Note: Descriptions are shown in the official language in which they were submitted.
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BI~IALLY ORIENTED PO~YETHYLENE FILMS
This invention relates to a biaxially oriented f~lm of an ethylene based polymer. More
specifically, this invention relates to a film layer made from thermoplastic ethylene based
homopolymer or random copolymer produced by a metallocene catalyst systern, wherein the film
layer is stretched in two directions for biaxial orientation.
Polymeric films are widely used, both in industrial m~m-f~ctllring processes and in the non-
industr~al sector for the wholesale and retail delivery of goods to the consumer market. By far the
greatest quantity of polymeric film, in film and bag form, is designecl for, use and con~l~mPd by the
non-industrial sector.
Currently, films composed of ethylene based thermpolastic polymers domin~te certain of
these con~-m~.r market applications -- such as the market for household disposables, trash bags and
liners; overwrap films and bags for laundry and dry cleaning goods; and shipping and callyuul bags
for retail merch~n~ ing of non-perishable goods. In other aspects ofthe c~n~ ..e~ goods delivery
market, ethylene based polymer films only weakly compete, if at all, with other more expensive
polymer films such as plasticized polyvinylchloride films and/or polypropylene films -- such as in
the heat-shrink wrap film market for the taut-co.lloul fit wrapping of various items, particularly
peli~l,ables such as cuts of meat, poultry, and fish. Yet for other applications, such as for pac~ ing
of produce, package constructions for cereals, dry foods, and snack foods ethylene based polymer
films compete somewhat in certain circl-m~t~nces of these applications. At the wholesale level
wherein films are used to unitize pallets and cases of goods to f~.ilit~te their shipment to a retail
market current ethylene based polymer films are ess~nfi~lly uncorl-p~ilive with shrink-wrap and
cling films of plasticized PVC or polypropylene.
Heat sh.il~le films and packages thereof have gained substantial acceptance for wholesale
and retail p~ ging of food products and goods. In the retail envir~ nment, p~cL-ing films used to
p~ L ~ food products and non-food products must be of high optical clarity (i.e., highly l~
and of low haze and, plc:r~l~ly high gloss) to provide an ~esthetically pleasing product and must
be sufficiently strong and resilient to provide the l-~cess~,y protection from normal h~ntlli~
Depending upon the nature of the packaged goods, the film must also possess proper barrier
properties with respect to permeation by water vapor, oxygen, and/or carbon dioxide.
S~lcces~fi-l heat shrink films and packages must satisfy a multiplicity of requirements
imposed by both the pa~ gin~ producer and the pa~ ging user. Of primary importance is the
capability of the film or package to physically survive intact through the process of being filled,
ev~c~te-l, sealed and heat shrunk. The film package must also be strong enough to survive the
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material h~nrlling involved along the distribution line to the Illtim~te user. In p?(~ ging foods such
as meat, it is also highly desirable, even necessary, that the package film serve as a barrier to
moderate and control in~sion of gaseous materials (oxygen and carbon dioxide) ~om the
surrounding environment. Of particular importance for p~ ~ng of meat products is that a film
provide an effective barrier to the infusion of oxygen, since oxygen is well known to cause spoilage
of meat. For bundling or unitizing non-perishable goods wherein oxygen perm~o~tiQn may be of less
con ~n, the shrink wrap lSLrn must often serve a barrier function against water vapor permeation and
should desirably possess a self-cling property.
Polyvinyl chloride (PVC) is now widely used for production of films used in over~.~pillg
applications in the pack~in~ field. PVC films are commonly used as an overwrap for trayed cuts
of meat and other fiood products in a retail environment such as a supermarket. PVC is desirable for
production of films for this service because it has excellent optics or clarity, good elasticity and
strength properties at use temperatures, and s~t;~ctc)ry elastic memory ability and elong~ticn
However, PVC resins have several disadvantages. PVC film has a poor resict~nce to physical abuse,
and thus, a PVC-based film package sometimes becomes leaky during .cl~ ..e..l PVC film tends
to tear along the edges of a sealed overwrap tray, for instance, if rubbed during transit by another
tray or an enclosing carton. Furthermore, unless the PVC resin is pl~ctici~erl films thereof are
generally not "heat shrinkable," which means that a~er stretching the film while heated followed
by cooling, under a la~er r~he~ting an unplasticized PVC film tends not to return to its original
unstretched dimension. If plasticized to enable the production of a heat shrinkable film, then
.cignific~nt concern has to be given to the nature of the pl~cti~i7er used, its quantity and co~ aLi~ility
with the PVC resin and migration of such plasticizer from a film thereof and its suitability for
contact with foodstuffs.
The p~qck~ging industry, particularly for perishable food products and individually wl~ped
non-food items, desires a film having the advantages of PVC, but without the disadvantages
described above. For such applications the film should be of high clarity, tear r~ci~t~nt, puncture
~ec~ , and the film should exhibit resi~t~nce to deformation or good recovery from d~lmalion
and satisfy food law requirements.
The use of films of vinylidene chloride-vinyl chloride copolymer, introduced under the
trademark Saran, is presently popular due to the physical characteristics of a film produced from
such resin. It is clear, has a high tensile sll eng~ll, forms a good barrier against moisture and vapor
ion, is mildew resi.~t~nt has good conroll.lability to the item being wrapped, may be
form~ ted for high slip, and it possesses an inherent self-adherability or l'cling. 'l
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Vinylidene chloride-vinyl chloride copolymer films are typically formed by extruding the
resin in the form of a blown tube which is then super cooled. This process of forming the initial film
layer may cause a certain degree of orientation to occur in the film layer but ~due to the relaxation
properties of this resin) such residual orientation as may remain in the film layer due to its bubble
extrusion r~ ing is not typical of the morphology that subsequent stretching of the film layer in a
m~hine direction (MD) infl~lcçs Such vinylidene chloride-vinyl chloride copolymer blown films
may be subseqllently oriented by stretching in a longitll-lin~l or m~fhine direction as a second
bubble while the so produced film layer is still warm and soft after which it is cooled.
Unfortunately, a vinylidene chloride-vinyl chloride copolymer oriented film does not
respond well to later applications of heat. Heat causes in~t~nt~n~ous film shrinkage because
clienL~Lion is lost, heat seals with such films are not smooth, reslllting in a weak seal. Furthermore,
hot melt coatings of this oriented film with another polymeric material to produce a film composite
or multilayer film having more desirable characteristics is not possible. The oriented film loses its
~,lienLalion upon contact with a hot melt. In lamination, the hot press rolls usually cause the oriented
film to react similarly.
