Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE II~IQ'E~NTION
The present invention pertains to compositions for
making packaging film, and more particularly to a thin
polymeric film suitable for replacing polyvinyl chloride
film especially in packaging applications.
Polyvinyl chloride (PVC) has lone been used in many
applications in the packaging art. One particularly wide-
spread application for PVC is the use of such material as an
overtrrap material far trayed retail cuts of meat and other
l0 food products in a retail environment such as a supermarket.
. PVC has several desirable properties for this use. Far
example, it has excellent burn-through resistance, optics
and good elasticity and stretch properties at use
temperatures.
Unfortunately, PVC also has several disadvantages,
including the production of hydrogen chloride gas during
heat sealing and the generally corrosive effects of such
gases in the packaging room. Extractables from the PVC into
the packaged food product have also become of concern.
It would be of great benefit to the packaging industry,
and particular to applications requiring an instore film for
overwrapping trayed food products, to provide a film with
many of the advantages of PVC but without the disadvantages
described above.
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In-addition to the foregoing, it is desirable to
provide a film as aforesaid without the disadvantages of
PVC, and to provide such a film which is cross-linked
without irradiation. In the applications described above
the film material is wrapped around the product and sealed
as on a hot plate. Without cross-linking the film might
burn through on the hot plate and therefore cross-linking is
desirable to provide high heat resistance. Irradiation is
customarily used to provide cross-linking, but this
procedure is inconvenient, expensive and inefficient and
also difficult to perform on very thin films.
It is also desirable to provide such a film which is
stretch oriented to provide improved properties. Processes
for producing oriented films and oriented films themselves
are known in the art.
U.S. Patent No. 3,456,044 (Pahlke) mentions thin films
of thicknesses less than i mil such as 0.5 mils, and _
discloses a double bubble method for biaxially orienting
thermoplastic films, including the steps of producing a
primary tubing which is inflated by introducing air into the
interior thereof, and a cooling ring 22, as well as squeeze
rolls 34 and 28, with rolls 34 having a greater speed than
rolls 28. Between the two pairs of squeeze rolls is a
reinflated secondary bubble. If annealing is desired, the
tubing can be reinflated to form a bubble 70.
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U.S. Patent No. 3,555,604 (Pahlke) is a patent based on
a divisional application which was derived from the same
priority application as the '044 patent described above, and
discloses the same information described above for the '044
patent.
U.S. Patent No, 4,258,166 (Canterino et al.) discloses
a uniaxially oriented plastic film material with improved
strength and clarity in the direction of orientation
preferably comprising homopolymers and copolymers of
ethylene.
U.S. Patent No. 4,355,076 (Gash) discloses monoaxially
oriented polypropylene film laminated to a monoaxially
oriented high density polyethylene film, the films produced
by for example tubular blowing.
U.S. Patent No. 4,440,824 (Bonis) discloses a
thermoformable coextruded multilayer structure useful for
thermoforming into containers, the structure having
polyolefin coextruded with a high impact polystyrene layer.
A five layer structure is shown.
U.S. Patent No. 4,464,439 (Castelein) discloses a
coextruded laminate having a sheet of polypropylene and a
sheet of a mixture of high impact polystyrene, crystalline
polypropylene, and styrene/dienic monomer block copolymer.
U.S. Patent No. 4,879,177 (Boice) discloses a
monoaxially oriented shrink film having a core layer of
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butadiene styrene copolymer, outer layers of ethylene
propylene copolymer, and intermediate binding layers of
ethylene copolymer.
U.S. Patent No. 5,21,666 (Schirmer et al.)
discloses polymeric,, oriented films made by the use of a
combination of a hat blown process and a blown bubble
process.
SUMMARY OF THE INVENTION
The present invention provides a cross-linked,
stretch oriented film which can be included i.n a
thermoplastic film useful as an overwrap material for trayed
retail cuts of meat and other food products in a supermarket
or other retail environments. Further, the present
invention provides a film as aforesaid that can be cross-
linked without irradiation. Ss.-.ill further, the present
invention provides a film as aforesaid which has good burn-
through resistance. Additiona_Lly, the present invention
provides such a film with other desirable characteristics,
such as good optical properties.
The inventor has discovered that a blend of a
polyolefin, a dime polymer and a transition metal catalyst
cross-links in the presence of oxygen. This provides a film
with heat resistance, especially burn-through resistance.
The film of the present invention is a cross-linked,
stretch-oriented film with heat, resistance and good optical
pr~opert ies .
The film can desirably be provided as a laminate
with an outer sealable layer and preferably with an
intermediate adhesive layer.
