Note: Descriptions are shown in the official language in which they were submitted.
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SLIP-CLING STRETCH FILM
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of United States Patent
Application
Serial No. 11/941,425, filed on November 16, 2007, which is a continuation-in-
part of
United States Patent Application Serial No. 11/763,639, filed on June 15,
2007, each of
which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] Embodiments of the present invention generally relate to a multilayer
stretch
film. More specifically, embodiments of the present invention relate to a
multilayer
stretch film having a high slip surface and an aggressive cling surface for
wrapping
items or loads for ease of transport.
Description of the Related Art
[0003] The use of thermoplastic stretch wrap films for the over-wrap packaging
of
goods, and in particular, large rolls of paper or carpet, is a commercially
significant
application of polymer film. Monolayer stretch wrap films have identical
surfaces on
both sides, while multilayer stretch films can have a non-cling surface and a
cling
surface. The non-cling surface generally does not cling to itself and prevents
adjoining
wrapped goods from sticking to one another. The cling surface enables the film
to stick
to itself to prevent unwrapping during transport. However, as the non-cling
surface
decreases in coefficient of friction, the cling surface must increase its
cling
aggressiveness to enable the film to stick to itself when wrapped around
goods.
[0004] Thus, there is a need for an improved slip-cling stretch film having a
high slip
surface and an aggressive cling surface.
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SUMMARY OF THE INVENTION
[0005] Embodiments of the present invention relate to a multilayer stretch
film
having a high slip surface and an aggressive cling surface for wrapping items
or loads
for ease of transport.
[0006] In one embodiment, a multilayer film comprises a slip layer comprising
at
least polypropylene and high-density polyethylene blend, a cling layer
comprising at
least a styrenic block copolymer, and an intermediary layer comprising at
least linear
low-density polyethylene resin or its blend.
[0007] In another embodiment of the present invention, a multilayer film
comprises
a first surface having a coefficient of friction at least less than about 0.9,
comprising at
least polypropylene and high-densily polyethylene, a second surface having a
cling
force to the first surface at least greater than about 5 g/in, comprising at
least a styrenic
block copolymer, and a core layer, positioned between the first surface and
the second
surface, comprising at least linear low-density polyethylene resin.
BRIEF DESCRIPTION OF THE DRAWING
[0008] So the manner in which the above recited features of the present
invention
can be understood in detail, a more particular description of embodiments of
the
present invention, briefly summarized above, may be had by reference to an
embodiment, which is illustrated in the appended drawing. It is to be noted,
however,
the appended drawing illustrates only a typical embodiment of embodiments
encompassed within the scope of the present invention, and, therefore, is not
to be
considered limiting, for the present invention may admit to other equally
effective
embodiments, wherein:
[0009] Figure 1 depicts a cross-sectional view of a film in accordance with
one
embodiment of the present invention.
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[0010] The headings used herein are for organizational purposes only and are
not
meant to be used to limit the scope of the description or the claims. As used
throughout
this application, the word "may" is used in a permissive sense (i.e., meaning
having the
potential to), rather than the mandatory sense (i.e., meaning must).
Similarly, the words
"include", "including", and "includes" mean including but not limited to. To
facilitate
understanding, like reference numerals have been used, where possible, to
designate
like elements common to the figure.
DETAILED DESCRIPTION OF THE INVENTION
[0011] Figure 1 depicts a cross-sectional view of a film in accordance with
one
embodiment of the present invention. The film 100 generally comprises a slip
layer 102,
a cling layer 104, and at least one intermediaiy layer 106 (also referred to
herein as a
"core" layer) positioned between the two surface layers. The slip layer 102
comprises
between about 5% by weight and about 20% by weight of the film 100. The cling
layer
104 comprises between about 3% by weight and about 20% by weight of the film
100. In
many of the embodiments, the balance of the film 100 is the intermediary layer
106.
