U.S. patent application number 10/770339 was filed with the patent office on 2005-08-04 for multilayer high clarity shrink film comprising monovinylarene-conjugated diene copolymer.
Invention is credited to Brown, J. Mark, Gange, Dale E., Keane, J. Alex.
Application Number | 20050166551 10/770339 |
Document ID | / |
Family ID | 34808308 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050166551 |
Kind Code |
A1 |
Keane, J. Alex ; et
al. |
August 4, 2005 |
Multilayer high clarity shrink film comprising
monovinylarene-conjugated diene copolymer
Abstract
We disclose a shrink film comprising a first layer comprising a
monovinylarene-conjugated diene copolymer; a second layer
comprising low density polyethylene (LDPE) and linear low density
polyethylene (LLDPE); and a third layer comprising a
monovinylarene-conjugated diene copolymer; wherein the second layer
is disposed between the first layer and the third layer. We also
disclose methods of using the shrink film to prepare bundled or
fully enclosed groups of objects.
Inventors: |
Keane, J. Alex; (The
Woodlands, TX) ; Gange, Dale E.; (Pompano Beach,
FL) ; Brown, J. Mark; (Bartlesville, OK) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON, P.C.
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Family ID: |
34808308 |
Appl. No.: |
10/770339 |
Filed: |
February 2, 2004 |
Current U.S.
Class: |
53/442 |
Current CPC
Class: |
B32B 2307/746 20130101;
B32B 27/302 20130101; B32B 27/08 20130101; B32B 27/32 20130101;
B32B 7/12 20130101; B32B 2307/736 20130101; B32B 27/30
20130101 |
Class at
Publication: |
053/442 |
International
Class: |
B65B 053/02 |
Claims
What is claimed is:
1. A shrink film, comprising: a first layer comprising a
monovinylarene-conjugated diene copolymer; a second layer
comprising low density polyethylene (LDPE) and linear low density
polyethylene (LLDPE); and a third layer comprising a
monovinylarene-conjugated diene copolymer; wherein the second layer
is disposed between the first layer and the third layer.
2. The shrink film of claim 1, wherein the first layer and the
third layer together comprise from about 10 wt % to about 40 wt %
of the shrink film, and the second layer comprises from about 30 wt
% to about 80 wt % of the shrink film.
3. The shrink film of claim 1, further comprising a first tie layer
between the first layer and the second layer, a second tie layer
between the third layer and the second layer, or both.
4. The shrink film of claim 3, wherein the first tie layer
comprises an ethylene-vinyl acetate copolymer (EVA) or an
anhydride-modified EVA.
5. The shrink film of claim 3, wherein the second tie layer
comprises an ethylene-vinyl acetate copolymer (EVA) or an
anhydride-modified EVA.
6. The shrink film of claim 1, wherein the
monovinylarene-conjugated diene copolymer is a styrene-butadiene
block copolymer.
7. The shrink film of claim 1, wherein the first layer, the third
layer, or both further comprise polystyrene (PS).
8. The shrink film of claim 1, wherein the first layer, the third
layer, or both further comprise an antiblock agent, a slip agent,
or both.
9. The shrink film of claim 8, wherein the antiblock agent is a
high impact polystyrene (PS).
10. The shrink film of claim 1, wherein the LDPE is a clarity-grade
LDPE.
11. The shrink film of claim 1, wherein the LLDPE is a
metallocene-catalyzed LLDPE (mLLDPE).
12. A method of bundling a group of objects, comprising: wrapping
the group of objects with a shrink film comprising a first layer
comprising a monovinylarene-conjugated diene copolymer; a second
layer comprising low density polyethylene (LDPE) and linear low
density polyethylene (LLDPE); and a third layer comprising a
monovinylarene-conjugated diene copolymer; wherein the second layer
is disposed between the first layer and the third layer and the
shrink film has a higher shrink in a first direction than in a
second direction, to yield a wrapped group of objects, and heating
the wrapped group of objects to a temperature and for a duration
sufficient to shrink the shrink film, to yield a bundled group of
objects.
13. The method of claim 12, wherein in the shrink film the first
layer and the third layer together comprise from about 10 wt % to
about 40 wt % of the shrink film, and the second layer comprises
from about 30 wt % to about 80 wt % of the shrink film.
14. The method of claim 12, wherein the shrink film further
comprises a first tie layer between the first layer and the second
layer, a second tie layer between the third layer and the second
layer, or both.
15. The method of claim 12, wherein in the shrink film the
monovinylarene-conjugated diene copolymer is a styrene-butadiene
block copolymer.
