Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 31 5062
MULTILAYER FILM CONTAINING
VERY LOW DENSITY POLYETHYLENE
FIELD OF THE INVENTION
This invention relates to a coextruded,
heat shrinkable, thermoplastic multilayer film
suitable for use in the manufacture of bags for
packaging fresh red meats and processed meats. In
particular, this invention relates to a coextruded
heat shrinkable multilayer film having a first outer
layer of very low density polyethylene, a core layer
of vinylidene chloride-methyl acrylate copolymer,
and a second outer layer of very low density
polyethylene.
BACXGROUND OF THE INVENTION
The Meat Packing Industry may be commonly
divided into three segments. They are ~resh meats,
~rozen meats and processed meats. This invention
relates to fresh red meats, such as beef and pork,
which is distinct from fresh white meat, such as
poultry. This invention also relates to processed
meats.
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In the Fresh Red Meat Industry, the cattle
and swine are slaughtered and broken down into
primal and subprimal meat cuts. The primal and
subprimal meat cuts are large cuts of meat. They
are smaller than a side of beef, for example, but
larger than the ultimate cut which is sold at retail
to the consumer. A primal cut comprises the entire
section of a side of beef, such as the rib section
or the rum~ roast section, while a subprimal cut
comprises only a portion of such a section. Primal
and subprimal cuts are prepared at the slaughter
house and are then shipped to a retail meat store,
or to an institution such as a hospital, hotel or
restaurant, where they are butchered into small cuts
of meat suitable for the individual consumer.
The Processed Meat Industry takes various
portions of the animal carcasses and processes these
portions under varying conditions to produce
~inished meat products which may be used directly by
the consumer. Products may include ham, smoked
picnics, smoked butts, corned beef, turkey breast,
and various sausage products such as frankfurters,
smoked sausage links, bologna, salami, and the
like. These products may be packaged in consumer
portions or they may be packaged in bulk for
shipment to a retail meat store, restaurant or
hotel~ Bulk shipments may include such items as ham
chunks, cooked turkey breasts, bologna chubs, long
bologna for delicatessen sale, rings of bologna,
corned bee~ brisket, smoked picnics, smoked butts
and linked products such a~ smoked sausage.
When fresh red meat cuts, such as roast or
cib sections, and bulk processed meats are prepared
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for shipment or storage, they are usually packaged
in such a way that air ti.e., oxygen) is prevented
from contacting the meat and moisture is preven~ed
from leaving the meat. This is done in order to
minimize spoilage and discoloration during shipping
and handling. One desirable way to package fresh
red meats and processed meats so as to protect them
fcom contact with air and from moisture loss is to
shrin~ package them with a packaging material that
has good oxygen and moisture vapor barrier
properties. One such shrink packaging material that
has good oxygen and moisture vapor barrier
properties is polyvinylidene chloride film.
Vinylidene chloride-vinyl chloride copolymers are
commonly referred to as PVDC.
While vinylidene chloride-vinyl chloride
copolymer film has excellent barrier properties, in
actual practice, when PVDC is used as a monolayer
film, it must be plasticized in order for the film
to have adequate abrasion resistance and flexibility
at storage temperatures of, for examele, 30 to
50F. Unfortunately, the addition of plasticizer
sufficient to provide the requisite low temperature
properties to the PVDC monolayer film has a
significant adverse effect on the barrier properties
of the film. While increasing the thickness of the
film from the conventional thickness of 1.5 to 2.0
mils, to 5 mils or more, for instance, would improve
the bacrier properties of the film, it would be
economically undesirable to use a monolayer film of
PVDC having a thickness of 5 or more mils. Also, if
such thick films were employed, bag~ made from the
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film would be difficult to gather and clip closed at
the open end.
One approach to the provision o~ a f.ilm
having barrier pro~erties which are better than
those of the 1.5 to 2.0 mil monolayer PVDC f ilm
previous~y used for shrink packaging meat, is to
emeloy a multilayer f ilm, one layer of which is
vinylidene chloride-vinyl chloride copolymer having
a minimum amount of plasticizer. The other layer or
layers of such multilayer films are selected so as
to provide the requisite low temperature properties
and abrasion resistance which ~re lacking in the
vinylidene chloride-vinyl chloride layer containing
little or no plasticizer.
In providing such a film, however, it must
be recognized that good oxygen and moisture vapor
barrier proeerties, abrasion resistance, and low
temperature propertie~ are not the only requirements
for a film that is to be used for shrink packaging
erocessed meats and erimal and subprimal meat cuts.
The film mu~t have been biaxially stretched in order
to produce shrinkage characteristics sufficient for
the ~ilm to heat-shrink within a specified range of
percentages, e.g., from about 15 to 60 percent at
about 90C, in both the machine and the tran6verse
directions. tConventionally, the term "MD" refers
to machine direction and the term "TD" refers to
transverse direction.) The film must be heat
sealable in order to be able to fabricate bags from
the film and in order to heat seal the oeen ends of
the fabricated bags after insertion of the meat
product. The heat sealed seams of the bags must not
pull apart during the heat shrinking operation, and
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the film must resist puncturing by shar~ bone edges
during the heat shrinking oeeration.
Also, there must be adequate adhesion
between the several layers of the film so that
delamination does not occur, either during the heat
shrinking operation or during exposure of the film
to the relatively high temperatures that may be
reached during shipping and storage of the film in
the summertime. Delamination is the phenomenon
where layers of the multilayer film are readily
separable or easily pulled apart from face to face
integrity with no tearing of t~e individual layers
of film.
In order to overcome problems of
delamination, it is known in the prior art to use
adhesive layers between layers which do not
otherwise adhere to each other with the required
bonding strength under normal conditions of
commercial use. For example, polyethylene and
polypro~ylene are known to have poor lamination
characteristics when in face to face relationship
with the oxygen barrier layers conventionally used
in bags o~ multilayer films used for packaging
primal or subprimal meat cuts o~ fresh red meat, and
in bags for packaging processed msats. Adhesive
layers are often used in compensation for such poor
lamination characteristics.
By way of illustrating multilayer film
containing adhesive layers, Shirmer U.S. Patent No.
4,448,792 teaches a cook-in shrink bag fabricated
from a multilayer film having a first heat sealing
and food contacting layer composed of propylene
homopolymer or copolymer; a second heat shrinkable
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layer comeosed of a blend of pcoeylene homopolymer
or copolyme~ and butylene homopolymer or co~olymer:
a third adhesive layer comeosed of irradiatively
cross-linkable ethylene co~olymer: a fourth oxygen
barrier layer comerising vinylidene chloride
copolymer; a fifth adhesive layer of irradiatively
cross-linkable .ethylene copolymer: and a sixth
optical clarity layer comerising propylene
homopolymer or copolymer; wherein the entire six
layer film has been irradiated either before or
after the multilayer film has been biaxially
stretched. Among the several keY benefits
enumerated by Schirmer, the completely irradiated
six layer film provides bags having structural
integrity in that the bags resist delamination and
their heat seals have high temperature resistance,
while at the same time the bags are acceptable for
food contact in terms of minimum levels of
extractables. Cook-in bags made of this multilayer
film will maintain seal integrity and will resist
delamination when submerged in water at 80C for 12
hours.
While the use o~ adhesive layers, as taught
in such prior art, is effective in avoiding problems
of delamination, it i5 an undesirable solution.
~irst of all, additional equipment is required, and
the process becomes more complex with the need for
new extruders and extrusion dies. Additionally, the
addition of adhesive layers will generally make the
film thicker. The production of a thicker film may
be avoided, however, by ~educing the thickness of
the other film layers or by controlling the
thickness of the adhesive layers so that they are
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very thin. Since the oxygen barrier layer thickness
may not be reduced without the los6 of barrier
effectiveness, it becomes necessary to only reduce
the thickness of the outer layers, and this can
cause a reduction in heat sealability and/or a
reduction in puncture resistance. Thus, it is
ereferred not to reduce the thickness of the outer
film layers, but to control the adhesive layers to a
minimum thickness which is effective in bonding the
layers sufficiently to avoid delamination problems.