[0012] The slip layer 102 generally comprises a polymer composition yielding a
low
coefficient of friction. The coefficient of friction of the slip layer 102 is
at least less than
about 0.9. In one embodiment, the coefficient of friction of the slip layer
102 is about
0.5.
[0013] The slip layer 102 generally comprises a polypropylene (PP) and high-
density
polyethylene (HDPE) composition. The PP may be provided in a range of about
60% by
weight to about 99% by weight, and the balance of the composition comprises
HDPE.
In one embodiment, the PP is provided in about 80% by weight and the HDPE is
provided in about 20% by weight of the slip layer 102. In another embodiment,
the PP
is provided in about 75% to about 95% by weight, the HDPE is provided in about
5% to
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about 25% by weight, and the balance comprises at least one resin or additive,
such as
abrasion-resistant resins, colorants, stabilizers, UV absorbers, and the like.
[0014] In one exemplary embodiment, the PP is a polypropylene copolymer
plastic
material commercially available from Basell Polyolefins of Hoofddorp, The
Netherlands, under the name Basell Pro-fax SA861. The PP has a density of
about 0.900
g/cm3, and a mass-flow rate (MFR) of about 6.50 g/10 min, as determined by
ASTM
test methods D792 and D1238, respectively.
[0015] In another exemplary embodiment, the HDPE is a high-density
polyethylene
plastic material, commercially available from the Westlake Chemical
Corporation of
Houslon, '1'exas, ui ler llte naine Westlake HDPE HC 6008. The HDPE has a
density of
about 0.962 g/cm3, and a MFR of about 8.2 g/10 min, as determined by ASTM test
methods D1505 and D1238, respectively.
[0016] The cling layer 104 generally comprises a polymer composition yielding
a
high cling. The cling force of the cling layer 104 to the slip layer 102, when
wrapped
around a good or load, is at least greater than about 5 g/in. In one
embodiment, the
cling force of the cling layer 104 to slip layer 102 is about 50 g/in.
[0017] The cling layer 104 generally comprises a styrenic block copolymer
(SBC).
The SBC may be provided in a range of about 50% by weight to about 100% by
weight.
In one embodiment, the SBC is provided in about 100% by weight of the slip
layer 102.
In another embodiment, the SBC is provided in about 50% to about 95% by
weight, and
the balance comprises at least an additional resin or additive, such as UV
protective
additives, puncture resistant resins, and the like.
[0018] In one exemplary embodiment, the SBC is a styrenic block copolymer
material commercially available from Kraton Polymers LLC of Houston, Texas,
under
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the name Kraton MD6718 Compound. The SBC has a density of about 0.9 g/ cm3,
and a
MFR of between about 2 g/10 min and about 25 g/10 min.
[0019] The cling layer 104 may further comprise additional resins and
additives. In
one embodiment, the cling layer 104 comprises a puncture-resistant resin. In
another
embodiment, the cling layer 104 may also comprise a low-density polyethylene
(LDPE)
resin having a density of between about 0.91 g/ cm3 and 0.94 g/ cm3, a LLDPE
resin
having a density of between about 0.915 g/cm3 and 0.925 g/cm3, or a very low-
density
polyethylene (VLDPE) resin having a density of between about 0.88 g/cm3 and
0.915
g/cm3.
[0020] In another embodiment of the present invention, the cling layer 104 may
additionally comprise an ethylene-octene copolymer resin having a density
between
about 0.850 g/cm3 and about 0.900 g/cm3. In an exemplary embodiment, the
ethylene-
octene copolymer is a copolymer, commercially available from the Dow Chemical
Company of Midland, Michigan, under the name Dow Affinity KC-8852. The
ethylene-
octene copolymer has a density of about 0.875 g/cm3 and a MFR of about 3.0
g/10 min,
as determined by ASTM test methods D792 and D1238, respectively.
