 |
Description  |
|
|
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 quantities 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 achieve 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 layer
containing titanium dioxide is used to form a cold seal receptive 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. Pat. No. 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 butene and any other polymerizable monomer having ethylenic
unsaturation. One surface of the film can be subjected to
electro-discharge treatment. Slip agents and antiblocking agents can be
added to the film.
2. U.S. Pat. No. 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 and a metallic layer on the surface of the
adherent layer remote from the substrate. 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. Pat. No. 4,692,380 of 09-08-87 to D. Reid discloses a metallized,
biaxially oriented polypropylene film which exhibits 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. Pat. No. 4,741,950 of 05-03-88 to L. Liu et al. discloses an
oriented, opaque, alpha-olefin laminate film having a non-blocking 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-blocking 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 sealable.
5. U.S. Pat. No. 4,758,396 of 07-19-88 to B. Crass et al. discloses
biaxially oriented film having a single ply or an opaque 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 promoting, dye coating, and metallizable layers.
The film of this prior patent has a density of 0.6 grams per cubic
centimeter.
6. U.S. Pat. No. 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 sealable 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 U.S. Pat. No. 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. Pat. No. 5,026,592 of 06-25-91 to S. Janocha et al. discloses an
opaque, 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. Pat. No. 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 by 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 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-opaque 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 baked 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 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 terpolymers, such as
those containing ethylene and or propylene.
DETAILED DESCRIPTION OF THE INVENTION
The Core 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 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 amides of fatty acids having from about 12
to 20 carbon atoms such as those of glycols , and glycerine, e.g.,
glycerol monostearate.
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 receptive layer is: (a) about 60% to 90% by weight of an
ethylene-propylene random 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. Preferably, the slip agent should not migrate
out of the cold seal receptive layer since such migration has an adverse
effect on the subsequently applied cold seal adhesive composition.
Suitable 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 synthetic rubber 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.degree. 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 COF is so great that smooth unwinding of
the film is virtually impossible.
Illustrative of suitable 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 beads, calcium carbonate, and the like.
Such solid slip agents are generally provided in the form of approximately
spheroidal particles having a particle size range of from about 0.5 to
about 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 about 0.01% to 1% by weight such as 0.2 to 0.5%. A preferred kinetic
coefficient of friction on the cold seal receptive layer side is about
0.28. The coefficient of friction is measured by ASTM: D-1894-87.
The cold seal receptive layer will have titanium dioxide dispersed therein.
The quantity of particulate titanium dioxide in 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 from
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 or 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
weight of ethylene and 90% to 99.5% by weight of propylene. One such
random copolymer is that of ethylene and propylene containing less than
10% by weight of ethylene, e.g. 2% to 8%. As with the core, such olefinic
layer is transparent and substantially free of cavitation, voids,
opacifying expansions, and fillers.
The metallizable layer can optionally also contain 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 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
MacBeth 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 i 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
aluminum metal layer has been placed. The total thickness of such film is
about 0.7 to 1.15 mil.
Manufacture of the film is carried out by known coextrusion processes. This
includes coextruding through a flat film die the 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 skins is
stretched by conventional techniques to orient the film, prior to corona
discharge and deposition of a metallic layer on to the metallizable skin
layer. Orientation may be 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 t | | |
