Note: Descriptions are shown in the official language in which they were submitted.
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R.I.C-2341 05-31-94
RTAXTAT.T.y ORl~Nl~ POLYPROPYL~N~ METAT.T.T7.ED
WHITE FILM FOR COLD SEAL APPLICATIONS
FIELD OF THE INVENTION
This invention relates to an oriented polyolefinic laminated
film which has a core, a titanium dioxide containing white-
partially opaque cold seal receptive polyolefinic skin layer on one
side of the core, and a metallized surface on the other side of the
core. The titanium dioxide in cooperation with the obverse
metallized surface of the film provides a strong white-opaque
appearance when viewing the cold seal receptive layer. In a
preferred form, the film is biaxially oriented and contains
significant ~uantities of isotactic polypropylene homopolymer.
BACKGROUND OF THE INVENTION
! a) Field of the Invention
White biaxially oriented polypropylene films have
traditionally been produced using a cavitated/expanded core in
order to ~chieve opacity. The cavitated/expanded core weakens the
core which is generally the thickest layer of such films.
In the invention of this application, a thin polyolefinic skin
laye containing titanium dioxide is used to form a cold seal
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rec~ptive layer which, in cooperation with the metallized surface
on the opposite side of the core layer, enhances the white
coloration of the titanium dioxide containing layer. Such films
have better physical properties, e.g., tensile strengths, since the
core has not been modified for opaqueness.
The cold seal receptive polyolefinic layer is on one side of
the core and preferably a polyolefinic skin layer which is
subsequently metallized is on the other side of the core. Each of
the skin layers has a thickness of less than one-fifth of that of
the core. Thus, the core can contain a large percentage of a high
tensile strength polyolefin such as isotactic polypropylene
homopolymer or high density polyethylene homopolymer whereas the
skin layers are thin and provide for a film which has high tensile
strength for its overall thickness.
(b) Discussion of the Prior Art
1. U.S. Patent 4,230,767 of 10-28-80 to T. Isaka, et al.
discloses an oriented heat sealable film of a polypropylene polymer
base layer and a surface layer or layers of a copolymer of ethylene
and propylene and a copolymer of ~utene and any other polymerizable
monomer having ethylenic unsaturation. One surfae of the film can
be subjected to electro-discharge treatment. Slip agents and
antiblocking agents can be a~ded to the film.
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2. U.S. Patent 4,357,383 of 11-02-82 to M. Howden et al.
discloses a multi-layer metallized film of an alpha-olefin having
on at least one surface an adherent layer of random copolymer of
ethylene with 0.5 to 15% of 3-6 carbon alpha-olefin a~d a metallic
layer on the surface of the adherent layer remote from the
su~strate. Among a number of options, this film can also have a
heat seal layer on the surface opposite from the metallized layer.
3. U.S. Patent 4,692,380 of 09-08-87 to D. Reid discloses a
metallized, ~iaxially oriented polypropylene film which exhi~its
good adhesion between the polypropylene and the metal coating. The
core has on at least one surface a propylene-ethylene copolymer
which contains no slip agent and which has been subjected to corona
treatment.
4. U.S. Patent 4,741,950 of 05-03-88 to L. Liu et al. discloses
an oriented, opaque, alpha-olefin laminate film having a non-
blockin~ first surface, a smooth lustrous second surface which is
intended for further film processing operations such as
metallization and an expanded core to provide opacity. The core
can optionally also contain additional filler for opacity and the
anti-~locking skin layer optionally contains 2% to 8% of titanium
dioxide to confer enhanced opacity. Among other shortcomings this
patent uses voids in the core which adversely affect the mechanical
properties and the anti-blocking layer is not heat seala~le.
