Note: Descriptions are shown in the official language in which they were submitted.
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F-5135-L
MULTI-LAYER HEAT-SEALABLE POLYPROPYLENE FILMS
This invention relates to multi-layer, heat-sealable
polypropylene films which possess good slip properties and good
adhesion to water based adhesives such as water based acrylics,
urethanes and vinylidene chloride polymer (PVDC).
Polypropylene films possess a number of desirable
characteristics including excellent optical properties such as
transparency and brilliance, satisfactory mechanical properties
such as tensile strength and Young's modulus, and substantial
1o non-toxic and odorless properties. Accordingly, polypropylene
films are widely used as packaging materials, especially for
foods. One drawback of polypropylene films, however, is that
they possess poor heat sealability. To remedy this, it is
widely known to laminate a low-temperature heat-sealable resin
15 to one or both sides of the polypropylene film by coating,
laminating or co-extruding. Such heat-sealable resins include,
for example, middle and low density polyethylenes,
ethylene-propylene copolymer, and terpolymers of ethylene,
butene and propylene.
2o Another drawback of unmodified polypropylene films is that
they exhibit relatively poor slip characteristics, i.e., they
exhibit high film to film coefficients of friction which makes
it difficult to utilize them in automatic packaging equipment.
The poor slip behavior of a film will interfere with its use in
25 automatic processing equipment since the film must pass freely
through the fabricating machine (e. g., heat sealer, bag maker,
bag loader or filler, bag opener, overwrap package) for it to
operate properly and reproducibly. In order to overcome the
slip problems in heat-sealable films, it is common to
3o incorporate one or more of several conventional slip additives,
e.g., oleamide, stearic acid, erucamide and the like, in the
heat-sealable film.
F-5135-L - 2 -
Although crystalline polypropylene films exhibit relatively
low vapor permeability, it is often desired to further increase
their gas and vapor barrier properties, especially for
applications in which the films are being used to package
products such as food items which are sensitive to, or attacked
by, oxygen or moisture. It is well-recognized in the art that
an effective means for increasing the gas and vapor barrier
properties of oriented polypropylene films is to coat such
films with polymers of vinylidene chloride.
It is important, when coating polypropylene films with such
vinylidene chloride polymer compositions, to ensure that the
adhesion of the coating layer to the polypropylene substrate is
adequate. For example, in many packaging applications, it is
necessary for the coated film to be heat sealed either to
itself or to other films to form a tightly closed package. If
the coating adhesion to the base film is inadequate, the
packages are liable to prematurely open when subjected to
stress.
It has been the common understanding in the art that, to
2o attain adequate adhesion between polypropylene film surfaces
and water based adhesives, the film surfaces must be subjected
to well known pretreatment operations such as, for example,
treatment by corona discharge, flame or oxidizing chemicals.
Other widely practiced means for improving the adhesion of the
water based adhesives are the coating of the polypropylene film
surface with specific primers or the co-lamination of the
polypropylene film with an adhesion promotion film. Both
coating or colamination methods, however, entail additional
processing steps which increase the manufacturing costs of the
3o films.
Even with pretreatment of the polypropylene film, such as
by corona discharge, the adhesion of the water based adhesives
to the polypropylene film surface will often not be
satisfactory when the polypropylene film contains slip
F-5135-L - 3 _ 2o109rC~,'~
additives such as erucamide. Such slip additives tend to bloom
or migrate to the surface of the polypropylene film where they
act to greatly increase the variability of the bonds between
the film and the water based adhesives.
There is therefore a need for a polypropylene film which
can run well in packaging machines and which can also form good
bonds with water based adhesives. Therefore, a film which
exhibits the combined properties of low coefficient of friction
and good adhesion to water based adhesives and which can be
made in a single manufacturing step is greatly desired.
