Note: Descriptions are shown in the official language in which they were submitted.
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HEAT-SHRINKABLE PACKAGING
BACKGROUND OF THE INVENTION
This application relates to U.S. Patent Publication No. 2004/0166261.
This invention relates to the shrink packaging of articles, particularly food
articles
such as poultry, cheese, primal or subprimal meat cuts, fresh red meat and
other
processed meat, fruits, vegetables, breads and food products. Shrink packaging
refers to
the use of a packaging film manufactured in such a way that when it is exposed
to a
certain amount of heat, it will contract, preferably in both directions,
reducing its overall
surface area. When this type of film is wrapped around an object, sealed
around its edges
and passed through a heated shrink tunnel where the package is exposed to an
elevated
temperature, the film will react to the heat and contract around the object.
Depending on
the respective application, the air trapped within the package may be
evacuated prior to
final sealing, or small holes may be provided through the film to allow air to
escape
during the heat shrinking process. This process results in an attractive skin-
tight package.
Articles packaged using shrink packaging are numerous and can include food
articles,
such as frozen pizzas, cheese, poultry, fresh red meat, and processed meat
products.
The shrink packaging of food articles such as poultry, cheese, fresh red meat,
and
processed meat products requires tough, puncture resistant, yet flexible, film
materials
suitable for use in fabricating individual heat-shrinkable packaging
receptacles, such as
pouches and bags for packaging such food articles. Generally, the shrink
packaging
method of food articles is predicated upon the heat-shrinking property of the
receptacle
by placing a given food article or articles into an individual receptacle,
evacuating the
receptacle to remove air so the receptacle collapses, heat sealing across the
receptacle's
opening or mouth to close the receptacle and thereafter exposing the
receptacle to
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a heat source such as a flow of hot air, infrared radiation, hot water, and
the like, thereby causing the
receptacle to shrink and come into intimate contact with the contours of the
food article or articles.
The packaged article prepared by this packaging method has an attractive
appearance which adds
to the commodity value of the wrapped article, its contents are kept in a
hygienic condition, and it
allows shoppers to examine the quality of the contents of the packaged
article. Packaging in this
fashion also excludes air from the package to prolong shelf life.
This invention relates generally to packaging and specifically to hermetically
heat sealable,
easy open, heat-shrinkable packaging for food products.
It is common practice package articles such as food products in thermoplastic
films or
laminates to protect the product to be packaged from abuse and exterior
contamination and to
provide a convenient and durable package for transportation and sale to the
end user. Shrink
packaging of food products has become extensively used due to its many
advantageous properties,
e.g., strength, compactness, content security, purge resistance, the
attractive appearance of the
packed article, etc., which add to the commodity value of the packaged
article. Shrink packaging
refers to the use of a packaging film manufactured in such a way that when it
is exposed to a certain
amount of heat, the film will contract in at least one direction along its
length or width, preferably
in both directions, reducing its overall surface area. When articles are
packaged in this type of film,
air in the package is usually evacuated and the package is typically passed
through a heated shrink
tunnel where the package is exposed to an elevated temperature which causes
the film to react to the
heat and contract around the object. This process results in an attractive
skin-tight package. Articles
packaged using shrink packaging are numerous and can include food articles,
such as frozen pizzas,
cheese, poultry, fresh red meat, and processed meat products as well as
nonfood industrial articles
such as wooden blinds, CD's, etc.
Many food products, such as poultry, fresh red meat, cheeses, and processed
meat products,
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are packaged in individual, pre-manufactured bags of heat-shrinkable film.
Typically,
individual bags or pouches for packaging food articles include one to three
sides heat
sealed by the bag manufacturer leaving one side open to allow product
insertion and a
final seal performed by the food processor. Such individual bags are typically
manufactured from shrink films by producing a seamless tube of heat-shrinkable
film
having a desired diameter, heat sealing one end of a length of the tubular
film and cutting
off the tube portion containing the sealed portion, thereby forming an
individual bag. The
bag formed thereby, when it is laid flat, has a bottom edge formed by the heat
seal, an
open mouth opposite the sealed bottom and two seamless side edges formed by
the fold
produced when the tube is laid flat. Another method of forming bags from a
seamless
tube comprises making two spaced-apart transverse seals across the tube and
cutting open
the side of the tube. If flat sheets of film are used, bags are formed
therefrom by heat
sealing three edges of two superimposed sheets of film or by end-folding a
flat sheet and
sealing two sides. U.S. patents describing known heat shrinkable bags include
U.S.
Patent Nos. 6,511,688, 5,928,740, and 6,015,235. U.S. Patent Application
Publication
No. 2004/0166261, in the name of Thomas Schell et al., filed on February 20,
2003,
entitled "HEAT-SHRINKABLE PACKAGING RECEPTACLE" discloses individual
heat-shrinkable bags formed from a sheet of film, preferably in a continuous
process,
wherein opposing side edges of the sheet are sealed longitudinally to form a
tube
member, which is then sealed and cut transversely to close an end of the tube
member
thereby forming a backseamed bag.
The known bags for heat-shrink packaging include strong factory and final
closing
seals to prevent the heat sealed seams from pulling apart during the heat
shrinking
operation, or during the handling and transport of the packaged article.
Although the
strong heat seals provide protection against unwanted seal failure, such seals
also make it
difficult for the end user to open the package. Accordingly, there is needed
an improved
heat-shrinkable packaging receptacle that includes seals of sufficient seal
strength to
survive the heat shrinking process and handling and resist spontaneous
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opening due to residual shrink forces, yet includes at least one heat seal
that is readily openable by
application of force without requiring use of a knife or cutting implement and
without uncontrolled
or random tearing or rupturing of the packaging materials, e.g., away from the
seal area, which may
result in opening in undesired location or in sudden destruction of the
package and inadvertent
contamination or spillage of the contents of the package.
Typically, individual bags or pouches for packaging food articles include one
to three sides
heat sealed by the bag manufacturer leaving one side open to allow product
insertion. Such
individual bags are generally manufactured from shrink films by producing a
seamless tube of heat-
shrinkable film having a desired diameter and heat sealing one end of a length
of the tubular film
and cutting off the tube portion containing the sealed portion, thereby
forming a bag which, when
it is laid flat, has a bottom edge formed by the heat seal, an open mouth
opposite the sealed bottom
and two seamless side edges formed by the fold produced when the tube is laid
flat. Another method
of forming bags from a seamless tube comprises making two spaced-apart
transverse seals across
the tube and cutting open the side of the tube. If flat sheets of film are
used, bags are formed
therefrom by heat sealing three edges of two superimposed sheets of film or by
end-folding a flat
sheet and sealing two sides.
Manufacturing bags from a seamless tube requires that the tube be extruded to
a specified
width for the intended end use. Thus, fabricating small diameter tubes for
small width bags does
not utilize the full capacity of the film manufacturing equipment and is thus
not economical.
Seamless tube sizes are also limited by the manufacturing equipment in how
small the width can be
made. The manufacture of individual bags by superimposing two sheets and
sealing about three
edges requires costly machinery to handle the separate sheets, properly align
the sheets and provide
seals around the several edges. Additionally, having a third sealed edge (four
sealed edges when
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closed) increases the risk of a seal failure during the shrinking process.
Folding a sheet of film and
sealing two sides creates a double thickness of film at the seals which
undesirably protrude from the
side of the finished package.
Accordingly, although the known shrink bags meet many of the requirements for
packaging
applications, a need still exists for an improved heat-shrinkable bag
structure that can be
economically fabricated and sealed using standard bag sealing machinery at the
place of packaging.
SUMMARY OF THE INVENTION In accordance with the present invention, there is
provided an
individual end-sealed packaging receptacle, such as a bag, formed from a sheet
of heat-shrinkable
film having a first edge and an opposing second edge. The packaging receptacle
includes a first seal
bonding the first edge and second edge to define a tube member having a first
bag wall, a second bag
wall, first and second opposing lay-flat bag edges, an end and an open mouth.
The packaging
receptacle includes a second seal through the first and second bag walls,
extending laterally across
the width of both the first and second walls and thereby closing the end.
The present invention also provides an easy opening heat-shrinkable bag to be
heat sealed
to a closed condition to contain and protect a product disposed therein. At
least one heat seal is
peelable and readily openable by application of force. The bag is formed from
a sheet of film having
a first side, an opposing second side, an outer surface and an inner surface.
The bag includes a first
seal longitudinally joining the first side and the second side, thereby
defining a tube member. The
tube member, when laid flat, includes a first bag wall, a second bag wall, a
first bag edge, an
opposing second bag edge, an open mouth and an end. The bag includes a second
seal extending
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laterally across the tube member adjacent the end, thereby sealing the first
and second bag walls
together and closing the end. A product receiving chamber is defined between
the first and second
bag walls, the second seal and the open mouth. Preferably, the first seal
comprises a lap seal and is
at least one peelable heat seal.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 illustrates a schematic view of an end-seal, shrink bag having a lap
seal according
to the present invention, in a slightly open position from a lay-flat
position.
FIG. 2 illustrates a transverse cross-sectional view of the bag illustrated in
FIG. 1, taken
through section 2-2 of FIG. 1.
FIG. 3 illustrates a schematic view of an end-seal, shrink bag having a fin
seal according to
the present invention, in a slightly open position from a lay-flat position.
FIG. 4 illustrates a transverse cross-sectional view of the bag illustrated in
FIG. 3, taken
through section 4-4 of FIG. 3.
FIG. 5 illustrates a schematic view of an end-seal, shrink bag having a butt-
seal according
to the present invention, in a slightly open position from a lay-flat
position.
FIG. 6 illustrates a transverse cross-sectional view of the bag illustrated in
FIG. 5, taken
through section 6-6 of FIG. 5.
FIG. 7 illustrates a preferred three-layer film structure for forming bags
according to the
present invention.
FIG. 8 is a schematic representation of a preferred method of manufacturing
films for use
with the present invention.
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FIG. 9 illustrates a preferred seven-layer film structure for forming bags
according to the
present invention.
FIG. 10 illustrates a schematic view of a film suitable for making a peelable
sealed heat
shrink bag according to the present invention.
FIG. 11 illustrates a schematic view of a preferred embodiment of a heat-
shrinkable bag
according to the present invention, in a substantially lay-flat position.
FIG. 12 illustrates a fragmentary cross-sectional view taken along lines A-A
of FIG. 11
depicting an enlarged, not to scale, lap seal area of a preferred film for use
in fabricating the bag
illustrated in FIGS. 11, 13 and 14.
FIG. 13 illustrates a fragmentary cross-sectional view taken along lines B-B
of FIG. 11
depicting an enlarged, not to scale, end seal area of a preferred film.
FIG. 14 illustrates schematic view of another preferred embodiment of a heat-
shrinkable bag
according to the present invention having a pull flap.
FIG. 15 illustrates a transverse cross-sectional view of the bag illustrated
in FIG. 14, taken
through section C-C of FIG. 14.
FIG. 16 illustrates a cross-sectional view taken along lines D-D of FIG. 15,
depicting an end
seal.
FIG. 17 illustrates yet another bag according to the present invention having
a fin seal
backseam.
FIG. 18 illustrates a cross-sectional view of the bag illustrated in FIG. 17,
taken through
section E-E.
FIG. 19 illustrates an enlarged fragmentary cross-sectional view of the seal
portion of FIG.
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18 detailing a preferred film structure.
FIG. 20 illustrates another bag embodiment according to the present invention
having a butt-
seal backseam.
FIG. 21 illustrates a cross-sectional view of the bag illustrated in FIG. 20,
taken through
section F-F.
FIG. 22 illustrates another bag according to the present invention having a
peel strip.
FIG. 23 illustrates a cross-sectional view of the bag illustrated in FIG. 22,
taken along section
G-G.
FIG. 24 is a schematic illustration of a preferred method of manufacturing
films for use with
the present invention.
FIG. 25 is a schematic illustration of a preferred method of manufacturing
bags according
to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment
of the heat-shrinkable packaging receptacle of the present invention is shown
in FIGS. I and 2
generally as bag 10. The bag 10 is formed from a sheet of heat-shrinkable film
11 having a first edge
12, a second edge 14, a top surface 13 and a bottom surface 15. The bag 10
includes a first seal 16
bonding the first and second edges 12 and 14 in an overlapping arrangement, or
lap seal, from the
top of the bag to the bottom. A tube member 18 is formed, shown in FIGS. 1 and
2 in a partially
lay-flat orientation, having a first bag wall 20, a second bag wall 22, a
first bag edge 24, a second
bag edge 26, an opening 28 and a bag end 30. In other words, the first and
second edges 12 and 14
are placed in an overlapping arrangement and a seal, such as a heat seal, is
provided between the top
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surface 13 of the first edge 12 and bottom surface 15 of the second edge 14
such that the top surface
13 of the first edge 12 is sealed in face-to-face contact with the bottom
surface 15 of the second edge.
The bag 10 includes a second seal 32 provided through the first and second bag
walls 20 and 22 and
extending laterally across the bag 10 from the first bag edge 24 to the second
bag edge 26, thereby
closing the bag end 30 and defining a product receiving chamber 34.
