Note: Descriptions are shown in the official language in which they were submitted.
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POULTRY SHRINK BAGS WITH ANTIBLOCK ADDITIVES
Background of the Invention
The present invention relates to multilayer polymeric film comprising three
layers wherein an antiblock additive may be present in layers one, two and
three.
These film structures are useful in the packaging of meats and poultry.
Polymeric materials have many applications in packaging structures. They are
used as films, sheets, lidstock, pouches, tubes and bags. These polymeric
materials
may be employed as a single layer or one or more layers in a structure.
Unfortunately,
there are countless polymeric materials available. Furthermore, resin
suppliers
frequently have a tendency to claim many more applications for a product than
the
product is actually suitable for. In addition, in view of the specialized
applications
and processing problems that are encountered despite the suppliers claims, one
skilled
in the art can not tell whether a particular resin will be suitable for an
application
unless tested. However, for various reasons there are frequently drawbacks to
the use
of many of these polymeric materials. For example, ethylene vinyl alcohol is
an
excellent oxygen barrier material for use in packaging food products. However,
this
polymeric material can be affected by moisture that is present in the
atmosphere or the
packaged product. As a result, it is frequently found that some polymeric
materials
are better for certain applications than others.
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One area where there is a need for suitable resins in film applications is in
the
area of heat shrinkable films. Heat shrinkable polymeric films are commonly
used in
packaging meats, particularly primal meat cuts and other large pieces of meat
or
poultry. While this description will detail the usage of films for packaging
meat and
meat by-products, it will be understood that these films are also suitable for
packaging
a myriad of other products, both including food products and non-food
products.
Some of the films embodying the present invention are intended to be used by
meat or poultry packers in the form of heat shrinkable bags with one opened
end,
which bags are closed and sealed after insertion of the meat or poultry. After
the
product is inserted, air is usually evacuated from the package and the open
end of the
bag is closed. Suitable methods of closing the bag include heat sealing, metal
clips,
adhesives etc. Heat is applied to the bag once sealing is completed to
initiate
shrinkage of the bag about the meat.
In subsequent processing of the meat, the bag may be opened and the meat
removed for further cutting of the meat into user cuts, for example, for
retail cuts or
for institutional use.
Suitable shrink bags must satisfy a number of criteria. Many bag users seek a
bag that is capable of surviving the physical process of filling, evacuating,
sealing and
heat shrinking. For example, during the shrinking process great stress can be
placed
on the film by the sharp edges of bone in the meat or poultry. The bag must
also have
sufficient strength to survive the material handling involved in moving the
large cuts
of ineat. or poultry, which may weigh twenty-five pounds or less, along the
distribution system.
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Because many food products including meat deteriorate in the presence of
oxygen and/or water, it is desirable that the bags have a barrier to prevent
the infusion
of deleterious gases and/or the loss or addition of moisture.
Conventional packaging for many products has frequently been made of
multiple layer films having at least three layers. These multiple layer films
are usually
provided with at least one core layer of either an oxygen barrier material
such as a
vinylidene chloride copolymer, ethylene vinyl alcohol, a nylon or a metal foil
preferably aluminum. Heat shrinkable meat bags, for example, have generally
used
vinylidene chloride copolymers. The copolymer of the vinylidene chloride may,
for
example, be a copolymer with vinyl chloride or methyl acrylate. Collapsible
dispensing tubes have generally used one or more foil layers. The foil layers
in
addition to supplying an oxygen barrier also provide the dispensing tube with
"deadfold", i.e., the property of a collapsible dispensing tube when squeezed
to
remain in the squeezed position without bouncing back.
Outer layers of films used in packaging food products can be any suitable
polymeric material such as linear low density polyethylene, low density
polyethylene,
ionomers including sodium and zinc ionomers, such as Surlyn~'. In conventional
shrink bags, the outer layers are generally linear low density polyethylene or
blends
thereof. Suitable outer layers for meat bags are taught by U.S. Pat. No.
4,457,960 to
Newsome.
U.S. Patent No. 4,894,107 to Tse et al. commonly assigned to American
National Can discloses novel films and processes for making them. The films
are
characterized by having first and second layers whose compositions have a
significant fraction of ethylene vinyl acetate (EVA). A third layer of
vinylidene
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chloride copolymer (VDC-CP) is disposed between the first and second layers.
The
composition of at least one of the first and second layers is a blend of 10%
by weight
to 90% by weight linear low density polyethylene (LLDPE) and 90% to 10% EVA.
These polymeric films are useful as heat shrinkable polymeric films. The film
may be
unoriented or oriented. Oriented films may be optionally cross-linked.
While conventional films have been suitable for many applications, it has been
found that there is a need for films that are stronger and more easily
processed than
conventional films. In meat bags, there is a need for films and bags that have
superior
toughness and sealability and the ability to undergo cross-linking without
undue
deterioration. Thus, it is an object of the present invention to provide
improved
structures, iricluding single and multi-layer films, sheets, lidstock,
pouches, tubes and
bags. In particular, structures for use in shrink bags wherein the shrink bags
are
capable of withstanding production stresses and the shrink process.