16. A method of fully enclosing a group of objects, comprising:
wrapping the group of objects with a shrink film comprising a first
layer comprising a monovinylarene-conjugated diene copolymer; a
second layer comprising low density polyethylene (LDPE) and linear
low density polyethylene (LLDPE); and a third layer comprising a
monovinylarene-conjugated diene copolymer; wherein the second layer
is disposed between the first layer and the third layer and the
shrink film has substantially similar shrink in both a first
direction and a second direction, to yield a wrapped group of
objects, and heating the wrapped group of objects to a temperature
and for a duration sufficient to shrink the shrink film, to yield a
fully enclosed group of objects.
17. The method of claim 16, wherein in the shrink film the first
layer and the third layer together comprise from about 10 wt % to
about 40 wt % of the shrink film, and the second layer comprises
from about 30 wt % to about 80 wt % of the shrink film.
18. The method of claim 16, wherein the shrink film further
comprises a first tie layer between the first layer and the second
layer, a second tie layer between the third layer and the second
layer, or both.
19. The method of claim 16, wherein in the shrink film the
monovinylarene-conjugated diene copolymer is a styrene-butadiene
block copolymer.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to the field of
monovinylarene-conjugated diene block copolymers. More
particularly, it concerns multilayer shrink films comprising such
copolymers and polyethylenes.
[0002] A variety of shrink films have been available for the
packaging industry. In bundling a group of objects, a film is
wrapped around the group of objects and then heat is applied,
typically in a heat tunnel, and the film shrinks, unitizing the
contents and providing rigidity and protection during handling. In
bundling a group of objects, the film used generally only
substantially shrinks in one direction, and thus the ends of
bundled packages are only enclosed by the shrinking of the loose
film edges, which produces what the industry calls a "bullseye."
Also known is fully enclosing a group of objects, which involves
generally the same technique, with a difference in using a film
which generally shrinks in both directions.
[0003] Films known commercially for bundling a group of objects or
fully enclosing a group of objects include monolayer polyethylene
(PE) films, which have limitations of clarity and gloss due to the
nature of the polyethylene molecule. Various types and grades of
ethylene homo- and copolymers have been used.
[0004] Among the clearest known PE films are low density
polyethylene (LDPE) films. However, LDPE films do not have
sufficient strength and puncture resistance for some packaging
applications.
[0005] In order to overcome the lower strength of LDPE films, films
containing a blend of both LDPE and linear low density polyethylene
(LLDPE) have also been used commercially. An LDPE+LLDPE film
generally has increased strength relative to an LDPE film, but
often have reduced clarity and shrink. The increased strength of an
LDPE+LLDPE film has allowed reductions in the thickness of the
films, which may improve clarity and reduce film costs, but reduces
film stiffness.
[0006] In order to increase the stiffness and strength of thinner
films, triblends of LDPE, LLDPE, and high density polyethylene
(HDPE) have been used commercially. While an LDPE+LLDPE+HDPE film
does have increased stiffness relative to an LDPE+LLDPE film, it
generally has both lower clarity and lower gloss. It is the nature
of HDPE to produce a film with higher haze and poorer gloss.
[0007] The current state of the art regarding polyethylene films
involves the use of coextruded polyethylene films. These films may
comprise LDPE outer layers and blends of LLDPE+HDPE in the core.
Such films are reasonably glossy and clear, and have the stiffness
to process in commonly available shrink bundling machinery.
[0008] However, in the interests of reducing cost, reducing
materials consumption, and providing improved products, a need
remains for materials with good visual properties (i.e., high gloss
and low haze), good physical properties (i.e., higher strength and
stiffness), good shrink properties, or some combination
thereof.
SUMMARY OF THE INVENTION
[0009] In one embodiment, the present invention relates to a shrink
film comprising a first layer comprising a
monovinylarene-conjugated diene copolymer; a second layer
comprising low density polyethylene (LDPE) and linear low density
polyethylene (LLDPE); and a third layer comprising a
monovinylarene-conjugated diene copolymer; wherein the second layer
is disposed between the first layer and the third layer.
[0010] In another embodiment, the present invention relates to a
method of bundling a group of objects, comprising wrapping the
group of objects with a shrink film as described above, wherein the
shrink film has a higher shrink in a first direction than in a
second direction, to yield a wrapped group of objects, and heating
the wrapped group of objects to a temperature and for a duration
sufficient to shrink the shrink film, to yield a bundled group of
objects.
[0011] In an additional embodiment, the present invention relates
to a method of fully enclosing a group of objects, comprising
wrapping the group of objects with a shrink film as described
above, wherein the shrink film has substantially similar shrink in
both a first direction and a second direction, to yield a wrapped
group of objects, and heating the wrapped group of objects to a
temperature and for a duration sufficient to shrink the shrink
film, to yield a fully enclosed group of objects.