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The final laminate may have a thickness of from
0.5 - 5 mils, and desirably may be used a: a thin laminate
in thicknesses from 0.5 - 2 mils.
In one aspect, the invention provides a cross-
linkable film, comprising a) two outer layers comprising a
polyolefir~ or a styrE:ne polymer or copolymer; and b) a layer
intermediate the outer layers comprising a. blend of a
polyolefin, a dime polymer and a transition metal catalyst,
wherein the film cross-7.inks in the presence of oxygen.
In a further aspect, the invention provides a
cross-linkable, oriented film, comprising: a) a core layer
comprising a blend of a polyolefin, a diene polymer, and a
transition metal catalyst; b) two outer layers comprising a
styrene polymer or copolymer; and c) t:wo intermediate
layers, bonding the core layer to respective outer layers,
comprising a polymeric adhesive.
BRIEF DESCRIPTION OF THE DRAWING
The present invention may be further' understood by
reference to the accompanying drawing:
which is a schematic cross-section of a preferred
embodiment. of a multi-layered film in accordance with the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cross-linked, stretch-oriented film of the
present invention offers considerable advantages. It is
cross-
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linked without irradiation and obtains high heat resistance.
Further, it is stretch-oriented to achieve desirable
properties and may readily be formed into an advantageous
laminate.
The polyolefin is preferably VLDPE (very low density
polyethylene) or LLDPE (linear, low density polyethylene),
although others can readily be used, as polypropylene and
copolymers of ethylene and propylene. Homogeneous ethylene
alpha olef in copolymers can also be used in connection with
this invention.
The diene polymer blended with the polyolefin provides
unsaturated groups. Typical diene polymers include
octadienes, hexadienes, 1,4-polybutadiene, I,2-
polybutadiene, non-conjugated dienes and heptadienes.
Preferably, the transition metal catalyst is in the
form of a salt, with the metal thereof selected from the
first, second or third transition series of the Periodic _
Table. Suitable metals include, but are not limited to,
manganese II or III, iron II or III, cobalt II or III,
nickel II or III, copper I or II, rhodium II, III or IV, and
ruthenium. The oxidation state of the metal when introduced
is not necessarily that of the active form. The metal is
preferably iron, nickel or copper, more preferably manganese
and most preferably cobalt. Suitable counterions for the
metal include, but are not limited to, chloride, acetate,
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stearate, palrnitate, 2-ethylhexanoate, neodecanoate or
naphthenate. Particularly preferable salts include cobalt
(II) 2-ethylhexanoate and cobalt (II) neodecanoate. The
metal salt may also be an ionomer, in which case a polymeric
counterion is employed. Such ionomers are well known in the
art.
In accordance with the present invention, it has been
found that a blend of a polyolefin and a di me polymer
cross-links in the presence of oxygen.
The blend is preferably stretch oriented before cross-
linking. For example, the blend is coextruded through a die
in the conventional manner and the extruded film hot blown
to form a blown bubble. In accordance with standard
processing, an air cooling ring may be positioned
circumferentially around the blown bubble to cool same as it
exits the die. The blown bubble is melt oriented in both
the machine and transverse directions using various blow up _
ratios, but preferably the bubble is hot blown to a blow-up
ratio of between 1.5 and 8. If desired, one may immediately
reinflate the bubble after cooling into a secondary bubble
and then expand same to impart orientation of the material
in primarily the transverse direction, primarily the
longitudinal direction, or in both the transverse and
longitudinal directions. The bubble is collapsed in a set
of pinch rolls and transferred to a take-up roll. This
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procedure is shown, for example, in I.T.S. patent No.
5,219,66F.
Naturally, alternate methods may be utilized for
providing the stretch oriented film.
The cross-linkable, stretch oriented film of the
present invention may be desirably farmed into a multi-layer
laminate. In a preferred constructian, outer layers 12 as
shown in Figure 1 may comprise a polyalefin or desirably an
ethylene polymer or copolymer such as ethylene vinyl acetate
copolymer (EVA). One may also desirably use styrenic
polymers and copolymers, for example, styrene butadiene
copolymer, such as that commercially available from Phillips
under the trade-mark KR-10 having a butadienE=_ content of
TM
25o by weight of the copolymer, or KK-36 (for fatty food
contact). Desirably, styrene butadiene copolymers (SBC) are
used for the outer layers, especially block copolymers
containing a major portion (greater than 500) of styrene and
a minor portion (less than 50%) of butadiene comonomer.
These materials provide an optimum balance of stiffness and
flexibility to the film.