Although the first seal 16 is illustrated as being positioned between the
first and second tube
edges 24 and 26 and running parallel thereto, one skilled in the art will
appreciate in view of the
present disclosure that the position of the first seal 16, when the bag 10 is
in a lay-flat orientation,
may be any desired position from first edge 24 to second edge 26 of either
first or second bag walls
20 and 22, as well as being positioned at either of the first and second bag
edges 24 and 26. The
second seal 32 is illustrated as being straight and extending perpendicular to
the first seal 16;
however, the skilled artisan will appreciate that the second seal 32 may take
any shape, so long as
the second seal 32 operates to close the bag end 30 and thereby define a
product receiving chamber
34. For example, common seal configurations include straight, or linear, seals
which usually extend
perpendicular to the tube edges 24 and 26 (the tube edges 24 and 26 generally
extend parallel to each
other), and also include nonlinear or curved edges, e.g., such as those
described in U.S. Patent No.
5,149,943, which patent is hereby incorporated by reference in its entirety.
Both linear and
nonlinear seals may be made by any suitable sealing method known, including
hot bar and impulse
sealing.
A second embodiment of the heat-shrinkable packaging receptacle of the present
invention
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is illustrated in FIGS. 3 and 4 generally as bag 110. The bag 110 is formed
from a sheet of heat-
shrinkable film 111 having a first edge 112, a second edge 114, a top surface
113 and a bottom
surface 115. The bag 110 includes a first seal 116 bonding the first and
second edges 112 and 114
in an abutting arrangement, or fin seal, thereby defining a tube member 118.
To form the first seal
116, the first and second edges 12 and 14 are brought together such that the
bottom surface 115 of
both the first and second edges 112 and 114 is placed in face-to-face contact
and a seal, such as a
heat seal, is provided therebetween. The tube member 118 is shown in FIGS. 3
and 4 in a partially
lay-flat orientation, defining a first bag wall 120, a second bag wall 122, a
first bag edge 124, a
second bag edge 126, an opening 128 and a bag end 130. The bag 110 includes a
second seal 132
provided through the first and second bag walls 120 and 122 extending
laterally across the bag 110
from the first bag edge 124 to the second bag edge 126, thereby closing the
bag end 130 and
defining a product receiving chamber 134.
Again, although the first seal 116 has been illustrated as being positioned
between the first
and second tube edges 124 and 126, one skilled in the art will appreciate in
view of the present
disclosure that the location of the first seal 116, when the bag 110 is in a
lay-flat orientation, may
be any desired position from first edge 124 to second edge 126 of either first
or second tube walls
120 and 122, as well as being positioned at either of the first and second
edges 124 and 126. Since
the first seal 116 forms a fin 117 that extends outwardly from the tube member
118, the first seal 116
is preferably positioned at a point between the first and second tube edges
124 and 126 at or near the
middle of a bag wall. In this manner, the fin 117 may be folded over flat
against the respective bag
wall from which it extends and the second seal 132 and final closing seal (not
shown) will operate
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to maintain the fin 117 in such a folded position. This advantageously
eliminates an unwanted,
unaesthetic fin seal at the side edge of a packaged product. Similar to second
seal 32, the second seal
132 is illustrated as being straight and extending perpendicular to the first
seal 116. The skilled
artisan will appreciate that the second seal 132 may take any shape, such as a
curved shape, so long
as the second seal 132 operates to close the bag end 130 and thereby define a
product receiving
chamber 134, as described with respect to the second seal 32.
Another embodiment of the present invention is illustrated in FIGS. 5 and 6
generally as bag
210. Bag 210 is formed from a sheet of heat-shrinkable film 210 having a first
edge 212, a second
edges 214, an inner surface 213 and an outer surface 215. The bag 210 includes
a first seal 216
comprising a butt-seal, that bonds the first and second edges 212 and 214 in a
longitudinally abutting
relationship with or without directly bonding surfaces of the first and second
edges 212 and 214
together. The first seal 216 preferably includes a butt-seal tape 217, one
side of which is sealed to
the outer surface 215 of the first edge 212 by seal 216a, while an opposing
side of the tape 217 is
sealed to the outer surface of the second edge by seal 216b, seals 216a and
216b being in regions
adjacent to and along the first and second edges 212 and 214. The first seal
216 defines a tube
member 218, shown in FIGS 5 and 6 in a partially lay-flat orientation, having
a first bag wall 220,
a second bag wall 222, a first bag edge 224, a second bag edge 226, an opening
228 and a bag end
230. The bag 210 includes a second seal 232 provided through the first and
second bag walls 220
and 222 extending laterally across the bag 210 from the first bag edge 224 to
the second bag edge
226, thereby closing the bag end 230 and defining a product receiving chamber
234.
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The film used to fabricate the bags of the present invention may be multilayer
or monolayer
flexible, heat-shrinkable film manufactured by any known process. For example,
in commercial
poultry packaging operations, monolayer films made from polyethylene and/or
ethylene-vinyl
acetate copolymers, and multilayer films containing polyethylene and/or
ethylene-vinyl acetate
copolymers are used extensively. Likewise, in the packaging of fresh red meat
and processed meat
products, multilayer heat-shrinkable films containing polyethylene and/or
ethylene-vinyl acetate
copolymers in one or more layers of the films are commonly employed. Preferred
films may also
provide a beneficial combination of one or more or all of the below noted
properties including high
puncture resistance (e.g. as measured by the ram and/or hot water puncture
tests), high shrinkage
values, low haze, high gloss, and high seal strengths. The film and/or bags
may also include an
indicia, such as they may be printed. For example, bags according to the
invention may preferably
include an indicia indicating that the bag includes a bone-containing product.
It may be desirable
for applications wherein the film is printed to corona treat the film surface
to improve ink adhesion.
Since corona treated surfaces do not normally heat seal as well as untreated
surfaces, it may be
desirable to corona treat only those portions that will not form part of a
heat seal or to limit the
treated area of the film to minimize adverse interaction with later sealed
areas. For example, a center
portion of the film may be corona treated, while those portions along each of
the machine direction
edges of the film are not. In this manner, those portions along each machine
direction edge, that are
sealed together to form the first seals 16 or 116 as described above, are not
corona treated and should
not be adversely affected.
Preferably, the film may have an unrestrained shrinkage of at least 20% in at
least one
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direction and more preferably 35% or more in one or both the machine and
transverse directions.
Free shrink is measured by cutting a square piece of film measuring 10 cm in
each of the machine
and transverse directions. The film is immersed in water at 90 C for five
seconds. After removal
from the water the piece is measured and the difference from the original
dimension is multiplied
by ten to obtain the percentage of shrink.
Although the films used in the bag according to the present invention can be
monolayer or
multilayer films, the bags are preferably formed of a multilayer film having 2
or more layers; more
preferably 3 to 9 layers; and still more preferably 3 to 5 to 7 layers. Since
the inventive bags are
primarily intended to hold food products after evacuation and sealing, it is
preferred to use a
thermoplastic film which includes an oxygen and/or moisture barrier layer. The
terms "barrier" or
"barrier layer" as used herein means a layer of a multilayer film which acts
as a physical barrier to
moisture or oxygen molecules. Advantageous for packaging of oxygen sensitive
materials such as
fresh red meat, a barrier layer material in conjunction with the other film
layers will provide an
oxygen gas transmission rate(O,GTR) of less than 70 (preferably 45 or less,
more preferably 15 or
less ) cc per square meter in 24 hours at one atmosphere at a temperature of
73_F (23_C) and 0%
relative humidity.
The bags 10, 110 and 210 are preferably fabricated continuously from a
continuous sheet or
roll stock. The roll stock is slit to a desired width and fed to the bag
making equipment, the machine
direction edges of the film are brought together and sealed longitudinally,
either with a lap seal (bag
10), fin seal (bag 110) or butt-seal (bag 210) to form a continuous single-
seamed tube, or tube
member. A transverse seal is made across the tube and the section including
the transverse seal is
severed from the continuous tube to form the individual bag.
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The type of first seal 16, 116 or 216 incorporated into the bags of the
present invention will
need to be taken into account when selecting a suitable film. Generally, heat
seals are made by
supplying sufficient heat and pressure between two polymeric film layer
surfaces for a sufficient
amount of time to cause a fusion bond between the polymeric film layers.
Common methods of
forming heat seals include hot bar sealing, wherein adjacent polymeric layers
are held in face-to-face
contact by opposing bars of which at least one is heated, and impulse sealing,
wherein adjacent
polymeric layers are held in face-to-face contact by opposing bars of which at
least one includes a
wire or ribbon through which electric current is passed for a very brief
period of time to cause
sufficient heat to cause the film layers to fusion bond. Less area is
generally bonded with an
impulse seal relative to a hot bar seal, thus the performance of the film's
sealing layer is more
critical. However, an impulse seal is generally more aesthetic since less area
is used to form the
bond. First seal 16, or lap seal, requires that the top surface 13 and bottom
surface 15 be capable of
forming a suitable heat seal therebetween. If a first seal 116, or fin seal,
is to be formed, only the
bottom surface 115 need be capable of forming a suitable heat seal, since the
interfacial bond will
be formed between the same surface or layer. If a first seal 216, or butt-
seal, is formed, then both
the top surface and bottom surfaces must be capable of forming a suitable heat
seal. Likewise, the
butt-seal tape 217, must also be capable of forming a suitable heat seal with
the top surface or a
suitable adhesive must be employed to adhere the tape 217 to the top surface
13 or bottom surface
15, depending on whether the tape 217 is place on the inside or outside of the
bag 110.
A preferred multilayer barrier film structure for use with the present
invention is shown in
FIG. 7 generally as 40. When an oxygen barrier layer 42 is needed, it is
usually provided as a
separate layer of a multilayer film, most commonly as a core layer sandwiched
between an inner heat
sealing layer 44 and an outer layer 46, though additional layers may also be
included, such as tie or
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adhesive layers as well as layers to add or modify various properties of the
desired film,
e.g., heat sealability, toughness, abrasion resistance, tear-resistance, heat
shrinkability,
delamination resistance, stiffness, moisture resistance, optical properties,
printability, etc.
Oxygen barrier materials which may be included in the films utilized for the
inventive
bags include ethylene vinyl alcohol copolymers (EVOH), polyacrylonitriles,
polyamides
and vinylidene chloride copolymers (PV DC). Preferred oxygen barrier polymers
for use
with the present invention are vinylidene chloride copolymers or vinylidene
chloride with
various comonomers such as vinyl chloride (VC-VDC copolymer) or methyl
acrylate
(MA-VDC copolymer), as well as EVOH. A specifically preferred barrier layer
comprises
about 85% vinylidene chloride-methyl acrylate comonomer and about 15%
vinylidene
chloride-vinyl chloride comonomer, as for example described in Schuetz et al.
U.S. Pat.
No. 4,798,751. Suitable and preferred EVOH copolymers are described in U.S.
Patent
No. 5,759,648.
The inner heat sealing layer 44 is generally provided on a side of the barrier
layer
42 that becomes the inner surface 38, or bottom surfaces 15 and 115 shown in
Figs. 1-6,
of the bags 10, 110 or 210. Other film layers may optionally be incorporated
between the
barrier layer 42 and the inner heat sealing layer 44 as previously noted.
Substantially
linear copolymers of ethylene and at least one alpha-olefin as well as
copolymers of
ethylene and vinyl esters or alkyl acrylates, such as vinyl acetate, may be
usefully
employed in one or more layers of the film, and may comprise monolayer and
multilayer
thermoplastic films. Preferably, the inner heat sealing layer comprises a
blend of at least
one ethylene-a-olefin copolymer (EAO), with ethylene vinyl acetate (EAO:EVA
blend).
Suitable a-olefins include C3 to C10 alpha-olefins such as propene, butene-1,
pentene-1,
hexene- 1, methylpentene- 1, octene- 1, decene-1 and combinations thereof. The
heat seal
layer is optionally the
CA 02458136 2004-02-19
thickest layer of a multilayer film and may significantly contribute to the
puncture resistance of the
film. Another desirable characteristic affected by this layer is the heat seal
temperature range. It is
preferred that the temperature range for heat sealing the film be as broad as
possible. This allows
greater variation in the operation of the heat sealing equipment relative to a
film having a very
narrow range. For example, it is desirable for a suitable film to heat seal
over a broad temperature
range providing a heat sealing window of 80 F or higher.
The outer layer 46 is provided on the side of the barrier layer opposite the
heat sealing layer
44 and acts as the outer surface 39. In the instance when a lap seal, such as
the first seal 32 of bag
is incorporated into a bag structure, the outer layer 46 must be heat seal
compatible with the inner
heat sealing layer. Other polymer layers may optionally be provided between
the barrier layer and
the outer layer as previously discussed. The outer layer may comprise an
ethylene-a-olefin
copolymer (EAO), ethylene vinyl acetate copolymer (EVA) or blends thereof.
EAOs are
copolymers predominately comprising ethylene polymeric units copolymerized
with less than 50 %
by weight of one or more suitable a-olefins which include C3 to C,o alpha-
olefins such as propene,
butene- 1, pentene- 1, hexene- 1, methylpentene- 1, octene- 1, decene- 1.
Preferred alpha-olefins are
hexene-1 and octene-1. Recent developments for improving properties of a heat-
shrinkable film
include U.S. Pat. No. 5,403,668, incorporated herein, which discloses a
multilayer heat-shrinkable
oxygen barrier film wherein the film outer layer is a four component blend of
VLDPE, LLDPE,
EVA and plastomer. LLDPE, or linear low density polyethylene, is a class of
ethylene-alpha olefin
copolymers having a density greater than 0.915 g/cm3. VLDPE, also called ultra
low density
polyethylene (ULDPE), is a class of ethylene-alpha olefin copolymers having a
density less than
16
CA 02458136 2011-01-13
0.915 g/cm3 and many commercial VLDPE resins are available having densities
from
0.900 up to 0.915 g/cm3. Plastomers are generally EAOs having densities below
0.900
g/cm3. U.S. Pat. No. 5,397,640 discloses a multilayer oxygen barrier film
wherein at least
one outer film layer is a three component blend of VLDPE, EVA and a plastomer.