[0012] The present invention can provide shrink films having visual
properties (such as gloss and haze), physical properties (such as
strength and stiffness), or shrink properties comparable to or
superior to known shrink films of the same or similar thickness,
such as shrink films having an LDPE/LLDPE+HDPE/LDPE three-layer
structure.
DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a cross-sectional view of a portion of a film
according to one embodiment of a shrink film according to the
present invention.
[0014] FIG. 2 shows a cross-sectional view of a portion of a film
according to another embodiment of a shrink film according to the
present invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0015] In one embodiment, the present invention relates to a shrink
film, comprising:
[0016] a first layer comprising a monovinylarene-conjugated diene
copolymer;
[0017] a second layer comprising low density polyethylene (LDPE)
and linear low density polyethylene (LLDPE); and
[0018] a third layer comprising a monovinylarene-conjugated diene
copolymer;
[0019] wherein the second layer is disposed between the first layer
and the third layer.
[0020] Unless specified to the contrary or apparent from the plain
meaning of a phrase, the word "or" has the inclusive meaning. The
adjectives "first," "second," and so forth are not to be construed
as limiting the modified subjects to a particular order in time,
space, or both, unless specified to the contrary or apparent from
the plain meaning of a phrase. A "copolymer" is used herein to
refer to any polymer comprising at least two types of units, e.g.,
two types of units, three types of units, etc.
[0021] The basic starting materials and polymerization conditions
for preparing monovinylarene-conjugated diene copolymers are
disclosed in, e.g., U.S. Pat. Nos. 4,091,053; 4,584,346; 4,704,434;
4,704,435; 5,227,419; 6,265,484; and 6,265,485.
[0022] "Monovinylarene," as used herein, refers to an organic
compound containing a single carbon-carbon double bond, at least
one aromatic moiety, and a total of 8 to 18 carbon atoms, such as 8
to 12 carbon atoms. Exemplary monovinylarenes include, but are not
limited to, styrene, alpha-methylstyrene, 2-methylstyrene,
3-methylstyrene, 4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene,
4-ethylstyrene, 4-n-propylstyrene, 4-t-butylstyrene,
2,4-dimethylstyrene, 4-cyclohexylstyrene, 4-decylstyrene,
2-ethyl-4-benzylstyrene, 4-(4-phenyl-n-butyl)styrene,
1-vinylnaphthalene, 2-vinylnaphthalene, and mixtures thereof. In
one embodiment, the monovinylarene is styrene. A unit of polymer,
wherein the unit is derived from polymerization of a monovinylarene
monomer, is a "monovinylarene unit."
[0023] "Conjugated diene," as used herein, refers to an organic
compound containing conjugated carbon-carbon double bonds and a
total of 4 to 12 carbon atoms, such as 4 to 8 carbon atoms.
Exemplary conjugated dienes include, but are not limited to,
1,3-butadiene, 2-methyl-1,3-butadiene, 2-ethyl-1,3-butadiene,
2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 3-butyl-1,3-octadiene,
and mixtures thereof. In one embodiment, the conjugated diene can
be 1,3-butadiene or isoprene. A unit of polymer, wherein the unit
is derived from polymerization of a conjugate diene monomer, is a
"conjugated diene unit."
[0024] A "monovinylarene-conjugated diene copolymer" is a polymer
comprising monovinylarene units and conjugated diene units. The
polymer can be a block copolymer, that is, can comprise one or more
blocks, wherein each block comprises monovinylarene units or
conjugated diene units. Any particular block can comprise either or
both monovinylarene units or conjugated diene units. If it
comprises both, it can be a random block, a tapered block, a
stepwise block, or any other type of block.
[0025] A block is "random" when the mole fractions of conjugated
diene units and monovinylarene units in a section of the block are
substantially the same as the mole fractions of conjugated diene
units and monovinylarene units in the entire block. This does not
preclude the possibility of sections of the block having regularity
(i.e., appearing non-random), but such regular sections will
typically be present at no more than about the level expected by
chance.
[0026] A block is "tapered" when both (a) the mole fraction of
conjugated diene units in a first section of the block is higher
than the mole fraction of conjugated diene units in a second
section of the block, wherein the second section of the block is
closer to a given end of the block and (b) condition (a) is true
for substantially all sections of the block. (Depending on the size
of the sections being considered, condition (a) may not be true for
all sections, but if so, will be not true at no more than about the
level expected by chance).