Core layer 10 preferably comprises the bT.end of the
present invention, i.e. a blend of a polyolefin and a diene
polymer, and a transition metal catalyst. Mor a than one
polyolefin can be used in the blend, as well as more than
one diene polymer. The term "polymer" includes herein not
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only homopolymers, but copolymers and tet-polymers as well
provided they blend with the other campc~nent~s to provide a
cross-linkable blend.
T_n the embodiment Shawn in the drawing, the outer
layers 12 are banded to the core layer 10 by means of
intermediate layers 14 comprising, for example, a polymeric
adhesive and preferably a copolymer of ethylene, and more
preferably an ethylene vinyl acetate copalyme:r (EVA). Other
polymeric materials, including chemically modified
adhesives, can be used for layers 14 provided!, that they
process adequately :in processes such as those: discussed
herein. Blends of polymeric materials and polymeric
adhesives may also be used for intermediate layers.
For layers of the present inventian which contain SBC,
TM
anti-fog/plasticizing agents, such as Atmer 645 and/or Atmer
1010 are preferably included, desirably i.n amounts between
about 0.5 and 10% by weight of the layer or layers. The
intermediate layers 14 of the present invention also
desirably may contain such agents in the aforesaid amounts.
The multi-layer film of the present invention is preferably
prepared by coextrusion techniques as discussed hereinabove.
The present invention will be more readily understood
from a consideration of the following illustrative examples.
Three sample rolls of film were produced, by the
process described in U.S. Patent No. 5,219,666.
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The construction of each ov these three films was as
follows:
Example 1 SBC/EVA/BLENLi 1/EVA/SBC
Example 2. SBC/EVA/BLE2JD 2/EVA/SBC
Example 3
(Comparatives SBC/EVA/VLDPE/EVA/SBC
In the examples, "SBC" is a styrene butadiene copolymer
available from Phillips under the trade-mark KK36-2. The
SBC materials of the skin layers included about 2% each, by
weight of the layer, of ATMER 1010 and ATMER 645; both Atmer
materials are used as antifog agents, and are available from
ICI.
"EVA" is an ethylene vinyl acetate copolymer, and in
the examples produced in accordance with the invention, the
EVA actually comprised a blend of 50% (by weight) of an EVA
resin, and 50% (by weight) of an EVA master batch. The EVA
TM
resin was Elvax 3165 available from DuPont. This resin has
a vinyl acetate content of about 18% by weight. The EVA
master batch comprised 92%, by weight of the master batch,
of Elvax 3165, and 4% each, by weight of the master batch,
of ATMER 1010 and ATMER 645.
The BLEND 1 of Example 1 was made up of:
TM
- 72% by weight of a VLDPE (DEFD 1015-8 available
from Union Carbide) which has a density of about
.900 and a melt index of about .1;
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'rM
- 20°s by weight of the blend layer of RH830, a 1,2-
polybutadiene available from ,7SR; anc~
- 8s by weight of an LLDPE roaster batch, i.e. a blend
of 40°s by weight of a linear low densir_y
polyethylene, and 30°s each, by weight, of ATI~IER 1010
and ATbtER 64540, the LLDPE master batch provided
cornrnercially under the trade-mark Santec 23-222.
The BLEND 2 of Example 2 was like BLEND 1, but also
included 5v, by weight of the blend, of cobalt decanoate
master batch, wherein a cobalt decanoate material was
compounded with EVA resin. In the cobalt decanoate master
batch, EVA (9~ VA) comprises 97.7°s of the master batch, and
the cobalt decanoate cornprised 2.3~ of the roaster batch.
The core layer 10 of Example 3 contained very low
density polyethylene, the DEFD 1015 rnaterial available from
Union Carbide.
Samples of a stretch olefin film made in accordance
with the method disclosed in U.S. Patent 5,219,666, where
produced to deterrnine if cross-linking would occur by an
oxidation reaction. These samples were analyzed for percent
gel in general accord with standard ASTM procedure for
determining gel content and inferent sally t;he degree of
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cross-linking of the film. All samples were extracted in
boiling toluene, vacuum dried and re-weighed. The samples
were extracted a second 21 hours to assure complete
solubility of all soluble portions. The results were as
follows:
Example 1 Example 2 Exam~ale 3
Gel ~ 0 14.7 0
(ASTM D-2765-84)
The cross-linkable material of the invention, as
illustrated in Example 2, is very useful in producing a heat
resistant material useful in over~arap applications such as
those described above.
This invention may be embodied in other forms or
carried out in other ways without departing from the spirit
or essential characteristics thereof. The present
embodiment is therefore to be considered as in all respects
illustrative and not restrictive, the scope of the invention .
being indicated by the appended claims, and all changes
which come within the meaning and range of equivalency are
intended to be embraced therein.
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