Alternatively, the outer layer may be formed of other thermoplastic materials
as for
example polyamide, styrenic copolymers, e.g., styrene-butadiene copolymer,
polypropylene, ethylene-propylene copolymer, ionomer, or an alpha olefin
polymer and in
particular a member of the polyethylene family such as linear low density
polyethylene
(LLDPE), very low density polyethylene (VLDPE and ULDPE), high density
polyethylene (HDPE), low density polyethylene (LDPE), an ethylene vinyl ester
copolymer or an ethylene alkyl acrylate copolymer or various blends of two or
more of
these materials.
In general, the monolayer or multilayer films used in the heat-shrinkable bags
of
the present invention can have any thickness desired, so long as the films
have sufficient
thickness and composition to provide the desired properties for the particular
packaging
operation in which the film is used, e.g., puncture-resistance, modulus, seal
strength,
barrier, optics, etc. For efficiency and conservation of materials, it is
desirable to provide
the necessary puncture-resistance and other properties using the minimum film
thicknesses. Preferably, the film has a total thickness from about 1.25 to
about 8.0 mils;
more preferably from about 1.75 to about 3.0 mils.
Suitable films for use with the present invention are disclosed in U.S. Patent
No.
5,928,740. The `740 patent discloses a heat sealing layer comprising a blend
of a first
polymer of ethylene and at least one a-olefin having a polymer melting
17
CA 02458136 2011-01-13
point between 55 to 75 C.; a second polymer of ethylene and at least one a-
olefin having
a polymer melting point between 85 to 110 C and a third thermoplastic polymer
having a
melting point between 115 to 130 C which is preferably selected from the group
of
ethylene homopolymers such as HDPE and LDPE, and ethylene copolymers with at
least
one a-olefin; and optionally and preferably a fourth polymer such as a
copolymer of
ethylene with an alkyl acrylate or vinyl ester having a melting point between
80 to 105 C,
preferably 90 to 100 C. The `740 patent also discloses a preferred biaxially
oriented,
heat-shrinkable three-layer barrier film embodiment for use with the present
invention.
The three-layer barrier film embodiment comprises an inner heat sealing layer
as
described above in conjunction with a barrier layer preferably comprising a
polyvinylidene chloride (PVDC) or vinylidene chloride methylacrylate copolymer
(VDC-
MA or MA-saran) or EVOH layer and an outer layer formed of at least 50 wt. %,
and
preferably at least 70%, of a copolymer of ethylene with at least one alpha-
olefin or at
least one vinyl ester or blends thereof. Also, preferred EVAs will have
between about 3%
and about 18% vinyl acetate content.
Preferred films for use with the present invention are disclosed in U.S.
Patent
No. 6,815,023. The `023 patent discloses monolayer and multilayer films having
at least
one layer comprising at least a three-polymer blend, optionally including a
fourth
polymer, comprising: (a) a first polymer having a melting point of 80 to 98 C,
preferably
80-92 C, comprising a copolymer of ethylene and hexene-1; (b) a second polymer
having a polymer melting point of 115 to 128 C comprising ethylene and at
least one a-
olefin; and (c) a third polymer having a melting point of 60 to 110 C
comprising a
copolymer of ethylene with an alkyl acrylate or vinyl ester; and optionally
(d) a fourth
polymer having a melting point of 80 to 110 C (preferably of 85 to 105 C),
preferably
selected from the group of ethylene homopolymers such as HDPE and LDPE, and
ethylene copolymers with at least one a-olefin. The inventive blend finds
utility as an
inner heat sealing layer in many multilayer
18
CA 02458136 2011-01-13
embodiments. In a preferred three, four or five-layer embodiment, an oxygen
barrier
layer of a vinylidene chloride copolymer, a polyamide or EVOH is between a
layer of the
inventive blend and either a layer comprising at least 50% by weight of an EAO
or at
least one vinyl ester or blends thereof, or another layer comprising the
inventive blend.
Additional preferred films for use with the present invention are disclosed in
U.S.
Patent No. 6,777,047. The `047 patent discloses multi-layer barrier
embodiments formed
of a flexible, thermoplastic, biaxially stretched, heat-shrinkable film having
at least one
layer comprising a blend of at least three copolymers comprising: 45 to 85
weight percent
of a first polymer having a melting point of from 55 to 98 C comprising at
least one
copolymer of ethylene and at least one comonomer selected from the group of
hexene-1
and octene-1; 5 to 35 weight percent of a second polymer having a melting
point of from
115 to 128 C comprising at least one copolymer of ethylene and at least one a-
olefin; and
to 50 weight percent of a third polymer having a melting point of from 60 to
110 C
comprising at least one unmodified or anhydride-modified copolymer of ethylene
and a
vinyl ester, acrylic acid, methacrylic acid, or an alkyl acrylate; where the
first and second
polymers above have a combined weight percentage of at least 50 weight percent
based
upon the total weight of the first, second and third polymers; and where the
bag film has a
total energy absorption of at least 0.70 Joule and a shrinkage value at 90 C
of at least
50% in at least one of the machine and transverse directions. A barrier layer
formed of
any suitable oxygen barrier material or blend of materials, for example,
ethylene-vinyl
alcohol copolymer (EVOH) or copolymers of vinylidene chloride (VDC) such as
VDC-
vinyl chloride (VDC-VC) or VDC-methylacrylate (VDC-MA) may be used. Preferably
the barrier layer comprises a blend of 85 wt.% VDC-MA and 15 wt.% VDC-VC. The
outer layer is preferably an EVA-VLDPE blend, and more preferably an EVA-VLDPE-
19
CA 02458136 2011-01-13
plastomer blend. The `046 patent also discloses a film comprising a flexible,
thermoplastic film having at least one layer comprising a blend of at least
two polymers
comprising: 5 to 20 weight percent of (i) an ionomer polymer, e.g., an
ethylene-
methacrylate acid copolymer whose acid groups have been neutralized partly or
completely to forma salt, preferably a zinc or sodium salt; 5 to 95 weight
percent of (ii) a
copolymer of ethylene and at least one C6 to C8 a-olefin, having a melting
point of from
55 to 95 C, and a MW/Mõ of from 1.5 to 3.5; 0 to 90 weight percent of (iii) a
copolymer
of ethylene and at least one C4 to C8 a-olefin, having a melting point of from
100 to
125 C; and 0 to 90 weight percent of (iv) a copolymer of propylene and at
least one
monomer selected from the group of ethylene and butene-1, where the copolymer
(iv) has
a melting point of from 105 to 145 C; 0 to 90 weight percent of (v) a
copolymer of
ethylene and at least one monomer selected from the group of hexene-1, octene-
1 and
decene-1, where the copolymer (v) has a melting point of from 125 to 135 C;
and
polymers (ii), (iii), (iv), and (v) have a combined weight percentage of at
least 80 weight
percent based upon the total weight of polymers (i), (ii), (iii), (iv), and
(v); and wherein
the film has a total energy absorption of at least 1.2 Joule. Optionally, the
same blend
may be used as an inner heat sealing layer for a bag film.
Further preferred films for use with the present invention are described in
U.S.
Pat. No. 5,302,402 to Dudenhoeffer et al., U.S. Pat. No. 6,171,627, Lustig et
al. U.S. Pat.
No. 4,863,769, and U.S. Pat. No. 6,015,235 to Kraimer et al.
In a preferred embodiment of the present invention, the heat-shrinkable bag is
formed of a three-layer film. The three-layer film is preferably a biaxially
oriented film
including a barrier layer disposed between an inner heat sealing layer and an
outer layer,
as shown in FIG. 5. The inner heat sealing layer comprises a blend of about
37% of an
ethylene-vinyl acetate (EVA) copolymer such as ESCORENETM LD 701.ID available
from Exxon Chemical Co., Houston, Texas, USA; about 24% VLDPE resin such as
SCLAIRTM IOB available from Nova Chemicals, Ltd., Calgary, Alberta,
CA 02458136 2004-02-19
Canada (0.77 dg/min Melt Index and 0.911 g/cm' density); about 33% of a
plastomer, such as
EXACTTM 4053 available from Exxon Chemical Co., Houston, Texas, USA; about 4%
slip/processing aid, such as Spartech A27023 (slip/processing aid in a VLDPE
carrier resin); and
about 2% of a processing stabilizer such as Spartech A32434 (available from
Spartech Polycom of
Washington, Pennsylvania, U.S.A.). The barrier layer comprises a blend of
about 15% vinylidene
chloride-vinyl chloride and about 85% vinylidene chloride-methacrylate, such
as further described
in U.S. Patent No. 4,798,751. The outer layer comprises a blend of about 40%
of an ethylene-vinyl
acetate (EVA) copolymer such as ESCORENETM LD 701.ID; about 33% of a
plastomer, such as
EXACTTM 4053; about 25% of a VLDPE resin, such as SCLAIRTM lOB; and about 2%
of a
processing aid/slip concentrate in a VLDPE carrier, such as Ampacet 501236,
available from
Ampacet Corporation, Tarrytown, New York, USA. The inner layer, barrier layer
and outer layer
represent about 57.7%, 17.7% and 25.1 % respectively based on the total
thickness of the three-layer
film.
In another preferred embodiment of the present invention, the heat-shrinkable
bag is formed
of another three-layer biaxially oriented shrink film including a barrier
layer disposed between an
inner heat sealing layer and an outer layer, as shown in FIG. 5. The barrier
layer preferably
comprises a blend of about 15% vinylidene chloride-vinyl chloride and about
85% vinylidene
chloride-methacrylate such as further described in U.S. Patent No. 4,798,751.
The barrier layer
preferably comprises approximately 16.5% of the three-layer film's thickness.
The inner heat
sealing layer preferably comprises about 57.1% of the film's thickness and
comprises a blend of
about 35.1 wt. % of an ethylene-hexene-1 copolymer such as EXACTTM 9519 (
0.895 g/cm' and 2.2
dg/min Melt Index available from Exxon Chemical Co., Houston, Texas, USA);
about 36.5% of an
ethylene-octene-1 copolymer such as ATTANETM XU 61509.32 (a C2C8 (<10 wt. %
CH) VLDPE
having a density of about 0.912 g/cm' and 0.5 dg/min Melt Index available from
Dow Chemical Co.,
21
CA 02458136 2004-02-19
Midland, Michigan, USA); about 26.5% of an ethylene-vinyl acetate (EVA)
copolymer such as
ESCORENETM LD 701.ID (an ethylene-vinyl acetate copolymer available from Exxon
Chemical
Co., Houston, Texas, USA and reportedly having a density of 0.93 g/cm3, a
vinyl acetate content of
10.5 wt. %, a melt index of about 0.19 dg/min., and a melting point of about
97 _C); about 3% of
a slip/processing aid such as Spartech A50050 (1.9% oleamide slip and an
fluoroelastomer in a
VLDPE carrier resin); and about 2% of a processing stabilizer such as Spartech
A32434 (10%
DHT4A in VLDPE carrier resin available from Spartech Polycom of Washington,
Pennsylvania,
U.S.A.). The outer layer preferably comprises about 26.4% of the film
thickness and comprises
about 35 wt. % of an ethylene-hexene-1 copolymer such as EXACT TM 9519; about
35 % of a
ethylene-octene-1 copolymer such as ATTANETM XU 61509.32; about 27% of a EVA
copolymer
such as ESCORENETM LD 701.ID; and about 3% of a slip/processing aid such as
Spartech A50050
(available from Spartech Polycom of Washington, Pennsylvania, U.S.A.).
In another preferred embodiment, the film of the bag comprises a biaxially
oriented three-
layer heat-shrinkable film having an inner heat sealing layer made of a blend
of about 17 wt. %
ethylene-octene-1 copolymer such as ATTANETM XU 61509.32; about 18 wt. % EVA
such
ESCORENETM LD 701.ID; 58% of an ethylene-hexene-1 copolymer such as EXACTTM
9110 (0.898
g/cm3 density, 0.8 dg/min Melt Index and 89 C melting point); about 2% of a
processing stabilizer
such as Spartech A32434; and about 5% of a slip/processing aid such as
Spartech A50050. The outer
layer is about 19 wt. % ethylene octene-1 copolymer such as ATTANETM XU
61509.32; 18% EVA
(ESCORENETM LD 701 -ID); 60% of an ethylene-hexene-1 copolymer such as EXACTTM
9110; and
3% processing aid such as A50056. The barrier layer is 85% vinylidene chloride-
methyl acrylate and
about 15% vinylidene chloride-vinyl chloride. Preferably, the inner
layer:barrier layer:outer layer
thickness ratio is about 62:9:29.