Summary of the Invention
The structures of the present invention may be single or multilayer films,
sheets, lidstock, pouches, containers, tubes and bags where at least one layer
contains
a polymer, usually a copolymer, formed by a polymerization reaction in the
presence
of a single site catalyst such as a metallocene. Examples of such a polymer
are
ethylene and propylene polymers and copolymers thereof. One preferred
copolymer
is a copolymer of ethylene and an alpha olefin where such alpha olefin has a
carbon
chain length of from C3-C20. The structures of the present invention may also
include
blends of polymers and copolymers formed by a polymerization reaction with a
single
site catalyst or blends of a polymer and copolymer formed by a polymerization
reaction with a single site catalyst and another polymeric material. Examples
of
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suitable polymers for blending include: high and medium density polyethylene
(HDPE, MDPE), linear low density polyethylene (LLDPE), low density
polyethylene
(LDPE), ethylene vinyl acetate (EVA), ultra low density polyethylene (ULDPE or
very low density polyethylene VLDPE), and ionomers such as Surlyn4. Polymers
made from single site catalyst, preferably metallocene catalysts, provide
increased
strength, particularly seal, burst, impact and puncture as well as improved
optics and
faster bag making/sealing speeds.
The present invention may also be a multilayer structure of at least three
layers
wherein the core layer is a barrier layer. In one embodiment of the present
invention,
there may be a first outer layer of an ethylene or propylene polymer or
copolymer
formed by a polymerization reaction in the presence of a single site catalyst,
a barrier
layer and a second outer layer of a polymeric material. The second outer layer
may be
an ethylene or propylene polymer or copolymer formed by a polymerization
reaction
in the presence of a single site catalyst or a layer of another polymeric
material such
as high density polyethylene, medium density polyethylene, linear low density
polyethylene, ultra low density polyethylene, low density polyethylene,
ethylene vinyl
acetate, an ionomer or blends thereof. The first outer layer may also be a
blend of the
ethylene copolymer with another suitable polymeric material such as described
above.
A preferred polymer formed by a single site catalyst is a copolymer of
ethylene and an
alpha olefin such as 1-octene. Additional layers such as adhesive layers or
other
polymeric layers may be interposed in the structure between one or both of the
outer
layers or on top of one or both of the outer layers. The structure of the
present
invention may be rendered oriented either uniaxially or biaxially and cross-
linked by
any suitable means, such as for example irradiation or chemical cross-linking.
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Brief Description of the Drawings
Figure 1 is a side view of a three layer structure of the present invention.
Figure 2 is a side view of a five layer film of the present invention.
Figures 3-6 are examples of the structure of metallocene catalysts which could
be used in the polynZerization of the polymer used in the structures of the
present
invention.
Detailed Describtion of the Invention
The structures of the present invention include films, sheets, lidstock,
pouches,
containers, tubes and bags. These structures may be a single layer or multi-
layer
structure. The structures are comprised of polymers that have been polymerized
in the
presence of a single site catalyst, such as a metallocene. A metallocene is a
complex
organometallic molecule typically containing zirconium or titanium, with a
pair of
cyclic alkyl molecules. More specifically, metallocene catalysts are usually
compounds with two cyclopentadiene rings fixed to the metal. These catalysts
are
frequently used with aluminoxanes as a cocatalyst or an activator, one
suitable
aluminozane is a methaluminoxane (MAO). Besides, titanium and zirconium,
hafnium may also be used as the metal to which the cyclopentadiene is bonded.
Alternative metallocenes may include Group IVA, VA and VIA transition metals
with
two cyclopentadiene rings. Also mono-cyclopentadiene rings or silyl amides may
alternatively be in the metallocene instead of two cyclopentadienes. Other
metals to
which the cyclopentadiene may be attached may include the metals in the
lanthanide
series. Figures 3, 4, 5 and 6 show representative metallocenes that are
suitable single
site catalysts.
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While the reaction mechanism is not completely understood, it is believed that
the metallocene, single site catalyst confines the copolymerization reaction
to a single
site over the polymer thus controlling comonomer placement and side chain
length
and branching. The copolymers formed from metallocene single site catalysts
are
highly stereo regular products with narrow molecular weight distribution. The
metallocenes can be used to polymerize ethylene, propylene, ethylenic and
acetylenic
monomers, dienes and carbon monoxide. Comonomers with ethylene and propylene
include styrene, substituted styrene and 1,4-hexadiene. The metallocene single
site
catalysts are capable of producing isotactic polymers and syndiotactic
polymers, i.e.,
polymers in which the crystalline branches alternate regularly on both sides
of the
back bone of the polymer. There are two general types of single site catalyst
reactions. The first are nonstereoselective catalysts reactions which have
been
developed by Exxon and Dow and which are used to make Exxon's Exact resins and
Dow's constrained geometry catalyst technology (CGCT) resins. See Figs. 3 and
4.
The second type of reactions are stereoselective catalysts developed by
Hoechst and
Fina for stereo specific polymerization particularly of polypropylene and
other olefms
such as 1-butene and 4-methyl-l-pentene. See, e.g., Figures 5 and 6.