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R.I.C-2341 05-31-94
5. U.S. Patent 4,758,396 of 07-19-88 to B. Crass et al. discloses
biaxially oriented film having a single ply or an opa~ue core layer
of a multiply film composed essentially of polypropylene. The
opaque core layer includes 10-40% of filler. Titanium dioxide is
mentioned as one of the fillers. In addition to the polypropylene
opaque single ply or multiply layered film such film can include
functional layers such as heat sealing, cold sealing, adhesion
pro~oting, dye coating, and metallizable layers. The film of this
prior patent has a density of 0.6 grams per cubic centimeter.
6. U.S. Patent 4,997,700 of 03-05-91 to L. Bothe et al. discloses
a metallizable, heat sealable, biaxially oriented multilayer film
which comprises a polypropylene core layer, a first polyolefin
heat-sealable surface layer and a second polyolefin metallizable
surface layer. The metallizable layer comprises a propylene-
ethylene copolymer containing from 1.2 to 2.8 ~ of ethylene. The
heat seala~le layer comprises ethylene containing polymers,
copolymers of ethylene and propylene or copolymers of propylene and
1-butane as well as other olefinic mixtures. In contrast to
densities of about 0.85 to 9.5 grams per cubic centimeter for the
films of this invention, the density of the 4,997,700 patent is
recited as 0.8 grams per cubic centimeter or less, particularly
0.65 grams per cubic centimeter or less.
7. U.S. Patent 5,026,592 of 06-25-91 to S. Janocha et al.
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R.I.C-2341 05-31-94
discloses an opague, biaxially stretched oriented, polyolefinic
multilayered film comprising an opaque core layer and two
transparent skin layers. The opaqueness is attained by the use of
inorganic fillers in the core layer. Titanium dioxide,is mentioned
along with many different fillers as suitable. Among many different
options the top layers can be sealable or non-sealable and
metallized or non-metallized.
8. U.S. Patent 5,091,236 of 02-25-92 to L. Keller et al.
discloses a multilayer opaque, biaxially oriented polymeric film.
The film structure includes: (a) a core layer having voids which
cause a significant degree of opacity; (b) a layer adhering to the
core layer including up to about 12% of titanium dioxide: and a
titanium dioxide-free non-voided thermoplastic skin layer adhering
to the other side of the core layer. The titanium dioxide improves
opacity and whiteness of the film.
9. HERCULES WTF 503 is a non-sealable film having a white-opaque
oriented polypropylene core and a metallizable surface designed for
the insulation market. This product is sold ~y Hercules, Inc.
The present invention has the following main advantages over
the prior art:
1. A significant white-opaque appearance is achieved by
cooperation of the metallized layer with the layer containing the
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R.I.C-2341 05-31-94
titanium dioxide without the need for cavitation, expansion, or
inert filling of the core layer. This also eliminates the need for
white color printing of the film. This provides for the use of
smaller quantities of filler to attain the white-op,a~ue look as
compared to fillers in the core, provides better mechanical
properties, and is more economical.
2. The white pigmented layer provides excellent cold seal
receptivity and adhesion in spite of the relatively large
percentage concentration of filler in this thin layer.
3. The excellent mechanical properties such as high
stiffness and tensile strength of conventional polyolefin films,
e.g., biaxially oriented isotactic polypropylene homopolymer film,
are maintained versus cavitated, expanded, or filled opaque
polyolefin film.
4. Thinner gauges (higher yields) of laminated film can be
used versus cavitated/expanded, opaque alpha-olefin film thus
providing a cost advantage.
5. Excellent metal bond strength as well as excellent gas
barrier properties are obtained.
The films of this invention are particularly useful as
wrappers or packages for confectionery and ~aked items, e.g.,
candy bars, cup cakes, and frozen confections such as ice cream
bars. The consuming public appears to associate the white
coloration with sanitation and such white coloration helps mask oil
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and grease from the confectionery or baked items.
