US20050166551A1

US20050166551A1 - Multilayer high clarity shrink film comprising monovinylarene-conjugated diene copolymer

Info

Publication number: US20050166551A1
Application number: US10/770,339
Authority: US
Inventors: J. Keane; Dale Gange; J. Brown
Original assignee: FLEXSOL PACKAGING CORP; Chevron Phillips Chemical Co LP
Current assignee: FLEXSOL PACKAGING CORP; Chevron Phillips Chemical Co LP
Priority date: 2004-02-02
Filing date: 2004-02-02
Publication date: 2005-08-04

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.

Description

BACKGROUND OF THE INVENTION

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.
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.
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.
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.
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.
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.
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.
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

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.
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.
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.
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

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.
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

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.
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.
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.
“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.”
“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.”
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.
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.
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).
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.
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® (Chevron Phillips Chemical Co., The Woodlands, Tex.).
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.
The monovinylarene-conjugated diene copolymer can further comprise other units known in the art for inclusion in monovinylarene-conjugated diene copolymers.
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.
The polymerization process can be carried out in a hydrocarbon diluent at any suitable temperature in the range of from about −100° C. to about 150° C., such as from about 0° C. to about 150° 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.
The polymerization process can be carried out in the substantial absence of oxygen and water, such as under an inert gas atmosphere.
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.
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.
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.
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, tetramethylethylenediamine, tetraethylethylenediamine, N,N-di-methylaniline, N-methyl-N-ethylaniline, N-methylmorpholine, and mixtures thereof, among others.
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.
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.
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.
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® from Atofina Chemicals (Philadelphia, Pa.).
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.
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.
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.
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.
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.
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.
In the present invention, the monovinylarene-conjugated diene copolymer can be coupled or uncoupled, as described above.
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.
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.
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.
The shrink film also comprises a second layer comprising low density polyethylene (LDPE) and linear low density polyethylene (LLDPE).
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/cm³to about 0.924 g/cm³and 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.
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.
The LLDPE in the second layer can be any linear copolymer comprising ethylene units and α-olefin units. Typically, LLDPEs have densities of from about 0.915 g/cm³to about 0.924 g/cm³and 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.
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.
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.
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/cm³and 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 α-olefin having a density less than about 0.912 g/cm³), or other polyethylenes, as well as other additives.
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.
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.
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.
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.
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® (Dupont, Wilmington, Del.).
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.
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.
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.
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).
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.
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.
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 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.
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.
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.
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.
The result of the wrapping step is a wrapped group of objects.
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.
In another 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 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.
The group of objects can be any group of objects for which full enclosure is desired. The shrink film can be as described above.
The wrapping step can be as described above.
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.
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

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®, 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.

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%
Example 1B was evaluated and found to completely and satisfactorily shrink full cases of bottled water.

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%

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.



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%

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.
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.
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.

Claims

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.