The ethylene alpha olefins polymerized by a single site catalyst can have low
crystallinity and a density that ranges from 0.854 to 0.97 gm/cc. Although
this
density range is similar to conventional ethylene polymers, i.e., LDPE, LLDPE
and
ULDPE, the polymers in the structures of the present invention have a narrow
molecular weight distribution and homogeneous branching. The molecular weight
distribution of the prefened polymers may be represented by the formula
MWD=MW/Mõ=<2.5
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In addition, the melt processability of these polymers (I10/IZ) has a range of
about 5.5 to about 12 while conventional homogenous polymers are generally
less
than 6.5 at an MWD of 2. The melt tension of these polymers is in the range of
about
1.5 to 3.5 grams.
The MWD of these polymers may be determined using a Water's 150 GPC at
140 C with linear columns (1036 A-106 Ao) from Polymer Labs and a differential
refractometer detector.. Comparison of the MWD of a 1MI, 0.920 density CGCT
polymer with that of 1MI, 0.920 density conventional LLDPE illustrates the
very
narrow MWD of the CGCT polymers which usually have a MW/MN of approximately
2 compared to 3 or greater for LLDPE.
A preferred ethylene copolymer is a copolymer of ethylene and a C3 to C20
alpha olefin. A preferred copolymer is a low modulus ethylene octene copolymer
sold
by Dow. This copolymer is formed by Dow's constrained-geometry catalyst
technology which uses a single site catalyst such as cyclopentadienyl titanium
complexes. As best understood, Dow's constrained geometry catalysts are based
on
group IV transition metals that are covalently bonded to a
monocyclopentadienyl
group bridged with a heteroatom. The bond angle between the
monocyclopentadienyl
group, the titanium center and the heteroatom is less than 115 . When the
alpha olefin
is present in the copolymer in the range of about 10 to 20% by weight these
copolymers are referred to as plastomers. When the percent alpha olefin is
greater
than 20% these copolymers are called elastomers. The preferred ethylene octene
copolymer has the octene comonomer present in an amount less than 25%.
Exaxnples
of Dow ethylene octene copolymers have the following physical properties.
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DENSITY MOLECULAR MELT MELT MELT
/g cc WEIGHT DISTRIBUTION INDEX FLOW RATIO
STRENGTH
0.920 1.97 1.0 9.5
1.89
0.910 1.90 1.0 7.9
1.68
0.902 2.10 1.0 7.6
1.68
Molecular weight distribution is defined as the ratio of weight average
molecular weight to number average molecular weight. The lower the figure, the
narrower the molecular weight distribution. Melt flow ratio is defined as the
ratio of
melt index, as tested with a 10-kg load to the melt index with a 2.168-kg
load. The
higher the ratio, the more processable the material. Melt flow ratio is
defined as melt
tension measured in grams. The higher the number the greater the melt
strength.
Other suitable resins are the Exact resins sold by Exxon, these resins have
the
following characteristics:
TYPICAL PROPERTIES OF EXACT
MEDICAL GRADE POLYETHYLENES
VALUE BY GRADE
PROPERTY 4028 4022 4021 4023 4024 4027
Melt index (D1238)* 10 6 22 35 3.8 4
Density, g./cc. 0.880 0.890 0.885 0.882 0.885 0.895
(D-1505)
Hardness (D-2240)
Shore A 78 84 84 80 83 89
Shore D 29 35 36 27 35 39
Tensile strength
at break, p.s.i.
(D-638) 2220 1700 3260 620 2840 2200
Tensile elongation
at break, % (D-638) >800 >800 >800 >800 >800 >800
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VALUE BY GRADE
PROPERTY 4028 4022 4021 4023 4024 4027
Tensile impact,
ft.-lb./sq. in.
(D-1822) 145 130 350 280 300 340
Flexural modulus,
p.s.i. (D-790) 5040 4930 3980 3100 4180 7230
Vicat softening
point F.(D-1525) 138 168 158 138 158 181
The structure of the present invention is comprised of an ethylene, propylene,
or styrene polymer or copolymer formed by a polymerization reaction in the
presence
of a single site catalyst preferably a metallocene. Ethylene may be
copolymerized
with any suitable monomer such as C3 - C20 alpha olefm including propylene
butene-
1, 4-methy-l-pentene, 1-hexene and 1-octene. A preferred comonomer is 1-
octene.
The preferred ethylene alpha olefin copolymer of the present invention has a
density
e in the range of .880 gm/cc to about .920 gm/cc, a more preferred range of
.890 gm/cc
to about .915 gm/cc and a most preferred range of about .900 gm/cc to about
.912
gm/cc.
Figure 1 shows a cross section of a three layer coextruded structure. Layer 14
is the core layer which may be a barrier layer that minimizes the transmission
of
oxygen through the structure. Preferred barrier materials are polyvinylidene
chloride
copolymers such as copolymers of vinylidene chloride and vinyl chloride or an
alkyl
acrylate such as methyl acrylate. Other preferred barrier materials include,
ethylene
vinyl alcohol, nylon or a metal foil such as aluminum. Layer 14 may also be a
copolymer of ethylene and styrene formed using a single site catalyst in the
polymerization reaction. In addition, layer 14 may also be a polystyrene
formed by a
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polymerization reaction in the presence of a single site catalyst. One such
polystyrene
is the crystalline syndiotactic polystyrene sold by Idemitsu Petro-Chemical
Co.,
Tokyo, Japan.