Characteristics such as heat sealability are not the only criteria by which one evaluates a f~m
structure for suitability to shrink wrap products such as compact discs, pallets of individually
w~ped items, or the like. Strength and clarity are very important characteristics in such p~cL-~n~
To obtain these desirable shrink characteristics, conventional shrink bags are today constructed of
ethylene vinyl-acetate copolymers. These copolyrners often have a vinylidene chloride - vinyl
chloride or ethylene vinyl alcohol copolymer layer serving as a water vapor barrier. However, films
f~om these resins tend to be so~ and cloudy, rendering the film lln~T-it~hle with respect to appe~ance
and vulnerable to failure at conditions of operation due to the relatively high temperatures to which
it is exposed in the orientation and shrinking process.
By reason of their lower cost, ethylene homopolymers and copolymer resins and blends
thereof enjoy widespread use in the pRck~ing industry for certain applications. Typically,
polyethylene (PE) resins employed as such or as a blend component in pa~k~in~ are high density
ffIDPE), linear low density ~Il,DPF), low density (LDPE), vely low density (VLDPE) or ultra low
density (ULDPE). LLDPE, VLDPE~ and ULDPE are ethylene copolymer wherein, typically, the
comonomer is a C3-C20 alpha-olefin. The comonomer introduces br~n~hing into the polymer and
affects its density. These polyethylenes have been and still are typically made with traditional
~;iegler-type catalysts which contain di~ele,~t types of reaction sites res~ in~ in a polyethylene resin
co..~ , a broad range of molecules. For h~sL~ce, such polyethylene typically co- ~ polymers
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having a medium molecular weight with me-lium br~n~.hing fraction, a low molecular weight and
high branching fraction, and high molecular weight with low b~ c1~ g fraction. This results in
inconsistent properties, and sometimes, poor clarity when such resins are converted into fflms as
well as certain difflculties in production of the film itself.
Today, many oriented polymeric films are of multiple layer construction and col,l~,lised of
at least one barrier or oxygen layer, such as ethylene polyvinyl alcohol (EVOH) or vinylidene
chloride-vinyl chloride copolymer, and a of dirreren~ polyethylenes or of polyethylene layer such
as l~LDPE, HDPE, LDPE, VLDPE, or blends of polyethylene with ~cet~tes, such as ethylene-vinyl-
acetate (EVA). This is because EVAs having greater than about 85 wt % ethylene provide
substantial structural strength to a film during the second bubble orientation process, and are
especially beneficial for the orientation of tubular films. Such multiple layer films are forrned by
a number of ways, including coextrusion, l~min~tion and coating techniques.
A heat shrinkable film should be susceptible to orientation without distortion or se~lion
of the multiple layers which are typically present in films of this nature. The film layer must be
strong enough, at the orientation temperature, to w~ c~ d the stretching in either of the m~f h~
direction (MD) or transverse direction (TD) without the creaLioll of holes, tears~ or non-l-niform
zones of stretching In the case of blown tubular film, the film layer must be capable of sup~ul 1;.~
the ~ ;Lcllillg bubble during the orientation process. With multi-layer fflm constructions, each layer
of the film should be susceptible to orientation without fracture, separation, or creation of holes in
the layer. In p~c~ging use, the final film product should respond to heat rapidly enough for
commercial practicality, and yet must not exhibit such a level of shrink energy as would cause the
multiple layer film to pull apart or de-l~mir ~te during shrinkage under its own internal forces.
Preferably, oriented films used for shrink p~ck~in~ should be clear enough for a consumer
to visually inspect the packaged item prior to purch~cing If the p~clr~n~ iS cloudy (hazy) or not
sufflciently tl~lll~alGn~ or tr~n~luc~nt, a wrapped food item will appear to be undesirable to the
consumer. Thus, clarity is a product attribute widely sought in many types of polymer films for
certain application.
In the past, thermoplastic ethylene based polymers have not been adopted by the industry
for applications that require so~ness, but with good shape retention or recovery (s-l~back).
Polyethylene films, par~icularly those of a LDPE or LIJDPE, have traditionally been relatively sof[
or limp, having a low secant modulus, and softness is a desirable property in some applications, such
as for wl~ppil~g a meat product. This would appear to make polyethylene resins desirable for
production of films for such services, which due to its lower density and cost, would desirably
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displace PVC films from these services. However, polyethylenes have been largely lm~uccçc~fi~l
in repl~ring PVC in at least the meat wrap application, primarily because a polyethylene film retains
or recovers its shape poorly after handling, leaving unacceptable finge~ on the film. PVC
films, on the other hand, snaps back or recovers its shape a few mim1tes after h~ndlin~, providing
a fresh appearance.
- In yet other applications, such as in pack~in~ snack foods, such as potato chips, a film
should feel stiff to protect and/or convey the crisp and crunchy nature of the product. The so~n~es
of an ethylene based polymer film has rendered it undesirable for this service. Today the film
widely employed for snack food p~ck~gin~ is biaxially oriented poly~ulo~ylene (BOPP) film due to
its high degree of stiffness) its good optical properties of low h~e and high gloss, and good barrier
properties to water vapor tr~n~mi~.cion.
For fresh-cut produce, today modified atmosphere packages are desired. A modified
atmosphere package (MAP) is one wherein the film material of its construction has b.~a~hil.g
characteristics within specifically intPn(led ranges relative to the tr~nsmi~ihility of the film to
oxygen, carbon dioxide and water vapor. In a modified atmosphere package the tr~n~mi~ibility of
a film to these vapors must neither be too low nor too high but should instead cGl,~ond ot the
"re~il ~Lion" of the produce product for these vapors. For example, a film for MAP appliç~tion~
should have an oxygen L~ ",;~;;on rate (OTR) of about 300-1200 cc miV100 in2 atm/day, a carbon
dioxide tr~n~mi~ion rate (CO2TR) of about 1,000 to 5,000 cc miV100 in2atm/day and, to prevent
dry rot of produce, a water vapor tr~n~mi~.sion rate (WVTR) of less than 2.~ g miV100 in2day.
Resins widely used today for MAP applications are Ziegler catalyst produced VLDPE films despite
their limited OTR and poor haze properties and EVA films despite their poorer WVTR properties.