Multi-layer, heat-sealable polypropylene films have now
been found which exhibit low coefficients of friction and good
adhesion to water based adhesives including acrylics, urethanes
and PVDC, and which can be made in a single manufacturing
step. Such film structures consist essentially of an outer
heat sealable layer (A), a core layer (B) and an outer layer
(C),
the outer heat sealable layer (A) being coextensively
adherent to the upper surface of the core layer (B), and the
z0 outer layer (C) being coextensively adherent to the lower
surface of the core layer (B),
said outer layer (A) being formed from a polymer
composition (a) consisting essentially of heat sealable resin
compounded with one or more slip additives,
said core layer (B) being derived from a polymer
composition (b) consisting essentially of an isotactic
polypropylene homopolymer compounded with one or more slip
additives, and
said outer layer (C) being formed from a polymer
3o composition consisting essentially of isotactic polypropylene
homopolymer in the substantial absence of slip additives,
wherein the total amount of slip additive in said film
structure is effective to provide the outer surface of outer
layer (A) with a coefficient of friction sufficient for high
speed heat sealing packaging operations but insufficient to
cause substantial hazing of said structure.
CA 02010927 1999-09-08
F-5135-L _ 4 _
By virtue of the presence of slip additive in heat seal
layer (A), and the migration of slip additive from layer (B) to
the surface of layer (A), outer layer (A) possesses a low
coefficient of friction which allows for excellent
machinability of the films of this invention. Since the slip
additive is one which is incompatible with polypropylene, it
does not substantially migrate to the surface of outer layer
(C) and thus does not interfere with good film-water-based
adhesive bonding.
1o This invention therefore relates to such films and to such
films to which a water-based adhesive coating has been applied.
The isotactic polypropylene homopolymer of the core layer
(B) and outer layer (C) is preferably a polypropylene having a
density of from 0.88 to 0.94 g/cc (880 to 940 kg/m2) and a
15 melt flow index of from 1 to 10 g/10 mins. (1.67 x 10 6 to
16.7 x 10 6 kg/s) at 230'C/2.16 Kp/cm2 (0.0745 N/m2)
pressure (as measured in accordance with ASTM D 1238).
The heat sealable resin in outer layer (A) can be any of
the heat sealable copolymers, blends of homopolymers and blends
20 of copolymers) and homopolymer(s) heretofore employed for this
purpose. Illustrative of heat sealable copolymers which can be
used in the heat sealable layer are ethylene-propylene
copolymers containing from about 1.5 to about 10, and
preferably from about 3 to about 5, weight percent ethylene,
25 copolymers of propylene and butene-1 containing from about 5 to
about 40 weight percent butene-1, and
ethylene-propylene-butene-1 terpolymers containing from about 1
to about 10, and preferably from about 2 to about 6, weight
percent ethylene, from about 80 to 97, and preferably from
3~ about 88 to about 95, weight percent propylene, and from about
1 to about 20, and preferably from about 2 to about 15, weight percent
butene-1, wherein the total of the weight percent does not exceed 100
weight percent.
Both core layer (B) and outer heat-sealable layer (A) are
formed from polymer compositions containing slip additives
10
F-5135-L - 5 -
which are incompatible with polypropylene. The percentage of
the slip additive in the multi-layer structure should be such
as to provide the outer surface of outer layer (A) with a
coefficient of friction sufficient for high speed heat sealing
packaging operations but insufficient to cause substantial
hazing of the film structure. While the amount of slip
additive is best defined by the result to be accomplished, it
is preferred that this additive be included in the overall film
structure in an amount of about 0.02% to about 0.20% by weight
1o and even more preferred in amounts between about 0.025 and
about 0.10% by weight. In the preferred embodiment, the amount
of slip additive in the polymer composition from which core
layer (B) is made is less than the amount of slip additive in
the polymer composition from which layer (A) is made. More
preferably, the composition from which core layer (B) is made
contains about 400 to 800 ppm slip additive, and the polymer
composition from which outer heat-sealable layer (A) is made
contains about 1000 to 2000
ppm slip additive; in both cases the slip additive is
2o preferably erucamide.
Slip additive in core layer (B) exudes from that layer
through the outer heat-sealable layer (A) to the film's surface
by "blooming" as is understood by those of skill in the art.
In this manner, the additive present in core layer (B) becomes
available at the surface of layer (A) so as to beneficially
affect the coefficient of friction and anti-stick
characteristics of the film structure. The slip additive
preferentially blooms to the (A) layer and therefore is not
present on the outer homopolymer surface of the (C) layer and
3o does not adversely affect lamination bonds.