This in turn requires sophisticated equipment, which
is expensive, since the equipment must control the
adhesive layer thickness generally to a range of
from about 0.10 to about 0.15 mil. Moreover, the
adhesives themselves are generally very expensive,
and the cost of adhesive plus the cost of the new
equipment generally cause~ an increase in the price
of the bags produced from the multilayer film.
Thus, it is preferred to find a means for
strongly bonding the other layers directly to the
oxygen barrier layer without the use of prior art
adhesive layers.
It should be noted that the aforementioned
Shirmer U.S. Patent No. 4,448,792 teaches that both
polypropylene and blends of proQylene with another
polymer cegui~e an adhesive layer in order to bond
these layers acceptably to the oxygen barrier layer
of vinylidene chloride copolymer. ~owever, the
prior art also teaches such a need for adhesive
layers in regard to polyethylene.
Illustrative of a polyethylene, unblended
with any other polymer, which requires an adhesive
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layer in order to acceptably bond to the oxygen
barriar layer is U.S. Patent No. 4,640,856 to
Ferguson et.al. This patent discloses bags for the
packaging of fresh red meat (primal and subp~imal
meat cuts), cheeses, eoultry and othe~ food and
non-food products, wherein the bag is fabricated
from a multilayer thermoplastic shrink film having a
substrate layer of a very low density polyethylene
which has been extrusion coated with at least a gas
barrier layer comprising a copolymer of vinylidene
chloride or a hydrolized ethylene vinyl acetate
copolymec. This two layer film~ is then extrusion
coated with another layer of a thermoplastic polymer
to form at least a three laye! film having a core
layer of the gas barrier layer. Ferguson e~.al.
teach that an ethylene vinyl acetate layer should be
interposed between the surface layer of very low
density polyethylene and the gas barrier core layer
of vinylidene chloride coæolymer to promote adhesion
between the layers and to lessen any tendency of the
film to delaminate, since very low density poly-
ethylene does not adhere to vinylidene chloride
coeolymers as well as the ethylene vinyl acetate
does. Additionally, thi~ patent teaches that for
maximum delamination protection adhesive layers
should be used to bond outer layers of very low
density polyethylene to core layers of barrier film,
both when the oxygen barrier layer is a copolymer of
vinylidene chloride and when it is a hydrolyzed
ethylene vinyl acetate.
Illustrative of a polyethylene blended with
another polymer in one layer of a multilayer film,
wherein an adhesive layer is required in order to
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acceptably bond the blend layer to ~he oxygen
barrier layer. is U.S. 4,456,646 to Nishimoto
et.al. This pasent teaches that a multilayer film
for the packaging of meats and cheeses may be
fabricated of a core layer of a vinylidene chloride
copolymer with outer layers of a blend of ethylene
vinyl acetate and a linear low densi~y polyethylene
having a density of from 0.900 to 0.950 grams per
cubic centimeter. Nishimoto et.al. teach that the
linear low density polyethylene, which is a
copolymer of ethylene with a~ alpha olefin having
less than 18 carbon atoms, will not adhere to the
core layer of vinylidene chloride copolymer so that
the outer layers are apt to delaminate from the core
layers, pa~ticularly in an elevated temperature
environment. Acco~dingly, Nishimoto et.al. teach
that it is necessary to provide an adhesive layer
between the core layer of barrier film and each
outer layer containing the blend of ethylene vinyl
acetate and linear low density polyethylene.
We have found that not only are the
multilayer film structures of Perguson et.al. U.S.
Patent No. 4,640,856 undesirable from the standpoint
of the adhesive layer requirements, but in fact,
contrary to the teachings of Ferguson et.al., we
have discovered that a first outer layer of very low
density polyethylene will bond directly to a core
layer of oxygen bar~ier film comp~ising a vinylidene
chloride copolymer without the use of any adhesive
layer interposed the~ebetween, if the multilayer
film is a coextruded film. This matter will be
discussed more fully hereinafter.
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We have also found that not only are the
polyethylene blends of Nishimoto et.al. U.S. Patent
No. 4,456.646 undesira~le from the stand~oint of the
adhesive layer ~equirement, but they are also
undesirable because they cause the optical
pro~erties of the film to be unacceptably degraded.
Such polysthylene blends cause the haze value for
the multilayer film to increase to an unacceptable
level, and they cause the gloss value for the
multilayer f ilm to decrease to an unacceptable level.
The haze value is imeortant because it is
an indication of the ability of the film to transmit
light. A low haze value indicates a very clear film
which enables one to clearly see the contents of the
package. We find that the haze value must not
exceed 6.5% when packaging fresh red meat cuts and
proce6sed meats. This value is particularly
important in regard to the eackaging of processed
meats, since it is the individual consumer who is
viewing the package and deciding whether or not to
~ake a purcha~e.
The gloss value is important because it is
a measure of the shiny a~pearance of the film. A
high gloss value indicates that the packaged meat
product will have a very shiny highly attractive
appearance. We ~ind that the glo6s value should not
be below 70~ when packaging fresh red meat cuts and
processed meats. This value is pa ticularly
important in regard to the packaging of processed
meats, since it is the individual consumer who is
viewing the package at the point of purchase.
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Another important consideration in
evaluating multilayer films is the type and degree
of film curl which the film exhibits. Film curl is
an indication of the ease or the difficulty which is
experienced in opening a bag which has been
fabricated from the multilayer film. In order for a
multilayer film to be commercially acceptable. the
film must be capable of producing bags which are
easily opened by the bagging opeeator who place6 the
meat product into the bag on the production line of
the meat packing plant. 3ags which are difficult to
open cause delays in the baggi~g operation and
reeult in low production efficiency. The
significance of film curl will be discussed more
fully hereinafter.
In s~y thRn, it is an object of an a~t of the
present invention to provide a multilayer film
containing a core layer of an oxygen barrier and
outer layers of polyethylene, wherein the outer
layers are bonded directly to the core layer with no
adhe~ive layer~ interpo~ed therebetween.
It ie an ob~ect of an a~t of the p~nt
invention to provide such multilayer films, wherein
the haze and gloss properties of the multilayer film
are acceptable under conditions of commercial use.
It is an object of an aq~t of the present
invention to provide such multilayer films, wherein
the film~ are capable of fabrication into bags which
ace ea~ily oeenable under conditions of commercial
use, as indicated by the curl pro~erties of such
multilayer film~.
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It is an object of an aq~ of the present
invention to provide such multilayer films, wherein
no adhesive layers are employed tO bond the outer
layers to the core layer, and wherein the outer
layers provide imeroved strength characteristics and
improved puncture resi~tance to the film
SUMMARY OF THE INVENTION
The for-going ob3ectives of the present
invention may be achieved by providing a coextruded,
thermopla~tic, heat shrinkable, multilayer film
wherein, (a) said multilayer film comerises a ~irst
layer comprising very low density polyethylene, a
core layer comprising vinylidene chloride-methyl
acrylate copolymer, and a second layer compSising
vory low density polyethylene; and wherein, (b) said
fir~t lay-r is adhered directly to one side of said
core lay~r and said second layer i~ adhered directly
to th- oth-r ~id- of said cor- layer
In on- pr-ferred emboai~ent, the present
invention ~rovide~ this ~ultilayer film whorein the
first and ~-cond layers of very low density
polyethylen- are outer layers, and the core layer is
aonfin-d between these outer layers to provide a
thr-e layer fil~
In~another preferred embodiment, the core
layer o~ vinylidene chlorid--m-thyl acrylate is a
first cor- lay-r, and either the first layer or the
econd layer of very low density polyethylene is a
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second core layer confined between the first core
layer and an outer layer of thermoplastic polymer or
copolymer to provide a multilayer film having four
or more layers.