[0021] Further examples of useful cling additives include polyisobutylenes
(PIB)
having a number average molecular weight in the range from about 1,000-3,000,
preferably about 1,200-1,800, as measured by vapor phase osmometry, amorphous
atactic polypropylenes, e.g., those having an average molecular weight of
about 2,000,
and polyterpenes. Examples of cling-enhancing resins include, but are not
limited to,
ethylene-vinyl acetate (EVA) copolymers containing from about 5% to about 15%
by
weight copolymerized vinyl acetate and VLDPE resins having densities from 0.88
g/cm3 to 0.912 g/cm3 and melt indexes from 0.5 g/10 min to 10 g/10 min. The
optional
cling additives may be present in the cling layer 104 in a concentration of
from about
0.5% by weight to about 10% by weight of the cling layer 104 composition.
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[0022] Optionally, in some embodiments, polyethylene resins are utilized in
the
cling layer 104, and are further blended with minor amounts, e.g., up to about
20% by
weight total, of one or more other suitable resins to achieve a desired range
of physical
and/or mechanical properties in the film product. For example, suitable resins
such as
ethyl vinyl acetate copolymer, and LLDPE resins may be useful when blended
with the
cling layer 104. In many embodiments, conventional film additives such as
antioxidants, UV stabilizers, pigments, dyes, etc., are utilized in the cling
layer 104 of
the film 100.
[0023] The intermediary layer 106 generally comprises one or more layers of
polymeric compositions yielding necessary mechanical properties of the film
100, such
as extensibility, load containment, tear and puncture resistance, and the
like. In one
embodiment, the intermediary layer 106 comprises a linear low-density
polyethylene
(LLDPE). In another embodiment, the intermediary layer 106 further comprises a
white
pigment such as titanium dioxide.
[0024] The LLDPE may be provided in a range of between about 50% and 100% by
weight. In particular embodiments, the titanium dioxide may be provided
between
about 0.1% to about 5% by weight. In one embodiment, the LLDPE is provided in
about 98% by weight, and the titanium dioxide is provided in about 2% by
weight of
the intermediary layer 106. In another embodiment, the composition may
additionally
comprise conventional additives, resins, and the like, to provide the desired
performance characteristics.
[0025] In one exemplary embodiment, the LLDPE is a linear low-density
polyethylene material commercially available from Dow Chemical Company of
Midland, Michigan, under the name Dowlex 2045G. The LLDPE has a density of
about
0.920 g/cm3 and a MFR of about 1.0 g/10 min, as measured by ASTM D792 and
D1238,
respectively. In another exemplary embodiment, the LLDPE is a linear low-
density
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polyethylene material commercially available from Dow Chemical Company of
Midland, Michigan, under the name Tuflin HSE-7002. The titanium dioxide is
provided
as an element of a white concentrate. In one embodiment, the titanium dioxide
is
contained within a white concentrate from Ampacet Corporation of Tarrytown,
New
York, under the name Ampacet White 110359-C.
[0026] Alternative embodiments of the present invention provide an
intermediary
layer 106 comprising at least two or more individual layers. It is understood
that
embodiments of the present invention may comprise as many individual layers in
the
intermediary layer 106 as necessary or feasible to achieve desired material
properties in
a film 100. For example, at least one embodiment of the present invention
provides a
three-layer film having a single intermediary layer, whereas an alternative
embodiment
provides for up to a fourteen layer film, having twelve individual layers in
the
intermediary layer. Additional embodiments provide for any number of layers
contained within the range of the above-disclosed embodiments, as well as any
number
of layers feasible within the scope of the embodiments of the present
invention.
[0027] The individual layers of the intermediary layer 106 may comprise
various
optional material compositions. Such materials include, but are not limited
to, any
feasible grade of LLDPE, metallocene-catalyzed LLDPE, Low-Density Polyethylene
(LDPE), Mid-Density Polyethylene (MDPE), High-Density Polyethylene (HDPE), or
Polypropylene (PP), including homopolymers, copolymers or blends of the above.
[0028] In one embodiment, the intermediary layer 106 comprises three layers
108,
110, 112. In one such embodiment, the three layers comprise an A-B-A
structure,
wherein A is a layer of LLDPE and wherein B is a layer of LLDPE and a white
concentrate.