Generally, it is desired that the outer surface of outer
layer (A) exhibit a coefficient of friction (ASTM D 1894) of
less than about 0.45, preferably less than about 0.35 at room
temperature.
F-5135-L - 6 -
Slip additives which may be used in making the films of
this invention are those which are incompatible with
polypropylene, i.e., those which bloom to the surface from the
core and skin layers. Such additives are known to those
skilled in the art. Non-ionic surfactants, such as the amides
and carboxylic acids, are particularly of interest. Amides
which are preferred are the amides' of ca~box4y,,~;ic a.~jy~: having
at least five carbon atoms, for exampl,eh, mehenamide,
linolenamide, arachidamide, ricoinolamide, palmitamide,
1o myristamide, linoleamide, lauramide, capramide, pelargonamide,
caprylamide, oleamide, stearamide, N,N'-ethylene bisoleamide,
and the most preferred slip additive, erucamide. Carboxylic
acids which are useful include those having at least four
carbon atoms, for example, butyric, caproic, caprylic, capric,
~5 lauric, lauroleic, myristic, myristoleic, pentadecanoic,
palmitic, palmltoleic, mar aric stearic, oleic, linoleic
9 ~ ,
limolen~c, ricinoleic, 2,3-dihydroxystearic, 12-hydroxystearic,
behenie, el~ostearic, arachidic, 2-ecosenoic,
2-4-eicosadienoic, 2-docosenoic, 2-tetracosenoic,
20 2,4,6-tetracosatrienoic and the like.
The slip additive is preferably dry blended together with
the polypropylene resin of layer (B) or the heat-sealable resin
of layer (A) and then melt mixed. Alternatively, the additive
can be incorporated into a minor portion of the resin as a
25 master batch to form a high concentration mix of the additive
and the resin. This may then be diluted to the appropriate
proportion by the addition of more resin.
Outer layer (C) preferably contains an effective amount of
one or more anti-block agents; heat-sealable layer (A) also
3o preferably contains such agents. The anti-blocking agent
preferred for inclusion in these outer layers may be any
particulate inorganic material having a mean particle size
ranging from about 0.5 to 5 microns. One comr~ercially
available silica"(Kaopolite 1152;' available from Kaopolite,
* Trademark
s
_~~~ 0~2~
F-5135-L - 7 -
Inc.) has a mean particle size of 0.8 microns and another
~~(Sipernat 4~;* available from DeGussa Chemical Company) has a
mean particle size of 4.0 microns. Either material, or
mixtures thereof, can be employed. Metal silicates, silica
glasses, clays and numerous other finely comminuted inorganic
materials may also be used. The anti-blocking agent is
preferably present in from about 0.05 to 0.5 wt.%, preferably
about 0.1 to 0.3 wt.%, of the layers (A) and/or (C).
Microcrystalline wax is preferably incorporated into the
outer heat sealable layer (A) as its inclusion permits the use
of much lower amounts of slip additive than would otherwise be
required and thus results in films with superior appearance and
physical performance. This is so because slip additives such
as the amides contribute to a hazy appearance of films. Useful
waxes may be any of the known microcrystalline waxes. It is
preferred, however, that synthetic n-paraffinic waxes be used.
Preferably, the wax has a melting point between about 85°C and
about 165°C. The wax is preferably added in amounts between
about 5% to about 15% by weight of the heat seal layer, and
2o most preferably at about 10% by weight of that layer.
A further, preferred, additive for inclusion in the heat
sealable layer (A) is glycerol monostearate or other
monoglyceride which may preferably be included in amounts
between about 0.05 and 0.3% by weight of the layer and most
preferably at about 0.1% by weight.
The multi-layer films of this invention can be prepared
employing commercially available systems for coextruding
resins. The polymer compositions (a), (b) and (c) are
preferably coextruded with one another. The polymers can be
3~ brought to the molten state and coextruded from a conventional
extruder through a flat sheet die, the melt streams being
combined in an adapter prior to being extruded from the die.