On opposite sides of the core layer 14 of Figure 1 are layers 12 and 16. At
least one of these layers 12 is a polymer formed by a polymerization reaction
in the
presence of a single site catalyst. The remaining layer 16 may be any suitable
polymeric material or blends of material such as a polyester, co-polyester,
polyamide,
polycarbonate, polypropylene, propylene-ethylene copolymer, ethylene-propylene
copolymer, combinations of polypropylene and ethylene vinyl acetate copolymer,
ultra low density polyethylene, low density polyethylene, medium density
polyethylene, high density polyethylene, linear low density polyethylene
copolymers,
linear medium density polyethylene copolymer, linear high density polyethylene
copolymer, ionomer, ethylene acrylic acid copolymer, ethylene ethyl acrylate
copolymer, ethylene methyl acrylate copolymer, or ethylene methacrylic acid
copolymer.
In an alternate embodiment, the layer 12 may be a blend of a polymer formed
by a polymerization reaction in the presence of a single site catalyst and a
suitable
polymeric material such as is identified in connection with the description of
layer 16
above.
As seen in Figure 2, the structure may also include embodiments which have a
fourth layer 28 over the first layer 22 and a fifth polymeric layer 30 over
the third
layer 26. The composition of the fourth layer 28 may be selected from the same
group of materials from which the composition of the first layer 12 or third
layer 16 is
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selected, and the fifth layer 30 may also be the same composition as the first
layer 22
or the third layer 26.
In an alternate embodiment of Figure 2, the five layer structure may have a
first layer 28 similar in composition to layer 12 of Figure 1, i.e., the film
may have a
first layer of a polymer formed by the polymerization reaction with a single
site
catalyst or blends thereof with another suitable polymeric material. One or
both of the
second 22 and fourth 26 layers may be an adhesive layer.
The composition of adhesive layers 22 and 26 is selected for its capability to
bond the core or barrier layer 24 to the surface layers 28 and 30. A variety
of the well
known extrudable adhesive polymers adhere well to the core or barrier layer
24.
Thus, if for example layer 30 is a polypropylene, an adhesive polymer based on
polypropylene is desirably selected for layer 26. Examples of such adhesives
are the
extrudable polymers available under the trade designations Admer QF-500,
QF550, of
QF-551 from Mitsui Petrochemical Company, or Exxon 5610A2.
If the composition of layer 23 or 30 is an ethylene based polymer or
copolymer, an adhesive polymer based on ethylene is preferably selected for
layer 22,
including ethylene homopolymer and copolymers. Such a preferred adhesive
composition is an ethylene vinyl acetate copolymer (EVA) containing up to 25%
to
30% by weight vinyl acetate. Other ethylene based homopolymer and copolymers,
modified to enhance adhesion properties are well known under the trade names
of, for
example, Bynel and Plexar. Typical base polymers for these extrudable
adhesives are
the polyethylene LLDPE and the ethylene vinyl acetate copolymers. Such
adhesive
polymers, including the polypropylene-based polymers, are typically modified
with
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carboxyl groups such as anhydride. Also acceptable as adhesives are ethylene
methyl
acrylate copolymers (EMA).
Additional layers may also be present in the structures of the present
invention. For example, the present invention contemplates 4, 6, 7, 8, and
higher
numbers of layers in the film of the present invention and different
combinations of
layer structures may also be present. For example, there may be more than one
barrier
layer, i.e., two layers of polyvinylidene chloride copolymers, two layers of
foil or two
layers of ethylene vinyl alcohol (EVOH) or nylon. Alternatively, this may be a
layer
of EVOH and a layer of a polyvinylidene chloride copolymer or a polyamide or a
polystyrene and other combinations of the core materials. The additional
layers of the
present invention also encompass more than one polymer formed by the
polymerization reaction in the presence of a single site catalyst. The
polymers may be
in a layer alone or in the form of a blend. Suitable polymers for blending
with an
ethylene polymer formed in a polymerization reaction with a single site
catalyst
include other ethylene polymers formed in a polymerization reaction with a
single site
catalyst, low density polyethylene (LDPE), linear low density polyethylene
(LLDPE),
ultra low density polyethylene (ULDPE), EVA, ionomers, ethylene copolymers,
ethylene methyl acrylate (EMA), ethylene acrylic acid (EAA), ethyl methyl
acrylic
acid (EMAA), polypropylene (PP), ethylene normal butyl acrylate (ENBA),
ethylene
propylene copolymers (PPE). Suitable polymers for blending with a propylene
polymers formed in a polymerization reaction with a single site catalyst
include
ethylene propylene copolymers.
Preferred blends using EVA's are those having a lower melt index as they tend
to yield EVA layers having better hot strength. EVA's having higher VA content
tend
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to yield EVA layers having increased adhesion to for example, the vinylidene
chloride
copolymer layer. EVA's having virtually any arnount of VA will have better
adhesion
to the vinylidene chloride copolymer layer than an ethylene homopolymer.
However,
good interlayer adhesion is considered desirable in the invention, and thus,
steps are
usually taken to enhance adhesion where no unacceptable negative effect is
encountered. Thus, higher VA contents, in the range of 6% to 12% vinyl acetate
are
preferred, a melt index of less than 1 is also preferred. While blend amounts
are
shown herein in weight percent, VA contents are mole percent. Especially
preferred
EVA's have VA content of 7% to 9% and melt index of 0.2 to 0.8. Blends of
EVA's
to make up the EVA component of layers 16 and 18 are acceptable.