[0029] In one embodiment of the present invention, a multilayer film comprises
a
slip layer comprising a polypropylene copolymer and high-density polyethylene
blend,
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a cling layer comprising a styrenic block copolymer and ethylene-octene
copolymer
blend, and an intermediary layer comprising at least three individual layers,
comprising
at least two layers of linear low-density polyethylene, and a layer of a
linear low-
density polyethylene and additive blend.
[0030] Additives and/or resins may be provided in the intermediary layer 106,
such
as those discussed above with respect to the slip layer 102 and the cling
layer 104.
Additional resins include polyolefin homopolymers and copolymers suitable for
enhancement of mechanical properties such as tensile strength, elongation at
break, tear
and puncture resistance.
[00311 In order to ensure the film 100 meets the desired parameters in
accordance
with embodiments of the present invention, testing may be done to determine
its
physical properties. A parameter used to analyze the performance of such films
is the
force required to stretch the film to a desired percentage of its original
length. This
force is indicative of the load retention characteristics of the film. The
films of
embodiments of the present invention generally have a force to stretch the
film to 100%
of at least about 5 pli (pounds per linear inch). In many embodiments, the
films
generally have tensile strength of at least about 5000 psi and elongation to
break of at
least about 400%, as determined by ASTM D882.
[0032] Generally, embodiments of the film 100 are constructed according to
conventional practices. The film 100 may be manufactured utilizing blown-film
or cast-
film co-extrusion. In one embodiment, a cast-film manufacturing process
provides
resin materials are heated to their molten state, and their viscosities are
coordinated to
prepare multilayer films in a uniform manner. The molten materials are
conveyed to a
co-extrusion adapter that combines the molten materials to form a multilayer
co-
extruded structure. The layered polymeric material is transferred through an
extrusion
die opened to a predetermined gap commonly in the range of between about 0.05
in
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(0.13 cm) and 0.012 in (0.03 cm). The material is then drawn down to the
intended
gauge thickness by means of a primary chill or casting roll maintained at
between about
50 F to about 130 F. Typical draw down ratios range from between about 5:1 to
about
40:1.
[0033] The overall thickness of the stretch wrap film can vary widely
according to
end use specifications, but is generally in the range of the typical
thicknesses for stretch
wrap films. In one embodiment of the present invention, the overall film 100
thickness
is between about 0.012 mm to about 0.125 mm. In one embodiment, the film 100
thickness is about 0.025 mm.
[0034] Once manufactured, in some embodiments, the slip layer 102 and/or the
cling layer 104 of the film can be post-formation treated by implementing
operations
such as corona discharge, chemical treatment, flame treatment, etc., to modify
the
printability or ink receptivity of the surface(s) or to impart other desirable
characteristics thereto.
[0035] In accordance with embodiments of the invention, the film may be
provided
in a non-stretched, i.e., unoriented, or merely a modestly stretched state
prior to use.
The film 100 is capable of being stretched from at least between about 40% and
400%
during a final goods wrapping step.
[0036] In accordance with embodiments of the present invention, the film shp
properties are unexpectedly improved as the film is stretched. These
advantageous
results may be a proximate result of the specific PP and HDPE compositions
utilized in
the slip layer with several embodiments of the present invention.
Specifically, as the
film is stretched, microscopic patterns appear on the slip layer film surface
reducing
film smoothness as well as the contact area. As a result, the slip surface of
the described
embodiments visibly loses its relatively glossy appearance when the film is
stretched to
about 100%. In one example, the static COF changed from about 0.65 before a
stretch to
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about 0.49 after the stretch. As is understood by those of ordinary skill in
the art, low
friction is advantageous for ease of moving and for reduction of possible
damage to
items wrapped in a film in accordance with embodiments of the present
invention.
[0037] While the foregoing is directed to embodiments of the present
invention,
other and further embodiments of the invention may be devised without
departing
from the basic scope thereof.
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