22
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A preferred seven-layer film for use in fabricating bags according to the
present invention
is illustrated in FIG. 9 generally as film 60. The film 60 includes a first or
inner heat sealing layer
61 preferably comprising about 10% of the total mass of the film 60. The inner
heat sealing layer
61 preferably comprises a blend of about 94% EXACTTM 3139 (an ethylene-hexene
copolymer
having a reported Melt Index of 7.5 g/10 min and a density of 0.900 g/cm3);
about 4% Spartech
A27023; and about 2% Spartech A32434. A second layer 62, adjacent the first
layer 61 preferably
comprises about 42.2% of the total mass of the film and comprises a blend of
about 37%
ESCORENETM LD 701.ID; about 33% EXACT 4053; about 24% SCLAIRTM IOB; about 4%
Spartech A27023; and about 2% Spartech A32434. The film 60 further includes
first and second
tie layers 63 and 65, each of which individually preferably comprise about 5%
of the total mass of
the film 60 and further comprise about 100% of VORIDIANTM SP1330, an ethylene-
methyl acrylate
copolymer available form Voridian, a division of Eastman Chemical Company,
Kingsport,
Tennessee, USA. The film 60 includes a barrier layer 64 between the first and
second tie layers 63.
The barrier layer 64 preferably comprises about 17.7% of the total mass of the
film and comprises
a blend of about 85% vinylidene chloride-methyl acrylate and about 15%
vinylidene chloride-vinyl
chloride. Th film includes a third layer 66 that preferably comprises about
15.1% of the total mass
of the film 60. The third layer 66 comprises a blend of about 40% ESCORENETM
701.ID; about
33% EXACTTM 4053; about 25% SCLAIRTM 1OB; and about 2% Spartech A27339. The
film 60
includes a fourth layer or outer layer 67 that preferably comprises about 5%
of the total mass of film
60 and comprises a blend of about 98% EXACTTM 3139 and about 2% Spartech
A27339. The total
thickness of the film 60 is preferably about 2 mils or greater.
Advantageously, it may be desirable to utilize high Melt Flow Index polymers
in sealant
layer(s) of the film to aid in transversely sealing across the lap, butt or
fin seals. High Melt Flow
polymers, having a Melt Flow Index greater than about 5 dg/min. The higher
Melt Flow Index
23
CA 02458136 2004-02-19
polymers fill gaps, such as gaps 9a (FIG. 2), 9b (FIG. 4) and 9c (FIG. 6) that
may form due to the
dimensional difference encountered when the second seals 32, 132 or 232 are
between the first and
second bag walls in the area of the first seals 16, 116 and 216, more readily
than lower Melt Flow
Index polymers. For example, other, high Melt Index polymers, such as EXACTTM
3040, which
has a reported Melt Index of 16.5 g/10 min, could be used in the inner and
outer layers 61 and 67
of film 60 to replace the lower Melt Index ethylene-hexene copolymer.
The films selected to fabricate the inventive receptacles are preferably
biaxially oriented by
the well-known trapped bubble or double bubble technique as for example
described in Pahlke U.S.
Pat. No. 3,456,044. In this technique an extruded primary tube leaving the
tubular extrusion die is
cooled, collapsed and then preferably oriented by reheating and reinflating to
form a secondary
bubble. The film is preferably biaxially oriented wherein transverse (TD)
orientation is accomplished
by inflation to radially expand the heated film. Machine direction (MD)
orientation is preferably
accomplished with the use of nip rolls rotating at different speeds to pull or
draw the film tube in the
machine direction.
The stretch ratio in the biaxial orientation to form the bag material is
preferably sufficient
to provide a film with total thickness of between about 1.5 and 3.5 mils. The
MD stretch ratio is
typically 3:1-5:1 and the TD stretch ratio is also typically 3:1-5:1.
Referring now to FIG. 8, a double bubble or trapped bubble process is shown.
The polymer
blends making up the several layers are coextruded by conveying separate melt
streams 311 a, 311b,
and 311c to the die 330. These polymer melts are joined together and
coextruded from annular die
330 as a relatively thick walled multilayered tube 332. The thick walled
primary tube 332 leaving
the extrusion die is cooled and collapsed by nip rollers 331 and the collapsed
primary tube 332 is
24
CA 02458136 2011-01-13
conveyed by transport rollers 333a and 333b to a reheating zone where tube 332
is then
reheated to below the melting point of the layers being oriented and inflated
with a
trapped fluid, preferably gas, most preferably air, to form a secondary bubble
334 and
cooled. The secondary bubble 334 is formed by a fluid trapped between a first
pair of nip
rollers 336 at one end of the bubble and a second pair of nip rollers 337 at
the opposing
end of the bubble. The inflation which radially expands the film provides
transverse
direction (TD) orientation. Orientation in the machine direction (MD) is
accomplished
by adjusting the relative speed and/or size of nip rollers 336 and nip rollers
337 to stretch
(draw) the film in the machine direction. Rollers 337 also collapse the bubble
forming an
oriented film 338 in a lay-flat condition which may be wound on a reel 339 or
slit for
further processing close up.
The biaxial orientation preferably is sufficient to provide a multilayer film
with a
total thickness of from about 1.25 to about 8.0 mils, preferably 1.5 to 4 mils
or more,
preferably between 1.75 and 3.0 mils (44 to 76 ), and more preferably about
2.5 mils.
A preferred film and process for making film suitable for use in fabricating
bags
according to the present invention is described in each of U. S. Patent No.
6,815,023 filed
September 22, 1999 for "Puncture Resistant Polymeric Films, Blends and
Process";
6,777,046 filed November 1, 1999 for "Puncture Resistant High Shrink Film,
Blend and
Process"; and 6,777,047 filed July 6, 2000 for "lonomeric, Puncture Resistant
Thermoplastic Patch Bag, Film, Blend and Process".
For a monolayer film, the process is similar but utilizes a single extruder
(or
multiple extruders running the same polymeric formulation) to produce a
primary tube,
and biaxial orientation is sufficient to provide a monolayer film preferably
having a total
thickness of between 2 to 6 mil
CA 02458136 2011-01-13
or higher, and more typically from about 3.5 to 4.5 mils and is generally in
the same draw
ratio range as previously discussed, namely about 3:1 to 5:1 for both the MD
and TD.
Although not essential, it is preferred to irradiate the film to broaden the
heat
sealing range and/or enhance the toughness properties of the inner and outer
layers by
irradiation induced cross-linking and/or scission. This is preferably
accomplished by
irradiation with an electron beam at a dosage level of at least 2 megarads
(MR) and
preferably in the range of 3-5 MR, although higher dosages may be utilized,
such as for
thicker films. Irradiation may be provided on the primary tube or after
biaxial
orientation. The latter, called post-irradiation, is preferred and described
in Lustig et al.
U.S. Patent No. 4,737,391.
After orientation, the tubular film 338 is collapsed, slit open
longitudinally, laid
flat and wound on a reel 339 for use as rollstock. One skilled in the art will
appreciate
that the above method may be used to form the film, films may be made by
conventional
single bubble, blown film processes, and oriented or nonoriented sheets may be
made by
slot cast sheet extrusion processes with or without tentering to provide
orientation. One
skilled in the art will further appreciate that the flatwidth of the collapsed
tube will
determine the width of the sheet film that results therefrom. Thus, the
primary tube
dimensions and subsequent processing may be selected to provide a maximum
flatwidth
and film thickness for the desired application, thereby advantageously
maximizing the
production capacity of the film making equipment.
Advantageously, a bag maker may produce bags of various lengths and widths
from rolls of film rollstock by adjusting the width of the sheet (by slitting
or cutting
rollstock to a desired width) and the distances between the transverse end
seal and bag
mouth for a particular bag or series of bags. This advantageously avoids the
costly need
to produce specific widths of seamless tubes which are currently widely used
by meat
packers. Also the present invention permits cost savings
26
CA 02458136 2004-02-19
and manufacturing efficiencies by permitting creation of numerous widths and
lengths of bag from
standard rollstock, which was produced utilizing substantially 100% of the
film producing
equipment's capacity. This reduces the need to carry larger inventories of a
vast array of seamless
tube rollstock having different widths. The bag maker may simply slit film
rollstock to a desired
width and form a continuous tube member by longitudinally sealing opposing
side edges as
described for bags 10, 110 and 210. Bags of adjustable lengths may be made by
transversely sealing
and cutting through the tube member at a position spaced from the transverse
seal. The film may
also be made into a continuous tube member rollstock by longitudinally joining
opposing side edges
of a film as described above to form a continuous tube member, collapsing the
continuous tube
member and winding the collapsed continuous tube member on a reel. The
continuous tube member
rollstock may then be provided to the food processor, who then forms the
individual bags, such as
bags 10, 110 and 210. Such continuous tube member rollstock may have a lay-
flat width of up to
20 inches, advantageously greater than 20 inches, and more advantageously
greater than or equal to
22 inches.
Preferably, bag making is a continuous process, wherein the film is directed
to a bag making
assembly (not shown) where individual end-seal bags are made. As previously
stated, the rollstock
may be slit to a desired width with the unused portion re-wound for later use.
Bags are produced by
continuously bringing the opposing side edges of the film together and forming
a heat seal, such as
a lap seal or fin seal to form a continuous tube member, then making lateral,
or transverse, heat seals
across the tube member width at spaced intervals to weld the first and second
bag walls of the tube
member together. The tube member is severed preferably at the same time or
during the same step
that it is transversely heat sealed to form a bag as shown in Figure 1, 3 or
5. Typically as the
transverse seal is made for one bag a transverse cut forming the mouth of the
adjacent bag is being
made. This process forms a so called "end-seal" bag which, when it is laid
flat, has a bottom edge
formed by the transverse heat seal, an open mouth formed by the severed edge
and two side edges
27
CA 02458136 2004-02-19
formed by the fold produced when the tube member is laid flat. The transverse
heat seal should
extend across the entire tube member to ensure a hermetic closure. Each bag
being formed from a
length of the tube member will necessarily be formed by at least two, usually
parallel, spaced apart,
transverse cuts which cause a segment of the tube member to be made and one
transverse seal,
usually adjacent one of these cuts, will define a bag bottom which is located
opposing the bag
opening, which is formed by the distal cut. In typical production the member
tube is transversely
sealed and an adjacent transverse cut made as part of the same step and the
seal and this proximate
cut form a sealed end for one bag while the same cut also forms the mouth
opening for the adjacent
bag, and for that adjacent bag may be referred to as the distal cut. The
spacing between the lateral
seal and the point of severance, which may vary, will determine the length of
the bags formed. The
length of the bags can easily be varied by changing the distance between cuts.
The width of the bags
can also be easily varied by changing the width of the film by slitting the
standard rollstock. In
another embodiment of the invention, cuts and seals may be made alternately
and apart from each
other to form dual attached bags in saddle bag fashion.
The present invention advantageously provides for producing a heat-shrinkable
bag wherein
the bag manufacturer may produce multiple bag sizes (different lengths and
widths) from a single
film stock size, which advantageously maximizes film production efficiency by
eliminating the need
to manufacture different widths of seamless tubes. In other words, the present
invention allows the
bag manufacturer to produce one standard width of sheet film stock, such as
86, 94, 98, 104, 112,
126, 162 inch or greater, depending on the capacity of the film producing
equipment. This standard
sheet film stock may then be slit to a desired width, formed into a bag as
described herein, and the
remaining portion of the sheet film stock rewound for later use on another
job. Prior art bags require
the manufacturer thereof to produce different seamless tube sizes for each
size of bag produced,
thereby reducing production efficiency.
28
CA 02458136 2004-02-19
Unless otherwise noted, the following physical properties are used to describe
the invention,
films and seals. These properties are measured by either the test procedures
described below or tests
similar to the following methods.
Average Gauge: ASTM D-2103
Tensile Strength: ASTM D-882, method A
1% Secant Modulus: ASTM D-882, method A
Oxygen Gas Transmission Rate (O2GTR) : ASTM D-3985-81
Percent Elongation at Break: ASTM D-882, method A
Molecular Weight Distribution: Gel permeation chromatography
Gloss: ASTM D-2457, 45 Angle
Haze: ASTM D-1003-52
Melt Index: ASTM D-1238, Condition E (190 C) (except for propene-based (>50%
C3 content)
polymers tested at Condition L(230 C.))
Melting Point: ASTM D-3418, peak m.p. determined by DSC with a 10 C/min.
heating rate.
Vicat Softening Point (Vsp): ASTM D-1525-82
Seal Strength: ASTM F88-94
All ASTM test methods noted herein are incorporated by reference into this
disclosure.
Shrinkage Values: Shrinkage values are obtained by measuring unrestrained
shrink of a 10 cm.
square sample immersed in water at 90 C (or the indicated temperature if
different) for ten seconds.
Four test specimens are cut from a given sample of the film to be tested.
Specimens are cut into
squares of 10 cm length (M.D.) by 10 cm. length (T.D.). Each specimen is
completely immersed
for 10 seconds in a 90 C (or the indicated temperature if different) water
bath. The specimen is then
removed from the bath and the distance between the ends of the shrunken
specimen is measured for
both the M.D. and T.D. directions. The difference in the measured distance for
the shrunken
29
CA 02458136 2011-01-13
specimen and each original 10 cm. side is multiplied by ten to obtain percent
shrinkage
in each direction. The shrinkage of 4 specimens is averaged and the average
M.D. and
T.D. shrinkage values reported. The term "heat shrinkable film at 90 C" means
a film
having an unrestrained shrinkage value of at least 10% in at least one
direction.
Tensile Seal Strength (Seal Strength Test
Five identical samples of film are cut 1 inch (2.54 cm) wide and a suitable
length for the
test equipment e.g. about 5 inches (77 cm) long with a 1 inch (2.54 cm) wide
seal portion
centrally and transversely disposed. Opposing end portions of a film sample
are secured
in opposing clamps in a universal tensile testing instrument. The film is
secured in a taut
snug fit between the clamps without stretching prior to beginning the test.
