Note: Descriptions are shown in the official language in which they were submitted.
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This invention pertains to multiple layer packaging
materials and containers made with those packaging materials.
The containers are typically made by heat sealing facing
portions of films to each other about a common periphery to
form an enclosure, but leaving one side of the enclosure
open. A product is then inserted into the container and the
open sida of the container is sealed closed. The closure of
this last side completes ths closure of the container.
Altaxnately, a package may be made from, for example, a
receptacle made by thermoforming a sheet, with a non-formed
film used as a lid closure over the receptacle.
For use with some products, it is desired that the
packaged product be capable of being boiled, as in boiling
water, after the package is sealed closed with the ~roduct in
it. It is this particular market and problem that the
invention addresses.
There are available, to the market, certain shaet
structures which are capable of handling boiling
temperatures. These include, for example, structures which
are made for retort pouch applications. These structures are
typically of the order of 6 to 8 mils thick, and may contain
a metal foil. Another family of structures, of approximately
the same thickness includes an outer layer of nylon, a second
layer of ethylene vinyl alcohol, a third layer of nylon, a
fourth layer of adhesive and a fifth layer of a sealant
material.
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While the above structures may bs useful as regards
retort pouch applications, khey are prohibitively expensive
for use with less valuable products~ To the present time, it
is not sPen that there is available a film for making
boilable ~ontainers, such as bags, a~ an economical cost,
which containers may thus be used ~or lower priced products.
The invention provides a novel and economical ~ilm which
can be made into a boilable container and wherein the
contants of the container may expand significantly at boiling
temperature the container being capable of withstanding the
boiling process without rupture.
The invention also provides a packaging container which
can be sealed with a moderate amount of gas in it and
subsequently boiled without rupture of the package.
Further, the invention provides a method of packaging a
product in a container where it is acceptable to include an
amount of air in the container and where the air does not
cause rupture of the container on subseguent boiling of the
container.
More particularly, the invention provides a polymeric
film, wherein the first layer of the film is a nylon selected
from the group consisting of nylon 6,6 and combinations of
nylon 6,6 w~ith nylon 6 wherein the combinations have melting
point temperatures greater than the melting point temperature
of the corresponding nylon 6. The second layer is linear low
density polyethylene (LLDPE) and optionally up to 50% by
weight low density polyethylene, in blend composition. A
third layer of an adhesive pvlymer is disposed between the
first and second layers and adheres them to each other. The
composition of the third layer preferably has a substantial
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fraction of linear low density polyethylene in its base
resin. The bass resin way consist en~irely of linear low
density polyethylene.
Preferably the ~ilm is made by th~ process o~ coextru-
sion of the three layers to form a three layer structure,followed by liquid quenching of the three layer structure.
Additionally the invention provides containers made ~rom the
films, and particularly containers which are capable of being
boiled in water for at least about 10 minutes, preferably 30
minutes.
Preferably the functional characteristics of the
container are such that when product is inserted into the
container it is entirely acceptable to include low pressure
air, such as, for example, atmospheric pressure, or slightly
above, in the container; and wherein the container can be
boiled and withstand the increased pressure of the expanded
hot gases in the container.
The invention is also seen to be embodied in a method of
packaging a product. The first step in the method is that of
coextruding a three layer coextrudable film by a tubular
liquid quench process. It is significant that the film have
a first layer of nylon selected from the group consisting of
nylon 6,6 and combinations of nylon 6,6 with nylon 6 wherein
the combinations have melting point temperatures greater than
~5 the melting point temperature of the corresponding nylon 6.
A second layer comprises linear low density polyethylene. A
third functionally adhesive layer is between the first and
second layers. The second step is that of forming a
container by sealing portions of the film to each other in
face to face relationship about a common area to form a
container having one side, or end, left open. The third step
is that of putting a product in the package with or without
the addition of air at low pressure. The fourth step is
sealing the package closed. And the fifth step i5 boiling
the sealed package.
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FIGURE 1 is a cross-section of a film of this invention.
FIGURE 2 is a pictorial view of an open package made
from films of this invention.
FIGURE 3 is a closed and sealed package, and having
product therein, the package being made with films of this
invention.
FIGURE 4 is a cross-section taken at 4-4 of FIGURE 3 and
showing the relationship of the packaging materials, the
product, and any enclosed air.
FIGURE 5 ~hows the melting point temperatures of
combinations of nylon 6,6 and nylon 6.
