Note: Descriptions are shown in the official language in which they were submitted.
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"HEAT SEALABLE MULTILAYER FILM CONTAINING POLYVINYL ALCOHOL LAYER".
This invention relates to a multi-layer film structure
comprising an oriented polypropylene core layer, a
poly(vinyl alcohol) (PVOH) layer, and an acrylic coating.
oriented plastic film, specifically biaxially oriented
polypropylene film is widely used for packaging products,
particularly foods. No single unmodified polymeric film,
however, has the gas and moisture barrier characteristics
needed for packaging.
Thus, multilayer polymeric films have been designed
which have improved gas and moisture barrier properties.
In U.S. Patent No. 5,192,620 to Chu et al. a polypropylene
film is coated with a blend of an ethylene-acrylic acid
copolymer and poly(vinyl alcohol) (PVOH) to produce a
polymeric film having better gas barrier properties. The
film is then metallized to provide the film with better
moisture barrier properties.
Since PVOH, on its own, adheres poorly to the
preferred substrates, such as polypropylene, U.S. Patent
No. 5,153,074 to Migliorini suggests functionalizing the
substrate with a maleic anhydride modified propylene
homopolymer to act as a tie layer for the PVOH, see col. 1,
lines 49-62. For purposes of making a metallized film,
however, the Migliorini patent is only concerned with
ethylene vinyl alcohol copolymer (EVOH) which is not as
good an oxygen barrier as PVOH.
A polymeric film having a PVOH skin layer is described
in European Patent Application 461,772 A2. The moisture
barrier properties of the PVOH layer are improved by
employing crosslinked PVOH. There is no suggestion to
metallize the film because the crosslinking agent makes
metal adhesion unreliable; that is, it can vary from o to
70%.
The PVOH skins described in European Patent
Application No. 461,772 And U.S. Patent No. 5,192,620 are
applied through a solution coating process. Pure PVOH
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applied by solution coating has a tendency to pose blocking
problems when wound onto a roll. European patent
application 461,772 sought to remedy this problem by
incorporating a crosslinking agent in the PVOH solution.
Acrylic copolymers have been used as heat sealable
coatings for oriented thermoplastic films such as
polypropylene film. When thermoplastic film substrates are
coated with these heat sealable coatings, it is
particularly important to insure that the adhesion of the
coating layer to the film substrate is adequate. In many
packaging operations it is necessary for the coated film to
be heat sealed to itself or to other films to form tightly
closed packaging. If the coating adhesion to the base film
is inadequate, the packages may prematurely open when
subjected to stress.
In general, acrylic coatings do not adhere well to
certain polyolefin film surfaces, e.g., polypropylene, even
when the latter have been subjected to well known pre-
treatment operations such as treatment by corona discharge,
flame, or oxidizing chemicals. Accordingly, it has often
been found necessary to apply a thin intermediate primer
layer to the surface of the polyolefin base film before
applying heat sealable topcoats. Moreover, the acrylic
coatings themselves are not especially effective as oxygen
and aroma barriers.
It would be advantageous to provide an acrylic coated
polypropylene film which enjoys not only enhanced sealing
characteristics but enhanced oxygen barrier as well.
Moreo~rer, it is desirable to provide a method for reducing
the minimum seal temperature of a heat sealable film in
order to permit heat sealing at lower temperatures thereby
providing a wider operating range for sealing machinery.
The present invention relates to a multilayer heat
sealable film comprising:
(a) an oriented polypropylene film substrate layer;
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(b) a heat sealable acrylic polymeric coating, and
(c) a layer of a poly(vinyl alcohol) (PVOH) between
(a) and (b).
The resulting film exhibits excellent oxygen barrier,
as well as enhanced sealability evidenced by reduced
minimum seal temperature compared to such or similar films
lacking the PVOH component. Such minimum seal temperatures
can be no greater than 99~C, preferably, no greater than
96~C. For present purposes, the minimum seal temperature
is the temperature required to achieve a l00 gm/in seal.