After leaving the die orifice, the multi-layer film structure
is chilled and the quenched sheet is then preferably reheated
and stretched, e.g., 4 to 6 times in the machine direction at
* Trademark
2c~ l o ~I Z't
F-5135-L - g -
approximately 250°F (121°C) and subsequently, for example, 8 to
times in the transverse direction at approximately 320°F
(160°C). The outer surface of layer (C) is then preferably
treated by flame or corona to a surface activity of at least 36
dynes/cm (36 mN/m), preferably to approximately 40 dynes/cm
(40mN/m). The edges of the film can be trimmed and the film
wound onto a core. It is preferred that the thus-formed
structure be conditioned or equilibrated by holding the same
for a period of about one to three days at 100-125°F (37-52°C)
1o to promote migration of slip additive for coefficient of
friction development.
The films described above are advantageous because they
possess low coefficients of friction, enabling their use in
automatic packaging equipment, and because they are also
capable of forming good bonds with water based adhesives on the
flame- or corona-treated surface of outer layer (C), i.e., in
the ranges of about 80-150g (0.08 - 0.15 kg) (measured using an
" Instron~~tester, bonds pulled along machine direction).
The composition of the water based adhesive is not critical
2o to the practice of the invention. Commercially available
acrylics, urethanes and vinylidene chloride latexes may be
employed. Commercially available vinylidene chloride latexes
generally have a vinylidene chloride content of at least 50%
and preferably from about 75% to about 92% may be employed.
The other ethylenically unsaturated comonomers may include
alpha, beta ethylenically unsaturated acids, such as acrylic
and methacrylic acids; alkyl esters containing 1 - 18 carbon
atoms of said acids, such as methylmethacrylate, ethyl
acrylate, butyl acrylate, etc. In addition, alpha, beta
ethylenically unsaturated nitriles such as acrylonitrile and
methacryionitrile can be employed. In addition, monovinyl
aromatic compounds such as styrene , or vinyl chloride, may be
employed.
* Trademark
r~ ~ p~21
F-5135-L - 9 -
Specific vinylidene chloride polymer latexes
contemplated comprise: 82% by weight vinylidene chloride, 14%
by weight ethyl acrylate and 4% by weight acrylic acid.
Alternatively, a polymer latex comprising about 80% by weight
vinylidene chloride, about l7fo by weight methyl acrylate and
about 3% by weight methacrylic acid can likewise be employed.
The best mode for carrying out the instant invention
presently contemplated by the inventors is a film of the
following structure:
to (A) an outer heat-sealable layer of 90% propylene/
ethylene/butene-1 terpolymer and 10% microcrystalline wax;
about 1600 pp * erucamide; about 1000 ppm glycerol monostearate
"(Myverol 1806'; available from Eastman Ch *mical) and about 3100
ppm silica anti-block particles '(Syloid;' available from W.R.
Grace Corp.);
(B) a core layer of isotactic polypropylene
containing 400-800 ppm erucamide;
(C) An outer layer of isotactic polypropylene
containing about 2400 ppm silica anti-block particles with mean
2p particle size about 0.8 microns (Sipernat 44)*and about 3000
ppm silica anti-block particles with mean particle size about
4.0 microns 'tKaopolite 1152)°,*the outer surface of which layer
is corona treated to about 40 dynes/cm (40 mN/m).
This invention is further illustrated by the
following examples.
Example 1
Three films were made using the following procedure:
The manufacturing process consisted of coextruding the outer
layers (A) and (C) with the isotactic polypropylene core layer
(B). The core resins were~Fina~~~670C, which contains
erucamide, and a standard isotactic non-erucamide
polypropylene, for example"Fina"828 (Fina resins are available
* Trademark (each instance)
20 ~ o ~' 2~
F-5135-L - 10 -
from Fina Oil & Chemicals Co., Dallas, TX). The (A) layer was
melted and coextruded with the core and (C) layer. The (C)
layer was isotactic polypropylene containing 2400 ppm'~ipernat"*
44 and 3000 ppm~~Kaopolite~1152 antiblock particles. The (A)
layer was extruded in the same manner, and was 90%~~Chisso~*
terpolymer (propylene/ethylene/butene-1, available from Chisso
Co.) and 10% microcrystalline wax, as well as a total of 1600
ppm erucamide, 1000 pprt~~Myverol"1806 antistatic agent and 3100
ppm ~~yloid~~a.ntiblock particles.