SUMMARY OF THE INVENTION
This invention provides a white-opaque, oriented, and
metallized polyolefin film prepared from at least 2 polyolefin
layers. One layer is a cold seal receptive skin layer which
contains: (a) a slip agent in an amount sufficient to provide not
more than about an 0.4 coefficient of friction to the surface of
such layer and can have its surface physically modified such as by
chemical, flame, or corona treatment so as to increase its
receptivity to other coatings; and (b) from about 10 to 40% of
titanium dioxide. Another layer is the core layer which is
transparent and substantially free of cavitations, opacifying
expansions, or fillers. The obverse surface from the cold seal
receptive layer is metallized to an optical density of at least 1.5
whereby the white pigment in the cold seal receptive layer in
cooperation with the metallization gives a strong white-opaque
appearance to the cold seal receptive layer.
The polymers are predominantly that of alpha-olefins having up
to about 10 carbon atoms, e.g., homopolymers, copolymers, and
terpoly~ers, such as those containing ethylene and or propylene.
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R.I.C-2341 05-31-94
DETAILED D~SCRIPTION O~ THE INVENTION
The ~ore Layer
The polyolefinic core is transparent and free of fillers and
voids such as those caused by cavitation or expansion which produce
opaqueness. Generally, the core contains polymers of alpha-olefins
such as that polypropylene, e.g., isotactic polypropylene
homopolymer, polyethylene such as a high density polyethylene,
random or block copolymers of ethylene and propylene, and mixtures
of the foregoing. The isotactic polypropylene homopolymer used in
this invention will contain at least about 80% and preferably at
least 90% or 95% by weight of isotactic polypropylene units.
Illustratively, the core can be that of from about 90% to 99% by
weight of isotactic polypropylene homopolymer together with from
about 1% to 10% by weight of the core of high density polyethylene.
Another illustrative core layer composition is that of a random or
block copolymer of ethylene and propylene containing about 90% to
99.5% of propylene and 0.5% to 10% of ethylene based on the weight
of the copolymer with or without high density polyethylene such as
that of about 1% to 50% by weight of the core. A preferred core is
about 60 gauge in thickness and has a polyolefin composition of
about 97% of isotactic polypropylene homopolymer and 3% of high
density polyethylene based on the weight of the core.
Optionally, the core layer will contain an antistatic agent in
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R.I.C-2341 05-31-94
an amount sufficient to inhibit the build up of static
electricity, such as that which can occur during corona treatment
of the film. Such quantity can vary over a wide range such as
that of about 0.01 to 0.1% by weight of the core layer.
Illustrative of suitable antistatic agents there can be mentioned
fatty acid esters and a~ides of fatty acids having from about 12 to
20 carbon atoms such as those of glycols , and glycerine, e.g.,
glycerol ~onostearate.
The thickness of the core layer can vary over a wide range
such as that of from about 50 to 120 gauge, e.g. from about 55 to
80 gauge, and preferably about 60 gauge, i.e. 60 thousandths of an
inch.
The Cold Seal Receptive Layer
The polyolefinic cold seal receptive layer contains a polymer
of suitable alpha-olefins as well as particulate titanium dioxide
and a slip agent intimately admixed in the olefinic polymer. A
preferred composition for the cold seal recepti~e layer is: (a)
about 60% to ~0% by weight of an ethylene-propylene rando~
copolymer containing about 2~ to 10% by weight of ethylene; and
about 10% to 40% by weigh of titanium dioxide and (c) a slip or
antiblocking agent. Prefera~ly, the slip agent should not migrate
out of the cold seal receptive layer since such migration has an
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R.I.C-2341 05-31-94
adverse effect on the subsequently applied cold seal adhesive
composition. Suita~le slip agents are normally solid, e.g., solid
inorganic slip agents.
A number of advantages accrue when the polymer of the cold
seal receptive layer is an ethylene-propylene random copolymer
containing 2% to 10% by weight of ethylene and 90% to 98% by weight
of propylene. The layer of oriented film of such random copolymer
with the titanium dioxide dispersed therein does not have the
extensive cavitations and voids as is the case when the polymeric
layer is that of a higher melting polymer such as isotactic
polypropylene homopolymer. The oriented non-cavitated layer of
this random copolymer is advantageous since cavities and voids:
reduce mechanical strength; lower the film density; and increase
pump down time due to evacuating air from the micropores in the
film layer when the film is vacuum metallized. Additionally, the
oriented non-cavitated titanium dioxide containing ethylene-
propylene random copolymer layer has a higher surface gloss as
compared to the surface gloss of a traditional cavitated white
opaque layer of isotactic polypropylene homopolymer.