The present invention is particularly suited to providing
films having an ultimate seal strength (USS) which is the
highest seal strength attainable under practical heat
sealing conditions. For the purpose of comparison, a
sealing temperature of 270 D F is used. The present
invention can be used to obtain films having a USS of
greater than 200 g/in, preferably greater than 300 g/in, or
even greater than 400 g/in.
Preferably, the film further comprises a suitable
primer between the polypropylene film substrate layer and
said poly(vinyl alcohol) layer. The film may further
comprise a suitable primer between the poly(vinyl alcohol)
layer and the heat sealable acrylic polymeric coating.
In another aspect, the present invention relates to a
method for preparing an acrylic coated multilayer film
which comprises
i) providing an oriented polypropylene film core
layer;
ii) coating said oriented polypropylene film core
layer with PVOH on at least one side of said core layer to
provide a PVOH coated oriented polypropylene film; and
iii) coating the product of ii) with acrylic on said
side coated with PVOH.
The method can further comprise applying a suitable
primer to said polypropylene film core layer prior to step
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ii) and applying a suitable primer to said PVOH coating
prior to step iii).
The polypropylene film substrate layer used in the
present invention can comprise homopolymer polypropylene or
ethylene propylene copolymer containing predominantly
propylene. A preferred embodiment of said polypropylene
film substrate comprises a layer of homopolymer
polypropylene and a layer of the copolymer. In oriented
films of the present invention, the melt flow rate of the
polyolefin layer must not be so low that it is too stiff
and thus unorientable. For propylene ethylene copolymers,
it is preferred that the melt flow rate be from 2.5 to 6.0
grams per 10 minutes at 230~C and a load of 2,160 grams.
For polypropylene, it is preferred that the melt flow rate
be from 2.5 to 4.5. In this range, orientation of the
copolymer or the polypropylene results in the best
properties.
The extruded polypropylene film can be biaxially
oriented. Biaxially oriented film can be stretched 3.5 to
7.0 times, preferably 4 to 6 times, in the machine
direction (MD), and 5 to 15 times, preferably 6 to 12
times, in the transverse direction (TD). The overall
orientation (MD X TD) can range from about 25 to 60. After
orientation, the edges of the film can be trimmed and the
film wound onto a core.
PVOH is of a suitable grade which can be applied to
the oriented polypropylene film substrate through a
solution coating process, as described in European Patent
Application No. 461,772 or U.S. Pat. No. 5,192,620,
incorporated herein by reference. Poly(vinyl alcohol) is
typically produced by hydrolyzing poly(vinyl acetate).
Specifically, the hydrolysis reaction replaces the acetate
groups with alcohol groups. The more acetate groups that
are replaced, the greater the hydrolysis of the PVOH. It
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is believed that the presence of more alcohol groups (i.e.,
greater hydrolysis) provides better barrier properties.
However, even after hydrolysis of the PVOH, a certain
number of acetate groups remain attached to the PVOH
molecule. For example, in a 95% hydrolyzed PVOH
approximately 5% of the originally present acetate groups
remain attached to the molecule, whereas in a 99%
hydrolyzed PVOH approximately 1% of the originally present
acetate groups remain attached to the molecule.
Poly(vinyl alcohol) may be produced with various
viscosities and various degrees of hydrolysis. Viscosity
is typically a function of the molecular weight of the PVOH
molecule. Specifically, a solution of PVOH in which the
individual molecules are relatively large (i.e., a high
molecular weight PVOH) tends to have a higher viscosity
than a solution of PVOH in which the individual molecules
are relatively small (i.e., a low molecular weight PVOH).
It is believed Van der Waals forces develop between the
larger-sized molecules because such molecules tend to align
themselves with one another, thus increasing the viscosity
of the PVOH.
A poly(vinyl alcohol) such as Elvanol 71-30 (produced
by DuPont) is typically referred to as a medium viscosity,
fully hydrolyzed PVOH. Specifically, the degree of
hydrolysis of a fully hydrolyzed PVOH is about 98%.
Further, the viscosity of a medium viscosity grade PVOH
such as Elvanol 71-30 is about 30 cps at 4% solution and
20~C.
Another commercially available PVOH is Elvanol 75-15
(also produced by DuPont), which is a low viscosity, fully
hydrolyzed PVOH. Specifically, the degree of hydrolysis is
about 98~ and the viscosity is about 13 cps at 4% solution
and 20~C.