Because an ethylene based polymer is generally a lower cost product, it has long been seen
as a desirable goal to use a thermoplastic polyethylene for various uses that have been heretofore
d~"ni~ ed by higher cost polymers. However, ethylene-based polymers l1elelorole available have
lacked various plo~ ies required ~or a particular film service or were otherwise objectionable. For
some applications, such as snack food p~ck~gin~, polyethylene was con~id~red too soft and lacking
in stiffness. As a shrink wrap film, it has he}etofore been necess~ry to form such films from the
heretofore available polyethylene resins by a blown bubble extrusion process and then orient it by
a double bubble procedure. Formation of a biaxially oriented film of polyethylene by a slot-die
casting tenter frame stretching process in which the degree of orienl~Lioll may be more varied and
precisely controlled has not been practical due to the t~ckines~ and inadequate melt strength and
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excessive melt elasticity of a film layer of an ethylene polymer ae the conditions required for tenter
frame sLI~k~
Cast and blown bubble films prepared from the same ethylene polymer resin and to the same
gauge differ dramatically with respect to their haze and gloss properties. Cast films have
significantly lower haze and significantly higher gloss than do blown bubble films of the same
ethylene polymer resin. In either case, cast or blown bubble films, as the density of the ethylene
polymer resin of which the film is formed increases the haze of the film increases while its gloss
decreases; with the rate of increaseJdecrease of haze/gloss with increasing resin dinsity being
~i~nific~ntly greater for blown bubble than for cast films.
In some cases, wherein good film optics are requisites for use of a fflm, low haze/high gloss
properties may be obtained in a cast film of a low density (d < 0.940 g/cm3 ) ethylene polyrner
~LDPE or LLDPE); but such LDP~ or LLDPE films are attended with a lower set of physical
strength properties (tensile, elongation, tear rçsi~t~nce) than could be ~tt~inecl with a high density
(d 2 0.940 g/cm3) ethylene polymer (~PE). Even then, such good optics cannot be att~;ne-l with
such low density ethylene polymers in a blown bubble film. On the other hand, wherein the physical
~llw~Lh properties required of a film exceeds those that can be obtained with a low density ethylene
polymer, a high density ethylene polymer can be employed for the cast or blown bubble extrusion
of a film to attain the requisite film strength ~l U~JG~ lies but at a sacrifice of the film optics, with this
sacrifice being much greater for blown bubble films.
Today such ethylene based polymer films that have been proposed as biaxially oriented heat
shrinkable films are those prepared from a blend of a low density ethylene polymer of density from
about 0.91 to 0.93 glcm3 and a high density ethylene polymer of density from about 0.940 to 0.98
glcm3 as described for example in GB 937,807 and GB 1,279,714, or a low density ethylene
polymer and a very low density ethylene polymer of density from about 0.87 to 0.910 g/cm3, as
described for example in EP 029g750 and U.S. Patent 4,801,652. Even then such ethylene based
polymer blend films have not been widely adopted for certain services such as food p~ ~n~
because of certain real or perceived deficiencies in their properties and/or difficulties in their
production.
Film producers and converters recognize that there is a need for a polyethylene (ethylene
homopolymer and/or ethylene copolymer) that can be fabricated to a film forrn of high clarity and
high stiffness and/or high clarity and good snapback. Further, a film produced from any
polyethylene should have good impact and puncture resi~L~lce, increased tensile at yield and other
physical properties, and low levels of extractables so as to not impart unpleasant odors or tastes to
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a food product. Proper shrink properties are also desired for certain applications such as p?(r~in~
certain food products and goods, covering windows, and wl~pphlg meat pallets.
A film should have as few layers as possible, but with a desirable collll~ina~ion of propenies.
There is a need for a one-, two- or three- layer film structure that may be produced from a
thermoplastic ethylene based polymer resin that has good clarity, high sllellgLh~ and which can be
readily heat sealed with a strong seal res-llting and printed upon ~,vith plole~ilion for the plil~ g,
which may be biaxially oriented to yield a heat shrinkable film.
It has now been found that ethylene based polymers produced from a catalyst having
~cs.gnti~lly only one type of catalytic site may be fabricated into oriented films having h~ Iovt;d
pr~/p~l lies with respect to heat sealability, greater impact strength, puncture reci~t~nce, and optimal
optic properties such as h~e and gloss. It has further been found that an ethylene based
Lllel~lloplaslic polyrner produced with a metallocene catalyst may be fabricated into a film which can
meet the needs of the food paç~g~ng industry, the pallet shrink-wrap industry, and the market for
individual shrink wrapped goods.
This invention comprises a film layer formed by cast or blown bubble extrusion of a single
ethylene polymer resin which is then biaxially oriented by a tenter frame or double bubble procedure
to yield a biaxially oriented film layer that has good optical properties of low haze and high gloss --
like that of a cast extrusion LDPE film -- while c~llibiLillg high strength pl-)pellies -- like that of a
cast or blown bubble extrusion HDPE film.
It has been found that certain ethylene homopolymers and copolymers (ethylene based
polymers) made lltili7;ng metallocene catalysts, after being made into a film and subseqll~ntly
ori~.nted in at least two directions show a surprising ability to wi~ d such oriçnt~ti~ n, as well as
displaying ~nh~nceln~nts in important physical properties afl[er such orientation, when compared
to other previously known ethylene polymers in general, and linear low density polyethylenes
(LLDPE) in particular, most especially those polyethylenes made using traditional Ziegler-Natta
catalysts. In one embodiment of the present invention, the most striking improvements are
evidenced in clarity (~ iresled in haze and gloss), dart drop impact rç~icPn~ ~ puncture r~ e
oxygen and water vapor ~ s;on rates, tensile at yield, secant modulus, and the like.
In an embodiment ofthe present invention, extruded articles such as film, or other fabricated
articles made from such film, are comprised of an ethylene homopolymer or a copolymer of ethylene
and at least one C3-C20 alpha-olefin. The alpha-olefin of the copolymer is present in the range of
fi~m about 0.01 to about 6.0 mole percent ofthe copolymer, for C4-C20 alpha-olefins plc;rt;l~bly no
greater than 5.0 mole %. The ethylene based polymer will have a content of ethylene greater than
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90 wt%, a ratio of weight average molecular weight (M~,) to number average molecular weight (1!~)
of M,~ ~ 3, a ratio of a Z - average molecular weight (Mz) to weight average molecular weight
of M~ s 2, a composition breadth distribution index (CDBI) of 50% or preferably greater, such
as 55 %, a density in the range offrom about 0.915 to about 0.965 g/cm3, an I2l/I2 melt index ratio
(~TR) of less than about 22, a Dow Rheological Index (DRI) of C 0.3, and a peak melting point
(DSC) from about 103-135~C.