The test is
conducted at an ambient or room temperature (RT) (about 23 C) test
temperature. The
instrument is activated to pull the film via the clamps transverse to the seal
at a uniform
rate of 12.0 inches (30.48 cm) per minute until failure of the film (breakage
of film or
seal, or delamination and loss of film integrity). The test temperature noted
and lbs. force
at break are measured and recorded. The test is repeated for four additional
samples and
the average grams at break reported.
Ram Puncture Test
The ram puncture test is used to determine the maximum puncture load or force,
and
the maximum puncture stress of a flexible film when struck by a
hemispherically or
spherically shaped striker. This test provides a quantitative measure of the
puncture
resistance of thin plastic films. This test is further described in U.S.
Patent No.
6,815,023.
The following example is given to illustrate the invention and should not be
construed as limiting that which is described in the appended claims.
CA 02458136 2004-02-19
In the following example, the film composition was produced generally
utilizing the apparatus
and method described in U.S. Pat. No. 3,456,044 (Pahlke) which describes a
coextrusion type of
double bubble method and in further accordance with the detailed description
above. All layers were
extruded as a primary tube which was cooled upon exiting the die e.g. by
spraying with tap water.
This primary tube was then reheated by radiant heaters(although other means
known to those skilled
in the art, such as conduction or convection heating may be used) with further
heating to the draw
(orientation) temperature for biaxial orientation accomplished by an air
cushion which was itself
heated by transverse flow through a heated porous tube concentrically
positioned around the moving
primary tube. Cooling was accomplished by means of a concentric air ring. Draw
point
temperature, bubble heating and cooling rates and orientation ratios were
generally adjusted to
maximize bubble stability and throughput for the desired amount of stretching
or orientation. All
percentages are by weight unless indicated otherwise.
EXAMPLE 1
A puncture-resistant bag according to the present invention, as generally
illustrated in Figs. 1
& 2, was produced from a film comprising a coextruded three-layer biaxially
oriented shrink film
having (A) an inner heat sealing layer, (B) a barrier layer and (C) an outer
layer. The inner and outer
layers being directly attached to opposing sides of the barrier layer. The
three layers included the
following compositions:
(A) 33 wt. % EXACTTM 4053; 37% ESCORENET"' LD 701.ID; 24% SCLAIRTM 10B; 4%
Spartech A27023; and 2% Spartech A32434;
(B) a blend of about 85% vinylidene chloride-vinyl chloride copolymer and
about 15%
vinylidene chloride-methacrylate copolymer; and
31
CA 02458136 2004-02-19
(C) 33 wt. % EXACTTM 4053; 25 % SCLAIRTM l OB; 40% ESCORENETM LD 701.ID; and
2%
Ampacet 501236.
One extruder was used for each layer. Each extruder was connected to an
annular coextrusion
die from which heat plastified resins were coextruded forming a primary tube.
The resin mixture
for each layer was fed from a hopper into an attached single screw extruder
where the mixture was
heat plastified and extruded through a three-layer coextrusion die into the
primary tube. The
extruder barrel temperature for the barrier layer (B) was between about 250-
300F (121-149_C); for
the inner layer (A) and for the outer layer (C) were about 290-330_F(143-
165_C). The coextrusion
die temperature profile was set from about 320 to 350_F (163 to 177_C). The
extruded multilayer
primary tube was cooled by spraying with cold tap water 50-68 _F (about 10-20
_C).
A cooled primary tube of about 4 inches flatwidth was produced passing through
a pair of nip
rollers. The cooled flattened primary tube was inflated, reheated, biaxially
stretched, and cooled
again to produce a biaxially stretched and biaxially oriented film which was
slit open, laid flat to
form a sheet having a width of approximately 16 inches and wound on a reel.
The M.D. orientation
ratio was about 5:1 and the T.D. orientation ratio was about 4:1 . The draw
point or orientation
temperature was below the predominant melting point for each layer oriented
and above that layer's
predominant glass transition point and is believed to be about 68-85 _C. The
resultant biaxially
oriented film had an average gauge of about 2.5 mil and had an excellent
appearance.
The film was irradiated at a dosage level of about 5.0 MR. As previously
noted, although not
essential, it is preferred to irradiate the entire film to broaden the heat
sealing range and/or enhance
the toughness properties of the inner and outer layers by irradiation induced
cross-linking and/or
scission. Irradiation may be done on the primary tube or after biaxial
orientation. The latter, called
32
CA 02458136 2011-01-13
post-irradiation, is preferred and described in Lustig et al. U.S. Pat. No.
4,737,391. An
advantage of post-irradiation is that a relatively thin film is treated
instead of the
relatively thick primary tube, thereby reducing the power requirement for a
given
treatment level.
The film was unwound and slit to a width of 13 inches. The film was then fed
into
the bag making equipment to form a tube member having a continuous
longitudinally
extending lap seal. Bags according to the bag 10 depicted in FIG. I were
formed by
sealing laterally across the tube member and simultaneously severing the
sealed portion
from the continuous tube structure.
Various tests were performed on the film and/or resultant inventive bags. The
film
thickness was determined to be an average 2.1 mil. The lap seal was tested to
have a very
strong average seal strength of about 8,000 to 10,000 grams. The bag also had
an average
M.D. and T.D. heat shrinkability at 90 C of 48 and 48, respectively. The ram
puncture
results were likewise impressive. The puncture resistance of the 2.1 mil thick
film was
measured and had a maximum puncture force of 86 Newtons (N) and a total energy
to
failure of 0.9 Joules (J). This preferred bag has very good heat shrink
percentages which
are highly desirable for packaging cuts of fresh red meat and extremely good
puncture
resistance. Thus, an economical to produce heat shrinkable bag having puncture
resistance and strong seals has been made having a unique combination of
features and
commercial advantages previously unknown.
Advantageously, the bags 10 and 110 may be fabricated of nearly any dimensions
economically since the bags 10 and 110 are not formed from a seamless tube
that must be
generated to the desired width. The only limitation on size of fabricated bag
is the size of
a stock sheet films having great enough widths to meet the specifications.
Standard roll
stock films are available in widths in excess of 100 inches. The present
invention allows
a bag manufacturer to fabricate any size bag from the
33
CA 02458136 2004-02-19
same flat sheet of roll stock, up to the dimensional limits of the roll stock.
For example, if the roll
stock is 52 inches in width, a tube member can be fabricated having a lay-flat
width of
approximately 26 inches, less the amount of overlap or abutment in the first
seal 16 or 116 used. If
the manufacturer wishes to fabricate a bag having a lay-flat width of 18
inches, then the
manufacturer slits the standard roll stock to the appropriate width
(approximately 36 plus extra for
the area of the first seal 16 or 116). The unused portion slit form the
standard roll stock is rewound
for use making bags of another dimension(s). In this manner, standard roll
stock films can be
manufactured more economically because film manufacturing equipment may be run
at or near the
upper limits of film width production and thereby use nearly all the
equipments capacity.
Fabricating bags from seamless tubes requires that the film making equipment
be run at limited
capacities to form the different smaller width tubes. Additionally, the film
making equipment
requires costly set-up and breakdown between jobs of differing dimensions that
add significantly to
the cost of manufacturing the seamless tubes.
A preferred embodiment of the heat-shrinkable package of the present invention
is made from
a sheet 410 of heat shrinkable film 411 having a first side edge 412a and
opposing, second side edge
412b connected by a third side edge 412c and a fourth side edge 412d. First
side edges 412a and
second 412b are preferably parallel to each other when film 411 is in a long
flat planar state. Third
side edge 412c and fourth side 412d are preferably parallel to each other when
film 411 is in a lay
flat planar state. First and second side edges 412a, 412b are also preferably
perpendicular to third
and fourth side edges 412c, 412d when film 411 is in a lay flat planar state.
Film 411 has four
corners at the intersections of the four sides with first corner 412ac defined
by the junction of first
side edge 412a with third side edge 412c; second corner 412b defined by the
junction of first side
edge 412a with third side edge 412c; second corner 412bc defined by the
junction of second side
edge 412b with third side edge 412c; third corner 412ad defined by the
junction of first side edge
34
CA 02458136 2004-02-19
412a with fourth side edge 412d; and fourth corner 412bd defined by the
junction of second side
edge 412b with fourth side edge 412d. Film 411 has a top surface 413a
circumscribed by a perimeter
414 formed by sides 412a, 412c, 412b and 412d with an opposing bottom surface
413b also
circumscribed by said perimeter 414. FIG. 10 Depicts corner 412ad of film 411
turned upward to
reveal said bottom surface 413b.
Referring now to FIG. 11, a preferred embodiment of the present invention is
depicted generally
as a bag 415 made from said film 411 of FIG. 10. The bag 415 is formed by
overlapping the first
side edge 412a with the second side edge 412b and sealing preferably by heat
to produce a fusion
bond lap seal 416 defined by parallel spaced apart dotted lines 417a and 417b,
and third side edge
412c and fourth side edge 412d. It should be noted that while said lap seal
416 is depicted as a
continuous elongated rectangle extending from side 412c to side 412d, the
invention further
contemplates that the seal shape may vary and could, for example, form a wavy
line or zigzag shape
or other shapes as desired. Also, the width of the seal may be varied to be
thicker or thinner as
desired. Also the seal may optionally be made by alternatives or additional
means, including, e.g.,by
applications of suitable flue or adhesive material known in the art for
sealing together films. It is
further contemplated that said lap seal 416, while depicted as a continuous
lap seal 416 suitable for
forming a hermetic package, it is also contemplated that for some
applications, e.g., for certain
industrial or non-perishable items, a noncontinuous seal having, e.g., the
appearance of a dotted or
dashed line, may be employed. The intermittent seal embodiment permits air to
escape enclosure
during packaging operations where it is not desired to either apply a vacuum,
or seal with a trapped
bubble of air or other gas, or remove air by other means. Optionally, the
strength of the seal may
be varied by one skilled in the art in view of the teachings of the present
application by selection of
aforesaid parameters such as seal shape, thickness, continuous or intermittent
nature, material
selection type of and known parameter for varying the strength of different
types of seals, e.g., by
adjusting dwell time or temperature for producing heat seals. Such variations
and adjustments may
CA 02458136 2004-02-19
be made by those skilled in the art without undue experimentation.
Referring again to FIG. 11, lap seal 416 is preferably a heat seal forming a
fusion bond between
top surface 413a and bottom surface 413b of film 411. The overlapped sealed
film 411 defines a
tube member 418 in which top surface 413a of film 411 forms an inner film
surface 419 of said tube
member 418. A second seal 420 extends laterally across said tube member 418
adjacent the third
side edge 412c of film 411 thereby forming a closed bag end 421. A variety of
seals may be used.
Preferably second seal 420 will be a heat seal which fusion bonds the bag film
inner surface 419 to
itself. The second seal 420 by closing bag end 421 both forms a first bag edge
422 and opposing
second bag edge 423, and the second seal extends across the tube member 418
from the first bag
edge 422 to the second bag edge 423. The second seal may also employ a variety
of shapes,
thicknesses, structures, etc., as for the previously described lap seal 416.
The lap seal does not need
to be centered between edges 422 and 423 but preferably is positioned anywhere
therebetween.
Opposite the closed bag end 421 is a bag mount formed by lap sealed film under
fourth side
edge 412d through which a product (not depicted) may be placed into a product
receiving chamber
425 defined by tube member 418, closed bag end 421 and bag mouth 424. The
first bag edge 422
may extend from a first bag end corner 426 to a first bag mouth point 427 and
a second bag edge 423
may extend from a second bag end corner 428 to a second bag mouth point 429
such that bag 415
may be collapsed into a lay flat condition having first bag edge 422 and
opposing second bag edge
423. In a lay flat condition or a state close to lay flat such as depicted in
FIG. 11, bag end 421, bag
mouth 424 and connecting first bag edge 422 and second bag edge 423 defines a
first bag wall 430
and connected opposing bag wall 431. Tube member 418 has an inner surface 419
and an outer
surface 433. The first bag wall 430 has first bag wall first side 430a
proximate second side edge
412b and extending to second bag edge 423. The first bag wall 430 also has an
opposing first bag
wall seamed side 430b proximate first side edge 412a and extending to first
bag edge 422.
Preferably, the second seal 420 is provided in a manner such that the first
seal 416 is positioned
36
CA 02458136 2011-01-13
within one of the first and second bag walls 430 and 431, thereby forming a
"backseam"
of the bag. This provides one seamless bag wall and two seamless bag edges
that may
include printed images applied to the film before forming bags or after the
bag is formed.
Additionally, the second seal 420 may take any shape, whether straight or
curved, so
long as the second seal 420 operates to close the end 421. At least one of the
first seal
416 and second seal 420 comprises a peelable seal. "Peelable seal" and like
terminology
is used herein to refer to a seal, and especially heat seals, which are
engineered to be
readily peelable without uncontrolled or random tearing or rupturing the
packaging
materials which may result in premature destruction of the package and /or
inadvertent
contamination or spillage of the contents of the package. A peelable seal is
one that can
be manually peeled apart to open the package at the seal without resort to a
knife or other
implement to tear or rupture the package. In the present invention, the
peelable seal must
have a seal strength sufficient to prevent failure of the seal during the
normal heat-
shrinking process and further normal handling and transport of the packaged
article. The
seal strength must also be low enough to permit manual opening of the seal.
Preferably
seal parameters such as choice of materials and sealing conditions will be
used to adjust
the seal strength to the desired level for the particular package and
application.