- In a further preferred embodiment, both the
first layer and the second layer of very low density
polyethylene are core layers confined between the
core layer of vinylidene chloride-methyl acrylate
copolymer and two outer layers of thermoplastic
polymer or copolymer to provide a multilayer f ilm
having five or more layers.
Despite the foregoing~teachings of the
prior art, we have now discovered that multilayer
films having a core layer of an oxygen barrier of
vinylidene chloride-methyl acrylate copolymer may
contain layers of very low density polyethylene
which are bonded directly to the core layer without
the use of interposed adhesive layers. Further, we
have discovered that the multilayer films of this
invention will not have increased haze or diminished
gloss i~ comparison to p~ior art multilayer films
currently in commercial use. Additionally, we have
found that the multilayer films of this invention
have acceptable film curl, and that they have
improved tensile strength and improved puncture
resistance.
The multilayer films of the present
invention may be further characterized by the fact
that the first layer of very low density
polyethylene and the second layer of very low
density polyethylene will bond directly to the two
sides of the core layer of vinylidene chloride-
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methyl acrylate copolymer even though said first layer
and said second layer both have substantial freedom from
cross-linking bonds. However, it is also within the
scope of the present invention for said first layer and
said second layer to comprise very low density
polyethylene which contains cross-linking bonds.
Other aspects of this invention are as follows:
A coextruded, thermoplastic, heat shrinkable,
multilayer film wherein:
~a) said multilayer film comprises a first layer
comprising very low density polyethylene of density not
greater than about 0.915 grams per cubic centimeter, a
core layer comprising vinylidene chloride-methyl
acrylate copolymer having a vinylidene chloride content
of from about 85 to about 95 weight percent and a methyl
acrylate content of from about 5 to about 15 weight
percent all based on the weight of said copolymer, and a
second layer comprising very low density polyethylene of
density not greater than about 0.915 grams per cubic
centimeter; and,
(b) said first layer is adhered directly to one
side of said core layer without adhesive material
therebetween and said second layer is adhered directly
to the other side of said core layer without adhesive
material therebetween.
A coextruded, thermoplastic, heat shrinkable,
multilayer film wherein:
(a) said multilayer film comprising a first layer
comprising very low density polyethylene of density not
greater than about 0.915 grams per cubic centimeter, a
core layer comprising vinylidene chloride-methyl
acrylate copolymer having a vinylidene chloride content
of from about 85 to about 95 weight percent and a methyl
acrylate content of from about 5 to about 15 weight
percent all based on the weight of said copolymer, and a
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second layer comprising very low density polyethylene
of density not greater than about 0.915 grams per cubic
centimeter;
(b) said first layer is adhered directly to one
side of said core layer without adhesive material
therebetween and said second layer is adhered directly
to the other side of said core layer without adhesive
material therebetween; and
(c) said first layer and said second layer have
substantial freedom from cross-linking bonds.
A process of producing a thermoplastic, heat
shrinkable, multilayer film suitable for use in
packaging fresh red meats and processed meats, which
comprises coextruding a core layer comprising a
vinylidene chloride-methyl acrylate copolymer having a
vinylidene chloride content of from about 85 to about 95
weight percent and a methyl acrylate content of from
about 5 to about 15 weight percent all based on the
weight of said copolymer, a first layer adhered directly
to one side of said core layer without adhesive material
therebetween and comprising very low density
polyethylene of density not greater than about 0.915
grams per cubic centimeter, and a second layer adhered
directly to the other side of said core layer without
adhesive material therebetween and comprising very low
density polyethylene of density not greater than about
0.915 grams per cubic centimeter.
DETAILED DESCRIPTION
All embodiments of the present invention comprise a
coextruded multilayer film, suitable for use in
packaging fresh red meat cuts and processed meats, which
comprises a core layer of vinylidene chloride-methyl
acrylate copolymer, a first layer comprising very low
density polyethylene, and a second layer comprising very
low density polyethylene, wherein the first layer and
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14bthe second layer are bonded directly to the first and
second surfaces of the core layer without the use of
adhesive layers.
It must be emphasized at this point that our
invention is directed to coextruded multilayer films,
because we have discovered that high adhesion bonding of
very low density polyethylene directly to the core layer
of vinylidene chloride-methyl acrylate copolymer, without
the use of interposed adhesive layers, can be achieved
through coextrusion. This is because the coextruded
multilayer film is produced by joining the several
layers together while all layers are in the li~uid
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phase. This allows the various polyme~s at the
layer-to-layer interface of liquid to intermingle
slightly so ~hat when the liquids solidify, the
layers ale strongly bonded to each other.
This explains why U.S. 4,640,856 to
Ferguson et.al. teaches that adhesive layers are
~equired in order to bond very low density
polyethylene to the vinylidene chloride copolymer.
The experimental runs which produce the multilayer
film samples for the Examples iA this Patent,
wherein the oxygen barrier layer was a copolymer of
vinylidene chloride, were all produced by the
extrusion coating process which is exemplified by
U.S. Patent Ns. 3,741,253 to Brax et.al. In this
extrusion coating ~rocess, a substrate layer is
first extruded to provide a tubular film which will
become the inner layer of the tubular multilayer
film. After this base layer has been made, a melt
of the oxygen barrier layer comprising the copolymer
of vinylidene chloLide is extrusion coated on the
outer surface o~ the tubular film. After this
extrusion coating has solidified to provide a two
layer substrate tubular film, a third layer of a
melted polymer or copolymer is coated on the outer
surface of the two layer tubular film to provide a
three layer tubular film. Succeedinq layers of
other polymers or copolymers may be extruded on this
three layer tubular film to the extent that tubular
multilayer films containing more than three layers
are desired. It is due to the fact that the
succeeding polymer melts are coated upon a solid
film substrate in each instance, that causes the
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succeeding layers to have very poor bonding when
very low density polyethylene and vinylidene
chloride copolyme~s are extrusion coated to one
another. The polymer melt is unable tO penetrate
the solid surface of the film subst{ate sufficiently
to allow the two layers to slightly intermingle at
the layer-to-layer inte~face and thereby produce a
strong interface bonding when the melted coating
solidifies.
In all embodiments of the present
invention, the core layer of oxygen barrier film
comprises vinylidene chloride-methyl acrylate
copolymer. The vinylidene chloride content of the
copolymer preferably should not exceed about 95
weight percent. This is because, when the
vinylidene chloride content is greater ~han about 95
weight percent, the vinylidene chloride-methyl
acrylate copolymer is generally not extrudable in
presently known coextrusion systems. However, the
vinylidene chloride content p~eferably should not be
less than about 85 weight percent of the vinylidene
chloride-methyl acrylate copolymer in order to
maintain the level of methyl acrylate in the
copolyme~ at not greater than 15 weight pe~cent,
which is the maximum level of methyl acrylate
cur~ently allowed by the United States Food and Drug
~dministration for ~ood contact applications.
It is within the scope of the p~esent
invention for the core layer of oxygen barrier film
to comprise vinylidene chloride- methyl acrylate
copolymer. It is also within the scope of the
p~esent invention for the oxygen
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barrier co~e layer tO contain vinylidene chloride-
metnyl acrylate copolymer blended with another
oxygen barrier material, such as vinylidene
chloride-vinyl chlocide copolymer. It should be
noted, however, that multilayer films containing
blends of ~inylidene chloride-methyl acrylate
coeolymer with vinylidene chloride-vinyl chloride
copolymer, and having adjacent layers of thermo-
plastic polymers in general, are not a part of this
invention, but are a separate invention which is
claimed in oqY~xjnq Canadian application Serial No.
567,067 filed May 18, 1988 in the name of J.M. Schuetz.