Biaxially oriented film made from such ethylene based polymers will exhibit certain film
properties that are relatively sensitive to a change of polymer density, such as(a) Secant Modulus (MD/TD), which increases with increasing density;
(b) Tensile Strength at Yield, which increases with increasing density,
~ c) ~longation at Break, which decreases with increasing density:
(d) Oxygen Tr~n.~mi.ccion Rate (OTR);
(e) Water Vapor Transmission Rate ~WVTR);
film ~ul)el lies that are relatively sensitive to resin density changes from about 0.915 to about 0.930
g/cm3 and thereafter relatively insensitive to density c.h~ng~s beyond 0.930 g/cm, such as
(f~ Tensile strength at Break;
(g) Elmendorf Tear;
(h) Puncture Strength;
(i) Dart Impact Strength, which is greatest at the 0.917 to 0.922 g/cm3 density range,
and declines signific~ntly by comparison as density increases beyond 0.922 g/cm3:
and film properties that are somewhat in~n~itive tû a change in polymer density, such as
(j) H~e;
(k) Gloss;
(I) Shrinkage.
Further, with an ethylene based polymer of an app~-ol,liately selected density, a film may be
engineered to provide a target or desired property value relating to film strength, vapor barrier
~rup.,. Iies and the like by ap~; l u~ ialely controlling the conditions and degree of olie~ Lion illl~ Led
to it in the MD and TD.
In certain embodiments of the invention a biaxially oriented film comprised of such ethylene
based polymers will exhibit properties such as:
a) h~e in the range of from 0 to 5%;
b) gloss above about 65%;
-
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_9_
c) dart drop impact above 400 g/mil at a resin density of 0.917 g/cm3 and no lower than
1~0 at a resin density of 0.940 g/cm3;
d~ oxygen tr~n.cmi~ion rate below about 600 cc-mil/100 in2-atm/24 hours ~ 25~C:e) a tensile at yield of 2000 psi or greater (MD) or 5000 psi or greater (TD);
f) a secant modulus (1%) above about 40,000 psi (1!~), or above about 70,000 psi
(TD) and;
~) shrinkage of at least 10% in each of the oriented directions.
Those of ordinary skill in the art will understand that for a given polymer at a given temperature,
the more highly oriented film will shrink more than a film of lower orientation (at same Lempel~-Lu~e
and base polymer density~. Shrinkage will occur at a temperature range of 70~ to 90~C within 10
seconds, preferably the shrinkage occurs in a temperature range of 75 to 85~ within 5 seconds.
Shrink films made form such ethylene homopolyrners and ethylene-alpha-olefin copolymers
and articles made i~om these films will be particularly useful in applications where ...~;."i,;,-
~optical plupelLies~ secant modulus, impact rçsi~t~nce, puncture resi.ct~nce and the like are important.
Thus, one preferred embodiment ofthe present invention provides an ethylene based polymer
film having an oxygen permeability no greater than about 500 and typically about 150-450 cc/100
in2/24hr. The film comprises a biaxially oriented (BO) film having increased optical clarity,
puncture re~i~t~nce and tensile strength, said film comprising at least one layer of an ethylene
copolymer thermoplastic resin produced from a metallocene catalyst and having an ethylene content
of greater than 90 wt%, a density range from 0.915 g/cm3 to about 0.930 g/cm3. The film layer is
biaxially orientable to an extent of 5 x 8 or 7 x 7 (MD x TD). The biaxially oriented film (BOF) is
heat shrinkable and more preferably exhibits a 76% shrink in the machine direction. In addition,
the film is 100% recyclable.
Another preferred embodiment of the present invention also provides a biaxially oriented
(BO) film comprising at least one layer of linear low density ethylene-alpha-olefin copolymer
density of 0.915 to 0.g40 g/cm3 polymerized with a metallocene catalyst, which may heleafler be
re~.led to as a "m-LLDPE" resin. The m-Il.DPE film has improved optical clarity and physical
toughness for biaxial orientation, and optionally may comprise a second and third layer. In a
~ preferred embodiment, the BO-m-LLDPE-film includes a first layer and a second layer that are co-
cxtruded. The BO-m-LLDPE-film can also include a fiourth layer adhesively l~min~tecl to the third
layer.
The present invention also comprehends packages fabricated from the films of the invention
wherein the second or third layer is heat sealed. The invention fi~rther includes a method for
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-10-
p~cl~g-ng a product such as a compact disc or software package and shrink wl~pillg the product
in a ,.~a~ ge made from the BO-m-l,LPDE-film of the present invention. In addition, the invention
c."~ hends a method for wrapping an item, preferably a window frame or door frame colllp-ised
of the film of the present invention.
These and other features, aspects, and advantages of the present invention will become better
understood with regard to the following description, appended claims and accc,nlpa~l~/ing dl~W;I~
A radar plot or diagram is a multiple axis (2 3) graphs wherein each ra~i~tin~ axis from the
center point of its origin to its end point represents a range of values -- either actual or as a
el~;G,lLage of an assigned m~imllm value ~relative) -- with each axis leplcisç~ g the value range
of a different chemicaVmechanical/physical property of the final film product or the film resin, as
may be indicated on the plot. In this respect the differences and/or similarities between one film and
another, or the relationship between a change in resin pl~pw Ly to a change in a final film prop~ Iy
(àctual or relative3 may readily be viewed and assessed.
Fig. 1 is a radar plot by which the final film properties of an m-E-polymer BO film of 1.62
mil thickness and S x 8 orientation of this invention is compared on a relative basis to a blown but
ullo~ led m-E-polymer film prepared from the same resin, namely EXCFFn 350 D60.
Fig. 2 is a radar plot by which the final film p. ~,pe, Lies of an m-E-polymer (EXCE~L) 350
D60) BO film of 1.62 mil thickness and 5 x 8 orientation ofthis invention is coll~aled on a relative
basis to a commercially available film of biaxially oriented polypropylene (BICOR~B, from Mobil
Ch~.mic~l Co.).
Fig. 3 is a radar plot by which on a relative basis the final film pl ol)e~ Lies of an m-E-polymer
~EXCEED 301 ) BO film of 5 x 5 orientation of this invention is compared to a blown film of a ~igh
density polyethylene.
~ ig. 4 is a radar plot by which on a relative basis the final film properties of three BO films
of EXC~FFD resins of ~i~w~llL densities (0.917; 0.922; 0.926-g/cm3) are con,pa,cd; the 0.917 g/cm3
density resin BO film (350 D60) being of 5 x 8 BO and 1.62 mil gauge and the other fikns being of
6x6BOandO.6milgauge.