Many varieties of peelable seals are known in the art and are suitable for use
with the
present invention. Peelable seals are generally made from thermoplastic films
having a
peelable system designed therein. Suitable peelable films and/or peelable
systems are
disclosed in U.S. Patent Nos. 4,944,409 (Busche et al.); 4,875, 587 (Lulham et
al.);
3,655,503 (Stanley et al.); 4,058,632 (Evans et al.); 4,252,846 (Romesberg et
al.);
4,615,926 (Hsu et al.) 4,666,778 (Hwo); 4,784,885 (Carespodi); 4,882,229
(Hwo);
6,476,137 (Longo); 5,997,968 (Dries, et al.); 4,189,519 (Ticknor); 5,547,752
(Yanidis);
5,128,414 (Hwo); 5,023,121 (Pockat, et al.); 4,937,139 (Genske, et al.);
4,916,190
(Hwo); and 4,550,141 (Hoh). Preferred films for use in fabricating bags
according to the
invention
37
CA 02458136 2004-02-19
may be selected from multilayer, heat-shrinkable films capable of forming a
peelable seal. Preferred
films may also provide a beneficial combination of one or more or all of the
below noted properties
including high puncture resistance (e.g., as measured by the ram and/or hot
water puncture tests),
high shrinkage values, low haze, high gloss, high seal strengths and
printability. Since the inventive
bags may advantageously be used to hold oxygen or moisture sensitive articles
such as food
products after evacuation and sealing, it is preferred to use a thermoplastic
film which includes an
oxygen and/or moisture barrier layer. The terms "barrier" or "barrier layer"
as used herein means a
layer of a multilayer film which acts as a physical barrier to moisture or
oxygen molecules.
Advantageous for packaging of oxygen sensitive materials such as fresh red
meat, a barrier layer
material in conjunction with the other film layers will provide an oxygen gas
transmission
rate(OZGTR) of less than 70 (preferably 45 or less, more preferably 15 or less
) cc per square meter
in 24 hours at one atmosphere at a temperature of 73_F (23_C) and 0% relative
humidity. In an
alternative embodiment, the gas permeability is controlled to allow the escape
of CO,, e.g., for
packaging respiring foods such as cheese as described in U.S. Patent No.
6,511,688. Preferably, the
film has an unrestrained shrinkage of at least 20% (preferably at least 35%)
at 90_C at least one and
preferably both the machine (MD) and transverse (TD) directions. Unrestrained
(sometimes referred
to as "free") shrink is measured by cutting a square piece of film measuring
10 cm in each of the
machine and transverse directions. The film is immersed in water at 90-C for
five seconds. After
removal from the water the piece is measured and the difference from the
original dimensions are
each multiplied by ten to obtain the percentage of shrink in each respective
direction.
Oxygen barrier materials which may be included in the films utilized for the
inventive bags
include ethylene vinyl alcohol copolymers (EVOH), polyacrylonitriles,
polyamides and vinylidene
chloride copolymers (PVDC). For some applications nylon may provide useful
oxygen barrier
properties especially at low temperatures, e.g., as used with frozen foods.
Preferred oxygen barrier
polymers for use with the present invention are vinylidene chloride copolymers
or vinylidene
38
CA 02458136 2011-01-13
chloride with various comonomers such as vinyl chloride (VC-VDC copolymer) or
methyl acrylate (MA-VDC copolymer), as well as EVOH. A specifically preferred
barrier
layer comprises about 85% vinylidene chloride-methyl acrylate comonomer and
about
15% vinylidene chloride-vinyl chloride comonomer, as for example described in
Schuetz
et al. U.S. Pat. No. 4,798,751. Suitable and preferred EVOH copolymers are
described in
U.S. Patent No. 5,759,648.
A variety of peelable films and peelable sealing systems may be employed in
the
present invention. In a preferred embodiment, a film comprising a coextrusion
of at least
three layers (referred to as three layer peelable system to distinguish it
from systems using
one or more contaminated seal layers described below) having an outer layer,
an inner
heat seal layer and a tie layer disposed between the outer layer and the inner
heat seal
layer is used. In this preferred three layer system embodiment, the film
layers are selected
such that peeling occurs by breaking apart the tie layer and/or a bond between
the tie layer
and at least one of the outer and inner layers. Permanent, peelable, and
fracturable bonds
may be engineered into the coextrusion process, e.g., by providing two
adjacent first and
second layers having materials with a greater affinity for each other compared
to the
second layer and an adjacent third layer where this establishes a relatively
permanent
bond between the layers, when two materials have a lesser affinity for each
other. This
three layer structure establishes a relatively permanent bond between the
first and second
layer which have a greater affinity for one another than the second or third
layers which
have a lesser affinity where the second layer is common to both the first and
third layers
as a tie layer or connecting layer. Thus, the lesser affinity between the
second and third
layers relative to the first and second layers produces a relatively peelable
bond between
the second and third layers. Selection of the various materials determines the
nature of
the bond, i.e., whether it is permanent, peelable, fracturable or combinations
thereof.
Suitable polymers for use in the outer, tie and inner heat sealable layers
include both
poly-type
39
CA 02458136 2004-02-19
material such as ethylene homopolymers and copolymers as well as ionomer type
material.
Examples of suitable polymers include: ethylene vinyl acetate copolymer (EVA,
ethylene a-olefin
copolymers, linear low density polyethylene, low density polyethylene, very
low density
polyethylene (VLDPE), neutralized ethylene acid copolymer, plastomers,
ethylene acrylate
copolymer, ethylene methyl acrylate copolymer and zinc or sodium salts of
partially or completely
neutralized ethylene-methacrylate acid copolymers. The inner heat seal layer
beneficially uses heat
sealable materials. The tie layer is selected to have a relatively low peel
strength when peelably
bonded to one of either the outer layer or inner heat seal layer. The tie
layer is typically comprised
of a blend of about 5-30% polybutylene and another constituent, such as
ethylene vinyl acetate
copolymer, ethylene copolymers with C, - C8 alpha olefin, linear low density
polyethylene,
ionomers, neutralized ethylene acid copolymer or unneutralized ethylene acid
copolymer and
mixtures thereof. The term "polybutylene" as used herein includes having
polymeric units derived
from butene -1 as the major (75% polymeric units) components and preferably at
least 80% of its
polymeric units will be derived from butene -1. A preferred polybutylene is a
random copolymer
of butene -1 with ethylene having a reported density of 0.908 g/cm3 and a melt
index of 1.0 g/10 min.
and a melting point of 243 F, which is commercially available from Basell
Polyolefins Company,
N.V., The Netherlands, under the trade name PB 8640. In this preferred
peelable embodiment, the
heat seal formed between the inner heat seal layer and another layer to which
it is heat sealed,
whether part of another film or the same, should be permanent, i.e., should
have a seal strength
greater than the peelable bond between the tie layer and one of its adjacent
layers. The preferred
three layer coextruded peeling structure described above contemplates optional
additional layers to
product a film of 4, 5, 6, 7, 8, 9, 10 or more layers. It is further
contemplated that one or more
additional layers may be coextruded with the described three layers or
separately and that the
multilayer film structure may be formed not only by coextrusion, but also by
other methods well
known in the art such as coating lamination, adhesive lamination or
combinations thereof.
CA 02458136 2004-02-19
It is also contemplated that such one or more additional layers may be
adjacent to or between
any of the described three layers. In one embodiment of the invention the heat
seal layer may be
replaced by a permanent adhesive or glue that may or may not be applied hot or
in a melt state, liquid
state or otherwise. However, it is preferred to utilize a heat sealable layer.
It is also contemplated that a peelable seal using one or more so-called
"contaminated" surface
layers may be utilized where peeling occurs at a seal layer interface 432
rather than at an interior
layer of film 411. This type of peeling system suffers from disadvantage
associate with, e.g.,
controlling the diverging properties of providing high seal strength with
desirable low forms for
peelings, as well as problems of sealing under conditions which may adversely
affect seal integrity,
e.g., where an article being packaged deposits particulates, starch, fat,
grease or other components
which may lessen seal strength or hamper the ability to provide a seal of
desired strength such as a
strong hermetic fusion bond, e.g., by heat sealing. Such sealing systems are
often referred to as two
layer peeling systems, but may include 3, 4, 5, 6, 7, 8, 9, 10 or more layers
in the film structure.
Preferred peelable sealing films and peelable seal systems are disclosed in
U.S. Patent No.
4,944,409 entitled "EASY OPEN PACKAGE", the disclosure of which is
incorporated herein in its
entirety.
A preferred multilayer, barrier film structure for use in fabricating bags
according to the present
invention is illustrated in FIG. 12, which depicts an enlarged, end view of
the first seal 416 of FIG.
1 I made from the sheet of heat-shrinkable film 411. Layer thicknesses in FIG.
12 and other figures
presented herein are not to scale, but are dimensioned for ease of
illustration. A preferred easy to
peel heat shrinkable film 411 is a five layer coextrusion and includes from
inner surface 419 of the
tube member 419 (See FIG. 11) to an opposing outer surface 433.
(a) an inner surface heat sealing layer 434 preferably comprising a blend of
ethylene vinyl
acetate (EVA) and polyethylene;
(b) a barrier layer 435 preferably comprising a vinylidene chloride copolymer
(PVDC);
41
CA 02458136 2004-02-19
(c) a core layer 436 preferably comprising a blend of EVA and polyethylene;
(d) a tie layer 437 preferably comprising a blend of polyethylene and
polybutylene; and,
(e) an outer surface heat sealing layer 438 preferably comprising
polyethylene.
The thicknesses of each layer, based on the total thickness of the film 411,
may be typically
<50% inner surface heat sealing layer 434; <20% barrier layer 435; <28% core
layer 436; <15% tie
layer 437; and <15% outer heat sealing layer 438. The first seal 416 is made
by longitudinally heat
sealing the inner film surface 419 of film 411 to the outer film surface 433
along their respective
lengths, such that inner film surface 419 and outer film surface 433 overlap.
In this manner, a fusion
bond is made between the inner surface heat sealing layer 434 and the outer
surface heat sealing
layer 438. The peelable bond of the system is provided by the tie layer 437
and peeling occurs there,
e.g., at the tie layer interface with the outer surface heat sealing layer
438, and/or at the tie layer
interface with core layer 436 and/or between outer layer 438 and core layer
436. Thus, referring to
FIGS. 1 I and 12, the peelable portion of the film is on the outside of the
tube member 418, which
is preferable. This will insure that the first seal 416 is peelable, while the
second seal 420 and final
closing seal (not shown) are strong fusion seals between the inner surface
heat sealing layer 434 of
each bag wall 430 and 431.
Referring to FIG. 13, a fragmentary sectional view taken along lines B-B of
FIG. 11 illustrates
how a preferred embodiment of the invention works to create strong end seals
while permitting the
lap seal to function as an easy to open peel seal. In FIG. 13, film 411 has an
outer surface 433 with
consecutive layers therefrom of outer surface layer 438, tie layer 437, core
layer 436, barrier layer
435, and inner surface heat sealing layer 434. Referring to FIG. 11, the
second seal 420 is provided
across tube member 418 to collapse its surface 419 upon itself. Referring
again to FIG. 13, this seal
joins inner surface heat sealing layer 434 to itself with the peelable tie
layer 437 being positioned
distal from end seal interface 439. This preferred embodiment of the invention
depicted in FIGS.
11-13 combines (a) an end seal which mates like materials with strong seal
properties to each other
42
CA 02458136 2004-02-19
keeping distal the easily peelable tie layer 437 and (b) a lap seal having
peelable tie layer 437
proximate the outer surface heat sealing layer 438 and lap seal interface 432
, thereby providing an
easily peelable opening in bags or packages made using the described
configuration.
The film 411 is designed to control the film failure when peeled manually. Due
to the
composition of the peelable tie layer 437, its location proximate the lap seal
interface 432, and in the
case of the preferred three layer peelable system, the thinness and
composition of the outer surface
heat sealing layer 438; as the second side edge 412b is manually pulled
across, up and away from
the lap seal 416, a first rupture or tear will begin. This tear will propagate
from the heat seal at the
edge 417b of lap seal interface 432 through the outer heat sealing layer 438
thereof. If the peelable
bond is designed to occur at the tie layer 437, the continued application of
opening force causes: a
delamination or breaking of the adhesive bond, along the tie layer 437/outer
heat sealing layer 438
interface or along the tie layer 437/core layer 436 interface and/or causes
fracture of the tie layer 437,
or a combination thereof until the tear reaches the opposite side edge 417a of
the heat seal 416,
where the tear either propagates to edge 412a or back across the outer layer
438 and the bag is
thereby opened.
In general, the films used in the heat-shrinkable bags of the present
invention can have any
thickness desired, so long as the films have sufficient thickness and
composition to provide the
desired properties for the particular packaging operation in which the film is
used, e.g., peelable seal,
puncture-resistance, modulus, seal strength, barrier, optics, etc. For
efficiency and conservation of
materials, it is desirable to provide the necessary puncture-resistance and
other properties using the
minimum film thicknesses. Preferably, the film has a total thickness from
about 1.25 to about 8.0
mils; more preferably from about 1.75 to about 3.0 mils.