Still another commonly available PVOH is Elvanol 90-50
(also produced by DuPont), which is a low viscosity super
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hydrolyzed PVOH. The degree of hydrolysis in a super
hydrolyzed PVOH is about 99.5%. The viscosity of a low
viscosity grade PVOH such as Elvanol 90-50 is about 13 cps
at 4% solution and 20~C. Another commercial source of
extrudable PVOH is Vinex resin manufactured by Air Products
and Chemicals, Inc.
Commercially suitable coating processes include a
reverse direct gravure process and a smooth rod process.
As is known to those skilled in the art, the gravure
process typically produces a higher level of foam than the
smooth rod process. The tensoactive agent reduces the
degree of foaming, while simultaneously lowering the
surface energy of the coating solution. The combination of
reduced foaming and lower surface energy provides improved
processing characteristics which result in a barrier
exhibiting reduced transmission of oxygen, particularly at
high relative humidities.
With respect to the gravure process, the coating
solution of the present invention preferably includes
approximately 200 to 500 ppm of l-octanol and, more
preferably, about 250 ppm of l-octanol. With respect to
the smooth rod process, the coating solution preferably
includes from 5 to 50 ppm of 1-octanol. This lower level
of tensoactive agent provides improved processing
characteristics and reduces the likelihood that the
subsequently-formed oxygen barrier will suffer any negative
impacts from the inclusion of such agent in the solution.
The solution, which is preferably aqueous, is prepared
by adding the poly(vinyl alcohol) to cold water, which is
thereafter heated to a temperature sufficient to dissolve
the PVOH. The water and dissolved PVOH are then cooled.
The cross-linking agent (i.e., the glyoxal) is then added
to the cooled PVOH and water. Thereafter, an effective
amount of the tensoactive agent is added to the solution.
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It is this resultant solution that is then coated on the
polymeric substrate.
In a preferred embodiment, the aqueous solution
includes from 4% to 14% by weight of solid and, preferably,
from 5% to 10% by weight of solid. This solid content is
made up from 70% to 95~ by weight of poly(vinyl alcohol),
from 5% to 30% by weight of cross-linking agent and from 5
ppm to 0.5% by weight of octanol.
The enhanced stability of this aqueous solution, with
the use of low viscosity PVOH, allows for higher solids
content of the solution, thereby reducing the percentage of
water in the solution. Accordingly, the applied solution
is more readily dried. This reduced drying time results in
an energy savings and/or a speed increase from the coating
machinery. It is also believed to facilitate the cross-
linking process as well as providing a longer pot life for
the solution.
Particularly, once the coating is applied to the
substrate, the film is rolled through a drying oven. A
typical drying oven is approximately 60 feet long and
adapted to heat the film to approximately 130~C. The film
is rolled through the oven at speeds of about 1000 feet per
minute. As the film rolls through the oven, the water in
the applied coating is driven off which, in turn, increases
the concentration of the solid content. At some point
(i.e., at a particular concentration and temperature), the
cross-linking process is initiated. This cross-linking
process occurs rapidly and completely throughout the PVOH
layer such that the film is substantially 100% cross-linked
by the time such film leaves the drying oven.
A PVOH coating ranging from about 0.02 to 0.06 mil,
preferably, 0.010 to 0.040 mil, can be applied.
The heat sealable acrylic polymeric coating can, for
example, be derived from any of the terpolymeric
compositions disclosed in U.S. Pat. No. 3,753,769, the
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contents of which are incorporated herein by reference.
These coating compositions contain as a film forming
component a resin consisting essentially of an interpolymer
of (a) from 2 to 15, and preferably from 2.5 to 6, parts by
weight of an alpha-beta monoethylenically unsaturated
carboxylic acid selected from the group consisting of
acrylic acid, methacrylic acid, and mixtures thereof, and
(b) from 85 to 98, and preferably from 94 to 97.5, parts by
weight of neutral monomer esters, said neutral monomer
esters preferably comprising (l) methyl acrylate or ethyl
acrylate and (2) methyl methacrylate. These interpolymer
compositions are further characterized as preferably
comprising from 30 to 55 percent by weight of methyl
methacrylate when said alkyl acrylate is methyl acrylate,
and from 52.5 percent to 69 percent by weight of
methylmethacrylate when said alkyl acrylate is ethyl
acrylate. Such coating compositions can be applied to the
films herein in a variety of ways including in the form of
ammoniacal solutions.