The present invention comprises the utilization of ethylene based polymers having certain
minimllm characteristics with respect to their ethylene col,Le"l, molecular weight, both M~, and M",
their density; and the compositional uni~l-l-iLy of the ethylene based polymer resin: for the
production of a film layer which upon biaxial orientation yields a film article having a set of
physical/mechanical/chemical ~l (".c;, Lies requisite to the service required of such film for various
. .
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particular applications which heretofore have not been particularly serviceable by an ethylene based
polymer film.
The present invention concerns certain classes of biaxially oriented films fabricated from
ethylene based polymers made utili7:ing a metallocene catalyst system, articles made from these
films, and uses of the films and/or articles. For purposes of this application, such ethylene based
polymers include ethylene homopolymers and ethylene copolymers in which ethylene is the
predominate monomeric constituent by weight or mole %; specifically, ethylene comprises at least
and preferably more than 90 wt % of the ethylene based polymer while the comonomer content
thereof does not exceed 6.0 mole % of the ethylene based polymer and the CDBI of the polyrner
exceeds 50%. Preferably such ethylene based polymers comprise at least about 93 wt % ethylene
monomeric units and the comonomer content thereof does not exceed 5.0 mole % and more
preferably the comonomer content does not exceed about 3.3 mole % of the ethylene based
polymer. Such ethylene based polymers may hereafter be referred to generally as an "m-
polyethylene" or as an "m-E-polymer. "
The term "oriented" means that the m-E-polymer film has been stretched while in a sonened
state while at a temperature below that at which the film layer was initially extruded by a cast or
blown bubble technique, and "biaxially" means that the cast or blown bubble m-E-polymer film has
been stretched in both of a machine direction (MD) and a direction transverse to the .,.~
direction (TD), as described further below. The degree of biaxial orientation is denoted as the
percentage as compared to the original dimensions of the extrusion formed film layer that the film
layer is stretched; i.e., such as 100,200...600 % of its original dimension of length (MD) or width
(TD3. For example lxl orientation means that the initial film layer has been stretched to twice its
original dimension in each of the machine direction (MD-length) and transverse direction (TD-
width); 6x6 would denote a stretching to six times the original length and width flimen~i<)ns of the
initially formed film layer. Films produced of the particular ethylene based polymer in their
biaxially oriented form have unique characteristics, which may be manipulated by the degree of or
type of orientation, which make them well suited for use in certain service applications.
The te~m ethylene copolymer as used herein shall mean copolymers of ethylene and alpha-
olefins. Such alpha-olefins will generally have 3 to 20 carbon atoms. Polymers of ethylene and one
or more or these alpha-olefins are contemplated. Preferred alpha-olefins are butene-l, pentene-l,
4-methyl-1-pentene, hexene-l, octene-l and decene-l. Especially prt;~-led are butene-l, hexene-l,
and octene-l.
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Compared to films made from previously available ethylene homo and alpha-olefin
copolymer, the present invention provides an m-E-polymer film layer that is biaxially orientable
without difficulty by either of the double bubble or tenter frame processes to yield an oriented film
having superior optical properties, out.ct~nding dart drop impact resistance and puncture les;~l~nc~,
and a dramatically increased secant modulus. A biaxially oriented m-E-polymer film acco~.ling to
the present invention is a stiffer film than previously available polyethylene films, has improved
clarity, and is better able to withstand the rigors that a film encounters in service. Consequently,
articles made from these biaxially oriented m-E-polymer films (BO-m-E-polymer films) exhibit
superior properties, which allows the ethylene based polymer fil~}s of the present invention to be
designed for and used in applications for which polyethylene was previously considered
unattractive.
The present invention provides various articles of m~m-f~r~ re, typically for p~ ging~ and
methods for making these articles. Articles contemplated by the present invention include a shrink
wrap pallet film; a film for wrapping goods, packages for goods such as snacks, inr.lllfli~g potato
chips and dry cereals; shrink wrap films for meats and poultry; and typical film applications such
as diaper backsheets. Depending upon the film stiffnes.e required for a particular end use
application, the required ~li~.ess (secant modulus) may be enginePred into the final film product,
ranging from relatively soflL to relatively stiff, by control of the degree of orientation imparted to the
film in one or the other, or both, of the MD and TD. The present invention provides BO-m-E-
polymer films for shrink wrapping individually packaged items, with applications ranging from
p~ ging compact discs to providing poultry bags. Some of the BO-m-E-polymer films embodying
this invention are serviceable as heat shrinkable packages in which a product is inserted, air is
normally ev~c~l~ted, the open end of the bag is closed, such as by heat sealing, and heat is applied
to shrink the fflm for a tight and COIl~llllillg fit around the product.
In other service applications wherein heat shrinkability of a film is either not desired or is
an ;l~r~~ e~luential property that will not be employed in the use of the film -- services such as liners
for cereal box pack~ng snack food p~ck~ging, overwrapping of fresh cut produce, and the like --
by appropriate selection of m-E-polymer resin density and control of the degree of orientation
imparted to the film, final film products of a desired degree of stiffness and of a desired range of
~apor tr~n~mi~ibility to ~2~ CO2 and H20 may be produced that are structurally strong (tensile
.engl}ls, puncture ~l~engLhs, tear strengths) and of good optical properties (haze and gloss).