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FIGURE 1 shows a cross section of a three layer sheet
material used in this inven~ion. The composition of layer 12
is a nylon selected from the group consisting of nylon 6,6
and combinations of nylon 6,6 with nylon 6 wherein the
combinations have melting point ~emperatures greater than the
melting point temperature of the corresponding nylon 6.
Layer 12 is used on the outside o~ ~he package. Layer 14 is
used on the inside of the package~ Its composition is linear
low density polyethylene and optionally up to about 50% low
density polyethylene. Layer 16 is an adhesive layer which
adheres layers 12 and 14 to each other. Its composition is
preferably based on lin~ar low density polyethylene.
FIGURE 2 shows a container made o~ sheet material of
this invention, as in FIGURE 1. The container 18 is formed
from either two separate portions 20 of the sheet material,
or by folding a piece of the sheet material over onto itself
so tha~ the portions thereof are in fa~e to ~ace relation-
ship. Heat seals are then formed as at 22A, 22B, and 22C,
along portions of the periphery, leaving an open end 23 for
insertion of product. The sheet material may also be used as
lid stock for sealing to a pre-formed tray, or the like,
having a surface which is compatible for forming a seal with
the instant sheet material.
FIGURE 3 shows a container as in FIGURE 2 with the open
end 23 having been closed by the formation of the seal 22D.
FIGURE 4 shows a cross-section of the closed and sealed
container of FIGURE 3. Facing portions of the sheet
materials on opposite sides of the container, are seen to be
heat sealed together as indicated at 22A. The product 24
partially fills the package~ leaving an unfilled space 26
which may be occupied by a gas such as air.
Returning now to the film itself, it is seen that each
of the layers must be capable of withstanding boiling
temperatures in order for the film per se to survive the
processing. The composition of layer 12 is selected for its
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heat tolerance, and its toughness characteristics. Layer 14
is selected fox its capability to form strong heat seals
which will remain stro~g throughout the process o~ the
package being boiled, and being subjected to expansive
stresses caused by expanding gases in the package. Layer 16
must have good adhesion to both layers 12 and 14, and must
have enough heat resistance to withstand the boiling
temperatures and still retain a good adhesion. Furthex, the
compositions of the several layers must be compatible with
coextrusion from the die at a single temperature.
It has been found that the combination of properties
required in the film is satisfied in a film where layer 12 is
about 20~ of the thickness of th~ film. While lower frac-
tions of nylon may be acceptable, reduction to less than 15%
of the thickness is not preferred because the overall
strength of the package, as regards the capabilities to
tolerate the stresses of the coextrusion process and the
boiling temperatureO is unacceptably reduced. As the amount
of nylon is increased over 20%j the coextrusion process
becomes more and more di~ficult until at about 40% the
coextrusion cannot be carried out with certain equipment. So
the amount of nylon may range from about 15% to about 40% of
the thickness of the film.
As regards performance properties of the sealant layer
14, and especially regarding its functional compatibility
with the coextrusion process and the boiling container-
processing temperatures, it is preferred that the layer 14
composition be 100% linear low density polyethylene. ~he
linear low density polyethylene is especially preferred for
its flex crack resistance. In some embodiments, low density
polyethylene is incorporated into the layer 14 composition,
up to about 50% by weight, preferably up to about 40~.
From applicants' observations, it appears that the first
layer of nylon is functional to provide the primary strength
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to ~upport the coextruded tube in ~he preferred embodiments,
where the nylon has a high fraction of nylon 6, 6 and is about
20% or more Qf the thickness o~ the ~ilm. ~hen the amount of
nylon is reduced, or when the fraction of nylon 6,6 in
reduced in favor of nylon 6, it is preferred to include some
low density polyethylene in ths second lay~r. The
incorporation of low density polyethylene into the composi-
tion is believed to lend a degree of stability and support to
the bubble. So a fraction of the composition of layer 14 may
be low density polyethylene. Blend compositions over 50%
LDPE are less capable of withstanding the boiling process
temperatures, especially as measured in terms of stress crack
resistance, and so are not used.
The adhesive material in layer 16 is preferably a
carboxy modified linear low density polyethylene base
material. One such material which is believed to be an
anhydride modified linear low density polyethylene i8 sold by
Mitsui Company of Japan under the trademark NF-500. Other
adhesive materials may be used so long as they are compatible
with the extrusion processing conditions and the package
boiling conditions, and provide the needed adhesion through-
out the package life cycle described herein, and including
normal commercial handling.