The vinylidene chloride-vinyl chloride
copolymer, which may be blended with the vinylidene
chlocide-methyl acrylate coeolymer, will contain at
least about 65 weiqht percent, and not more than
about 95 weight percent, of polymerized vinylidene
chloride because, when the vinylidene chloride
content is less than about 65 weight percent, the
oxygen and moisture barriec property of the
copolymer is diminished. If che vinylidene chloride
content is more than 95 weight percent, the
vinylitene chloride-vinyl chloride copolymer is
generally not extcudable.
The vinylidene chloride-methyl acrylate
coeoly~er and the vinylidene chloride-vinyl chloride
copolymer each will preferably contain less than 5
weight percent of a plasticizer. the percentages
being based on the weight of the total blend, i.e.,
including the copolymer and all additives such as
the plasticizer. in order to maxi~ize the barrier
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~ropertie6 of the film~ Conventional plasticizers
such as dibutyl sebacate and eeoxidized soybean oil
may be employed therein.
All embodiments of the present invention
COntaiQ a ~irst layer comprising very low density
eolyethylene and a second layer comecising very low
density eolyethylene. Those skilled in the art
recogni2e that this is a specific species of
polyethylene. Several species of polyethylene are
commercially available, and these species may be
characte~ized as the product of a high pressure
catalytic pcocess or the product of a low pressure
catalytic process.
The high pressure process produces eolymers
which are highly branched, with higher densities
being an indication of shorter branches and~high
cry~tallinity. Such polymers are conventionally
classiried as low den~ity polyethylene, commonly
called l~LDpel~ which has a~density below about 0.925
g~ams per CUbiC centimeter, and high density
polyethylene, commonly called "HDPE", which has a
density greater than about 0.940 gram~ per cubic
centimeter. Polyethylenes having a den~ity in the
range o~ ~rom about 0.925 to about 0.940 grams per
cubic centimeter ac- commonly reterred to as medium
density polyethylene.
The ethylene may also bs polymerized in the
high pressure process with other monomers, such as
vinyl acetate, ethyl acrylate, or acrylic acid. The
copolymer with vinyl ac-tate is known as ethylene
vinyl acetate and it is commonly referred to as
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The low pressure process produces polymers
which are more linear in structu~e. Such polymers
are commonly classified as very low density
polyethylene, commonly called "VLDPE", which has a
density of from about 0.860 to about 0.915 grams per
cubic centimeter, and linear low density poly-
ethylene, commonly called ~LLDPE~, which has a
density greater than about 0.915 grams per cubic
centimeter.
Very low density polyethylene and linear
low density eolyethylene are copolymers of ethylene
with a higher alpha olefin. The higher alpha
olefins which can be poly~e~ized with ethylene to
produce the low modulus linear copolymers can
contain from three to eight carbon atoms. These
alpha olefins should not contain any branching on
any of their carbon atoms closer than two carbon
atoms removed from the double bond. Suitable alpha
olefins include p~opylene, butene-l, pentene-l,
hexene-l, 4-methylpentene-1, heptene-l and
octene-l. The preferred alpha olefins are
ropylene, butene-l, hexene-l, 4-methylpentene-1 and
octene-l.
In some instances, one or more dienes,
either conjugated or non-conjugated, may have been
present in the polymerization reaction mixture.
Such dienes may include, for example, butadiene,
1,4-hexadiene, 1,5-hexadiene, vinyl norbornene,
ethylidene norbornene and dicyclopentadiene.
The linear polyethylene produced by this
low pressure ca~alytic copolymerization, typically
has a melt index of from about 0.5 to about 2.5
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decigrams per minute. When the melt index i9 below
0.5 decigrams per minute, the film is difficult to
extrude, and resins having a melt index above 2.5
decigrams per minute are not film grade resins.
~ s previously noted hereinabove, those
copolymers having a density in the range of f~om
about 0.86 to about 0.9~5 grams per cubic centimeter
are commonly referred to as a very low density
polyethylene, while those having a density greater
than about 0.915 grams per cubic centimeter are
commonly referred to as linear low density
polyethylene. ~
In one preferred embodiment of the present
invention, the first layer of very low density
polyethylene, which is bonded directly to the oxygen
barrier core layer of vinylidene chloride-methyl
acrylate copolymer, provides the heat sealing layer
for the multilayer film. Where the film is produced
by coextrusion to provide a tubular multilayer film,
the first layer of very low density polyethylene
will be the inner layer of ~he tubular film.
In another preferred embodiment of the
pre~ent invention, the second layer of very low
density polyethylene, which is bonded directly to
the core layer Or vinylidene chloride-methyl
acrylate copolymer without the use of adhesives,
provides the second outer layer of a three laye~
~ilm embodiment. It has been found that using a
very low density eolyethylene in the second outer
layer of the multilayer film provides the film with
a puncture ~esistance which i9 improved over similar
films containing outer layers o~ ethylene-vinyl
acetate copolymers, and it also provides loading and
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-21-
shrink tunnel survival rates which are at least
equal to, if not su~e~io~ to, the survival rates of
those films which contain ethylene vinyl acetate
copolymer outer laye~s.
In an alte~na~e embodiment of the present
invention, however, the first layer of very low
density polyethylene, which is bonded directly to
the oxygen barrier core layer of vinylidene
chloride-methyl acrylate copolymer, provides a
second core layer which is confined between the
first core layer of oxygen barrier film and a first
outer layer of a heat sealable thermoplastic polymer
or copolymer. The heat sealable first outer layer
may comprise such films as an ionomeL, an ethylene
vinyl acetate copolymer, an ethylene-propylene
copolymer, and the like. Also suitable is
polypropylene blended with another polymer, such as
polybutene-l. Thus, this embodiment contemplates a
multilayer film having four layers, although more
than rOur layers are also possible.
In another alternate embodiment of the
present invention, the second layer of very low
den~ity polyethylene, which is bonded directly to
the oxygen barrier first core layer of vinylidene
chloride-methyl acrylate copolymer without the use
o~ adhesives, provides a second cora layer which is
confined between the first core layer and a second
outer layer which comprises a thermoplastic polymer
or copolymer. Thus, this embodiment of the present
invention also contemplates another multilayer film
having four layers, although multilayer films having
more than rOur layers are also possible.
,~
D-ZOOZO
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The addition of this fourth layer as a
second outer layer for the multilayer film of this
invention may be undertaken when it is desired to
improve the abrasion resistance of the film. This
can be accomplished by providing a second outer
layer comprising a thermoplastic polymer or
copolymer such as an ionomer resin, a propylene-
ethylene copolymer, a high density polyethylene, a
linear low density polyethylene, and the like, as
well as blends thereof.
In one preferred embodiment, the second
outer layer (the fourth layer)~comprises a blend of
a high density polyethylene with a linear low
density polyethylene or a very low density
polyethylene. ~8 the level of high density
polyethylene in the blend of the second outer layer
is increased, the abrasion resistance of the film
continues to increase. However, when the level of
high density polyethylene in the second outer layer
blend becomes greateL than about 5 weight percent,
the haze value of the film becomes unacceptable for
bags used in the packaging of processed meats.
Additionally, when the level of high density
polyethylene is increased to greater than 30 weight
percent of the second outer layer blend, the
shrinkage property of the film becomes unacceptable
for bags used in the packaging of fresh red meats
and eeocessed meats. Thus, the amount of high
density polyethylene in the blend should not exceed
30 weight percent in fresh red meat bags or 5 weight
percent in proce~sed meat bags. The high density
polyethylene useful in the ~econd outer layer has a
melt index of from about 0.1 to about 1.0 decigram
.,
D-Z0020
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-23-
per minute, and a density of from about 0.94 to
about 0.96 gram per cubic centimeter. Resins having
a melt index below 0.1 are not extrudable, and those
having a melt index above 1.0 produce films of
diminished strength.