Another embodiment of the present invention is illustrated in FIGS. 14 and 15,
generally as bag
415a. Identical reference numerals have been used with respect to elements of
Bag 415a, which are
also found in bag 415. Bag 415a further includes a pull flap 440. The pull
flap 440 is formed by
43
CA 02458136 2004-02-19
providing additional overlap by moving the first and second sides edge 412a
and 412b further apart
and positioning the first lap seal 416 such that a portion of the first bag
wall, first side 430a, that
overlaps the first bag wall second side 430b outside of the product receiving
chamber 425 is not
sealed to the second side 430b. The pull flap 440 may be readily grasped by
the end user and pulled
to easily open the package, without resort to a cutting instrument, as is
often required when opening
packages without a peelable system. Although shown as extending the entire
length of the bag 415a,
a skilled artisan will appreciate that the pull flap 440 may be cut to a
desired shape or that any other
known device known to aid initiation of peeling may be incorporated. The
preferred film illustrated
in FIGS. 10, 12 and 13 described previously is also preferred for use with bag
415a.
The alternative embodiment illustrated in FIGS. 14 and 15 has reversed the
location of the bag
mouth 424 and second seal 420 of FIG. 10 which is depicted in FIG. 14 as bag
mouth 424a and
second seal 420a.
Referring to FIG. 16, an illustration of the second seal 420a in cross-section
shows first bag wall
430 sealed to second bag wall 431 from first bag edge 422 to second bag edge
423 and across first
lap seal 416 which is located between first side edge 412a and second side
edge 412b. In the well
known heat sealing process opposing sealing bars or wires press together
layers of film under
elevated temperature and pressure for a time sufficient to cause a fusion bond
therebetween. These
heat seal bars may be rigid and/or flexible but generally are not supple or
not as supple as the film
being sealed. As depicted in FIG. 16, the second seal 420a has a seal
interface 439a which has two
possible points proximate first side edge 412a and second side edge 412b where
sealing pressure
may be reduced during the sealing operation sealing pressure may be reduced at
second seal interface
439a at a point 441 below edge 412b, and also at point 442 adjacent first edge
412a. It is also
possible that a void may exist, e.g., at point 442. In order to produce a
desired strong seal
particularly at points 441 and 442 as well as all along second seal interface
439a, sealing parameters
such as pressure, temperature, dwell time and heat sealing layer composition
may be adjusted as
44
CA 02458136 2004-02-19
desired. In particular, it has been found that use of a high melt index
polymer component in the heat
seal layer may be advantageous to fill potential voids. It may also be
advantageous to taper one or
both edges 412a and 412b to increase contact surfaces and/or pressure between
the overlapping films
particularly at points 441 and 442 and adjacent areas.
Another embodiment of the present invention is illustrated in FIG. 17,
generally as bag 415b.
Again, like elements include like reference numerals. Bag 415b includes a
first fin seal 516 joining
the first and second sides 430a and 430b of bag wall 430 such that the inner
film surfaces 419 of
each side are in a face-to-face abutment, having a fin seal interface 517. One
or both of the first and
second side edges 412a and 412b may extend outwardly beyond the first fin seal
interface 517 such
that a pull flap (not shown) is provided. Bag 415a (FIG. 14) is preferred over
bag 415b, since the
plane of the first seal 416 is parallel to the plane of the shrink forces
encountered during the heat-
shrinking process. The first fin seal 516 of bag 415b places the plane of the
heat seal perpendicular
to the shrink forces (as shown by arrows Z' and Z" in FIG. 19), which
increases the risk of seal
failure (premature peeling) during the heat-shrinking process. Additionally,
since the inventive
receptacles are advantageously fabricated from a single sheet or web of film,
then a fin seal
arrangement, such as first seal 516, requires that each seal of the receptacle
be a peelable seal. Also,
the second seal 420 and final closing seal (not shown) are also necessarily
peelable since the first and
second bag walls 430a and 430b are sealed with the film in the same abutted
relationship. For
example, FIG. 19 depicts an enlarged view of the first fin seal 516 shown in
cross-section showing
discrete layers of the preferred film discussed above with bags 415 and 415a.
Each wall 450 and 452
of the seal 516 includes a three layer peelable system (the tie layer 437)
equidistant from and
proximate to the sealed interface of sealant layer 438. Thus, it not only
cannot be predetermined in
which wall 450 or 452 the peel failure will occur, but all seals are easily
peeled and the shrink force
direction further reduces the ability to make strong seals. For all these
disadvantages this
embodiment is least favored.
CA 02458136 2004-02-19
Another embodiment of the present invention is illustrated in FIGS. 20 and 21
generally as bag
415c. Again, like elements include like reference numerals. Bag 415c includes
a first seal 616
comprising a butt-seal tape 641 comprising a butt-seal film 611 having a first
border 607, a second
border 609, a sealing surface 615 and an exterior surface 614. The first seal
616 includes a first heat
seal 618 longitudinally joining the first side 430a of bag wall 430 to the
first border 607 of the butt-
seal tape 641, and a second heat seal 619 longitudinally joining the second
side 430b of bag wall 430
to the second border 609 of the butt-seal tape 641. Thus, first and second
sides 430a and 430b are
joined in an abutting edge-to-edge relationship thereby forming bag wall 430
without a heat seal
directly there between. Preferably, the butt-seal film 611 comprises the same
film as described in
reference to bags 415, 415a and 415b described above and illustrated in FIGS.
10-19, with the
outer heat sealing layer 438 (FIG. 11) comprising the inner surface 615. Thus,
bag 415c may be
manufactured from a film that does not include a peelable system therein, but
includes a peelable
seal by means of the peelable system included in the butt-seal tape 641 used
to form the first seal
616. Conversely, the film 411 may preferably include a peelable system while
the butt-seal tape
641 does not, or both film 411 and butt-seal film 611 may include a peelable
system compatible with
the other. The butt-seal film 611 is preferably heat-shrinkable, but need not
be. A pull flap 440 may
be provided in the butt-seal tape 641 to provide an area for the consumer to
manually grasp and pull
to easily open the bag 415c. If the butt-seal tape 641 is sealed to the inner
surface 419 of the film
411, then a portion of the first or second sides 430a and 430b may extend
outwardly past the first
or second heat seals 618 and 619 to provide a pull flap for the consumer to
grasp. The second seal
420 is preferably a permanent seal made between the inner surfaces 419 of the
first and second bag
walls 430a and 430b.
Although depicted in FIG 20 as being sealed to the outer surfaces 415 of the
first and second
sides 412 and 414, one skilled in the art will appreciate that the butt-seal
tape 641 that forms the first
seal 616 may be placed on the inside of the bag 410c (not shown), whereby the
sealing surface 615
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CA 02458136 2004-02-19
is heat sealed to inner surfaces 419 of the first and second sides 430a and
430b. In this instance,
preferably at least one of the first and second sides 430a and 430b include a
portion that extends
outwardly beyond the heat seal to the butt-seal tape 641. Thus, the consumer
is provided with a pull
flap to grasp.
A further embodiment of the present invention is illustrated in FIGS. 22 and
23 generally as bag
415d. Like elements discussed above in connection with bags 415, 415a, 415b
and 415c have been
given the same reference numerals in bag 415d. Bag 415d includes a first seal
716 comprising a seal
strip 741 comprising a strip film 711 having an inside surface 714 and an
outward surface 715. The
seal strip 741 includes a first margin 718 longitudinally heat sealed to the
first side 430a by first heat
seal 720, such that the outward surface 715 is sealed in face-to-face contact
with the inner surface
419 of film 411. The seal strip 741 includes a second margin 719
longitudinally heat sealed to the
second side 430b by second heat seal 721, such that the inside surface 714 is
sealed in face-to-face
contact with the outer surface 433 of the second side 430b. A pull flap 440
may be provided by
including a portion of the strip film 711 that extends outwardly beyond second
heat seal 721 joining
the second margin 719 and the second side 430b. Alternatively, the first side
430a could be
provided with a portion that extends outwardly beyond the second heat seal
420.
Preferably, the strip film 711 includes a peelable system and comprises the
same film as
described in reference to bags 415, 415a and 415b described above and
illustrated in FIGS. 10-2 1,
with the outer heat sealing layer 438 (FIGS. 12-13) comprising the inside
surface 714. In this
manner, the heat seal 721 is peelable and the film 411 need not include a
peelable system.
Alternatively, the outer heat sealing layer 438 could comprise the outward
surface 715, such that heat
seal 720 is peelable. In this case, the film 411 need not include a peelable
system and the second
seal 420 maybe made permanent. Ina similar manner as described for bag 415c,
the strip film 711
may not include a peelable system while the film 411 does include a peelable
system, or both film
411 and strip film 711 may include compatible peelable systems. The strip film
711 is preferably
47
CA 02458136 2011-01-13
heat-shrinkable, but need not be.
The bags according to the invention are preferably fabricated continuously
from a
continuous sheet or roll stock as described in U.S. Patent Application
Publication No.
2004/0166261, in the name of Gregory Robert Pockat, et al., filed on February
20, 2003
entitled "HEAT-SHRINKABLE PACKAGING RECEPTACLE". The roll stock is slit to
a desired width and fed to bag making equipment, wherein the machine direction
sides of
the film are brought together and sealed longitudinally, either with a lap
seal (bags 415
and 415a) or a fin seal (bag 415b) to form a continuous single-seamed tube, or
tube
member. A transverse seal is made across the tube member and the section
including the
transverse seal is severed from the continuous tube to form the individual
bag. Generally,
heat seals are made by supplying sufficient heat and pressure between two
polymeric film
layer surfaces for a sufficient amount of time to cause a fusion bond between
the
polymeric film layers. Common methods of forming heat seals include hot bar
sealing,
wherein adjacent polymeric layers are held in face-to-face contact by opposing
bars of
which at least one is heated, and impulse sealing, wherein adjacent polymeric
layers are
held in face-to-face contact by opposing bars of which at least one includes a
wire or
ribbon through which electric current is passed for a very brief period of
time to cause
sufficient heat to cause the film layers to fusion bond. Less area is
generally bonded with
an impulse seal relative to a hot bar seal, thus the performance of the film's
sealing layer
is more critical. However, an impulse seal is generally more aesthetic since
less area is
used to form the bond.
The films selected to fabricate the inventive receptacles are preferably
biaxially
stretched or oriented by the well-known trapped bubble or double bubble
technique as for
example described in U.S. Patent Nos. 3,456,044 and 6,511,688. In this
technique an
extruded primary tube leaving the tubular extrusion die is cooled, collapsed
and then
preferably oriented by reheating, reinflating to form a secondary bubble and
recooling.
The film is preferably biaxially oriented wherein transverse (TD)
48
CA 02458136 2004-02-19
orientation is accomplished by inflation to radially expand the heated film.
Machine direction (MD)
orientation is preferably accomplished with the use of nip rolls rotating at
different speeds to pull
or draw the film tube in the machine direction. The stretch ratio in the
biaxial orientation to form
the bag material is preferably sufficient to provide a film with total
thickness of between about 1 and
8 mils. The MD stretch ratio is typically 3:1-5:1 and the TD stretch ratio is
also typically 3:1-5:1.
Referring now to FIG. 21, a double bubble (also know as a trapped bubble)
process is shown.
The polymer blends making up the several layers are coextruded by conveying
separate melt streams
61 la, 61 lb, and 611c to the die 630. These polymer melts are joined together
and coextruded from
annular die 630 as a relatively thick walled multilayered tube 632. The thick
walled primary tube
632 leaving the extrusion die is cooled and collapsed by nip rollers 631 and
the collapsed primary
tube 632 is conveyed by transport rollers 633a and 633b to a reheating zone
where tube 632 is then
reheated to below the melting point of the layers being oriented and inflated
with a trapped fluid,
preferably gas, most preferably air, to form a secondary bubble 634 and
cooled. The secondary
bubble 634 is formed by a fluid trapped between a first pair of nip rollers
636 at one end of the
bubble and a second pair of nip rollers 637 at the opposing end of the bubble.
The inflation which
radially expands the film provides transverse direction (TD) stretching and
orientation. Orientation
in the machine direction (MD) is accomplished by adjusting the relative speed
and/or size of nip
rollers 636 and nip rollers 637 to stretch (draw) the film in the machine
direction.
The biaxial orientation preferably is sufficient to provide a multilayer film
with a total thickness
less than 10 mil and typically from about 1.25 to 8.0 mils or more, preferably
less than 5 mil and
more preferably between 1.75 and 3.0 mils (44.5 to 76 ).
After orientation, the tubular film 238 is collapsed preferably to a flatwidth
of up to 80 inches,
typically between about 5-30 inches, slit open longitudinally, laid flat and
wound on a reel 239 for
use as rollstock. One skilled in the art will appreciate that while the above
described method may
be used to form the film, films may be made by other conventional processes,
including single
49
CA 02458136 2004-02-19
bubble blown film or slot cast sheet extrusion processes with subsequent
stretching, e.g., by tentering
to provide orientation. One skilled in the art will further appreciate that
the flatwidth of the
collapsed tube will determine the width of the sheet film that results
therefrom. Thus, the primary
tube dimensions and subsequent processing may be selected to provide a maximum
flatwidth and
film thickness for the desired application, thereby advantageously maximizing
the production
capacity of the film making equipment.
Advantageously, a bag maker may produce bags of various lengths and widths
from rolls of film
rollstock by adjusting the width of the sheet and the distances between the
transverse end seal and
bag mouth for a particular bag or series of bags. This advantageously avoids
the costly need to
produce specific widths of seamless tubes which are currently widely used by
meat packers and
which do not include a peelable seal. Also the present invention permits cost
savings and
manufacturing efficiencies by permitting creation of numerous widths and
lengths of bag from
standard rollstock. The bag maker may simply slit film rollstock to a desired
width and form a
continuous tube member by longitudinally sealing opposing sides as described
for bags 415, 415a
and 415b. Bags of adjustable lengths may be made by transversely sealing and
cutting through the
tube member at a position spaced from the transverse seal.