The end use service intended for the m-E-polymer film will in part inflllence the selection
of the ethylene polymer resin for production of the film layer and thereafter the conditions employed
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for its orientation in the MD and TD For example, films may be inten~1ed for service as a general
heat shrink wrap for non-perishable good or for perishable food goods such as meats, or the like
The int~nded service will then require as final properties of the film certain ~ i, ",, properties with
respect to ( 1 ) haze and gloss, depending upon whether user viewability of the packaged goods are
or are not of a practical concern; (2) oxygen and moisture barrier moderating properties, a particular
concern with respect to food packaging; (3) secant modulus (flex;bility - reci~t~nce to dero~ Lion),
to be geared to the ~ fel eilce of the end user market; (4) degree of shrinkability, to be geared to the
~lerele..ce of the end user market; (5) puncture reci~t~nce, tensile impact, tear strengths and
elongation properties of suitable minimum~ for service The requisite minim-lnn service l)lul)e-lies
required of the film for use in a particular application is then imparted to the film by proper selectiûn
of the film base m-E-polymer resin and the selection of the film forming and orientation con~ ion~
The m-E-Polymer Resins
Catalyst for polymerization of the ethylene based polymers here concerned are co~ -ised
of a transition metal component having at least one organo ligand which contains a cyclop~nt~ nyl
anion moiety through which the organo ligand bondingly coordinates to the transition metal cation
Such catalyst systems are now commonly referred to as "metallocene" (m) catalysts and many
examples of such metallocene catalyst systems have now been described in the artIn contrast to catalyst systems thel~erole known for alpha-olefin polymerization that utilize
a transitional metal component not having an organo ligand having a cyclopentadienyl anion moiety,
now co.l.nlollly referred to as conventional or traditional Ziegler-Natta (ZN) catalysts, metallocene
catalysts are e~senti~lly single cited catalysts whereas ZN catalysts are multi-sited catalysts that
generally produce a polymer resin having a great diversity of polymeric species By co--ll~L, an
ethylene-alpha-olefin copolymer produced by a metallocene catalyst is generally much more
uniform with respect to the polymeric species that comprise the res llfing m-E-poly-mer resin,
particularly with respect to the disparity between the differing molecular weight fractions thereof--
as; .d;c-,led by the ~ value of the m-E-polymer resins generally being s 3 0 -- and with respect
to the distribution of alpha-olefin comonomer between the dirrere--~ molecular weight fraction
thereof -- as indicated by a high comonomer distribution breadth index (CDBI) value of 50% and
higher In part, by reason of the greater compositional and molecule weight distribution uniro~ lly
achieved in an ethylene based polymer produced by a metallocene catalyst, the density of the
resulting m-E-polymer resins is subst~nt~ y a linear function of its mole % comonomer content and
densities of the m-E-polymer resin in the 0 915 to 0 965 g/cc range of interest for films of this
invention may be accomplished with an ethylene content of greater than 90 wt % and a comonomer
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content not exceeding about 6.0 mole %, and a comonomer content preferably not exceeding about
5.0 mole %, and more preferably of or less than 3.3 mole %. Further, such densities are achieved
in the m-E-polymer resin while the polymer backbone remains subst~nti~lly linear; that is, such
short chain branching (SCB) that occurs along the polymer backbone is due sllbst~nti~lly only to the
alpha-olefin comonomer content of the polymer. Accordingly, although the final density of the
ethylene copolymer varies somewhat depending upon the carbon number of the comonomer utili~e~,
the m~gnitude ofthis variation with C4-C20 alpha-olefin comonomers is not subst~nti~l the le-lui~ile
copolymer densities required of the m-E-polymer resin for films of this invention may readily be
achieved with low contents of comonomer such as the C3-C8 alpha-olefins, with butene-l and
hexene- 1 pl efel . ed as the comonomer by reason of their lower cost. Such m-E-polymers having
these requisites have recently become commercially available from E~on Chemical Company in
Baytown, Texas and are now identified by the trademark "EXCEED. "
Production of the m-E-Polymer Resins
The ethylene based polymers utilized in the present invention are preferably produced using
supported or unsupported metallocene catalysts. The polymers may be produced in many types of
reactors or reaction schemes, including, but not limited to, fluidized bed or stirred bed gas phase
reactors, slurry or bulk liquid reactors of tank or loop type, solution, or any other process practiced
for the polymerization or copolymerization of ethylene.
Specific metallocene-type catalysts are known to be useful for producing olefin polymers,
and such catalysts are described in U.S. Patent No. 5,324,800. For placing catalyst systems on a
supporting medium and using the resulting catalyst, see, for example, U.S. Patent No. 5,124,418.
Support te-~hnillues for metallocene-type catalysts for use in the pl ep~u alion of alpha-olefin polymers
are described in U.S. patent 5,240,894. While catalysts used for the examples which follow were
employed in a gas phase fluidized bed polymerization, catalysts for commercial use may be used
in other processes including for example, slurry and solution processes. U.S. Patent Numbers
5,324,800, 5,124,418; and 5,240,894 are hereby incorporated by .~;re.ence for purposes of U.S.
patent practice.
~ n one pl~rel,~d embodiment, a catalyst system c~ l;sing bis(1,3 methyl-n-butyl
cyclopentadienyl)~h~;onium dichloride activated with methyl ~ mo~c~ne (MAO~ is the catalyst of
choice. Such catalysts are outlined in copending U.S. Application Serial Number 08/466,587 which
is included herein by reference for purposes of U.S. patent practice.
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(~haracteristics of the m-E-Polvmer Resins
The ethylene polymers of the present invention will generally have a narrow molecular
weight distribution (MWD), as characterized by the ratio of weight average molecular weight (M~
to number average molecular number (M"), M",~,. These M,~ and ~" values are deterrnined by ~el
Permeation Chromatography (GPC). The MWD for the m-E-polyrners of the present invention is
less than or equal to 5; preferably ~ 3.5, more preferably ~ 3.0; and most pr~r~lled s 2.5.
Embodiments of these m-E-polymers will have a density in the range of from about 0.915-0.965,
preferably 0.917-0.95, more preferably 0.917-0.940 and most p}eferably from 0.920 to 0.94 g/cc.
Resins produced by the above referenced processes and catalysts will be homopolymers or
copolymers that contain greater than 90 wt % ethylene, prerel~bly 93 wt % ethylene or greater, and
may incorporate one or more alpha-olefins comonomers in the range of from about 0.01 mole
percent to about 6.0 mole %, pl~r~,~ly 5.0 mole % of less, and most pl~re,ably no more than about
3.3 mole % comonomen In a preferred embodiment, the c.,l,lollo.ller ranges from about 0.1 to about
3 mole percent.
Copolymers produced from a catalyst system having a single metallocene component have
a very narrow composition distribution - most of the polyrner molecules will have roughly the same
or comparable comonomer mole % content. Ziegler-Natta catalysts, on the other hand generally
yield copolymers having considerably broader composition distribution me~nin~ that comonomer
inclusion varies widely among the polymer molecules.
A measure of composition distribution is the "Composition Distribution Breadth Index"
("CDBI") as defined in U.S. Patent 5,382,630 which is hereby incorporated by reference. CDBI is
defined as the weight percent of the copolymer molecules having a comonomer content within 50%
of the median total molar comonomer content. The CDBI of a copolyrner is readily determined
utilizing will known te~ hniques for isolating individual fractions of a sample of the copolymer. One
such ter,hn que is Telllpe, ~L~Ire Rising Elution Fraction ~TREF~, as described in Wild, et al., J. Polv.