The films of this invention which are to be used to make
boilable pouches are typically of the order of 2 to 6 mils
thick. Substantially thinner films lack the strength to
Withstand the boiling temperatures. Substantially thicker
films are less flexible, and thus may be cracked by the
stress caused by gaseous expansion when the container is
boiled. Thicker films are also, economically speaking, more
expensive and thus not competitive. Within this framework,
layer 12 preferably represents about 15% to about 25% of the
thickness of the film. Layer 16 is preferably about 10% and
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no more than about 20% of the film thickness. Layer 14 is at
least 55% and no more than about 80% of the thickness of the
film.
A fundamental prohlem addressed in the invention herein
is that nylon ~, which has been used alone conventionally,
tends to become brittle when it is boiled, and yet a heat
resistant polymer is necessary as at least one of the layers
of the package. Thi~ brittleness can be overcome by mole-
cularly orienting the nylon 6 as described in U.S. patent
number 4,687,68~. Bu~ the nylon 6 is still somewhat
sensitive to the ~hermal stresses of the package environment.
~he inventors herein have now discovered that both the
brittleness of the film and its sensitivity to the thermal
environment may be overcome by incorporating nylon 6,6 into
the composition, and by using novel combinations of
processing parameters to accommodate that incorporation.
By using the preferred combination including nylon 6,6
in one outer layer and 100% LLDPE as the other outer layer, a
film having excellent clarity, heat resistance, and flex
crack resistance is obtained.
Heat resistance of the film iæ the temperature at which
the film begins to distort or melt. One competitive film is
an adhesive lamination of polyethylene terephthalate
laminated to LLDPE. While this film has improved heat
resistance of over about 200 C., the adhesive lamination
process makes it expensive compared to the economical
coextrusion process preferred in the instant i~vention. Also
the PET is more expensive than nylon. Another film,
disclosed in U.S. patent number 4,687,688 is a coextrusion of
nylon 6 with a blend of LLDPE and LDPE. As stated in that
application, the purpose of the LDPE is to improve the
tubular coextrusion processing by stabilizing the extruded
tube. The disadvantage of the LDPE is in reduced flex crack
resistance, as seen hereinafter. And while nylon 6 provides
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131~6~9
good abuse resistance, it has been ~ound~ as indicated in
above U.S. patent no. 4,687,688, that molecular orientation
of the film is necessary to overcome a tendency of the nylon
6 to become brittle upon the anticipated boiling processing
of the package. Even so, the ~eat resistance of the oriented
nylon 6 structure is only about 177 C., a~ compared to about
288 C. for the coextruded, unoriented films herein of 100%
nylon 6,6 in the first layer.
As indicated for the above U.S. patent no. 4,~87,688 the
primary function of the LDPE in that application was to
provide support/stability to the extruded tube. Indeed, the
presence of LDPE was tolerated as neces~ary, even though it
reduced the excellent flex crack resistance of the LLDPE
layer. And, apparently, it was the sealant layer o~ LLDPE
and LDPE which provided the structural melt strength for that
film extrudate, at a die temperature of about 215 C.
At that about 215 C. die temperature, the LLDP~ used
therein has a melt index of about 1Ø In order to extrude
the instant coextruded film herein, when using 100% nylon 6,6
~0 as the first layer composition, it is necessary to operate
the process at a temperature above the melting point of the
highest melting component, namely nylon 6,6. Thus, the die
temperature used herein is typically of the order of ~71 C.
Since it was necessary to include the LDPE in the LLDPE
layer at about 215 C., in the U.S. patent 4,687,688
structure including nylon ~, in order to stabilize the
extrudate tube, it would be expected that it would be
necessary to include aven more LDPE at this higher
temperature of about 271 C. To the contrary, it has
unexpectedly been found that the layer which is eventually
used as the sealant layer in the package may be 100~ LLDPE,
with all its excellent flex crack resistance, when the
composition of the layer on the other surface is 100% nylon
6,6; even though the extrusion processing temperature is much
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higher, at 271 C., than ~he temperature a~ which the nylon 6
film, which requires LDPE in the sealant layer, is
coextruded~ Additionally, ~he nylon 6,6 film need not be
molecularly oriented to avoid brittleness, though it can be
oriented if desired.