Preferably, bag making is a continuous process; shown schematically in FIG.
25, wherein the
film is directed to a bag making assembly (not shown) where individual end-
seal bags are made.
Film 411 is fed continuously from reel 639 and optionally slit to form a
desired width film 411 a and
an unused film 41 lb. Film 41 la is fed to a bag making assembly (not shown).
Unused film 41 lb
is rewound on reel 639b for later use, or may be fed to another bag making
assembly. The first and
second sides 430a and 430b of film 411a are brought together and sealed
longitudinally, preferably
in a first seal, e.g., lap seal 416 having an additional overlap portion that
will act as a pull flap, to
form a continuous backseamed tube member 418. The second seal 420 is provided
transversely
across the tube member 418 at a desired location spaced from the opening 424.
The tube member
CA 02458136 2004-02-19
418 is then (or preferably simultaneously) severed to separate the portion
containing the second seal
from the continuous tube, thereby forming bag 415. Typically as the transverse
seal is made for one
bag a transverse cut forming the mouth of the adjacent bag is being made. This
process forms a so
called "end-seal" bag which, when it is laid flat, has a bottom edge formed by
the transverse heat
seal, an open mouth formed by the severed edge and two side edges formed by
the fold produced
when the tube member is laid flat. The transverse heat seal should extend
across the entire tube
member to ensure a hermetic closure where such is desired. Each bag being
formed from a length
of the tube member will necessarily be formed by at least two, usually
parallel, spaced apart,
transverse cuts which cause a segment of the tube member to be made and one
transverse seal,
usually adjacent one of these cuts, will define a bag bottom which is located
opposing the bag
opening, which is formed by the distal cut. The spacing between the lateral
seal and the opening,
which may vary, will determine the length of the bags formed. The length of
the bags can easily
be varied by changing the distance between transverse seals and cuts. The
width of the bags can also
be easily varied by changing the width of the film by slitting the standard
rollstock.
Unless otherwise noted, the following physical properties are used to describe
the invention,
films and seals. These properties are measured by either the test procedures
described below or tests
similar to the following methods.
Average Gauge: ASTM D-2103
Tensile Strength: ASTM D-882, method A
1% Secant Modulus: ASTM D-882, method A
Oxygen Gas Transmission Rate (O2GTR) : ASTM D-3985-81
Percent Elongation at Break: ASTM D-882, method A
Molecular Weight Distribution: Gel permeation chromatography
Gloss: ASTM D-2457, 45 Angle
Haze: ASTM D-1003-52
51
CA 02458136 2004-02-19
Melt Index: ASTM D-1238, Condition E (190 C) (except for propene-based (>50%
C3 content)
polymers tested at Condition L(230 C.))
Melting Point: ASTM D-3418, peak m.p. determined by DSC with a 10 C/min.
heating rate.
Vicat Softening Point (Vsp): ASTM D-1525-82
Seal Strength: ASTM F88-94 (Standard Test Methods for Seal Strength of
Flexible Barrier
Materials)
All ASTM test methods noted herein are incorporated by reference into this
disclosure.
Shrinkage Values: Shrinkage values are obtained by measuring unrestrained
shrink of a 10 cm.
square sample immersed in water at 90 C (or the indicated temperature if
different) for five to ten
seconds. Four test specimens are cut from a given sample of the film to be
tested. Specimens are
cut into squares of 10 cm length (M.D.) by 10 cm. length (T.D.). Each specimen
is completely
immersed for 5-10 seconds in a 90 C (or the indicated temperature if
different) water bath. The
specimen is then removed from the bath and the distance between the ends of
the shrunken specimen
is measured for both the M.D. and T.D. directions. The difference in the
measured distance for the
shrunken specimen and each original 10 cm. side is multiplied by ten to obtain
percent shrinkage in
each direction. The shrinkage of 4 specimens is averaged and the average M.D.
and T.D. shrinkage
values reported. The term "heat shrinkable film at 90 C" means a film having
an unrestrained
shrinkage value of at least 10% in at least one direction.
Tensile Seal Strength (Seal Strength) Test
Five identical samples of film are cut 1 inch (2.54 cm) wide and a suitable
length for the test
equipment e.g. about 5 inches (12.7 cm) long with a 1 inch (2.54 cm) wide seal
portion centrally and
transversely disposed. Opposing end portions of a film sample are secured in
opposing clamps in
a universal tensile testing instrument. The film is secured in a taut snug fit
between the clamps
52
CA 02458136 2011-01-13
without stretching prior to beginning the test. The test is conducted at an
ambient or
room temperature (RT) (about 23 C) test temperature. The instrument is
activated to
pull the film via the clamps transverse to the seal at a uniform rate of 12.0
inches (30.48
cm) per minute until failure of the film (breakage of film or seal, or
delamination and loss
of film integrity). The test temperature noted and lbs. force at break are
measured and
recorded. The test is repeated for four additional samples and the average
grams at break
reported.
Ram Puncture Test
The ram puncture test is used to determine the maximum puncture load or force,
and the maximum puncture stress of a flexible film when struck by a
hemispherically or
spherically shaped striker. This test provides a quantitative measure of the
puncture
resistance of thin plastic films. This test is further described in U.S.
Patent No.
6,815,023.
Following are examples and comparative examples given to illustrate the
invention.
In all the following examples, unless otherwise indicated, the film
compositions
were produced generally utilizing the apparatus and method described in U.S.
Patent Nos.
3,456,044 (Pahlke) and 6,511,688 (Edwards, et al.) which both describe a
coextrusion
type of double bubble method and in further accordance with the detailed
description
above. In the following examples, all layers are extruded (coextruded in the
multilayer
examples) as a primary tube which is then cooled upon exiting the die e.g. by
spraying
with tap water. This primary tube is then reheated, and stretched and cooled
as taught in
the above patents.
EXAMPLE 2
A heat-shrinkable bag according to the present invention, as generally
illustrated in FIGS.
53
CA 02458136 2011-01-13
& 11, is produced from a film comprising a coextruded five-layer biaxially
oriented shrink film having from inner surface to outer surface, (A) an inner
heat
sealing layer, (B) a barrier layer (C) a core layer, (D) a tie layer and (E)
an outer heat
sealing layer. The inner and outer layers being directly attached to opposing
sides of
the barrier layer. The five layers included the following composition:
(A) 37 wt. % VLDPE; 24% EVA; 33 % plastomer (Exact 4053); 6%
processing aids;
(B) a blend of about 85% vinylidene chloride-vinyl chloride copolymer and
about 15% vinylidene chloride-methacrylate copolymer;
(C) 100 wt. % EMA
(D) 20 wt. % VLDPE; 33% plastomer (Exact 4053) and 20 wt. %
polybutylene; and,
(E) 40 wt. % VLDPE; 33% plastomer (Exact 4053); 25% EVA; 2% processing
air.
One extruder was used for each layer. Each extruder was connected to an
annular coextrusion die from which heat plastified resins were coextruded
forming a
primary tube. The resin mixture for each layer was fed from a hopper into an
attached
single screw extruder where the mixture was heat plastified and extruded
through a
five-layer coextrusion die into the primary tube under conditions similar to
those
disclosed in copending U.S. Application Publication No. 2004/0166261.
Although not essential, it is preferred to irradiate the entire film to
broaden the
heat sealing range and/or enhance the toughness properties of the inner and
outer
layers by irradiation induced cross-linking and/or scission. This is
preferably done by
irradiation with an election beam at dosage level of at least about 2 megarads
(MR)
and preferably in the range of 3-5 MR, although higher dosages may be employed
especially for thicker films or where the primary tube is irradiated.
Irradiation may be
done on the primary tube or after biaxial orientation. The latter,
54
CA 02458136 2011-01-13
called post-irradiation, is preferred and described in Lustig et al. U.S. Pat.
No.
4,737,391. An advantage of post-irradiation is that a relatively thin film is
treated
instead of the relatively thick primary tube, thereby reducing the power
requirement
for a given treatment level.
The film is unwound and slit to a desired width. The film is then fed into the
bag making equipment to form a tube member having a continuous longitudinally
extending lap seal. Bags according to the bag 415a depicted in FIG. 14 may be
formed by sealing laterally across the tube member and simultaneously severing
the
sealed portion from the continuous tube structure.
Various tests may be performed on the resultant inventive bags. The gauge
thickness will typically be a film thickness of less than 10 mil, and
preferably between
1.25 to 5.0 mil. The lap seal should typically have an average seal strength
of at least
2 kilograms per inch. The end seal will typically have an average seal
strength of at
least 3 kilograms. The bag will also have an average M.D. and T.D. heat
shrinkability
at 90 C of at least 20%, and preferably at least 40% in both directions,
respectively.
This preferred bag will have very good heat shrink percentages which are
highly
desirable for packaging cuts of fresh red meat and also have extremely good
puncture
resistance, yet advantageously incorporate a peelable seal heretofore not seen
in
individual food packaging bags. Thus an economical to produce, heat shrinkable
bag,
having a peelable seal, puncture resistance and strong end seals is provided
having a
unique combination of features and commercial advantages previously unknown.
The present invention advantageously provides an individual heat-shrinkable
bag having an easily peelable seal. Thus, the receptacles or bags of the
present
invention may be easily opened without resort to a knife or other
cutting/opening
instrument, which allows food
CA 02458136 2004-02-19
producers to offer a desirable, consumer-friendly package.
Another preferred embodiment of the present invention uses a 7-layer heat
shrinkable
film to produce backseamed material. This 7-layer film has several advantages
over 3 and 5
layer structures. Use of a polymeric having a high melt index greater than 2.0
dg/10 min, e.g., an
ethylene a-olefin copolymer such as Exact 4053 in the sealant layers helps
seal through creases
and wrinkles in the seal. This is important as the overlapped area creates a
crease in the seal.
Another advantage is use of a strong adhesive polymer, e.g., an ethylene
methylacrylate
copolymer (EMA) such as Emact SP 1330 (which reportedly has: a density of
0.948 g/cm'; melt
index of 2.0 g/lOmin.; a melting point of 93 C; is at softening point of 49 C;
and a
methylacrylate (MA) content of 22% as a PVDC tie layer to give improved
adhesion. This has
been shown to give a superior bond strength. EMA gives bonds over l OOg in the
finished film.
A preferred 7-layer structure has a first heat seal layer comprising an
ethylene a-olefin
copolymer (Exxon Exact 3139), a second peelable tie layer comprising a
polymeric blend having
between 15 to 35% each of EVA (Exxon 701.ID); ethylene butene -1 copolymer
(Exxon Exact
4053); ethylene octene -1 copolymer (Nova VLDPE 10B) and a third tie layer,
e.g., comprising
EMA (Voridian SP 1330); a fourth barrier layer, e.g., as described above in
Example 1; a fifth tie
layer, e.g., comprising EMA; a sixth intermediate layer comprising a blend of
20-45% each of
EVA ethylene-butene -1 copolymer and ethylene-octene -1 copolymer; and a
seventh outer
surface layer comprising an ethylene a-olefin copolymer, e.g., Exxon Exact
3139.
The above film is preferably 2 mils thick overall and has a layer thickness
ratio for the
first through seventh layers, respectively of 10:42:5:18:5:15:5.
The bags 415, 415a, 415b, 415c and 415d may be fabricated of nearly any
dimensions
56
CA 02458136 2004-02-19
economically since the bags are not formed from a seamless tube that must be
generated to the
desired width. The only limitation on size of fabricated bag is the size of
rollstock films.
Standard roll stock films are available in widths in excess of 100 inches. The
present invention
allows a bag manufacturer to fabricate any size bag from the same flat sheet
of roll stock, up to
the dimensional limits of the roll stock. For example, if the roll stock is 52
inches in width, a
tube member can be fabricated having a lay-flat width of approximately 26
inches, taking into
account the amount of overlap, gap or abutment in the first seal 416, 516, 616
and 716 used. For
example, if the manufacturer wishes to fabricate a lap seal or fin seal bag
having a lay-flat width
of 18 inches, then the manufacturer slits the standard roll stock to the
appropriate width
(approximately 36 plus extra for the area of the first seal 416 or 516). The
unused portion slit
form the standard roll stock is rewound for use making bags of another
dimension(s). In this
manner, standard roll stock films can be manufactured more economically
because film
manufacturing equipment may be run at or near the upper limits of film width
production and
thereby use nearly all the equipments capacity. Fabricating bags from seamless
tubes requires
that the film making equipment be run at limited capacities to form the
different smaller width
tubes. Additionally, the film making equipment requires costly set-up and
breakdown between
jobs of differing dimensions that add significantly to the cost of
manufacturing the seamless
tubes.
An easily peelable heat shrinkable film has been described above with respect
to end
sealed bags having seamless sides, it should be readily apparent in view of
the present disclosure
that side seal heat shrinkable bags and pouches made from a plurality of films
may also be
adapted to the present invention to provide easy to peel open heat shrinkable
receptacle. The
57
CA 02458136 2011-01-13
present invention may be utilized with heat shrinkable formed into a pouch as
described in U.S. Patent Nos. 6,015,235 (Kraimer, et al.) and 6,206,569
(Kraimer, et
al.).
While this invention has been described with reference to certain specific
embodiments, it will be recognized by those skilled in the art that many
variations are
possible without departing from the scope and spirit of the invention and such
variations are deemed to be within the scope of the invention claimed below.
58