The cold seal receptive layer will eventually have a layer of
polymeric composition applied thereto in order to effectuate the
cold sealing. This does not significantly affect the white-opaque
appearance. Cold seal compositions are generally natural or
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~ R.I.C-2341 05-31-94
synthetic ru~ber latex compositions which when applied to a
flexible film allow the film to be cohesively sealed about the item
being packaged by pressure and adequate dwell time at ambient
temperature. By "cold seal" herein is meant the sealing or bonding
of a film layer to itself at a temperature of less than about
150 F. Cold seal compositions are particularly useful in wrapping
products which are heat sensitive, such as confectioneries.
The cold seal receptive layer is also preferably treated to
modify its surface so that it has improved receptivity to the cold
seal composition which will later be applied thereto. Such surface
modification can be by conventional means such as chemical, flame
or corona treatment.
The cold seal receptive skin layer will contain an effective
amount of a slip agent to improve the mechanical properties of the
surface of this layer. Generally, it is desirable to reduce the
slip to a coefficient of friction (COF) of not more than about 0.4.
By this means, blocking of adjacent layers of film on mill rolls is
prevented and the film can readily be unwound for use or for
further processing. The quantity of slip agent, also referred to
herein as anti-blocking agent, is also sufficient to substantially
eliminate blocking of the film. In the absence of the slip agent,
the layer-to-layer CO~ is so great that smooth unwinding of the
film is virtually impossible.
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R.I.C-2341 05-31-94
Illustrative of suita~le non-migratory slip agents, also
referred to as anti-blocking agents for the cold seal receptive
layer, there can be mentioned: cross-linked silicone such as
TOSPEARL of Toshiba Silicone Co., Ltd., silica, silicates such as
magnesium silicate, clay such as kaolin, diatomaceous earth, talc,
glass ~aos, ~a~cl~ffl carbonate, and the like. Such solid slip
agents are ~2rally provided in the form of approximately
spheroidal particles having a particle size range of from about 0.5
to a~out 10 microns. A preferred slip agent for the cold seal
receptive layer is about 0.1% to 1% by weight of that layer of
about 1 to 4.5 micron cross-linked silicone.
The amount of the slip agent in the cold seal receptive layer
is that amount sufficient to prevent blocking and to provide good
machinability such as that which provides the film layer with a COF
of less than about 0.4, e.g., from abut 0.2 to about 0.4. The
quantity of the slip or anti-blocking agent normally used in this
film layer involved can vary from a~out 0.01% to 1% ~y weight such
as 0.2 to ~.5%. A preferred kinetic coefficient of friction on the
cold seal receptive layer side is a~out 0.28. The coefficient of
friction is measured by ASTM: ~-18~4-87.
The cold seal receptive layer will have titanium dioxide
dispe~sed t~erein. The ~uantity of particulate titanium dioxide in
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R.I.C-2341 05-31-94
the cold seal receptive layer will vary from about 10% to 40% by
weight of the skin layer such as about 15% to 35~. A concentration
of 25% of rutile titanium dioxide is preferred. The cold seal
receptivity of this layer is unaffected by incorporation of these
relatively large quantities of titanium dioxide.
The titanium dioxide containing layer is not opaque in and of
itself as the case with the expanded/cavitated cores but rather
appears opaque in cooperation with the metallized layer.
The thickness of the cold seal receptive layer will generally
vary fro~ about 4 to 10 gauge and particularly about 7 gauge. The
thicker gauge provides the greater amount of whiteness.