In summary then, the broadest aspect of the
multilayer film of this invention contemplates a
three layer film. However, it is contemplated that
one or more layers may be added to the outer surface
of either the first layer or the second layer of
very low density polyethylene to erovide a
multilayer film containing fou~ or more layers.
Moreover, it is also contemplated that one or more
layers may be added to the outer surface of both the
first layer and the second layer of very low density
polyethylene to provide a multilayer film containing
five or more layers.
The coextruded thermoplastic multilayer
films of this invention can be produced by known
techniques. For example, the multilayer films may
be prepared by coextruding the film layers through
an annular die to produce a primary tube, and then
biaxially stretching the multilayer tubular film in
accordance with the conventional "double-bubble~'
technique disclosed in Pahlke U.S. Patent No.
3,456,044. ~lternatiYely, the coextruded multilayer
film ~ay be slot cast and biaxially stretched by
tentering before the resulting sheet is fabricated
into bags.
When the coextruded, multilayer film of the
present invention has been produced, it may be
desirable to cross-link the multilayer film,
although cross-linking is not required in order to
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achieve high adhesion bonding of the very low
density polyethylene directly to the vinylidene
chloride-methyl acrylate core layer without the use
of adhesive layers thereinbetween. Cross-linking
may be undertaken in order to enhance the heat
sealing characteristics of the first outer layer of
very low den~ity eolyethylene, which is the inner
layer of the tubular film embodiments. Cross-
linking may also be undertaken in order to improve
the puncture resistance of the multilayer film~
While chemical cross-linkinq is feasible, we prefer
to crôss-link by irradiation. ,Chemical cross-
linking may be achieved by means such as organic
peroxide cross-linking, or by the addition of a
silane to the very low density polyethylene and the
subsequent reaction of the modified polyethylene
with a silanol condensation catalyst and water. We
prefer to cross-link by irradiation, since this
technique is less complicated and it entails a lower
cost than the chemical cross-linking methods.
Although the irradiation may be undertaken prior to
the biaxial stretching step, we prefer to irradiate
the multilayer film after biaxially stretching the
film. The film is preferably irradiated with
electrons at a dosage of from about 1 to about S
mega~ads, and more preferably at a dosage of f~om
about 2 to about 3 megarads.
In summary then, the multilayer films of
the pcesent invention may be characterized by the
fact that the first layer of very low density
polyethylene and the second layer of very low
density polyethylene will bond directly to the two
sides of the first core layer of vinylidene
.
.
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chloride-methyl acrylate copolymer even though said
first layer and said second layer both have
substantial freedom from cross-linking bonds.
However, it is also within the scope of the present
invention for said first layer and said second layer
to comprise verg low density polyethylene which
contains cross-linking bonds. Those skilled in the
art recognize that cross- linking, whether by
irradiation or by chemical means, will cause the
melt index o~ the cross-linked VLDPE layers to be
below the melt index of the original VLDPE resins
which are used respectively in~-the first layer of
VLDPE and in the second layer of VLDPE. Thus, the
term "contains cross-linking bondsl' means that the
melt index of the VLDPE layers is significantly
lowered during the pcocess of converting the
respective resins of the two VLDPE layers into the
multiIayer film products of this invention.
Similarly, the term "having substantial freedom from
cross-linking bonds" means that the melt index of
the VLDPR layers is not significantly lowered during
the process of converting the respective resins of
the two VLDPR layers into the multilayer film
products of this invention.
In one ere~erred embodiment of the present
invention, the coextruded multilayer film comprises
a biaxially stretched thermoplastic three layer film
hav~ing a total thickness of from about 1.75 mils to
about 4.5 mils, and preferably from about 2.0 mils
to about 3.0 mils. Films of less than about 1.75
mils thickness will generally not have the necessary
puncture resistance, and films of greater than about
4.5 mils will produce bags which will be somewhat
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1 3 1 5062
-26-
difficult to gather and clip closed or they may be
difficult to heat seal closed at efficient speeds on
heat sealing vacuum packaging machines. The heat
sealing ficst outer layer of ve~y low density poly-
ethylene will pceferably have a thickness of fcom
about 1.~ mils to about 1.8 mils; the core layer of
vinylidene chloride-methyl accylate copolymer will
preferably have a thickness of from about 0.25 mil
to about 0.45 mil; and the second outer layer of
very low density polyethylene will have a thickness
of from about 0.35 mil to about 2.0 mils, but
eceferablY from about 0.5 mil to about 1.0 mils.
In this three layer film embodiment, the
thickness of the first outer layer is preferably
within the aforementioned range in order to obtain
good seal strength and acceptable film shrinkage.
The thickness of the first core layer is preferably
within the aforementioned range in order to provide
adeguate oxygen barrier without detracting from
toughness properties, but the upper limit of 0.45
mil is based upon the extent of the barrier
protection which is required for the intended use
for the multilayer film. The thickness of the
second outer layer is preferably within the
aforementioned range in order to make up the total
film thickness and to pcovide properties of abcasion
resistance and puncture resistance.
As noted hereinabove, the present invention
contemplates two alternate embodiments of multilayer
~ilm having four layecs. The first alternate
embodiment comprises a coextruded multilayer film
containing a first outer layec of a heat sealable
thermoplastic polymer or copolymer; a first coce
D-Z0020
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31 5062
layer of an oxygen barrier material comprising
vinylidene chloride-methyl acrylate copolymer; a
second core layer comprising very low density
p~lyethylene, confined between said first outer
layer and said first core layer, and bonded directly
to said ~irst core layer without the use of
adhesives; and a second outer layer comprising ve~y
low density eolyethylene bonded directly to said
first core layer without the use of adhesives. The
second alternate embodiment comprises a coextruded
multilayer film containing a first core layer of
oxygen barrier material comprising vinylidene
chlocide-methyl acrylate coeolymer; a heat sealable
fi~st outer laye~ comprising very low density
eolyethylene bonded directly to said first core
layer without the use of adhesives; a second outer
layer comprising a thermoplastic eolymer or
coeolymer; and a second core layer comprising very
low density polyethylene. confined between said
Sirst core layer and said second outer layer, and
bonded directly to said ~irst core layer without the
use of adhesives.
In said Sirst alternate embodiment of the
pcesent invention, the coextruded multilayer film
comerises a biaxially stretched thermoelastic four
layer film having a total thickness of from about
2.0 mils to about 4.5 mils. The 2.0 mils lower
Iimit is established by the total thickness achieved
in adding the lower limit of thickness for the four
individual layers. As 2reviously noted, films
having a thickness greater than about 4.5 mils will
produce bags which will be somewhat difficult to
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1 31 50h2
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gather and clip closed or they may be difficult to
heat seal closed at efficient seeeds on heat sealing
vacuum packaging machines. The heat sealing first
outer layer of thermoplastic polymer or coeolymer
will preferably have a thickness of from abol~t 1.1
mils to about 1.8 mils; the first core layer of
vinylidene chloride-methyl acrylate copolymer will
preferably have a thickness of from about 0.25 mil
~o about 0.45 mil; the second core layer of very low
density ~olyethylene confined between said first
outer layer and said first core layer will
preferably have a thickness of-from about 0.35 mil
to about 2.0 mils; and the second outer layer of
very low density polyethylene will prefecably have a
thickness of from about 0.35 mil to about 2.0 mils,
but more pre~erably ~rom about 0.5 mil to about
1.0 mil.