[0027] A block is "stepwise" when a first section of the block
contains substantially all monovinylarene units of the block and a
second section of the block contains substantially all conjugated
diene units of the block.
[0028] In one embodiment, the monovinylarene-conjugated diene
copolymer is a block copolymer comprising styrene blocks and
butadiene blocks (a "styrene-butadiene block copolymer"). Exemplary
styrene-butadiene copolymers are commercially available under the
name K-Resin.RTM. (Chevron Phillips Chemical Co., The Woodlands,
Tex.).
[0029] The monovinylarene-conjugated diene copolymer can have any
proportion of monovinylarene units and conjugated diene units. In
one embodiment, the monovinylarene-conjugated diene copolymer has
from about 50 wt %:50 wt % monovinylarene units:conjugated diene
units to about 90 wt %: 10 wt % monovinylarene units:conjugated
diene units.
[0030] The monovinylarene-conjugated diene copolymer can further
comprise other units known in the art for inclusion in
monovinylarene-conjugated diene copolymers.
[0031] Generally, each block is formed by polymerizing the monomer
or mixture of monomers from which the desired units of the block
are derived. The polymerization process will generally be amenable
to a relative lack of change in process parameters between
different blocks, but the skilled artisan, having the benefit of
the present disclosure, may make some minor changes in process
parameters between different blocks as a matter of routine
experimentation. The following descriptions of the polymerization
process will generally apply to the formation of all types of
blocks in the inventive polymer, although certain descriptions may
be of more or less value to forming one or more of the types of
blocks in the inventive polymer.
[0032] The polymerization process can be carried out in a
hydrocarbon diluent at any suitable temperature in the range of
from about -100.degree. C. to about 150.degree. C., such as from
about 0.degree. C. to about 150.degree. C., and at a pressure
sufficient to maintain the reaction mixture substantially in the
liquid phase. In one embodiment, the hydrocarbon diluent can be a
linear or cyclic paraffin, or mixtures thereof. Exemplary linear or
cyclic paraffms include, but are not limited to, pentane, hexane,
octane, cyclopentane, cyclohexane, and mixtures thereof, among
others. In one embodiment, the paraffin is cyclohexane.
[0033] The polymerization process can be carried out in the
substantial absence of oxygen and water, such as under an inert gas
atmosphere.
[0034] The polymerization process can be performed in the presence
of an initiator. In one embodiment, the initiator can be any
organomonoalkali metal compound known for use as an initiator. In a
further embodiment, the initiator can have the formula RM, wherein
R is an alkyl, cycloalkyl, or aryl radical containing 4 to 8 carbon
atoms, such as an n-butyl radical, and M is an alkali metal, such
as lithium. In a particular embodiment, the initiator is n-butyl
lithium.
[0035] The amount of initiator employed depends upon the desired
polymer or block molecular weight, as is known in the art and is
readily determinable, making due allowance for traces of poisons in
the feed streams.
[0036] The polymerization process can further involve the inclusion
of a randomizer. In one embodiment, the randomizer can be a polar
organic compound, such as an ether, a thioether, or a tertiary
amine. In another embodiment, the randomizer can be a potassium
salt or a sodium salt of an alcohol. The randomizer can be included
in the hydrocarbon diluent to improve the effectiveness of the
initiator, to randomize at least part of the monovinylarene monomer
in a mixed monomer charge, or both. The inclusion of a randomizer
can be of value when forming a random or tapered
monovinylarene-conjugated diene block of the present polymer.
[0037] Exemplary randomizers include, but are not limited to,
dimethyl ether, diethyl ether, ethyl methyl ether, ethyl propyl
ether, di-n-propyl ether, di-n-octyl ether, anisole, dioxane,
1,2-dimetboxyethane, dibenzyl ether, diphenyl ether,
1,2-dimethoxybenzene, tetramethylene oxide (tetrahydrofuran or
THF), potassium tert-amylate (KTA), dimethyl sulfide, diethyl
sulfide, di-n-propyl sulfide, di-n-butyl sulfide, methyl ethyl
sulfide, dimethylethylamine, tri-n-ethylamine, tri-n-propylamine,
tri-n-butylamine, trimethylanine, triethylamine,
tetramethylethylenediami- ne, tetraethylethylenediamine,
N,N-di-methylaniline, N-methyl-N-ethylaniline, N-methylmorpholine,
and mixtures thereof, among others.
[0038] When forming a particular block, each monomer charge or
monomer mixture charge can be polymerized under solution
polymerization conditions such that the polymerization of each
monomer charge or monomer mixture charge, to form the particular
block, is substantially complete before charging a subsequent
charge. "Charging," as used herein, refers to the introduction of a
compound to a reaction zone, such as the interior of a reactor
vessel.