1o The three layer extrudate was quenched, reheated and
stretched 4-6 times in the machine direction at approximately
250°F (121°C). Subsequently, the MD stretched sheet was
stretched 8-10 times in the transverse direction at
approximately 320°F (160°C). The (C) layer was treated by
flame or corona to approximately 40 d/cm (40 mN/m) and was
wound into mill roll form. The film was then stored at
100-125°F (37-52°C) for 1-3 days to promote the migration of
erucamide.
Film 1-A - This film was a two-layer film having a
2o heat-sealable layer of 3.5% random ethylene propylene
conventional copolymer and a layer of isotactic polypropylene
which has no erucamide or other slip additives. The film was
corona treated on the homopolymer side and coated with PVDC
"(Morton' 2015) .
Film 1-B - This film was a three-layer structure with
an outer layer (i) containing 50% propylene/ethylene/butene-1
terpolymer, 40% of 3.5% random ethylene/propylene conventional
copolymer, and 10% microcrystalline wax plus antiblocking and
slip agents; a core layer (ii) of conventional isotactic
3o polypropylene with 400-700 ppm erucamide; and an outer
heat-seal layer (iii) of 90% propylene/ethylene/-butene-1
terpolymer, 10% microcrystalline wax plus slip and antiblocking
agents. * The layer (i) was corona treated and coated with PVDC
~~Grace 8600) .
* Trademark (each instance)
2~~~9~~
F-5135-L - 11 -
Film 1-C - This film was the same as Film 1-B except
that the outer layer (i) was 100% isotactic polypropylene plus
3000 pm Kaopolite 1152 and 2400 ppm Sipernat 44 antiblock
particles. The homopolymer layer was corona treated and coated
with PVDC (Grace 8600).
Saran lamination bond strengths of the laminates were
tested by cutting one inch strips of the laminates and testing
in an Instron tensile tester. Alternatively, a Sutter tester
could be used for determing lamination bonds. Properties of
to Films 1-A, 1-B and 1-C are presented in Table 1.
TABLE 1
Film Lamination COFa Saran Lamination
Bonds
1-A 0.7-1.0 100-400
grams/in
(kg/m) (0.028-0.039) (3.9-15.8)
1-B 0.25-0.45 10-150
grams/in
(kg/m) (0.0098-0.018) (0.39-5.9)
1-C 0.25-0.35 80-150
grams/in
(kg/m) (0.0098-0.014) (3.2-5.9)
a Film is laminated to itself
The data presented in Table 1 indicate that the
films 1-A and 1-B are unacceptable. Film 1-A exhibited
acceptable saran bond strengths, but its COF was too high for
packaging machine performance. Film 1-B exhibited acceptable
COF, but its PVDC bond strengths were too inconsistent. The
film of this invention, Film 1-C, exchibited both acceptable
3o high barrier saran bond strength and acceptable COF. All films
exhibited satisfactory wettability and adhesion.
Example 2
Additional films were manufactured using the general
method described in Example 1 to illustrate deficiencies of
films not having the structure of the claimed films.
20~Q9~~
F-5135-L _ 12 _
Film 2-A - This film was a three-layer film
comprising a core of isotactic polypropylene with no additives
and two outer layers of typical random copolymer or terpoiymer
heat-sealable layer. Silicone fluid was added to the outer
layers for lubricity.
Film 2-B - This film was an ABA structure with the
core (B) layer containing 2000 ppm erucamide, and the (A)
layers being conventional 3.5~ random ethylene/propylene
copolymers
1o Properties of these films are presented in Table 2.
TABLE 2
Film Lamination COFa Saran Lamination
Bonds
2-A (g/in) 0.30-0.40 0-25
(kg/m) (0.012-0.016) (0.98)
2-B (g/in) 0.20-0.50 20-100
(kg/m) (0.0079-0.020) (0.79-3.9)
a Film is laminated to itself
The data in Table 2 indicate that Film 2-A exhibited
Zo good coefficient of friction but very poor PVDC bonds. Film
2-B exhibits good coefficient of friction but its PVDC bonds
are too inconsistent. These examples highlight the need to
minimize the amount of erucamide in the outer (C) layer and the
need to have a sufficient concentration in the other surface
for acceptable packaging machine performance.