Other preferred structures of the invention are represented by a multiple
layer
polymeric film having three layers and not including a barrier layer.
In a preferred embodiment, the first outer layer comprises a blend of ethylene
vinyl acetate copolymer (EVA) and linear low density polyethylene (LLDPE)
wherein
the LLDPE polymer is fonned via a polymerization reaction in the presence of a
single site catalyst. The core/middle layer comprises ethylene vinyl acetate
(EVA).
The second outer layer comprises a blend of LLDPE and low density polyethylene
(LDPE) wherein the LLDPE polymer is formed via a polymerization reaction in
the
presence of a single site catalyst. The first outer layer, the core/middle
layer and the
second outer layer may independently contain a color concentrate such as
titanium
dioxide dispersed in a polymer carrier resin such as LDPE when a
colored/pigmented
film is desired.
In a preferred embodiment of the invention, the multiple layer polymer film
comprises a first and second layer, the composition of said first layer
comprising a
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blend of ethylene vinyl acetate copolymer, linear low-density polyethylene and
a
second ethylene vinyl acetate copolymer and slip; the composition of said
second
layer comprising ethylene vinyl acetate copolymer; and a third layer
comprising a
blend of linear low-density polyethylene and low-density polyethylene said
second
layer being disposed between said first and third layer. A color concentrate
may
optionally and independently be present in layers one, two and three.
In a further preferred embodiment of the invention, the first layer is a blend
of
5% by weight to 100% by weight of ethylene vinyl acetate copolymer; 0% by
weight
to 50% by weight of linear low-density polyethylene, 0% by weight to 50% by
weight
of a second ethylene vinyl acetate copolymer, 0% by weight to 2% by weight of
slip,
and 0% by weight to 25% by weight of a color concentrate said second layer is
25%
by weight to 100% by weight of ethylene vinyl acetate copolymer and 0% by
weight
to 25% by weight of a color concentrate said third layer is a blend of 45% by
weight
to 100% by weight of linear low density polyethylene, 0% by weight to 30% by
weight of low density polyethylene and 0% by weight to 25% by weight of a
color
concentrate.
In further preferred embodiments of the invention, layers one, two and three
may independently contain a color concentrate. The color concentrate is
defined as a
pigment dispersed in a polymer. When a white pigmented layer is desired, the
color
concentrate is titanium dioxide dispersed in a polymer carrier resin such as
LDPE.
This particular color concentrate is exemplified by CM64224, CM64226 and
CM82143 made by Equistar.
In a still further preferred embodiment of the invention, the first layer is a
blend of 25% by weight to 90% by weight of ethylene vinyl acetate copolymer,
5% by
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weight to 25% by weight of linear low density polyethylene and 5% by weight to
25%
by weight of a second ethylene vinyl acetate copolymer 0% to 1% by weight of
slip
and 0% by weight to 25% by weight of a color concentrate. The second layer is
a
blend of 25% to 100% by weight of ethylene vinyl acetate and 0% to 25% by
weight
of a color concentrate and the third layer is a blend of 55% to 100% by weight
linear
low density polyethylene 0% to 20% by weight of low density polyethylene and
0%
by weight to 25% by weight of a color concentrate wherein said second layer is
disposed between said first and third layers.
In a still further preferred embodiment of the invention, the first layer is a
blend of 80% by weight of ethylene vinyl acetate copolymer, 10% by weight of
linear
low density polyethylene and 9.65% by weight of a second ethylene vinyl
acetate
copolymer and 0.35% by weight of slip additive; the second layer is 100% by
weight
of ethylene vinyl acetate; and a third layer is a blend of 82% by weight of
linear low
density polyethylene and 18% by weight of a color concentrate and 0% by weight
of
low density polyethylene wherein said second layer is disposed between said
first and
third layers.
In a still fiirther preferred embodiment of the invention, the composition of
the
first layer is 80% by weight of ethylene vinyl acetate copolymer, 10% by
weight of
linear low density polyethylene, 9.65% by weight of a second ethylene vinyl
acetate
copolymer and 0.35% of slip; the second layer is 100% by weight of ethylene
vinyl
acetate; and the third layer is a blend of 90% by weight of linear low density
polyethylene and 10% by weight of low density polyethylene.
In a preferred embodiment of the present invention, the film structure
comprises three layers; the first layer comprises a blend of ethylene vinyl
acetate
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copolymer, linear low-density polyethylene, and a second ethylene vinyl
acetate
copolymer, a second layer comprises an ethylene vinyl acetate copolymer and a
third
layer comprises a blend of linear low-density polyethylene and low-density
polyethylene with said second layer being disposed between said first and
second
layer. A color concentrate, a slip additive and an antiblock additive may
optionally
and independently be present in layers one, two and three.