The resulting unoriented film containing 100% nylon 6,6
as the first layer has excellent flex crack resistance as
compared to previous films, and as indicated in Table 1
below. The flex crack resistance is defined as the number of
pinholes in an 8" x 11" sample that is Gelbo flexed for 900
cycles at a rate of 40 cycles/minute. This is a highly
abusive test which exceeds the normal level of abuse in
typical packaging life cycla. The fewer the number of
pinholes, the better the indicated package performanca.
Table 1
Yilm Average Number
of Pinholes
PET/Adhesive/LLDPE 5.8
Nylon 6/Adhesive/LLDPE 3.8
Nylon 6~Adh. Tie/LLDPE-LDPE blend 6.0
(coextruded, oriented)
Nylon 6.6/ Adh. Tie/LLDPE 2.0
(coextruded, unoriented)
(This invention)
Table l shows that both the adhesive laminated PET film
and the coextruded nylon 6 film have substantially more of
the objectionable pinholes than films containing 100% nylon
6,6 as the first layer, in this abusive test. The adhesive
laminated nylon 6 film is better than adhesive laminated PET,
but it still has more pinholes than films using the nylon
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6,6; by a factor of 1.9 ~o 1, and is economically encumbered
by the adhesive lamination process.
Thus the films of the invention are both economically
and functionally superior to other similar ~ilms.
Based on the knowledge of typical extrusion temperatures
of LLDPE at 215 C. and nylon 6,6 at 271 C., it would not be
expected that these materials could be coextruded in a
tubular coextrusion process. It would be expected that the
LLDPE might be degraded at this much higher temperature and
that, in any event, it would have such a high melt flow rate
that the tube could not be supported. On the contrary, no
degradation is experienced, and the extruded tube is readily
maintained in the extrusion process.
Thus the preferred process is a tubular rapid quench
coextrusion. Rapid quenching of the extru~ate is desired to
preserve the amorphous nature of the molecular arrangements
in the layers, which is beneficial for the subsequent boiling
process. Rapid quenching is typically achieved by passing
the extruded tube through a water ring. Alternatively, the
tube may be subjected to cold gases, such as cryogenic gases,
to cool it quickly.
Typical processing temperatures are 249 C. at the
outlet of the extruders processing LLDPE for layer 14 and tie
layer 16. Comparable temperature at the outlet of the
extruder processing the nylon 6.6 of layer 12 is 271 C. The
three layers are joined at the die and typically exit the die
at a temperature of 316 - 321 C.
Conventional amounts of conventional additives are used
as desired.
Films of the invention may be made into packages by
sealing together facing portions of two separate sheets of
the material, or by folding over a portion of the sheet
material and heat sealing it to itself about a contiguous
periphery in order to form a container as seen in FIGURE 2.
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After the container is formed, the package may have product
inserted therein through the open end 23. The container may
then be evacuated and sealed, or air may be le~t in the
container for cushioning purposes, or for other purposes.
In an alternate construction, a package may be
fabricated from a combination of a thermoformed receptacle
and a film closure, where especially the film closure, and
prefer-ably both the receptacle and the film are structured
in accordance with~the sheet materials of this invention~
In cases where a substantial amount of air is left in
the container, the boiling of the container causes the
expansion, not only of any water which may be contained in
the container, which water turns into steam as boiling
temperatures are approached, but also the expansion of any
contained air. Significant stresses are placed on the
container during the boiling operation by the combination of
the steam, and any air which may be left in the container.
The container is susceptible to being ruptured during the
boiling because of the pressure applied by this heated steam
and air.
And so a functional and performance test of containers
of the invention is provided by ~orming containers from the
film, filling them partially full with water, and sealing
then leaving a substantial amount of air in the package along
with the water. The sealed package is then boiled for 30
~inutes at standard atmospheric boiling temperatures and
conditions. As the container internal temperature increases,
the gaseous pressure inside the container increases as it
expands, applying substankial force to the container,
particularly at its seals. Containers which burst or
otherwise break or leak during the boiling processing, of
course, are considered failures, and unacceptable. Con-
tainers which survive the processing substantially intact are
considered success~ul, and acceptable.
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In layer 12, th~ preferred composition is essentially
100% nylon 6,6. Nylon 6,6 in pre~erred, as compared to nylon
6, because the nylon 6,6 exhibits greater toughness, and can
better tolerate the high temperatures to which the film is
exposad, both in the process of forming the film, and the
process of packaging the product and sterilizing the filled
and sealed package.