Similarly useful are copolymeric coating compositions
prepared exclusively from the foregoing neutral monomer
esters. These coating compositions are advantageously
applied to the film laminates in the form of emulsions.
An adhesive tie layer can be used to enhance adhesion
between the PVOH layer and the polypropylene film substrate
layer when such layers are incompatible in their natural
state. The adhesive layer when used can be any of a number
of proprietary materials such as described in U.S. Pat. No.
4,561,920. Suita~le adhesives include CXA-3036 (TM) (an
ethylene-vinyl acetate copolymer available from DuPont), a
high density polyethylene based adhesive, e.g., 8ynel 4003
(TM) available from DuPont, the Admer (TM) adhesives from
the Mitsui Petrochemical Company, e.g., NF500A (TM) and NF
550A (TM), and the Plexar (TM) family from USI Chemicals,
e.g., a maleic anhydride grafted LDPE, Quantum Plexar 201
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(TM). Suitable maleic anhydride modified polyolefins
include maleic anhydride modified polypropylene homopolymer
or copolymer which are especially suitable for use as
adhesive tie layer in the present invention. Such
materials result from the reaction between maleic anhydride
and the thermal degradation product of polypropylene or
polypropylene copolymer. Examples of this material are
disclosed in U.S. Pat. No. 3,480,580, the contents of which
are incorporated herein by reference in its entirety.
Particular attention is directed to Examples 3, 4, and 6 of
this patent. A commercially available maleic anhydride
modified polypropylene is Epolene E-43(TM), from Eastman
Kodak Company of Rochester, NY.
The adhesive tie layer can be dispensed with by
including an adhesion promoter in the polyolefin layers as
described in U.S. Pat. No. 4,650,721, which is incorporated
herein by reference. That patent discloses polyolefin
layers (polypropylene) which contain a maleic anhydride
modified olefin polymer.
PVOH coating is preferably applied over an appropriate
primer interface to assure adequate bonding to the base
substrate. A second bonding primer layer can also be
applied onto the PVOH layer to provide adequate bonding for
the acrylic layer. Typical primers for such purpose
include an aqueous polyethyleneimine solution.
Polyethyleneimine primer is commercially available and is
generally applied as a 0.1%-0.6% by weight
polyethyleneimine solution in water or organic solvent.
The use of PEI as a primer or adhesive for polymeric
coatings to films substrates is well known to the art, as
disclosed in U.S. Pat. No. 3,230,135, incorporated herein
by reference.
Epoxy polymers and polyurethanes are also useful as
primers. Such primer compositions are disclosed in U.S.
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Pat. Nos. 4,447,494, 4,681,803, and 3,023,125, which
patents are incorporated herein by reference.
The PVOH layer, preferably after priming, is coated
with the acrylic polymeric coating by conventional
teçhniques as noted above. Such coating can be carried out
either before or after, preferably after, the PVOH layer is
combined with the polyolefin core layer.
In one embodiment, the present invention relates to a
method of making a biaxially oriented composite barrier
film having two or more layers which can be conducted in a
continuous manner.
First, a polypropylene sheet is formed by extrusion.
As the sheet exits the die, it is immediately cooled by use
of a cooling drum or a water bath to a temperature of about
40 to 50~C. Immediately after cooling, the sheet can be
fed into an apparatus for MD orientation of the plastic
material. Any such apparatus can be used in the present
invention. In one embodiment, the composite sheet is fed
into a set of differential speed heated rollers to stretch
the sheet in the longitll~in~l direction to a degree ranging
from greater than 1:1 and less than 2:1, preferably about
1.2:1 to 1.5:1, say, e.g., 1.3:1. Next, the sheet can be
fed to a tenter frame where it is stretched in the
transverse direction to a degree of greater than 5:1,
preferably from 5:1 to 12:1, e.g., 8:1 to 9:1. MD
orientation is generally conducted by preheating the film
at 130~C to 145~C, stretching in the same temperature
range, and annealing at about 115~C to 125~C. Preheating
for TD orientation is advantageously done at 160~C to
175~C, stretching at 145~C to 160~C, and annealing at 155~C
to 165~C.