In a'fiu-ther preferred embodiment of the present invention, the film
structure
comprises three layers; the first layer comprises a blend of ethylene vinyl
acetate
copolymer, linear low-density polyethylene, a second ethylene vinyl acetate
copolymer, a slip additive and an anti-block additive; a second layer
comprises an
ethylene vinyl acetate copolymer and a third layer comprises a blend of linear
low-
density polyethylene and low-density polyethylene with said second layer being
disposed between said first and second layer. A color concentrate may
optionally and
independently be present in layers one, two and three.
In a still further preferred embodiment of the present invention, the first
layer
is a blend of about 5% by weight to about 100% by weight of ethylene vinyl
acetate
copolymer; about 0% by weight to about 50% by weight of linear low-density
polyethylene, about 0% by weight to about 50% by weight of a second ethylene
vinyl
acetate copolymer, about 0% by weight to about 2% by weight of slip additive
about
0% by weight to about 10% by weight of antiblock additive said second layer is
about
25% by weight to about 100% by weight of ethylene vinyl acetate copolymer and
about 0% by weight to about 2% by weight of a slip additive said third layer
is a blend
of about 45% by weight to about 100% by weight of linear low density
polyethylene,
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about 0% by weight to about 30% by weight of low density with said second
layer
being disposed between said first and second layer.
In the preferred embodiment of the present invention the antiblock additive is
selected from Ampacet 7012124 or Ampacet 10579 both additives are products of
Ampacet Corporation of Mount Vernon, New York. The antiblock additive may be
present in the film structure as a single antiblock additive or as a blend of
antiblock
additive. A preferred range of antiblock additive for the film structures of
the present
invention is about 0% by weight to about 10% by weight of the layer; a further
preferred range of antiblock additive is about 2% by weight to about 5% by
weight of
the layer; and a most preferred percentage of antiblock additive is about 3%
by weight
of the layer.
Ethylene vinyl acetate resins suitable for the practice of this invention are
exemplified by ESCORENE (LD-318) and ESCORENE (LD-712) from Exxon
Chemical.
EXCORENE LD-318 is a 9.0% vinyl acetate copolymer film resin. This
resin has the following properties:
Resin Properties ASTM Method Units (Sl) Typical Valuez
Melt Index Exxon Method g/10 min. 2.0
Density Exxon Method g/cm3 0.930
Vinyl Acetate Exxon Method % by wt. 9.0
Melting Point Exxon Method F( C) 210 (99)
ESCORENE LD-712 is a 10.0% vinyl acetate copolymer film resin. This
resin has the following properties:
Resin Properties ASTM Method Units (Sl) Typical Value3
Melt Index Exxon Method g/10 min. 0.35
Density Exxon Method g/cm3 0.931
Vinyl Acetate Exxon Method % by wt. 10
Melting Point Exxon Method F( C) 207 (97)
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ESCORENE LD-318 is suitable for the 9.65% of EVA of layers one and
four and ESCORENE LD-712 is suitable for the remaining ethylene vinyl acetate
of
layers one and two.
Linear low density polyethylene resin suitable for the practice of this
invention
is exemplified by DOWLEX 2267A and DOWLEX 2247A from Dow Chemical
Company.
DOWLEX 2267A has the following properties:
Physical Properties ASTM Method Values('j:English (Sl)
Resin Properties
Melt Index, gm/10 min D 1238 0.85
Density, gm/cc D 792 0.917
Vicat Softening Point, F ( C) D 1525 208 (98)
DOWLEX 2247A has the following properties:
Physical Properties ASTM Method Values('):English (Sl)
Resin Properties
Melt Index, gm/10 min D 1238 2.3
Density, gm/cc D 792 0.917
Vicat Softening Point, F ( C) D 1525 210 (99)
Alternatively, Exxon's EXCEED 363C32 linear low density polyethylene
resin may also be used in the structure of the present invention. EXCEED
363C32
has the following properties:
Resin Propertiesl Units (Sl) Typical Value3
Melt Index g/10 min. 2.5
Density g/cm3 0.917
Melting Point F ( C) 239 (115)
In the preferred embodiment of the present invention, the low density
polyethylene resin suitable for the practice of this invention is exemplified
by
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PETROTHENE NA204-000 from Equistar Chemical, L.P. PETROTHENE has
the following properties:
Value Units ASTM Test Method
Density 0.918 g/em3 D1505
Melt Index 7.0 g/10 min. D 1238
Melting Point 106.5 C
Slip additive is a coefficient of friction additive which is selected from
erucamide and other fatty acid amides.
The structure of the present invention may be formed by any conventional
process. Such processes include extrusion, coextrusion, extrusion coating,
extrusion
lamination, adhesive lamination and the like, and combinations of processes.
The
specific process or processes for making a given film which is neither
oriented nor
cross-linked can be selected with average skill, once the desired structure
and
compositions have been determined.
When the structure of the present invention is a film, the film may also be
oriented either uniaxially or biaxially. Orientation can also be done by any
conventional process for forming multiple layer films. A preferred process
includes
the steps of coextrusion of the layers to be oriented, followed by orientation
in one of
the conventional processes such as blown tubular orientation or stretch
orientation in
the form of a continuous sheet; both being molecular orientation processes.