[0039] Though not to be bound by theory, if an initiator is
included in a charge, a block will typically form either de novo or
by addition to the end of an unterminated, previously-formed,
block. Further not to be bound by theory, if an initiator is not
included in a charge, a block will typically only form by addition
to the end of an unterminated, previously-formed, block.
[0040] A coupling agent can be added after polymerization is
complete. Suitable coupling agents include, but are not limited to,
di- or multivinylarene compounds; di- or multiepoxides; di- or
multiisocyanates; di- or multiimines; di- or multialdehydes; di- or
multiketones; alkoxytin compounds; di- or multihalides, such as
silicon halides and halosilanes; mono-, di-, or multianhydrides;
di- or multiesters, such as the esters of monoalcohols with
polycarboxylic acids; diesters which are esters of monohydric
alcohols with dicarboxylic acids; diesters which are esters of
monobasic acids with polyalcohols such as glycerol; and mixtures of
two or more such compounds, among others.
[0041] Useful multifunctional coupling agents include, but are not
limited to, epoxidized vegetable oils such as epoxidized soybean
oil, epoxidized linseed oil, and mixtures thereof, among others. In
one embodiment, the coupling agent is epoxidized soybean oil.
Epoxidized vegetable oils are commercially available under the
trademark Vikoflex.RTM. from Atofina Chemicals (Philadelphia,
Pa.).
[0042] If coupling is to be performed, any effective amount of the
coupling agent can be employed. In one embodiment, a stoichiometric
amount of the coupling agent relative to active polymer alkali
metal tends to promote maximum coupling. However, more or less than
stoichiometric amounts can be used for varying coupling efficiency
where desired for particular products.
[0043] Following completion of the coupling reaction, if any, the
polymerization reaction mixture can be treated with a terminating
agent such as water, carbon dioxide, alcohol, phenols, or linear
saturated aliphatic mono-dicarboxylic acids, to remove alkali metal
from the block copolymer or for color control.
[0044] After termination, if any, the polymer cement (polymer in
polymerization solvent) usually contains about 10 to 40 weight
percent solids, more usually 20 to 35 weight percent solids. The
polymer cement can be flashed to evaporate a portion of the solvent
so as to increase the solids content to a concentration of about 50
to about 99 weight percent solids, followed by vacuum oven or
devolatilizing extruder drying to remove the remaining solvent.
[0045] The block copolymer can be recovered and worked into a
desired shape, such as by milling, extrusion, or injection molding.
The block copolymer can also contain additives such as
antioxidants, antiblocking agents, release agents, slip agents,
fillers, extenders, dyes, or the like.
[0046] In one embodiment, the antiblocking agent is a high impact
polystyrene (HIPS), by which is meant a composition comprising any
graft copolymer of styrene and butadiene. By "graft copolymer" is
meant polystyrene produced by polymerizing styrene in the presence
of an unsaturated rubber wherein the rubber becomes dispersed
throughout the polystyrene in the form of discrete domains. In one
embodiment the unsaturated rubber is polybutadiene.
[0047] In the present invention, the monovinylarene-conjugated
diene copolymer can be monomodal, that is, a population of
copolymer molecules can have one peak in a histogram of the
population's molecular weight distribution, or it can be polymodal,
that is, have two or more peaks in a histogram of the copolymer
molecules' population's molecular weight distribution.
[0048] In the present invention, the monovinylarene-conjugated
diene copolymer can be coupled or uncoupled, as described
above.
[0049] As stated above, the first layer and the third layer
comprise a monovinylarene-conjugated diene copolymer. In a further
embodiment, either or both of the first layer or the third layer
can further comprise polystyrene (PS). As used herein,
"polystyrene" refers to any homo- or copolymer comprising styrene
units. The first layer and the third layer can each independently
comprise from 0 wt % PS to about 75 wt % PS. In one embodiment, the
first layer and the third layer can each independently comprise
from 0 wt % PS to about 50 wt % PS.
[0050] In the present invention, the monovinylarene-conjugated
diene block copolymer can have any number of tapered blocks. In one
embodiment, the monovinylarene-conjugated diene block copolymer has
zero tapered blocks. In another embodiment, the
monovinylarene-conjugated diene block copolymer has at least one
tapered block.
[0051] The first layer and the third layer can be identical in
composition, or can differ in composition, such as by use of
different monovinylarene-conjugated diene copolymers, different
proportions of monovinylarene units and conjugated diene units in
the copolymers, the presence or absence of different additives
(such as PS), or other differences as will be apparent to the
skilled artisan having the benefit of the present disclosure.
