Note: Descriptions are shown in the official language in which they were submitted.
20~3707
CHEESE PACKAGING LAMINATE
The present invention is directed to a laminated
structure useful in the packaging of food items, particularly
soft cheeses.
Soft cheeses such as Camembert and Brie are current-
ly packaged in a variety of different materials including
polyethylenes with and without ethylene/vinyl-acetate
copolymers, polypropylenes, nylon/polyethylene laminates, and
polyester/polyethylene laminates. The oxygen and carbon
dioxide transmission rates of such structures are of primary
importance in the packaging of many soft cheeses, as well as
other food items which require a packaging material of high
gas permeability such as many fruits and vegetables. High
oxygen permeability is of primary importance in the packaging
of cheeses which, like Brie, require an aerobic atmosphere to
support fungal growth on the surface of the cheese. Other
cheeses respire COz in amounts that require a permeable
packaging material in order to preclude gassing of the pack-
age. In the case of produce packaging, regulation of the
transmission of 2 and CO2 minimizes enzymatic browning
of cut surfaces and the damaging effects of product anaerobio-
sis.
However, in most cases, materials having the de-
sired oxygen and carbon dioxide transmission rates required
in the above-discussed applications also have minimal abuse
resistance and machinability properties. Conversely, film
structures which are abuse resistant and machinable generally
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lack the gas permeability and sealing properties required by
such applications.
The present application requires a structure which
combines heat resistance, low extensibility and strength with
high gas permeability and sealing. To prevent drying of the
packaged food product, a low water vapor transmission rate is
also required.
Such requirements were met by the structure of U.S.
Pat. No. 4,935,271 to Schirmer which discloses a multilayer
laminate for use in the packaging of lettuce and the like
having a first impermeable film of propylene homopolymer or
copolymer having perforations defined therein, and a second
permeable film bonded to the first film which includes a
bonding layer of ethylene vinyl acetate copolymer and an
outer heat sealable layer. In that laminate, machinability
is provided by the polypropylene film which is perforated in
order to render it permeable. The gas permeable second film
contributes the properties of low moisture transmission and
sealability to the overall structure. However, for the
Schirmer structure, disposition of the soft, flexible second
film adjacent the relatively stiff, heat set polypropylene,
results in curling of the overall structure and, thus, poor
machining performance.
SUMMARY OF THE INV~ENTION
It is thus an object of the present invention to
provide an improved material for use in the packaging of soft
cheeses which has a low moisture permeability and a high gas
permeability.
It is still another object of the present invention
to provide such an improved material for the packaging of
cheese and the like which is machinable, having heat resis-
tance, non-extensibility and strength.
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It is a further object of the present invention to
provide a packaging laminate having high gas permeability in
addition to strength, heat resistance, and low extensibility
which precludes moisture transmission and tends to remain in
a planar configuration during processing.
These, as well as other objects, are achieved by
providing a multilayer laminate comprising a first component
of a strong, nonextensible film having perforations defined
therein, and a second component film, bonded to the first
component film, having high permeability and low moisture
transmission and including a bonding layer and a layer which
imparts stiffness to the second film.
Such objects are further achieved by providing a
method of making a multilayer laminate comprising providing a
perforated film preferably comprising a propylene homopolymer
or copolymer, coextruding a second film having a bonding
layer and an interior styrene butadiene layer, and bonding
the second film to the perforated film.
DEFINITIONS
The term "ethylene copolymers" as used herein re-
fers to copolymers of ethylene and vinyl acetate, alkyl
acrylate or alpha-olefin. Also within the scope of the
present definition are chemically modified derivatives of
these materials.
The tern "EVA" as used herein designates
ethylene/vinyl-acetate copolymers.
The term "linear low density polyethylene" (LLDPE)
as used herein includes that group of ethylene/alpha-olefin
copolymers having limited side chain branching and which fall
into a density range of 0.916 g/cc to 0.940 g/cc. Sometimes
linear polyethylene in the density range from 0.926 g/cc to
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0.940 g/cc is referred to as linear medium density polyethyl-
ene (LMDPE). Typical brand names are Dowlex from Dow Chemi-
cal Company, Ultzex and Neozex from Mitsui Petro Chemical
Company, and Sclair from duPont. The alpha-olefin copolymers
are typically butene-1, pentene-1, hexene-1, octene-1, etc.