The
double bubble technique disclosure in Pahlke, U.S. Patent No. 3,456,044 is
suitable
for use in producing the film of this invention. The films may also be formed
by a
tubular water quench process. In this process the film may be extruded
downwardly
as a tube formed by an annular die, and carried into a water quench tank,
generally
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with a cascade of water on the outside surface providing initial cooling. The
flattened
tape is withdrawn from the quench bath, is reheated (normally in a second
water bath)
to its orientation temperature, is stretched in the machine direction between
two sets
of rolls that are so rotated as to establish a linear rate differential there
between, and is
simultaneously oriented in the transverse, or cross-machine, direction as an
inflated
bubble trapped between the nips of the rolls. In accordance with conventional
practice, the film will usually be cooled by air in the orientation zone.
The film of the present invention may also be oriented and/or cross-linked.
The first step is the formation of a multiple layer film. The formation of the
multiple
layer film, is usually most easily accomplished by coextrusion of the desired
layers.
Other formation processes are acceptable so long as the resulting oriented
film at the
conclusion of fabrication processing is a unitary structure.
The second step is orienting the multiple layer film. One method for
accomplishing orientation is by heating the film to a temperature appropriate
to
molecular orientation and molecularly orienting it. The film may then be
optionally
heat set by holding it at an elevated temperature while its dimensions are
maintained.
The orientation step is preferentially carried out in line with the first
step, which is the
film formation step of the process.
The third step is subjecting the formed and oriented multiple layer film, to
electron beam irradiation.
The amount of electron beam irradiation is adjusted, depending on the make-
up of the specific film to be treated and the end use requirement. While
virtually any
amount of irradiation will induce some cross-linlcing, a minimum level of at
least 1.0
megarads is usually preferred in order to achieve desired levels of
enhancement of the
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hot strength of the film and to expand the range of temperature at which
satisfactory
heat seals may be formed. While treatment up to about 50 megarads can be
tolerated,
there is usually no need to use more than 10 megarads, so this is a preferred
upper
level of treatment the most preferred dosage being 2 to 5 megarads.
The third step of subjecting the film to electron beam irradiation is
performed
only after the multiple layer film has been formed, and after molecular
orientation, in
those embodiments where the film is molecularly oriented. It should be noted
that, in
the irradiation step, all of the layers in the film are exposed simultaneously
to the
irradiation sources, such that irradiation of all the layers of the film takes
place
simultaneously.
In one embodiment of the process, the second step of orientation may be
omitted and the unoriented multiple layer film may be cross-linked by
irradiation
treatment to produce a cross-linked, unoriented, multiple layer film.
EXAMPLES
Multilayer films may be prepared according to the present invention.
Biaxially stretched three layer films may be prepared by a "double bubble"
process
similar to that disclosed in U.S. Patent No. 3,456,044 by coextruding the
following
compositions through a multilayer die, biaxially stretching the coextruded
primary
tube. The films may also be irradiated if desired.
EXAMPLE 1
Layer 1 - Copolymer of ethylene and an alpha olefin such as 1-Hexene or 1-
Octene formed by the polymerization reaction in the presence of a
single site catalyst or metallocene (hereinafter CEO)
Layer 2 - Vinylidene chloride - methyl acrylate (VDC-MA) copolymer
Layer 3 - Polyolefm. This film may be biaxially stretched and if necessary
irradiated.
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EXAMPLE 2 EXAMPLE 3 EXAMPLE 4
Layer 1 CEO CEO CEO-EVA Blend
Layer 2 VDC-MA VDC-MA VDC-MA
Layer 3 ULDPE-EVA Blend CEO CEO-EVA Blend
EXAMPLE 5 EXAMPLE 6 EXAMPLE 7
LAYER 1 CEO CEO CEO-EVA Blend
LAYER 2 Nylon Nylon Nylon
LAYER 3 CEO ULDPE-EVA Blend CEO-EVA Blend
EXAMPLE 8 EXAMPLE 9
LAYER 1 Polyolefin Polyolefin
LAYER 2 Styrene copolymer formed Propylene copolymer
by the polymerization reaction formed by the
with a single site catalyst Polymerization
reaction with a single
site catalyst
LAYER 3 Polyolefin Polyolefin
EXAMPLE 10 EXAMPLE 11 EXAMPLE 12
LAYER 1 CEO CEO CEO-EVA Blend
LAYER2 CEO EVOH EVOH
LAYER 3 CEO ULDPE-EVA Blend CEO-EVA Blend
EXAMPLE 13 EXAMPLE 14 EXAMPLE 15
LAYER 1 CEO CEO CEO-EVA Blend
LAYER 2 Tie Tie Tie
LAYER 3 PVDC Copolymer PVDC Copolymer PVDC Copolymer
or EVOH or EVOH or EVOH
LAYER 4 Tie Tie Tie
LAYER 5 ULDPE-EVA Blend CEO CEO-EVA Blend
EXAMPLE 16
LAYER 1 EVA-ULDPE
LAYER 2 ULDPE or CEO
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LAYER 3 PVDC Copolymer or EVOH
LAYER 4 EVA
LAYER 5 CEO or blend of CEO and EVA
The following examples may also be prepared in accordance with the present
invention:
EXAMPLE 17
Meat Film - Forming Web
Formed by TWQ Process
(Tubular Water Quench Process)
LAYER 1 Nylon
LAYER 2 Tie
LAYER 3 EVOH
LAYER 4 Tie
LAYER 5 CEH or CEO
CEH is a copolymer of ethylene and 1-Hexene formed by the polymerization
reaction in the presence of a single site catalyst or a metallocene. Other
alpha olefins
can be polymerized with the ethylene also.