The oriented polypropylene sheet is thereafter primed
with a suitable epoxy or PEI primer and then solution
coated with PVOH barrier layer to provide a PVOH layer of
0.015 to 0.035 mil. The PVOH coating of the sheet is
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thereafter primed to provide suitable bonding between PVOH
and acrylic heat seal layer which is thereafter applied by
the above-described process.
Finally, the sheet, either before or after coating
with acrylic, can optionally be subjected to treatment in
the known manner, e.g., corona discharge treatment or flame
treatment, to improve its surface characteristics, e.g.,
printability.
The multilayer heat sealable film of the invention can
10have an overall thickness ranging from 0.50 to 2.0 mil.
The films of this invention are suitable in a variety
of applications in which moisture and gas barrier
properties are desired. The films are particularly useful
in food packaging.
15The invention is illustrated by the following non-
limiting examples in which all parts are by weight unless
otherwise specified.
EXANPLE 1--OPP/PEI/PVOH (Comparative)
Sample 1 was produced. A solution of Elvanol 71-30,
Parez 613 (a methylated melamine formaldehyde) and ammonium
chloride was coated onto a polymeric substrate of biaxially
oriented polypropylene approximately .75 mils thick. The
solution contained 6% by weight of solid. In turn, the
solid contained approximately 83~ by weight of PVOH,
approximately 15% by weight of methylated melamine
formaldehyde and approximately 2% by weight of ammonium
chloride.
The substrate was treated with a poly(ethyleneimine)
primer prior to application of the coating. The coating
was applied to the polypropylene substrate using a smooth
rod process. The coated substrate was rolled through a
dying oven about 60 feet in length at about 800 feet per
minute, heating the film to 130~C and initiating cross-
linking. The film was then measured for oxygen
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transmission at 0% relative humidity, minimum seal
temperature, and ultimate seal strength. As can be seen
from the results set out in the Table below, seal
properties were unacceptable.
EXANP~B 2--OPP/PFI/ACRYLIC (Comparative)
A polymeric substrate of biaxially oriented
polypropylene 0.75 mils thick was coated with an acrylic
coating solution prepared in accordance with Example l of
U.S. Pat. No. 3,753,769, which is incorporated herein by
reference in its entirety.
The substrate was treated with a poly(ethyleneimine)
primer prior to application and drying of the coating.
The film was then measured for oxygen transmission at 0%
relative humidity, minimum seal temperature, and ultimate
seal strength. As can be seen from the results set out in
the Table below, oxygen transmission properties were
unacceptable.
EXAMPLE 3-OPP/PEI/PVOH/EPOXY/LOW TEMPERATURE SEAL COATING
~l00% CROBS-~TN~n PVOH)
Sample 3 was produced. A solution of Elvanol 90-50,
Glyoxal 40, available from Freedom Textile, Charlotte, NC
and l-octanol was coated onto a poly(ethyleneimine) primed
polypropylene substrate by a reverse direct gravure
process. The solution contained 8% by weight of solid. In
turn, the solid contained approximately 85% by weight of
PVOH, approximately 15% by weight of cross-linking agent
and 2s0 ppm of l-octanol. The resulting material was
primed with an epoxy primer and coated with a low
temperature seal coating (LTSC) produced as follows:
Low temperature seal coating composition was prepared
by adding to an aqueous solution or fine dispersion of 25
wt.% of an ammonium salt of a copolymer of 80 wt.% of
ethylene and 20 wt.% of acrylic acid, sold by Michelman as
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Primacor 4983, varying amounts of sodium hydroxide (NaOH),
poly(oxymethylene) sorbitan monooleate anti-static agent
(A-S), sold as Glycosperse 0-20, microcrystalline wax
having an average size of about 0.12 to 0.2 micron (MWX)
sold by Michelman as 41540, and melamine-formaldehyde
cross-linking agent (M-F) sold as Cymel 385. In addition,
0.4 phr of talc and 0.1 phr of fumed silica having an
average particle size of about 3 to 5 microns sold as
Syloid 72 were also added to each composition. All the
components were added as an aqueous dispersion or solution
except the anti-static agent which was added as a pure
liquid. Water was then added to bring the final coating
composition to a solids content (% SOL) of 12 wt%.