[0052] The shrink film also comprises a second layer comprising low
density polyethylene (LDPE) and linear low density polyethylene
(LLDPE).
[0053] The LDPE in the second layer can be any branched homopolymer
containing ethylene units. Typically, the LDPE has a density of
from about 0.922 g/cm.sup.3 to about 0.924 g/cm.sup.3 and a melt
index (MI) from about 0.25 g/10 min to about 2.0 g/10 min (ASTM
D1238). LDPE can be made by any process known in the art.
[0054] In one embodiment, the LDPE is a clarity-grade LDPE. By
"clarity-grade" is meant an LDPE having an MI greater than about
1.0 g/10 min and a haze less than about 5% for a 1 mil thick film
consisting of the LDPE.
[0055] The LLDPE in the second layer can be any linear copolymer
comprising ethylene units and .alpha.-olefin units. Typically,
LLDPEs have densities of from about 0.915 g/cm.sup.3 to about 0.924
g/cm.sup.3 and MI values from about 0.5 g/10 min to about 1.5 g/10
min (ASTM D1238), although this is an observation and not a
statement limiting the present invention. In one embodiment, the
a-olefin is selected from the group consisting of 1-butene,
1-pentene, 1-hexene, 1-heptene, and 1-octene.
[0056] The LLDPE can be produced by any technique, such as
Ziegler-Natta polymerization or metallocene-catalyzed
polymerization, both of which are known in the art. In one
embodiment, the LLDPE is produced by metallocene-catalyzed
polymerization. A metallocene-catalyzed LLDPE can be referred to
herein as "mLLDPE." We have observed that mLLDPE typically has a
narrower distribution of polymer molecular weights and lower haze
than LLDPEs prepared by other techniques, although this is an
observation, and not a statement limiting the present
invention.
[0057] Any proportion of LDPE to LLDPE can be used in the second
layer. In one embodiment, the second layer comprises greater than
about 50 wt % LDPE.
[0058] The second layer can also comprise other materials, such as
other polymers, for example, high density polyethylene (HDPE; an
ethylene homopolymer having a density greater than about 0.940
g/cm.sup.3 and an MI of from about 0.25 g.10 min to about 1.5
g/10min (ASTM D1238)), very low density polyethylene (VLDPE; a
copolymer of ethylene and an .alpha.-olefin having a density less
than about 0.912 g/cm.sup.3), or other polyethylenes, as well as
other additives.
[0059] In the shrink film, the first layer and the third layer
together can comprise from about 10 wt % to about 40 wt % of the
shrink film. This wt % is the total over both layers. The first
layer and the third layer can comprise equal weight portions of the
shrink film, or they can comprise unequal weight portions of the
shrink film. The second layer can comprise from about 30 wt % to
about 80 wt % of the shrink film. As will be apparent to the
skilled artisan having the benefit of the present disclosure, the
total wt % of the three layers cannot exceed 100 wt % of the shrink
film. In the event the total wt % of the three layers is less than
100 wt %, it will be apparent that the shrink film comprises one or
more additional layers.
[0060] As stated above, the second layer is disposed between the
first layer and the third layer. It can be directly disposed
therebetween, or a tie layer or layers can be used to facilitate
adhesion between the second layer and either or both of the first
layer and the third layer.
[0061] A cross-sectional view of a portion of a shrink film
according to one embodiment of the present invention is shown in
FIG. 1. The first layer 10 and the third layer 12 sandwich the
second layer 14 (i.e., the second layer 14 is directly disposed
between the first layer 10 and the third layer 12). FIG. 1 is not
necessarily to scale.
[0062] A cross-sectional view of a portion of a shrink film
according to another embodiment of the present invention is shown
in FIG. 1. In this embodiment, tie layer 20 facilitates adhesion
between the first layer 10 and the second layer 14, and tie layer
22 facilitates adhesion between the third layer 12 and the second
layer 14. FIG. 2 is not necessarily to scale.
[0063] In one embodiment, the shrink film further comprises a first
tie layer between the first layer and the second layer, a second
tie layer between the third layer and the second layer, or both.
The tie layer or each tie layer, if more than one, can
independently comprise an ethylene-vinyl acetate copolymer (EVA) or
an anhydride-modified EVA. An exemplary anhydride-modified EVA is
Bynel.RTM. (Dupont, Wilmington, Del.).