The terms "very low density polyethylene" (VLDPE)
and "ultra-low density polyethylene" (ULDPE) as used herein
refer to ethylene/alpha-olefin copolymers which have a densi-
ty of less than about 0.915 g/cc and, more specifically,
usually 0.912 g/cc or below and may be below 0.89 g/cc.
Typical VLDPE resins are those designated DFDA by Union Car-
bide and are believed to principally or usually have butene
or isobutene as a comonomer. The very low density polyethyl-
enes as compared to LLDPE, usually have significantly higher
copolymer content and distinctly different properties making
A them a distinct class of polymers. Typically, resins desig-
nated "ULDPE" resins come from Dow and are believed to have
octene as the comonomer. There is a slight difference in
properties which is thought to be attributable to the
comonomer. As used herein the term "linear
ethylene/alpha-olefin copolymer having a density of less than
0.915 g/cc" encompasses both VLDPE and ULDPE.
The term "butadiene styrene copolymer" (BDS) is
used herein to denote thermoplastic copolymers, especially
block copolymers containing a major portion (greater than
50%) of styrene and a minor proportion (less than 50%) of
butadiene comonomer.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be further understood by refer-
ence to the drawing figures, wherein:
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FIG. 1 is a schematic cross sectional view o~ one
embodiment of the multilayer laminate of the present inven-
tion; and
FIG. 2 iS a schematic cross sectional view of anoth-
er embodiment of a multilayer laminate in accordance with the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to a laminate for
use in the packaging of soft cheese and other items which
require a packaging material having the properties disclosed
herein. The packaging of soft cheeses requires a packaging
material having a low moisture transmission rate and a high
gas transmission rate. Further, automation of the process
requires the material to be machinable. Machinability may be
defined as including the properties of strength, low extensi-
bility, and heat resistance. Further, for a film to be ma-
chinable it must be capable of maintaining a substantially
planar configuration during many types of handling and pro-
cessing. Typically, those materials having high gas permea-
bilities do not demonstrate adequate machinability and vice-
versa. The present invention meets all of the prerequisites
of this application by first providing a machinable film
which is strong, non-extensible and heat resistant, but also
impermeable and nonsealable, and then perforating it, thereby
rendering it permeable. In order to provide sealability
while maintaining permeability and providing for a moisture
impermeable structure, a permeable, heat sealable film is
bonded to one side of the perforated film. Curling of the
final laminate structure, which is expected when a highly
permeable film is laminated onto a strong, non-extensible
film, is precluded by including a stiffness imparting layer
within the otherwise soft, permeable film. Relative gas and
moisture transmission rates of the ultimate structure are
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determined by the size and number of perforations in the
first, impermeable film, as well as the thickness and permea-
bility of the second film.
Referring to Figure 1, the multilayer laminate 100
includes an first film 12 having perforations 14 therein. In
the preferred embodiments, layer 12 comprises a propylene
homopolymer (PP), although a propylene copolymer such as
ethylene propylene copolymer (EPC) may be used. Various
homopolymsrs and copolymers of polypropylene are commercially
available and well known in the art. One film especially
preferred for the present invention is a commercial film
designated PY, supplied by the Cryovac Division af W. R.
Grace & Co.-Conn. Also within the scope of the present inven-
tion are other machinable films. Generally, such films are
first stretch-oriented and then heat set, that is, held in a
stretched configuration and heated to some temperature higher
than the original orientation temperature but below the melt
temperature such that shrinkage will occur at such tempera-
ture and not at the original orientation temperature. Such
processing provides strength, low elongation and heat resis-
tance. In addition to polypropylenes, polyesters and nylons
may be oriented ~nd heat set to provide the properties needed
for good machinability required by the present invention.