The Metallizable Layer
When a separate, i.e., second polyolefinic skin layer, is used
for the metallizable surface instead of the core, such layer can be
that of an alpha-olefin homopolymer or copolymer such as one having
2 to 3 carbon atoms, e.g., a homopolymer of isotactic
polypropylene, random nr block ethylene-propylene copolymer and
mixtures thereof. The random or block copolymers for this layer
will generally contain from about 0.5% to 10% by weigh of ethylene
and 90% to 9g.5% by weight of propylene. One such random copolymer
is that o~ ethylene and propylene containing less than 10% by
weight of ethylenel e.g. 2% to 8~. As with the core, such olefinic
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R.I.C-2341 05-31-94
layer is transparent and substantially free of cavitation, voids,
opacifying expansions, and fillers.
The metallizable layer can optionally also contai~n an adhesion
promoting agent, particularly with the isotactic homopolymer of
propylene to improve adhesion to the vacuum metallized metal
surface. Such adhesion promoters include powdered polyamides, e.g.
Nylon 6, clay, random copolymers of 1 to 4 carbon atoms as well as
polar polymers such as those containing acrylic or methacrylic acid
in a quantity sufficient to enhance adhesion of the metal to the
treated skin surface. Such quantity can vary over a broad range
such as from about 0.1% to 1~ by weight of the metallizable skin
layer. A preferred metallizable layer has a thickness of about 5
gauge and has a polyolefin composition which consists of ethylene-
propylene random copolymer containing about 3~ by weight of
ethylene.
The metallization is preferably achieved by placing a vacuum
deposited metal layer, preferably aluminum, on to the polyolefinic
corona treated skin layer or core surface of the film. In many
applications, a thin transparent polymeric film, e.g.,
polypropylene, is placed over the metallization in order to protect
it from abrasion, etc.
The amount of metal deposition on the treated second skin
14
R.I.C-2341 05-31-94
layer should be sufficient to provide an optical density to the
film of at least 1.5 such as 1.75 or 2.0, and preferably 2.4.
Optical density is measured with a Mac~eth TD. 904 Optical
Densitometer. Such densitometer has an optical density,range of 0
to 4 with 0 being 100% light transmission and 4 being 0% light
transmission. An optical density of 2 has a light transmission of
1%. Preferred metallizable film of this invention having a heat
sealable layer, a core, and a metallizable layer, prior to corona
treatment and metallization, will have a light transmission of
about 74%. Thus, it can be seen that the titanium dioxide in the
sealing layer does not have much effect on opaqueness.
In structures wherein the metallizing is on a separate
polyolefin layer from that of the core, the thickness of such layer
will generally vary from about 1 to 10 gauge and preferably about
5 gauge.
The preferred film has a core layer, a cold seal corona
discharge treated receptive layer on one side of the core, and, on
the other side of the core, a corona treated metallizable layer to
which a vacuum deposited alu~inu~ met~l layer has been placed. The
total thickness of such film is about 0.7 to 1.15 mil.
Manufacture of the fil~ is carried out by known coextrusion
processes. This includes coextruding through a flat film die the
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R.I.C-2341 05-31-94
melt corresponding to the individual layers of the film, cooling
the film obtained by coextrusion in order to harden it, orienting,
e.g. biaxially stretching, the film, heat-setting the stretched
film and corona treating the metallizable surface layer and
generally the cold seal receptive layer. The film can also be made
by the blown film or double bubble orientation process.
Preferably, the cold seal receptive layer is coextruded on to
a bulk core layer of a conventional, non-pigmented, non-cavitated
and non-expanded, alpha-olefin of 2 to 4 carbon atoms. Also, the
skin layers of an alpha-olefin having 2 to 4 carbon atoms suitable
for further treatment are coextruded on opposite sides of the
core.
The composition of the cold seal receptive layer is preferably
produced employing the masterbatch method, e.g., the titanium
dioxide is first intimately dispersed with the alpha-olefin
polymer, generally a block or random copolymer of 2 to 4 carbon
atoms, and such mixture is then mixed in with the remaining
ingredients of the heat sealable film layer. A masterbatch which
applicants have found to be particularly suitable is P8555SC of A.
