / 95%MCPE 1 .5/.5/.5
+ 5%MB2
3 94%MCPE 1 + 6%MB 1 / SB C 1 / 94%MCPE .5/.7/.
I + 6%MB 1 5
4 95%MCPEl + 5%MBI 1 SBCl / 95%MCPEI + .5/.7/.5
5%MBl
5 95%MCPE 1 + 5%MB 1 ! SBC2 / 95%MCPE I .6/.9/.5
+ 5%MB 1
6 95% MCPEl + 5% MBl J 90%SBC2 + 10%PS .5/.81.5
/
95%MCPE1 + 5%MBl
7 95%MCPE1 + 5%MB1! SBCI / EPC / SBC1 / .31.1/.7/.1/.3
95%MCPE I + 5%MB 1
8 95%MCPEl + 5%MB1/ SBC1 / EPC / SBC1 / .4/.3/.8/.3/.3
95%MCPEI + 5%MBI
9 95%MCPEI ~- 5%MBI/ SBC1 / EPC / SBCI .3/.2/.5/.2/.3
/
95%MCPEI + 5%MBI
96%MCPE 1 + 4%MB I / EPC / 96%MCPE 1 . 5/.8/.
+ 4%MB 1 5
I 1 96%MCPE 1 + 4%MB I / EPC / 96%MCPE 1 .4/.5/.4
+ 4%MB 1
TABLE II
Ex. Longi- Trans- KineticHaze Gloss 02TR COZTR MVTR
' No. tudinal verse COF
(%) (at (cc/m2(cc/m2 (gm/I00
45} in 2
Modulus Modulus 24hr.24hr. 24hr.
STP)
(psi) (psi) STP) STP)
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WO 97/28964 PCT/US9'7/00694
1 90,0000 69,000 O.I7 6.I 73 7,30025,000 1.8
2 76,000 55,000 0.13 11.9 60 6,50023,000 1.5
3 99,000 74,000 0.12 6.0 74 7,20025,000 1.6
4 93,000 72,000 0.44 4.3 77 7,80025,000 1.9
I10,000 90,000 0.43 6.5 73 5,90023,000 1.6
6 112,000 88,000 0.43 6.3 7I 6,60024,000 1.9
7 87,000 75,000 0.17 6.4 72 3,70014,000 0.81
8 106,000 85,000 0.36 5.9 75 3,20013,000 0.74
9 87,000 67,000 0.36 5.6 74 5,60017,000 I.2
54,000 54,000 0.13 6.3 70 4,60014,000 0.72
1I 48,000 52,000 0.14 6.4 68 6,90018,000 1.01
Tables III and IV, below, Oz-concentration data (Table III), and C02-
concentration
data (Table IV) taken from two independent evaluations of the films of
Examples 3, 5, 6, and
I 1. In both studies, one pound broccoli florets were packaged in the films,
for the purpose
5 of ascertaining and comparing the product shelf life in the films. In both
cases, materials
containing styrenelbutadiene copolymer in the core, i.e., the films of
Examples 3, 5, and 6,
were effective in maintaining good product quality while providing the desired
aesthetics and
machinability properties needed for a retail application of a packaged product
prepared using
a vertical-form-fill-and-seal ("VFFS") process. The analysis of packaged
products utilizing
10 the films of Examples 3, 5, and 6 revealed ideal gas transmission rates,
with particular regard
for carbon dioxide and water vapor transmission rates. Studies suggest that
the best product
quality, in terms of appearance, flavor, and odor, is obtained if throughout
the self life there is
an average 02-concentration of 1 to 3 percent, and an average COa-
concentration of 5 to i 0
percent. It is believed that these properties are of significant benefit to
extending the shelf life
of highly perishable commodities such as fresh-cut broccoli.
As is apparent from the data set forth in Table III, the OZ-concentration of
the ,
various packages was similar, with the differences, in most cases, having no
statistical
significance. However, as set forth in Table IV, the COZ-concentrations inside
the packages
utilizing the films of Examples 3, 5, and 6 range from about 40 to 59 percent
of the CO~-
2 0 concentration inside the package made using the film of comparative
Example 11. This result
is advantageous for the films according to the present invention, since a high
COZ-
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WO 97/28964 PCT/US97/00694
concentration in a packages produces an in off flavor as well as an off odor.
In fact, during
the study it was observed that packages made from the comparative film of
Example I I
developed an ofI=odor faster and more pronounced than packages made from the
films of
Examples 3, 5, and 6. Also, it was noticed that packages made from films of
Examples 3, 5,
a 5 and 6 had significantly less moisture therein, because of their higher
water vapor transmission
rate which prevents slime growth on the product.
TABLE III
(Percent Mean Oz-Concentration Measured in
One Pound Samples of Broccoli Florets
Packaged in Various Films and Stored at 40°F)
Elapsed Days Film of Film of Film Film of
Example 3 Example of Example
5 Example 11
6
3 1.2 1.6 1.2 1.2
7 1.5 1.1 1.2 1.1
14 20 I.3 1.2 i.l
TABLE IV
(Percent Mean C02-Concentration Measured in
One Pound Samples of Broccoli Florets
2 0 Packaged in Various Films and Stored at 40°F)
Elapsed Film of Fiim of Film of Film of
Days
Example Example 5 Example Example
3 6 11
3 5.7 ' 7.1 6.8 12
7 5.0 5.2 4,6 11
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WO 97/28964 PCT/L1S97/00694
14 4.3 4.6 3.8 9.6
In addition to the OZ and COZ concentration data provided immediately above,
"machinability" tests were also performed. These machinability tests were
designed to
determine how the film performed in FFS machinery. From the machinabilty test
results,
which were qualitative, it was determined that films possessing a modulus of
from about
80,000 to 100,000 psi, and a coefl'lcient of fiiction ("COF") less than 0.4,
performed best in
terms of formability and tracking. Formability refers to the ability of the
film to form around
the forming tube without wrinkling; tracking refers to the ability of the film
to be forwarded
through the FFS without the side edges of the film "wandering" to a degree
that the back seal
cannot consistently be formed.