A coextruded second component film 16 is also de-
picted in Figure 1. A web of film 16 is bonded to one side
of film 12. It should be noted that although it is within
the scope of the present invention to bond a web of film 16
to both sides of film 12, such is not necessary and is, gener-
ally, not preferred. A second web of film 16 is not required
to "balance out" the present structure and prevent curling of
a machinable film about a relatively flexible, permeable
film. Instead, the present film 16 is stiffened by an interi-
or layer 20, discussed in greater detail below. However,
lamination of a web of film 16 to both sides of film 12 may
be preferred when it is desirable to provide heat sealable
layers on both outer surfaces of the final structure such as
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when a BDS copolymer is employed for stiffness imparting
layer 20, a further sealing layer is not required because of
the wide sealing range provided by such material. Note that
in Examples 3 and 5 below, an outer BDS layer serves as a
heat sealable layer. In order to provide the requisite stiff-
ness for the present application either two or more BDS lay-
ers are required, or, if only one BDS layer is provided, it
must be a relatively thick layer, comprising from about _%
to about _ % of the total thickness of film 16.
In Figure 1, an outer heat sealable layer is desig-
nated at 22. Layer 22 is preferably an ethylene vinyl ace-
tate copolymer and can be comprised of the same material as
layer 18. A preferred EVA is Elvax 3165 from DuPont, which
has a vinyl acetate content of about 18%. Layer 22 can also
be a very low density polyethylene (VLDPE) or a VLDPE/EVA
blend.
Referring to Figure 2, a multilayer laminate 200
like that of Figure 1 is shown, but in which a filler layer
24' is disposed between reduced BDS layers 20'. Such struc-
ture is preferred because it provides the stiffening proper-
ties of the thick BDS layer 20 of Figure 1 above at less
expense as the filler layer may be composed of a polymeric
material which is less expensive than BDS. Such filler layer
is preferably an ethylene~alpha-olefin copolymer. The pre-
ferred resin for layer 24' is a VLDPE such as DFDA 1137 sup-
plied by Union Carbide, although other materials having high
gas permeability, such as high vinyl acetate EVA copolymers
or EVA/VLDPE blends may also be employed.
Bonding reduced BDS layers 20' to filler layer 24'
are intermediate tie layers 26', each comprising a polymeric
adhesive and preferably a copolymer of ethylene, and more
preferably an ethylene vinyl acetate copolymer (EVA). A
preferred EVA is one having a vinyl acetate content at about
18% by weight of the copolymer. Other polymeric materials
may be used for layers 26' as long as they provide the neces-
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when lap sealing, which requires the sealing of one surfaceof the laminate to the opposite surface, is required. Howev-
er, while the provision of film 16 on only one side of film
12 allows only for fin sealing, wherein one surface of the
laminate is sealed to itself, such sealing method advanta-
geously subjects only the heat resistant film 12 to contact
with the sealing jaws.
The film 16 of the present invention must include a
bonding layer 18 for bonding film 16 to film 12. Preferably,
bonding layer 18 comprises an ethylene vinyl acetate
copolymer such as Elvax 3165, an 18% vinyl acetate EVA from
DuPont. However, other materials which provide film 16 with
a corona laminatable surface are also within the scope of the
present invention. In a preferred embodiment, layer 18 com-
prises a blend of EVA and antiblock, preferably about 80% of
Elvax 3130, a 12% vinyl acetate EVA from DuPont, and about
20% of an antiblock masterbatch such as EPE-8160, a 90% poly-
ethylene, 10% Syloid blend from Teknor Color. In an alterna-
tive embodiment, bonding layer 18 is composed of a high vinyl
acetate, low melting point EVA such as Alathon 3180 available
from DuPont, having a melt index of between 23 and 27
grams/10 minutes, a vinyl acetate content by weight of be-
tween about 27% and 29%, and a density of between about 0.948
g/cc and 0.954 g/cc.