Combinations o~ nylon 6.6 and nylon 6 are beneficially
used to the extent they tolerate higher temperatures, and are
tougher than, a corresponding nylon 6 composition. Since the
toughness of the film is improved by the incorporation of
even small amounts of nylon 6,6 into the composition, the
desirability of a given combination of nylon 6,6 and nylon 6,
over a corresponding 100% nylon ~ composition, depends
primarily on the melting point temperature of the given
composition of nylon 6.6 and nylon 6 as compared to that of
the 100% nylon ~. To the extent the melting point of the
combination is greater than the melting point of the corre-
sponding nylon 6, the composition is an improvement over
plain nylon 6, and is preferred, as compared to the films
disclosed in U.S. patent number 4 r 687, 688 .
As the amount of nylon 6 in the composition is decreased
in favor of nylon 6,6, the need for the LDPE in layer 14 to
support the coextrusion process is commensurately decreased,
until, as the composition approaches 100% nylon 6,6, the
coextrusion process is supported adequately without use of
LDPE in layer 14. The extrusion processing temperature is,
of course, adjusted to accommodate the melting properties of
especially the nylon composition of layer 12.
FIGURE 5 shows, in general representation, the melting
point temperatures of combinations of nylon 6,6 and nylon 6.
The combinations represented are blends (Curve A), random
copolymers (Curve B), and block copolymers (Curve C~. As
sPen in FIGURE 5, blends of nylon 6,6 and nylon 6 having
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melting points above the melting points of the corresponding
nylon 6, and thus advantageous for use in the instant
invention are those containing greater than about 20~ nylon
6,6. Random copolymers having melting points above the
melting point of the corresponding nylon 6 are thosP having
greater than about 80% nylon 6,6. Finally, block copolymers
having melting points above the melting point temperature of
the corresponding nylon 6 are those having greater than about
26% nylon 6,6.
Thus the criteria for determining whether a composition
containing nylon 6,6 and nylon 6 is suitable for use in this
invention rests heavily on the melting point of the composi-
tion; and usually the melting point criteria is the deter-
mining factor governing whether a given film is satisfactory
for use herein by having heat tolerance greater than the heat
tolerance of nylon 6 alone. To that end, acceptable
combinations of nylon 6,6 and nylon 6, as represented in
FIGURE 5, are
blends >about 20% nylon 6,6
random copol~mers >about 80% nylon 6,6
block copolymers >about 26% nylon 6,6
The threshold ratio does vary somewhat, depending on the
properties of the specific nylon 6,6 and the specific nylon 6
selectad. This acknowledges that both nylon 6,6 and nylon 6
represent families o* polymers having a range of physical
properties, including melting point temperatures. Combina-
tions of the blends, the block copolymers and the random
copolymers can be used so long as their heat tolerance is
greater than that of the corresponding nylon 6.
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THE EXAMPLES
Three layer films wore made by the tubular water quench
process. The first layer was Dupont Zytel 42 nylon 6,6, on
the outer surface o~ the tube. The second layer on the inner
surface o~ the tube was Dow 2056 LLDPE. The third, and
intervening, adhesive layer was Mitsui's Admer NF-500, an
LLDPE-based adhesive polymer. The first layer of nylon 6,6
was 20~ of the thickness of the film. The third layer of NF-
500 was lO~ of the thickness of the film. The second layero~ LLDPE was 70% of the thickness of the film. Total film
thicknesses of the example films ranged from 2.0 mils to 5.5
mils.
All the films ran well in the tubular extrusion process.
Extrusion die tempera~ure at the die outlet was about 271 C.
The coextruded ~ilm was water quenched at a temperature of
43 C. and was wound up. Portions of the film were formed
into packages as seen in FIGURE 2. The formed packages were
6 inches wide and 6 inches high and were filled with .25
liter of water and closed leaving a substantial amount of air
in the package. The closed and sealed packages were boiled
in water at atmospheric pressure for 30 minutes and the
packages observed. Packages made according to the above
described examples all survived the boiling process without
failure, and remained substantially intact.
Thus it is seen that the invention provides novel and
economical films, of the order of 2 to 6 mils thickness,
which have the capability of withstanding a boiling process
while experiencing internal pressure exerted by steam and
axpanding hot air.
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The invention further provides a method of packaging a
product in a sealed container with an included quantity of
air in the container, wherein ~he subsequent boiling of the
container does not cause rupture of the container; and
wherein the thickness of the container walls is of the order
of 2 to 6 ~ils~
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