In the second alternate embodiment of the
present invention, the coextruded multilayer film
comprises a biaxially stretched thermoplastic four
layer film having a total thickness of from about
2.0 mils to about 4.5 mils. The 2.0 mils lower
limit is established by the total thickness achieved
in adding the lower limit of thicknesg for the four
individual layers. A~ 2reviously noted, films
having a thickness greater than about 4.5 mils will
produce bags which will be somewhat difficult to
gather and clip closed or they may be difficult to
heat seal closed at efficient speeds on heat sealing
vacuum packaging machines. The heat sealing first
outer layer of very low density polyethylene will
preferably have a thickness of from about 1.1 mils
to about 1.8 mils; the first core layer of
D-20020
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-29-
vinylidene chlo~ide-methyl acrylate copolyme~ will
p~eferably have a thickness of from about 0.25 mil
to about 0.45 mil; the second core layer of very low
density polyethylene confined between said f i~8t
core layer and the second outer layer will
preferably have a thickness of f~om about 0.35 mil
to about 2.0 mils; and the second outer layer of
thermoelastic polymer or coeolymer will preferably
have a thickness of from about 0.35 mil to about 2.0
mils, but more preferably from about 0.5 mil to
about 1.0 mil.
In both alternate embQdiments comprising
coextruded four layer films, the thickness of the
first outer layer is preferably within the
afo~ementioned range in order to obtain good seal
strength and acceptable film shrinkage. The
thickness of the first core layer is preferably
within the aforementioned range in order to provide
adeguate oxygen barrier without detracting from
toughness properties, but the ueper limit cf 0.45
mil is baset upon the extent of the barrier
protection which i5 reguired or the intended use
for the multilayer film. The thickness of the
second core layer is preferably within the
above-indicated range in order to enhance the
puncture resistance of the film without being too
costly. The thickness of the second outer layer is
preferably within the aforementioned range in order
to make up the total film thickness and to provide
properties of abrasion resistance and puncture
resistance.
In addition to providing good heat sealing
when the film is fabricated into bags, multilayer
.
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films of the eresent invention have good shrink
properties, good abrasion resistance and good
toughness. Thus, these films have utility in many
eackaging applications. However, in a preferred
embodiment, these films are fabricated into bags ~or
the packaging of primal and suberimal meat cuts of
fresh red meat, and for the packaging of e~ocessed
meats.
Such bags may be eroduced from the
multilayer films of this invention by any suitable
method, such as by heat sealing the side ant/or
bottom edges. For instance, i~ the film of this
invention is produced in the form of a tubular film,
bags can be produced therefrom by heat sealing one
end of a length of the tubular film, or by sealing
both ends of the tube end and then slitting one edge
to form the bag mouth. If the film of this
invention is made in the form of flat sheets, bags
can be formed therefrom by sealing three edges of
two sueerimeosed sheets of film. When carrying out
a heat sealing opelation, the surfaces which are
heat sealed to each other to form seams are the said
first outer layers of the films of this invention.
Thu~, for example, when forming a bag by heat
sealing one edge of a length of tubular film, the
inner surface of the tube, i.e., the surface which
will be heat sealed to itself, will be the said
first outer layer of the film. Accordingly, the
first outer layer of the film becomes the inner
surface Or the bag and the second outer layer of the
~ilm becomes the outer surface of the bag.
The invention is further illustrated by
Examples which are eresented hereinafter.
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The resins which were used in making the
multilayer films of the Examples are identified as
follows:
EthYlene Vinvl Acetate (EVA)
EVA-l: DQDA 6832
Vinyl Acetate 11 Wt.%
Melt Index 0.25 dg./min.
Union Carbide Corporation;
Danbury, CT
EVA-2: ELVAX 3135X _~
Vinyl Acetate 12 Wt.%
Melt Index 0.35 dg./min.
E. I. DuPont de Nemours ~ Co.,
Inc.; Wilmington, DE
EVA-3: ENRON 3507C
Vinyl acetate 5 Wt.S
~elt Index 0.25 dg./min.
USI Chemicals Company
Cincinnati, OH
EVA-4: A blend of EVA-2 and EVA-3
75 Wt.t EVA-2. 25 Wt.% EVA-3
Contains about 9 Wt.% vinyl acetate
VinYlidene~Chloride-Vinvl Chloride Co~olvmer(VC-VDC~
VC-VDC: Kureha F Resin
Vinyl Chloride 29 Wt.%
Molecular Weight 125.000
Kureha Chemical Industry Co., Ltd.
Tokyo, Japan
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Vinylidene Chloride-Methyl AcrYlate Co~olYmer(MA-VDC)
MA-VDC: XU 32023
Methyl Acrylate B wt.%
Molecular Weight 105,000
Dow Chemical Co.; Midland, MI
VinYlidene Chloride CoPolvmer Blend
Bl~nd: 75 Wt.% MA-VDC
25 Wt.~ VC-VDC
Linear Low DensitY Pol~eth~lene ~LLDPE~
LLDPE-l: ~S 7028
Melt Index l.O dg./min.
Density 0.918 gm.~cc.
Ethylene-Hexene Copolymer
Union Carbide Corporation; Danbury,
CT
LLDPE-2: Dowlex 2045
Melt Index l.O dg.~min.
Density 0.920 gm.~cc.
Ethylene-Octene Copolymer
Dow Chemical Co.: Midland, MI
VerY Low Densit~ PolYethYlene (VLDPE~
VLDPE-l: DFDA 1137
Melt Index l.O dg./min.
Density 0.905 gm./cc.
Ethylene-Butene Copolymer
Union Carbide Corpocation; Danbury,
CT
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IM
VLDPE - 2: Dowlex 4002
~elt Index 2.5 dg./min.
Density 0.912 gm./cc.
Ethylene-Octene Copolymer
Dow Chemical Co.; Midland, MI
VLDPE-3: Dowlex 4001
Melt Index 1.0 dg./min.
Den~ity 0.912 gm./cc.
Ethylene-Octene Coeolymer
Dow Chemical Co.; Midland, ~$
VLDPE-2 and VLDPE-3 are commonly called
"ultra low density eolyethylene~' or ~'ULDPE~ by the
9ew Chemical Company. Since these two resins have
densities which are below 0.915 gms./cc., for
purposes of this invention they are very low density
polyethylene. There~ore, we have identified these
resin~ as VLDPE-2 and VLDPE-3 for eurposes of
clarity and consistency in the Examples whi~h follow.
The pcoperties of the resin~ and of the
produced therefrom may be determined by the
~ollowing methods:
Den~ity:
ASTM D-1505 - Plaque is conditioned ~or one
hour at 100C to aeproach equilibrium
crystallinity - ~eeorted as gms/cm .
Melt Index (MI):
ASTM D-123~ - Condition E - measured at
190C.
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Haze:
~STM D-1003, Procedure A.
Gloss:
~STM D-523, 45 Angle.
Tensile Strength and Elongation At Break:
ASTM D-882, Procedure A
Shrinkage Values:
Values are obtained by measuring
unrestrained shrink at 90C for five
seconds. Four test specimens are cut from
a given sample o~ the film to be tested.
The specimens are cut~to 10 cm. in the
machine direction by 10 cm. in the
transverse direction. Each specimen is
completely immersed for S seconds in a 90C
wate bath. The distance between the ends
of the shrunken specimen is measured. The
difference in the measured distance for the
shrunken seecimen and the original 10 cm.
is multiplied by ten to obtain the percent
of shrinkage $or the specimen. The
shrink~ge for the four specimens is
averaged for the MD shrinkage value of the
given film sample, and the shrinkage for
, the four specimens is averaged for the TD
shrinkage value.
Dynamic Puncture:
The dynamic puncture-impact test procedure
is used to compare films for their
resistance to bone puncture. It ~easures
the energy required to puncture a test
sample with a sharp triangular metal point
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made to simulate a sharp bone end. A
Dynamic Ball Burst Tester, Model No. 13-8,
available from Testing Machines, Inc.,
Amityville, Long Island, New York, is used,
and a 3/8 inch diameter conical tip is
installed on the tester probe arm for use
in this test erocedu~e. The conical tip
has the configuration of a right circular
cone, and the angle between the cone axis
and an element of the conical surface at
the vertex is about 65. Six te~t
specimens approximately 4 inches square are
prepared, a samele is placed in the sample
holder, and the 2endulum is relea~ed. The
puncture energy reading is recorded. The
test is repeated until 6 samples have been
evaluated. The results are calculated in
cm-kg per mil of film thickness and are
ave~aqed.