Also required in film 16 is a layer 20 which im-
parts stiffness to the otherwise pliant permeable film. As
discussed above, it is this stiffness imparting layer which
precludes curling of the final laminate structure 10. A pre-
ferred material for layer 20 is a butadiene styrene (BDS)
copolymer, although any material having the combined proper-
ties of stiffness and high gas permeability is within the
scope of the present invention. One example of a commercial-
ly available resin for use in layer 20 is a BDS copolymer
designated DK 10, one of the K-resin series available from
Phillips Chemical Company. It should be noted that, although
layer 20 is illustrated as an interior layer in Figure 1,
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sary high gas permeability required by the present inven-
tion. Blends of polymeric materials and polymeric adhesives
can also be used for intermediate layers 26'. Further, it
should be noted that tie layers may be employed as needed in
film 16 for bonding adjacent layers which may otherwise be
susceptible to delamination during handling. For example, in
Example 2 below, tie layers are employed between an interior
BDS layer and each of the bonding layer and the heat sealable
layer.
Generally, perforated film 12 may be made by extrud-
ing a polypropylene, ethylene propylene copolymer, polyester
or nylon film, orienting and heat setting the film and perfo-
rating same by means well known in the art such as flame or
needle perforation. Multilayer film 16 can be made by stan-
dard coextrusion techniques.
A web of multilayer film 16 is bonded to one side
of perforated film 12 preferably by corona treating the bond-
ing surfaces of both film 12 and film 16, and then, under
some heat and pressure, adhering the two webs together.
Although it is within the scope of the present invention to
corona treat only film 16 or only film 12, optimum lamination
is achieved when both bonding surfaces are treated. The bond
between the two films is designated at 40 in the drawings.
Other bonding techniques, including the use of
conventional lamination adhesives, may also be used. Howev-
er, bonding techniques in which a separate adhesive is uti-
lized may be less desirable in that such adhesive may block
the perforations of film 12.
In the preferred embodiment, packaging film of the
present invention is especially suitable for use in connec-
tion with Kartridg-Pak or other types of vertical form fill
seal machinery.
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The invention may be further understood by refer-
ence to Table 1 indicating specific laminate structures made
in accordance with the present invention.
TABLE 1
EXAMPLE
1 PP//EVA/BDS/EVA~tie)/VLDPE~EVA(tie)/BDS~EVA
2 PP//EVA/EVA(tie)/BDS/EVA(tie~¦EVA
3 PP//E~A/BDS¦EVA(tie)¦VLDPE/EVA~tie)/BDS¦EVA/BDS
4 PP//EVAIBDS/EVA
PP//EVA/BDS
In the laminate of Example 3, about 20~ of the EVA
of bonding layer comprised an antiblocking agent masterbatch
which had been preblended with the EVA prior to extrusion.
The antiblocking agent used included 90% low density polyeth-
ylene blended with 10% of a colloidal silica. Such structure
was tested for water vapor transmission and 2 and CO2
permeability. The water vapor transmission rate averaged
about 2.73 grams/100 in.2 over 24 hrs. at 100F and
100%RH. The oxygen transmission rate averaged about 4858.9
cm3/mZ-atm.-24 hrs. at 73F. The carbon dioxide transmis-
sion rate averaged about 30204.0 cm3/m2-atm.-24 hrs. at
73F
Although the outer permeable films, generally 16,
of the present invention are represented as composites of
varying numbers of layers, it is to be understood that the
relative thickness of any film 16 is not necessarily a func-
tion of the number of layers comprising such. For example,
the permeable film of Example 4 above may have a thickness as
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great as that of Example 1. In Example 1, the VLDPE core is
included in order to add bulk between the stiffness imparting
BDS layers at a reduced cost. In Examples 4 and 5 the BDS
layer is thickened in order to provide the thickness and
stiffness of the film of Example 1.
Although the present invention has been described
by reference to the specific embodiments and examples, a
latitude of modification change and substitution is intended
in the f~regoing disclosure, those skilled in the art would
readily understand that modifications may be made by one
skilled in the art after a review of this description without
departing from the spirit and scope of the claims which fol-
low.
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