The films of Examples 3, 5, 6, 10 and 11 were compared on a Hayssen Ultima CMB
model 12/19HPR vertical FFS machine. All films performed acceptably. However,
the
films of Examples 3, 5 and 6 out-performed film of comparative Examples 10 and
11, i.e., in
terms of forming and tracking. The films of Examples 3, 5 and 6 Iayed flatter
on the seal
bars, and therefore, relative to the films of comparative Examples 10 and 11,
had significantly
fewer wrinkles in the seal areas. Furthermore, relative to the films of
comparative Examples
10 and 1 l, the films of Examples 3, 5, and 6 exhibited superior tracking,
resulting in a more
consistent back seal. However, in order to seal through wrinkles when they
occurred, the
filins of Examples 3, 5, and 6 did require either (a) a longer sealing time,
or (b) more heat.
2 0 The film of Example 3 machined slightly better than the films of Examples
5 and 6, because
the film of Example 3 had a lower coefficient of friction.
Table V, below, provides the commercial name, density, melt index, and
supplier
identity for various resins used in the above examples.
2 5 TABLE V
Component Commercial DensityMelt Index Supplier Identity,
Identity Name g/cc (g/lOmin)
MCPEI AFFINITY 0.902 1.0 (ASTM D- The Dow Chemical
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WO 97/28964 PCT/CTS97/00694
PL 1880 (TM) 1238 conditionCo., 2040 Dow
E) Center,
Midland, MI 48674
MCPE2 EXACT 3028 Exxon Chemical
Co.
(TM) P.O. Box 3272
Houston, TX 77253-
3272
EPC ESCORENE~ 0.895 3.8 (ASTM D- Exxon Chemical
Co.
PD 9302 1238 conditionP.O. Box 3272
L)
Houston, TX 77253-
3272
SBCI K-Resin~ 1.01 7.5 (ASTM D- Phillips 66 Co_
KK36 1238 conditionP.O. Box 58966
G)
Houston, TX 77258-
8966
SBC2 Styrolux~ 1.01 8.5 (ASTM D- BASF Corporation
684D 1238 condition3000 Continental
G)
Drive, N.
Mount OIive, NJ
07828
SBC 3 KR05 Styrene Phillips 66 Co.
Butadiene P.O. Box 58966
copolymer Houston, TX 77258-
8966
MB 1 POLYBATCH 1.10 11 {ASTM D-1238A. Schulman, Inc.
~ FSU-93-E Condition E) Akron, Ohio
MB2 POLYBATCH 1.15 No Data A. Schulman, Inc.
~ FSU-255E Akron, Ohio
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WO 97128964 PCTlUS97/00694
Example 12 tComparative~
A multilayer film produced by Cypress Packaging, Inc. of Rochester, N.Y. was
obtained and compared with the a film according to the present invention, more
particularly, the films of Examples 3, 5, and 6, above. The comparative film,
from
Cypress Packaging, Inc., was believed to be the same as, or substantially the
same as,
the film disclosed in Example 2 of PCT WO 94/25271, in the name of Cypress
Packaging, Inc., published 10 Nov. 1994, naming D.C. Fischer et al. as
inventors. Table
VI, below, provides the arrangement and composition of the various film
layers, as well
as the OTR of the comparative film.
Film Film Layer Composition Film Layer OTR
of and
Example Structure Thickness cc/mZ/day (STP)
No.
12 SBC3/MCPE2ladhesive/MCPEZ0.?/0.2/0.10/1.05600
The film of Example 12 (comparative) has been reported as often being
diff=icult
to machine as a result of edge curl, which has been discovered to be directly
related to
the asymmetrical nature of this film. Additionally, the edge curl prevents
film splicing
and complicates film threading as a result of poor film flatness. Fin sealing
is also
required as a result of the asymmetry of this comparative structure; the film
of Example
12 (comparative) was not suitable for the formation of a lap seal. Relative to
lap seals,
fin seals are more difFicult to make, and require greater film width to
compensate for any
2 0 inconsistencies in film tracking. The edge curt of this comparative film
caused the film
to exhibit diminished consistency of back seal formation, resulting in missed
seals or
insufficient sealing area.
In contrast, the film of Examples 3, 5, and 6 of the present invention is
symmetrical and flat. It spliced and threaded easily, and could be used for
the formation
2 5 of lap seals. There is a predominant opinion in the produce and vertical
equipment
industries that materials comprising polyethylene as the outer layer of the
package are
not machinable on vertical FFS systems equipped with constant heal seal bars
because of
CA 02245678 1998-08-06
WO 97128964 PC7YUS97/00694
the belief that such structures would adhere to the seal bar limiting if not
preventing the
ability to seal. This phenomenon is overcome by the use of TEFLON
polytetrafluoroethylene coated seal bars and air cooling of the seal area
following seal
initiation. This allows for constant heat sealing of a balanced structure
comprised of
ethylene/alpha-olefin copolymer as outer layers of the film. Sealing is
determined by the
interaction of time, temperature, and pressure. The film of the invention
requires less
seal time and temperature (25° to 50° F) than the comparative
material.
Although the present invention has been described with reference to particular
means, materials, and embodiments, it should be noted that the invention is
not to be limited
to the particulars disclosed, and extends to all equivalents within the scope
of the claims.
31