Schulman, Inc. which contains about 50% by weight of titanium
dioxide and about 50% of ethylene-propylene random copolymer.
The laminated film structure comprising the core and adherent
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R.I.C-2341 05-31-94
skins is stretched by conventional techniques to orient the film,
prior to corona discharge and deposition of a metallic layer on to
the metalliza~le skin layer. Orientation may ~e effected
uniaxially, by stretching the film in one direction or biaxially,
by stretching the film in each of two mutually perpendicular
directions in the plane of the film. Biaxial orientation is
preferred.
The degree to which the film is stretched depends to some
extent on the ultimate use for which the film is intended.
Preferably, the film is stretched to between about 4 to 6.5 times
its original dimension in the longitudinal direction and about 5 to
10 times in the transverse direction. The longitudinal stretching
is expeditiously carried out with the aid of two rolls running at
different speeds according to the stretch ratio desired and the
transverse stretching with the aid of a corresponding tenter frame.
After stretching the polymeric film is normally l'heat set",
while restrained against shrinkage at a temperature above the glass
transition te~perature of the polymer and below its melting point.
Prior to deposition of the metallic layer onto the separate
adherent metallizable layer or the exposed surface of the core
itself when a separate layer is not used, the exposed surface
thereof is subjected to a physical surface-modifying ~reatment to
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R.I.C-2341 05-31-g4
improve the bond between that surface and the subse~uently applied
metal which forms a metallic layer. A preferred treatment is to
subject the exposed surface to a high voltage electrical stress
accompanied by corona discharge. The film is then ,heat aged to
impart further dimensional stability to the film.
Deposition of the metallic layer onto the skin layer may be
effected by conventional metallizing techniques, but preferably by
a vacuum deposition process in which the metal is evaporated on to
the receptive polyolefin surface in a cham~er ~aintained under
conditions of high vacuum. Suitable metals include nickel, copper,
silver, gold, zinc, and preferably aluminum. As mentioned earlier,
a thin transparent polymeric film can be adhered over the
metallization in order to protect it such as from abrasion in
handling of a finished package made from the composite film. The
composite film which has been produced in this manner is wound up
in the customary manner with the aid of a wind-up unit.
Preferred films of this invention can have tensile strengths
of about 19,000 psi in the machine direction and 38,000 psi in the
transverse direction.
A preferred film of this invention has: (a) a core layer with
a thickness of about 58 gauge composed of about 97% by weight of
isotactic polypropylene homopolymer, 3% by weight of high density
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R.I.C-2341 05-31-94
polyethylene and 300 parts per million of glycerol monostearate;
(b) a corona treated (white) cold seal receptive layer of 75% by
weight of an ethylene-propylene random copolymer containing 3% to
5% by weight of ethylene, 25% of rutile titanium dioxide and 4,000
parts per million of 3 micron cross-linked silicone on one side of
the core; and (c) on the other side of the core, a layer of 100% by
weight of an ethylene-propylene random copolymer containing 3% of
ethylene wherein the layer is treated on its surface with corona
discharge and subsequently a film of aluminum is vacuum deposited
thereon wherein the aluminum provides an optical density of about
2.0 to the film. Such film will typically have the following
properties: a nominal thickness of 0.7 to 2.0 mil; a yield of
44,000 to 26,600 square inches per pound; a tensile strength of
about 17,000 pounds per square inch in the machine direction and
about 43,000 pounds per square inch in the transverse direction; an
elongation of about 120% to 125% in the machine direction and about
40~ to 30% in the transverse direction; a kinetic (film/film)
coefficient of friction of about 0.40: an optical density of about
2.4; and an oxygen transmission of about 5 cubic centimeters per
100 square inches in 24 hours at 75 F and 60% relative humidity.
All parts and percentages given in this specification and
claims is by weight unless the context clearly indicates otherwise.
19