Sci. Poly. Phys. Ed.. vol. 20, p. 441 (1982~ and U.S. Patent No. 5,008,204, which are incorporated
herein by reference.
To determine CDBI, a solubility distribution curve is first generated for the copolymer. This
rnay be ~compli~h~d using data acquired from the TREF techni~ue described above. This solubility
distribution curve is a plot of the weight fraction of the copolyrner that is solubilized as a function
of temperature. This is converted to a weight fraction versus composition distribution curve. For
the purpose of simplifying the correlation o~ composition with elution temperature all fractions are
~umPd to have an Mn 2 15,000, where Mn is the number average molecular weight of the fraction.
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Low weight fractions generally represent a trivial portion of the polyrner. The rem~in-1~.r of this
description and the appended claims m~int~qin this convention of ~ min,E~ all fractions have ~ 2
15,000 in the CDBI measurement.
From the weight fraction versus composition distribution curve the CDBI is determined by
establishing what weight percent of the sample has a comonomer content within 25 % of each side
of the median comonomer content. Further details of dt;~el-,~ilnng the CDBI of a copolymer are
known to those skilled in the art. See, for example, PCT Patent Application WO 93/03093,
published February 18, 1993. The m-E-polymers employed in the films of this invention have
CDBI's equal to or greater than 50% the range of 50-98%, usually in the range of 50-70% and most
typically in the range of 55-60%.
These m-E-polymer resins have a melt index (MI) in the range of from about 0.5 to about
10, preferably in the range offrom about 1.0 to 5.0, and more preferably from 1 to 4.0 dg/min. The
MI range fior film production of an m-E-polyrner via a blown tube technique is preferably from about
0.8 to about 2.0; for cast film production the MI range of the m-E-polymer is preferably from about
0.75 to 4.0; preferably 1 to 5.0, more preferably 1 to 4. Choice of melt index for the m-E-polymer
will generally be driven by the type of extrusion process and the specific eqllirm~nt in use as well
as the end use for films and/or subsequent use in converting operations.
Food law compliance can be an important criterion for articles made from these resins, such
compliance is usually directly a~ected by the extractable content of an article made from a resin.
Using an n-hexane reflux procedure, a standard of the U.S. Food and Drug ~rtminietration as noted
in 21 C.F.R. 177.1520, the maximum extractables level ofthe products ofthe present invention
is expected to be less than about 5 wt%, pr~ bly less than about 4 wt%, and most preferably less
than about 3 wt%.
The m-E-polymers used in this invention have an I2l/I2 or melt index ratio (M~) less than
35, generally in the range of from about 16 to 22. DRI ranges from about 0 to 0.4, preferably from
about 0 to 0.25, more preferably from 0 to 0.2, and most prefel~bly from about 0 to 0.15. A
definition of DRI and test methods for it are described in the publication ANTEC '93 Procee-lin~,
INSITETM Technology Polymers (ITP) - New RJ~les in f*e Polyolefns Struc~ure/Rheology
Rela~ionship of Ethylene o~-olef n Copolymers, New Orleans, LA, May 1993.
The EX(~ m-E-polymer resin product now available from Exxon Ch~mic~l Companyis a metallocene catalyst produced ethylene based copolymer. The processing of an EXCEEDTM
resin may be performed in a manner which is similar to that of conventional LLDP~ with min;m~t
e~uipment conversion required. A thermoplastic film made from an EXS~ resin has
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demonstrated a 50 percent increase in puncture resistance and a 40 percent increase in tensile
strength, as compared to a conventional polyethylene fflm.
One grade of EXCEEDTM is a copolymer of ethylene and hexene-1 and is a linear polymer
and a unique type of linear low density polyethylene. This metallocene produced m-E-polymer has
a narrow molecular weight distribution (M~), typically less than 3.0 while having useful weight
average molecular weights (Mw) of greater than 10,000 and less than 500,000, and a narrow range
of short chain branching (SCB) of about 12 and less than 30 SCB/1000 carbon atoms. The
EXCE~3~TM class of polyethylenes (wherein the comonomer is a C4-C8 alpha-olefin) have a
substantial absence of low molecular weight and high comonomer content molecules, a substantial
absence of high molecular weight and low comonomer content molecules, as indicated by CDBI
2 50 %; a narrow molecular weight distribution, and slightly lower melt strength than tr~-litinn~l
linear ethylene polymers, and a slightly flatter shear rate viscosity curve.
Makin~ A ~ilrn Layer
Blown films produced with an annular die and air cooling and cast films using a slot die and
a chill-roll for cooling are both acceptable techniques for making a film layer of the m-E-polymer
resin according to the present invention. Any technique may be used, provided the res-llting film
meets the specifications stated herein.
Additionally, various additives including pigm~nts, plasticizers (?~, tackifiers, anti-static
agents, anti-fogging agents, antioxidants or other additives are also cont~mpl~ted and may be
inc}uded in the resins and/or films made therefrom.
Multilayered structures may be plc;rellcd in some applications. Such structures in~ le, but
are not limited to, coextruded films, and laminated films. I ~min~ted films can indude not only one
or more film layers based on m-E-polymers of the present invention, but other film layers as well,
including but not limited to, polyester, polyamide, polypropylene, other polyethylenes, Saran~,
ethylene vinyl alcohol, and the like. Methods of l~min~tion include extrusion l~min~tion, adhesive
l~min~tiC)n, heat lall,i.la~ion, and the like. Other materials may be 1~.,;"~1~e~1 to final films structures
of this inveniton, including films based on embodiments of the present invention, for instance paper,
minllm foil, paperboard, woven and non-woven materials.
~ Iso contemplated are films where one or more layers are at least partially cross-linked by
radiation. Such radiation and the techniques tO achieve it are well known to those of ol~lhl~y skill
in the art. These ter.hniq~es include both gamma ~cobalt 57) and x-ray radiation, electron beam and
ultraviolet.
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The film of the present invention can be any suitable thickness, depending upon the end use
for the film. Typical thicknesses may range from about 3 microns to about 0.030 inches (0.25 mm)
for a thin film to about 1 to 2 mm if a thick film is desired. The film can be obtained in a rollstock
format for automatic form, fill and seal equipment or as ~rt;r~ led bags or pouches.