[0064] The shrink film can be produced by any technique known in
the art of monolayer and coextruded film making. Such techniques
include milling, coextrusion, blow molding, injection molding, or
cast molding. Generally, the shrink film can be produced by blown
or cast film techniques. For example, the shrink film can be
produced using conventional extrusion techniques such as a
coextruded cast film. In coextrusion, two or more polymers are
simultaneously extruded through one die. Two or more extruders are
used simultaneously to feed the die. In this process, various
polymer melts are introduced into the die under conditions of
laminar flow such that there is no intermixing, but bonding occurs
at the interface between the film layers.
[0065] In a cast process, molten material flows from a flat die
across the width of the line and onto a chilled drum, which cools
the molten material. It is then trimmed and wound on a final drum
into rolls of film. In one embodiment, orientation can be
introduced into the film by stretching the film prior to winding on
the final drum. In another embodiment, orientation can be
introduced by stretching as the material is pulled from the
die.
[0066] In a blown film process, while the extrusion process
upstream of the die is similar to the cast process, the die and
downstream are different. In the blown film process, the die is
annular (circular) and typically points upward. This produces a
cylindrical tube, which can then be closed at the top (collapsed),
resulting in a flattened tube; or the tube can be inflated and
stretched to introduce orientation. This tube can have its edges
removed and then be wound into separate rolls of film.
[0067] Generally, the shrink film can have a machine direction (the
direction in which the shrink film comes off the production
apparatus) and a transverse direction (the direction perpendicular
to the machine direction).
[0068] During or after preparation of the shrink film, it can be
oriented, that is, stretched in at least one direction. One example
of orienting is post-resin conversion on a tentering frame,
although other techniques can be used. If stretched in one
direction, the film can be stretched in either the machine
direction or the transverse direction. Typically, a cast film has a
higher shrink in the machine direction than in the transverse
direction, but this is solely an observation of typical films, and
not a limiting description of the invention.
[0069] In one embodiment, a typical shrink film according to the
present invention can have a thickness of about 0.5 mil to about
3.0 mil, and at such a thickness it can have visual properties
(such as gloss and haze), physical properties (such as strength and
stiffness), or shrink properties comparable to or superior to known
shrink films of the same or similar thickness and not comprising
monovinylarene-conjugated diene copolymers.
[0070] In another embodiment, the present invention relates to a
method of bundling a group of objects, comprising:
[0071] wrapping the group of objects with a shrink film comprising
a first layer comprising a monovinylarene-conjugated diene
copolymer; a second layer comprising low density polyethylene
(LDPE) and linear low density polyethylene (LLDPE); and a third
layer comprising a monovinylarene-conjugated diene copolymer;
wherein the second layer is disposed between the first layer and
the third layer and the shrink film has a higher shrink in a first
direction than in a second direction, to yield a wrapped group of
objects, and
[0072] heating the wrapped group of objects to a temperature and
for a duration sufficient to shrink the shrink film, to yield a
bundled group of objects.
[0073] The shrink film can be as described above. In this
embodiment, the shrink film has a higher shrink in a first
direction than in a second direction. If oriented in one direction,
the first direction can be the machine direction or the transverse
direction. The second direction would then be the other of the
machine direction or the transverse direction.
[0074] Any group of objects for which bundling is desired can be
used in this method. In one embodiment, the group of objects is a
group of bottles, cans, or other discrete objects, optionally
contained in a tray.
[0075] In the wrapping step, the shrink film is disposed in a
substantially cylindrical manner around the group of objects. The
direction of disposing can be chosen as a routine matter for the
skilled artisan having the benefit of the present disclosure,
depending on the objects, the structure of the shrink film, and the
desired structure of the bundled group of objects.
[0076] The result of the wrapping step is a wrapped group of
objects.
[0077] After wrapping, the wrapped group of objects can be heated
to a temperature and for a duration sufficient to shrink the shrink
film. The temperature and the duration are a matter of routine
experimentation for the skilled artisan having the benefit of the
present disclosure. Because the shrink film of this embodiment has
a higher shrink in a first direction than a second direction, the
shrink film will typically only shrink in the first direction. In
one embodiment, the shrink in the first direction is at least about
40%. Shrinking will typically proceed until the film has shrunk in
the first direction to contact the group of objects.
[0078] In another embodiment, the present invention relates to a
method of fully enclosing a group of objects, comprising:
[0079] wrapping the group of objects with a shrink film comprising
a first layer comprising a monovinylarene-conjugated diene
copolymer; a second layer comprising low density polyethylene
(LDPE) and linear low density polyethylene (LLDPE); and a third
layer comprising a monovinylarene-conjugated diene copolymer;
wherein the second layer is disposed between the first layer and
the third layer and the shrink film has substantially similar
shrink in both a first direction and a second direction, to yield a
wrapped group of objects, and
[0080] heating the wrapped group of objects to a temperature and
for a duration sufficient to shrink the shrink film, to yield a
fully enclosed group of objects.