The characteristics of the film were thereafter
measured and the results are set out in the Table below.
EXAMPLB ~-OPP/P~I/PVOH/~POXY/ACRY~IC l100% CROSS-TTN~D
PVOH)
Sample 4 was produced by the procedure set out in
Example 3 except that the low temperature seal coating was
substituted by the acrylic coating of Example 2. The
characteristics of the film were thereafter measured and
the results are set out in the Table below.
EXAMPLE 5-OPP/PEI/PVOH/LT8C (10% CROSS-TT~ PVOH)
Sample 5 was produced. A solution of Elvanol 71-30,
Parez 613 (a methylated melamine formaldehyde) and ammonium
chloride was coated onto a polymeric substrate of biaxially
oriented polypropylene approximately .75 mils thick. The
solution contained 6% by weight of solid. In turn, the
solid contained approximately 83% by weight of PVOH,
approximately 15% by weight of methylated melamine
formaldehyde and approximately 2% by weight of ammonium
chloride.
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The substrate was treated with a poly(ethyleneimine)
primer prior to application of the coating. The coating
was applied to the polypropylene substrate using a smooth
rod coating process. The low temperature seal coating set
out in Example 3 was applied to the PVOH layer. The
resulting product contained l0~ cross-linked PVOH. The
characteristics of the film were thereafter measured and
the results are set out in the Table below.
EXAMP~E 6-OPP~PBI/PVOH/ACRYLIC ~10% CRO88-~INRED PVOH)
l0Sample 6 was produced by the procedure set out in
Example 5 except that the low temperature seal coating was
substituted by the acrylic coating of Example 2. The
characteristics of the film were thereafter measured and
the results are set out in the Table below.
15EXAMP~E 7-OPP/PEI/PVOH/EPOXY/ACRYLIC (10% CRO88-T-TNR~
PVOH)
Sample 7 was produced by the procedure set out in
Example 6 except that the PVOH coating was primed with an
epoxy primer prior to coating with the acrylic coating.The
characteristics of the film were thereafter measured and
the results are set out in the Table below.
EXAMPLE 8-OPP/PEI/PVOH/PEI/ACRY~IC (10% CRO88-TT~n PVOH)
Sample 8 was produced by the procedure set out in
Example 6 except that the PVOH coating was primed with a
polyethyleneimine (PEI) primer prior to coating with the
acrylic coating. The characteristics of the film were
thereafter measured and the results are set out in the
Table below.
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TABLE
- TO2 MST ULTIMATE
SEALS (q/in)
5 OPP/PEI/PVOH * <0.05 NIL NIL
OPP/PEI * 150. 96~C 600
/ACRYLIC
OPP/PEI/PVOH/ * 0.017 97~C 650
EPOXY/LTSC
(100% cross-linked PVOH)
OPP/PEI/PVOH/ * 0.010 97~C 40S
EPOXY/ACRYLIC
OPP/PEI/PVOH/LTSC * 0.175 99~C 212
(10% cross-linked PVOH)
lS OPP/PEI/PVOH/ACRYLIC * 0.062 96~C 418
(10% cross-linked PVOH)
OPP/PEI/PVOH/EPOXY * 0.079 98~C 218
ACRYLIC
(10% cross-linked PVOH)
OPP/PEI/PVOH/
PEI/ACRYLIC * 0.099 96~C 387
(10% cross-linked PVOH)
* All results in C.C./100 sq. in./24 hour, 0% rel.
humidity
~.
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It is apparent from the data set forth in the Table
that films having the combination of the PVOH and acrylic
coatings, including low temperature seal coatings, produced
in accordance with the present invention are superior to
those of the prior art.