Hot Wat3r Puncture:
Hot water puncture values are obtained by
perSorming the hot water puncture test as
Sollows. Water is heated to 90 + 1C. A
3/8 inch round wooden dowel is sharpened on
one end to a conical point. This sharpened
point has the configuration of a right
circulal cone, and the angle between the
cone axis and an element of the conical
surface at the vertex is about 60. This
sharp point is then rounded to a spherical
tip of about 1/16 inch diameter. The
wooden dowel is fastened to a wooden block
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so that the rounded point projects 1-1/2
inches beyond the end of the wooden bloc~.
A specimen about 3 inches wide in the
transverse direction (TD) and about ten
inches long is cut from the test sample
material. One end of the specimen is
placed on the end of the wooden block
opeosite the pointed dowel. The specimen
is wrapped around the end of the shareened
dowel and back to the wooden block on the
oppo~ite side, where it is secured. The
film thickness in the~area of contact with
the sharpened dowel is measured in order to
assure that the film specimen thickness is
truly representative of the given test
sample material. The specimen and pointed
dowel are quickly immersed into the hot
water and a timer is started. The timer is
stopped when the wooden dowel point
punctures the film specimen. The test
procedure is repeated five more times with
new 3 inch wide TD specimens from the given
test sample material. The time required
for penetration is recorded and then
averaged for the six TD specimens.
Athesion And Curl:
~lthough the p~operties of adhesion and
curl are separate and distinct properties
of mul~ilayer films, they are both
determined by a single test procedure. A
coextruded multilayer film in tubular form
having a nominal length of about 2 feet is
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cut to provide a sample having a straight
TD cut on one end and an arc TD cut on the
other end. The two cuts simulate a
straight bag mouth and an arcuate bag
mouth. The film sample is inserted into a
hot air circulating oven set at 120F.
After seven days at 120F, the sample is
removed from the oven and it is inspected
at both the straight and arcuate ends for
curl and delamination. In general, both
ends of the tubular sample will exhibit the
same degree of curl a~d delamination.
Delamination is generally found or not
found at the interface between the oxygen
barrier core layer and the adjacent layer.
The extent of delamination is reported as
"adhesion", and the adhesion is categorized
as being poor, fair, good or excellent.
~ilms having an adhesion of poor and fair
are unacceptable for packaging primal and
subprimal meat cuts and processed meats.
Curl is an indication of the ease or
difficulty which will be experienced when
opening bags fabricated of the multilayer
film of the given sample. An outward curl
i5 an indication that the bags will be
easily opened. An inward curl is an
indication of the degree of difficulty in
opening a bag. Curl is categorized as
slightly inward, moderately inward, tightly
inward, slightly outward, moderately
outward and tightly ou~ward. Tubing
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samples showing a tightly inward curl are
deemed unacceptable as p~oviding bags which
are too difficult to open. Tubing samples
which show a mode~ately inward curl, a
slight inwaLd curl, or any degree of
outward curl are deemed acceptable for the
fab~i~ation of bags.
The invection is now described fu~ther in
the followi~g Examples. In the Examples, all parts
and percentages are by weight unless otherwiee
indicated.
EXAMPLE 1
This Exam~le illustrates the production of
a first set of multilayer films under conventional
coextrusion operating conditions.
A first series of coextrusion runs was made
in ordec to produce three layer films containing an
oxygen barrier layer comprising the vinylidene
chloride-vinyl chloride copolymer defined herein-
above as VC-VDC. The first outer layer contained
ethylene vinyl acetate copolymer identified herein-
above as EVA-l, and the EVA-l was blended in eight
of ten runs with other copolymers of ethylene
identified hereinabove as LLDPE-1, VLDPE-l and
VLDPE-Z. The second outer layer contained the blend
of ethylene vinyl acetate copolymers which is
identified hereinabove as EVA-4.
The melted resins were extruded from a
multilayer annular die to produce the three layer
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film in a tubular form, wherein the first outer
layer was the inner layer of the tubular film. The
tubular film was extruded from the annular die to
form a pcimary tube which was then biaxially
stretched in acco~dance with a conventional
"double-bubble" technique similar to tha~ disclosed
in U.S. Patent No. 3,456,044 to Pahlke. The
biaxially stre~ched multilayer film was flattened
and the~ reeled ~or storage and samele evaluation.
The film produced by these runs was not icradiated.
The comeosition of each multilayer film
produced in these runs is set ~orth as Samples No.
~-1 through A-10 in Table 1. Note that all runs
~roduced films containing a core layer of VC-VDC and
an outer layer of EVA-4. The inner layer contained
blends of EVA-l with varying amounts of LLDPE-l,
VLDPE-l, and VLDPE-2, exceet that Samele No. A-l was
100 wt.S EV~-l and Sample No. A-10 was 100 wt.%
VLDPE-2. Sample No. A-l is a control sample which
i8 the equivalent of a film product which has been
in commercial use for many years in the packaging of
p~imal and suberimal meat cuts and processed meats,
having been sold first by Union Carbide Corporation
of Danbury, CT, and now being sold by Viskase
Corporation of Chicago, IL, under the product
identifications of PERFLEX ~ 52 8ag and
PERFLEX ~ 62 Bag.
The films produced in this series of runs
had a total thickness of 2.4 mils. The inner layer
was 1.4 mils in thickness, the core layer of VC-VDC
was 0.3 mils th~ick, and the outer layer of EV~-4 was
0.7 mils thick.
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E~AMPLE 2
This Example illustrates the production of
a second set of multilayer films under conventional
coextrusion operating conditions. These films
include one embodiment of the present invention.
~ second series of coextrusion runs was
made in order to eroduce three layeL films
containing an oxygen barrier layer comprising the
blend of vinylidene chlozide-vinyl chloride
copolymer with vinylidene chloride-methyl acrylate
copolymer which is defined hereinabove. The blend
comerised 75 wt.% of the methyl acrylate copolymer
and 25 wt.% of the vinyl chloride copolymer which
are both defined hereinabove. The process utilized
was the same as that set forth in Example 1.
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TABLE 1
ComPosition O~ Coextruded Films
SamPle No. Inner Laver Co~e La~er Outer Laver
A-l EVA-l VC-VDC EVA-4
A-2 15% LLDPE-l VC-VDC EVA-4
85S EVA-l
A-3 25S LLDPE-l VC-VDC EVA-4
75% EVA-l
A-4 15% VLDPE-l V~-VDC EVA-4
85% EVA-l
A-5 25% VLDPE-1 VC-VDC EVA-4
75% EVA-l
A-6 50% VLDPE-l VC-VDC EVA-4
50% EVA-l
A-7 15% VLDPE-2 VC-VDC EVA-4
85S EVA-l
A-8 25S VLDPE-2 VC-VDC EVA-4
75% EVA-l
A-9 50S VLDPE-2 VC-VDC EVA-4
50% EVA-l
A-10 VLDPE-2 VC-VDC EVA-4
B-l 50S LLDPE-2 Blend 50S LLDPE-2
50% EVA-l 50S EVA-4
B-2 VLDPE-3 Blend VLDPE-3
B-3 EVA-l Blend EVA-4
,,
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The coextruded biaxially stretched films were reeled
for storage and sample evaluation without
irradiation of the films.
The composition of each multilayer film
eroduced in these runs is set forth as Samples No.