[0081] The group of objects can be any group of objects for which
full enclosure is desired. The shrink film can be as described
above.
[0082] The wrapping step can be as described above.
[0083] The heating step can be as described above. Because the
shrink film of this embodiment has substantially similar shrink in
both a first direction and a second direction, the shrink film will
typically shrink in both directions. ("Substantially similar
shrink" in this embodiment means the shrink in the first direction
is no more or no less than about 2-fold greater or less than the
shrink in the second direction). In one embodiment, the shrink in
the first direction is at least about 40% and the shrink in the
second direction is at least about 40%. Shrinking will typically
proceed until the film has shrunk in both directions to contact the
package.
[0084] The following examples are included to demonstrate specific
embodiments of the invention. It should be appreciated by those of
skill in the art that the techniques disclosed in the examples
which follow represent techniques discovered by the inventor to
function well in the practice of the invention. However, those of
skill in the art should, in light of the present disclosure,
appreciate that many changes can be made in the specific
embodiments which are disclosed and still obtain a like or similar
result without departing from the spirit and scope of the
invention.
EXAMPLE 1
[0085] Several example and comparative films were produced. The
example films comprised an A/B/A structure, wherein the A layers
comprised styrene-butadiene block copolymer (K-Resin.RTM., Chevron
Phillips) and polystyrene, and the B layers comprised LDPE and an
mLLDPE. The films were oriented after production. In the following
tables, "MD" refers to machine direction and "TD" refers to
transverse direction.
1 Example 1A Thickness (mil) 2.4 Secant Modulus (psi) MD 96,000 TD
87,000 Shrink Ratio % MD 65% TD 0% Gloss % (45 degree) 102% Haze %
5% Example 1B Thickness (mil) 2.43 Secant Modulus (psi) MD 72,000
TD 68,000 Shrink Ratio % MD 69% TD 5% Gloss % (45 degree) 106% Haze
% 5%
[0086] Example 1B was evaluated and found to completely and
satisfactorily shrink full cases of bottled water.
2 Example 1C Thickness (mil) 1.6 Secant Modulus (psi) MD 69,000 TD
65,000 Shrink Ratio % MD 66% TD 17% Gloss % (45 degree) 101% Haze %
5%
[0087] Comparative Examples were generally C/D/C structures,
wherein the C layers comprised LDPE and the D layers contained
blends of LLDPE and HDPE. The Examples generally had higher gloss
and lower haze than the Comparative Examples, as well as higher
toughness at lower thickness.
3 Comparative Example C1A Comparative Example C1B Thickness (mil)
2.5 Thickness (mil) 2.59 Secant Modulus (psi) MD 33,000 Secant
Modulus (psi) MD 46,000 TD 38,000 TD 57,000 Shrink Ratio % MD 70%
Shrink Ratio % MD 75% TD 10% TD 0% Gloss % (45 degree) 69% Gloss %
(45 degree) 74% Haze % 11% Haze % 12% Comparative Example C1C
Comparative Example C1D Thickness (mil) 2.95 Thickness (mil) 2
Secant Modulus (psi) MD 41,000 Secant Modulus (psi) MD 36,000 TD
47,000 TD 43,000 Shrink Ratio % MD 76% Shrink Ratio % MD 65% TD 0%
TD 15% Gloss % (45 degree) 69% Gloss % (45 degree) 70% Haze % 11%
Haze % 10%
[0088] The data shows that the shrink films of the Examples had
increased stiffness while maintaining strength, allowing them to be
produced at thinner gauges. The stiffness of the Examples (secant
modulus between 65,000 and 96,000) exceeds the Comparative Examples
(secant modulus between 33,000 and 59,000). Gloss was about 69-74%
in the Comparative Examples, versus a much higher gloss from 101%
to 106% for the Examples. Haze in the Comparative Examples was
about 10-12%, versus the much lower gloss of about 5% in the
Examples.
[0089] In summary, the shrink film of the Examples had superior
visual properties and physical properties to the
LDPE/LLDPE+HDPE/LDPE shrink films of the Comparative Examples known
in the art.
[0090] All of the compositions, articles, and methods disclosed and
claimed herein can be made and executed without undue
experimentation in light of the present disclosure. While the
compositions, articles, and methods of this invention have been
described in terms of particular embodiments, it will be apparent
to those of skill in the art that variations may be applied to the
compositions, articles, and methods described herein without
departing from the concept, spirit and scope of the invention. All
such variations apparent to those skilled in the art are deemed to
be within the spirit, scope and concept of the invention as defined
by the appended claims.
* * * * *