EXAMPLES 18-20
Innerliner Films - These films can be formed either on a blown film line or by
using a tubular water quench.
LAYER 1 HDPE
LAYER 2 Blend of CEH or CEO and EVA and polybutylene
LAYER 1 HDPE
LAYER 2 CEH or CEO and polybutylene
LAYER 1 HDPE
LAYER 2 CEH or CEO
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EXAMPLES 21 and 22
Meat - Non Forming Top Web film
LAYER 1 PVDC coated PET
LAYER 2 Adhesive (lamination)
LAYER 3 CEO or CEH
This film may be produced by adhesive laminating a film formed of a
copolymer of ethylene and an alpha olefin with the PVDC coated PET film.
LAYER 1 PVDC coated PET
LAYER 2 LDPE - extrusion laminated
LAYER 3 LDPE/CEH or CEO coextrusion
This film can be formed by extrusion laminating a film of PVDC coated PET
or LDPE.
EXAMPLE 23
Layer 1- Blend of two or more copolymers of ethylene and an alpha olefin
polymerized in the presence of a single site catalyst or metallocene such
as CEO with either CEH or CEB. CEB is a copolymer of ethylene and
butene-1 formed by a polymerization reaction in the presence of a single
site catalyst or a metallocene.
EXAMPLE 24
Layer 1- Blend of a copolymer of ethylene and an alpha olefm formed by a
polymerization reaction in the presence of a single site catalyst or a
metallocene with polyethylene or other polyolefin such as EVA, EMA,
FAA, EMAA, ionomers, ENBA, PP or PPE.
The films of example 23 and 24 can either be single layer films or multi layer
films where additional layers are present on layer 1.
EXAMPLE 25
A three layer film was coextruded at a total caliper of 2.3 or 2.75 mil.
depending upon the end user requirements. The outer layer of the film is a
blend of
EVA, LLDPE and slip additive. The 80% EVA was Exxon LD 712.06; the 9.65%
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EVA was Exxon LD 318.92; the 10% LLDPE was Dow 2267 A-1 and the 0.35% slip
Reed Spectrum 1080823 U slip. The core layer, which was positioned between the
outer layer and the inner (sealant) layer, was 100% Exxon EVA LD 712.06. The
inner (sealant) layer was a blend of approximately 82% Exxon Exceed 363C32 and
18% color concentrate. The color concentrate was Equistar white CM 82143 or
Equistar cream CM 64224. Alternatively, if a clear film is desired, the inner
(sealant)
layer is a blend of 90% Exxon Exceed 363C32 and 10% Equistar LDPE NA204-000.
The color concentrate was fed into the inner layer via a calibrated color
auger. The,
thus, coextruded film was cooled and reheated to orientation temperature via a
hot
water bath (98 C) and biaxially oriented at a stretch ratio of 3.8 X 2.7/1.
After
orientation, the film was treated with 5.3 megarads of electron beam
irradiation and
fabricated into bags for the use in the packaging of various fresh and frozen
poultry
products. The bags were clip sealed on a Rota-Matic 1 rotary vacuum packaging
machine available from Tipper Tie, a division of Dover Industries Company. The
bags can also be heat sealed on commercial 8300 and 8600 rotary evacuator-
sealer
equipment available from the Cryovac division of W.R. Grace and Company.
EXAMPLE 26
A three layer film was coextruded at a total caliper of 2.75 mil. depending
upon the end user requirements. The outer layer of the film is a blend of EVA,
LLDPE antiblock additive and slip additive. The 77.60% EVA was Exxon LD
712.06; the 9.36% EVA was Exxon LD 318.92; the 9.7% LLDPE was Dow 2267 A-
1; the 0.34% slip was Reed Spectrum 1080823 U slip; and the 3.00% antiblock
was
Ampacet 7012124. The core layer, which was positioned between the outer layer
and
the inner (sealant) layer, was 100% Exxon EVA LD 712.06. The inner (sealant)
layer
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was a blend of approximately 84% Exxon Exceed 363C32 and 16% color
concentrate.
The color concentrate was Equistar white CM 82143 or Equistar cream CM 64224.
Alternatively, if a clear film is desired, the inner (sealant) layer is a
blend of 90%
Exxon Exceed 363C32 and 10% Equistar LDPE NA204-000. The color concentrate
was fed into the inner layer via a calibrated color auger. The, thus,
coextruded film
was cooled and reheated to orientation temperature via a hot water bath (98 C)
and
biaxially oriented at a stretch ratio of 3.8 X 2.7/1. After orientation, the
film was
treated with 5.3 megarads of electron beam irradiation and fabricated into
bags for the
use in the packaging of various fresh and frozen poultry products. The bags
were clip
sealed on a Rota-Matic 1 rotary vacuum packaging machine available from Tipper
Tie, a division of Dover Industries Company. The bags can also be heat sealed
on
commercial 8300 and 8600 rotary evacuator-sealer equipment available from the
Cryovac division of W.R. Grace and Company.
27