B-l through B-3 in Table 1. Note that these runs
produced multilayer films containinq an inner layer
and an outer layer of the same com~osition in
Samples No. B-l and B-2. Sample No. B-l had inner
and outer layers of a blend containing resin
LLDPE-2. which is defined hereinabove, and E~A-l in
equal amounts. Sample No. B-2~had inner and outer
layers of 100% VLDPE-3, which is defined
hereinabove. and this Sample illustrates one
embodiment of the present invention. Sample No. B-3
was a control sample similar to Sample No. A-l in
that it was the equivalent of the prior art
commercial product noted in Example 1.
The rilms produced in this series of runs
had a total thickness of 2.4 mils. The inner layer
had a thickness of l.g mils, the core layer was 0.3
mil thick. and the outer layer was 0.7 mil thick.
EXAMPLE 3
This example summarizes the key physical
prope~ties of the unirradiated reel stock multilayer
films produced in the series of coextrusion runs
defined in Examples 1 and 2, in regard to film
acceetability fo~ use in the packaging of primal and
subp~imal meat cuts and processed meats.
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--43--
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Specimens of the unirradiated reel stock
for each sample were evaluated for layer adhesion,
film cu~l. haze and gloss in ocder to determine the
acceptability of the various films in each of these
categories. The results are presented in Table 2.
Layer adhesion is a visual evaluation of
the degree of delamination, if any, found at the
interface between the core layer and the inner
layer. Delamination may be exhibited by actual
separation of the layers at the end cuts of the film
specimen, or by the aepearance of blisters at the
surface between the end cuts. ~The blisters are a
sign of layer separation in the body of the film
between the end cuts. Only the prior art control
Samples No. A-l and B-3, and new film Samples No.
A-6, B-l and B-2 showed acceptable adhesion.
Note that the prior art films of Samples
No. A-l and B-3, which contain inner and oute~
layers of EVA, showed an adhesion rating of Good,
while the inventive film of Sample No. B-2, which
contained inner and outer layers of VLDPE had an
adhesion rating of Excellent. Thus, the film
embodiment of this invention showed an adhesion
rating which was improved over the adhesion rating
of the prior art films. This is contrary to the
teachings of Ferguson et.al. U.S. Patent No.
4,640,856 which states that VLDPE does not adhere as
well as EVA does.
~ The evaluation of film curl showed that
; only the prior art commercial type films of Samples
No. A-l and B-3 had an outward curl, which is the
most desired type o~ curl for ease in opening the
mouth of a bag made of the multilayer film. For the
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1 31 5Q62
experimental multilayer films which were evaluated,
only samples A-4, B-l and B-2 had an acceptable
cu~l. Sample B-l had a slight inward curl, and
Samples A-4 and B-2 had a moderate inward curl. All
other samples had an unacceptable tight inward curl.
The differences in the direction o~ curl,
inward or outward, and in the degree of the curl are
caused by the differences that exist between the
inner layer an~ the outer layer. When LLDPE or
VLDPE is blended into the EVA-l of the inner layer,
the balance between the inner layer and the outer
layec becomes distorted and the film exhibits the
unacceetable tight inward curl of Samples No. A-2,
A-3 and A-5 through A-10.
Curl also has an influence on adhesion
since it can impose stresses on the layers which may
cause delamination. This is the probable reason why
only those samples which had both acceptable
adhesion and acceptable curl are Samples No. B-l and
B-2, where the inner and outer layers both had the
same composition.
The data in Table Z demonstrates that a
' multilayer film containing LLDPE or VLDPE in the
,' inner layer must also have the same composition in
the outer layer in order to assure that the ilm
will have acceptable adhesion and acceptable curl.
The data in Table 2 also shows that films
which contain a blend of EVA-l with LLDPE or VLDPE
exhibit unacceptable optical characteristics. The
only exception is Sample No. A-4 which showed
acceptable haze and acceetable gloss, but this
' sample had a low VLDPE content of 15 wt.S in the
3 inner layer. Also, this sample had an unacceptable
,,~
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1 3 1 5062
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adhesion. Sample No. A-lO containing lO0 wt.
VLDPE-2 in the inner layer and Samele No. B-2
containing lO0 wt.~ ~LDPE-3 in both the inner and
outer layers also had acceptable haze and acceptable
gloss. (As noted hereinabove, haze value must not
exceed 6.5% and gloss value must not be below 70%
for a film to be acceptable.)
In order for a multilayer film to be
acceptible for use in the packaging of fresh red
meat cuts and processed meats, the film must be
acceptable in all four catagories of adhesion, curl,
haze and gloss, and the data show that only three of
the film samples meet this standard. They are the
prior art Samples No. A-l and B-3, and the new
Sample No. B-2 which is an embodiment of our
invention.
Sample No. B-2 is also noteworthy, not only
for being the only new film meeting all standards
for film acceptability, but also because it exhibits
the best adhesion, the best curl, the best haze, and
the best gloss for all of the new film samples.
The data in Table 2 also indicate that
excellent adhesion and acceptable curl are obtained
if the core layer is a blend of 1S wt.% MA-VDC and
25 wt.t VC-VDC with lO0 wt.% VIDPE in both the inner
and outer layers. This blend demonstrates that the
core layer may contain a substantial quantity of
VC-VDC in the MA-VDC blend without adversely
effecting the superioE physical characteristics of
the multilayer films of this invention. A core
layer of lO0 wt.% MA-VDC will also give acceptable
adhesion and curl with inner and outer layers of lO0
wt.% V~DPE.
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EXAMPLE 4
This Example illustrates that the
embodiment of our invention which is represented by
Sample No. B-2, not only has acceptable adhesion,
curl, haze and gloss, but that it also has other
eroperties which make it suitable for the packaging
of erimal and subprimal meat cuts and processed
meats.
The inventive multilayer film Sample No.
B-2 and the prior art multilayer film of Samples No.
A-l and B-3 were evaluated for tensile strength,
elongation at break, shrinkage, hot water puncture,
and dynamic puncture. The test results are given in
Table 3.
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TABLE 3
Properties Of Coextruded
Unirradiated ~ultila~er Films
SampLe Sample Sample
No. A-l No. B-2 No. ~-3
Tensile stren8th. psi. 7,100/8,700 11,900~12,400 7,500~8,800
elOn~atiOn e Break, % 215/150 240/170 215/170
Shrinka~e @ 90'C, % 35/52 2~/32 37~53
Hot Water Puncture, sec.
@90-C 29 120+ 24
Dynamic Puncture,
cm.-k~./mil 2.0 2.7 2.1
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The inventive multilayer film Sample No.
B-2 had tensile strength, hot water puncture, and
dynamic puncture values which were improved over the
sameles of prior art film~ The shrinkage values
were less ehan the values for the prior art film,
but the shrinkage remained at an acceptable level.
Elongation at break was about the same for all three
samples, and ~as acceptable.
Although certain embodiments of this
invention have been described ~in detail, it is
contemplated that modifications thereof may be made
and some preferred features may be employed without
others, all within the spi~it and sco~e of the broad
invention. For example, although Sample B-2 had the
same very low density ~olyethylene on both sides of
the core layer of vinylidene chlorids-methyl
acrylate copolymer, it is possible for two different
VLDPE resins to be used, provided that their
physical characteristics must not be so different
that unacceptable curl or unacceptable adhesion
results. Additionally, it is contemplated that the
VLDPE of the first layer, or of the qecond layer, or
of both the first and the second layers, may
comprise a blend of VLDPE with one or more other
polymers or copolymers, proviaed that such blends
must not cau~e degradation of adhesion, curl, haze
or gloss to such an extent that the multilayer film
is rendered unacceptable. Further, those skilled in
the art will recognize that the multilayer films of
this invention may contain conventional additives
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such as pigments. antiblock agents, slip agents, and
the like.
The p~esent invention is now set ~orth with
particularity in the claims which follow. ~s used
in the claims. the term "polymer" includes
homopolymezs and copolymers.
! D-20020
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