Making Oriented Film
The metallocene produced polyethylene films of this invention can be heat-shrinkable, that
is an oriented m-E-polymer film can return to its original unstretched size when heated to its
son~ g point. The terms "oriP.nt~tion" and "oriented" describe a m~m7f~ch~re of metallocene m-E-
polymer heat shrinkable films. The m-E-polymer resin is heated to its flow or melting point and
extruded through a die into either tubular (blown bubble) or sheet (cast) form and then cooled from
its extrusion temperature. After cooling, the relatively thick extrudate is, as necessary, reheated to
a femperature range suitable to orient or align the crystallites and/or molecules of the base m-E-
polymer resin. An orientation temperature range for a given resin must be deterrnined. The
orientation temperature range is a range of temperatures in which the intermolecular configuration
of the resin is revised by physical alignment of the crystallites and/or molecules of the resin to
improve certain mechanical properties of the film, such as shrink tension as, for example, measured
in accordance with ASTM D-2838-81.
While the m-E-polymer extrudate film layer is within the orientation temperature range, it
is stretched, which changes interrnolecular configuration, and then cooled while in the ~Ll~Lcl~ed or
extended position. Cooling of the film stock while in the stretched position locks the crystallites
and/or molecules of the material into a desirable configuration, providing an oriented film. Upon
subsequent r~h~ating to its so~eninE point, as may occur during a p~f~k~glng operation, forces within
the film cause it to shrink es.s~nti~lly back to its original unstretched position. In this manner an
oriented film is heat shrinkable, providing a shrink wrap film.
When the stretching force is applied in one direction, uniaxial or monoaxial orie.nt&~tit n
results. When the stretching force is applied in two directions, biaxial orientation (BO) results. In
a continuous operation producing a rolled sheet of film, uniaxial orientation is typically provided
by running a downstream roller at higher revolutions per minute than an immediately u~ ea
roller, thereby ~ ching the film. This orients the film layer along its length in the m~hine
direction (MD). Biaxial orientation is provided by also stretching the film layer along an axis at an
angle to the uniaxial axis. Typically, biaxial orientation is provided by stretching in the m~chine
dir~lio,l and in a transverse direction (TD) at a right angle to the m~chine direction which is along
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the width of the film layer. Transverse stretching can be provided by a m~c hine called a TM Long
or by a machine called a tenter frame stretcher
A monoaxially oriented film typically refers to films oriented primarily in the lon~it~lt1in~1
direction. ~Iowever, some incident~l orientation can be present in the transverse direction. The term
can also refer to films oriented principally in the l~-s~else direction, such as can be provided in a
blown tubular film, with or without some incidental orientation in the lon~it-lAin~l direction. A
biaxially oriented film (BOF) for purposes of this application means a film the film layer of which,
after its initial forrnation, is then intentionally and specifically oriented in the lon~ rlin~l direction
(MD) and transverse direction (TD) by a degree that is beyond that which may be imparted a mere
blown bubble process of forming the initial film layer itself; with the longitudinal direction being
in the direction of the machine and transverse direction is the direction transverse to the m~r~hine
The biaxial stretching of the m-E-polymer film of the present invention can be carried out
simultaneously or s~lcceecively, with sllccçs~ive biaxial stretching being pl-ere~d, in which
stretching is first carried out longit~l(1in~liy then transversely. The p-~r~lled percentage of
sh.il~al~ility of a film of this invention in the machine direction is 76%. This provides a film having
good m~chin~bility and a wide sealing range.
Oriented m-E-polymer films may be produced by either post extruder manipulation of a
blown film through heating and orientation (often called "double bubble" or "trapped bubble"), or
by Inn~it-l-lin~l stretching of an extruded sheet followed by tentering techniques. Orienting the film
by a tentering technique is plerell~d since this generally produced a biaxially oriented film of
superior mechanical/physical properties compared to orientation by a double bubble technique.
Films a~er orientation are generally in the range offrom about 0.2 to about 10 mils (5.08 to 254 ,um)
thick (gauge), ,~ rel~ly from 0.5 to 5 mils, and most preferably from 0.6 to 3 mils. Films layers
formed by cast or blown bubble techniques to be produced into oriented structures will generally
I-ecçs~ ily be thicker prior to orientation and will range from about 4 to about 25 mils. Choice of
the l~ J~P~S (gauge) prior to orientation will depend on orientation equipment, degree of int~nrle~
orientation, and the properties intended of the film and/or resin. All such parameters are well within
the skill and knowledge of those of ordinary skill in the art to determine, as a matter of routine setup,
of a film line.
For purposes ofthis document, biaxial orientation will include orientations such as 2x2, 3x5,
5x~, 6x6, 7x7, ~x8 and the 1ike. These notations will indicate a coll,bil,aLion of film ~l~c;Lcl~ing in
both the machine direction (MD) and transverse direction (TD) where 2 or 3, for in~t~nce in-1ic~te
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200 or 300 percent difference in dimension after orientation. The arnount of orientation need not
be equal in both directions.
Film Properties
Compared to films made from previously known ethylene homopolymers copolymers orblends thereof, oriented m-E-polyrner films made according to the present invention will generally
be clearer, stif~er, stronger and exhibit greater shrinkage when heated. These stiffer, clearer and
stronger BO-m-E-polymer-films may be used to make packages for crisp snacks, ~esh meat, fresh
produce, consumer soflc and hard goods and the like.
Films made from the polymers of certain versions of the present invention may be used to
contain food articles such as meats and snacks for instance. Such m-E-polymer so be used to protect
and display articles of apparel and other consumer goods such as computer soIlw~ e, books, compact
discs snd a wide variety of magnetic storage media.
Various embodiments of the BO m-E-polymer films of the present invention have the
following properties and ranges:
Haze 0 to 5, preferably 0 to 3, more pl~rel~bly O to 2 %;Gloss grea~er than 70, pr~rel~bly greater than 80, more plert,l~bly
greater than 8~;
Film densities from 0.917 to 0.960 gtcm3;
Dart Drop Impact above about 400 g/mil at a resin desntiy of 0.917g/cm3;
Peak Puncture Strength greater than 40 Ibs, preferably greater than 4~ Ibs/mil, more
preferably above about 50 Ibs/mil;
Puncture Break Energy greater than about 25, preferably greater than 30 in-lb/mil;
Oxygen transmission (OTR)
less than 600, preferably less than about 550, and more
preferably less than about ~00 cc-rnil/lOOin2-atm/24 hrs; and
Water Vapor Tr~n.cmi.~ion Rate (WVTR)
less than about 1, preferably less than about 0.9, and more
preferably equal to or less than about 0.8 g-mil/100 in2-24
hrs.
With ethylene polymers of certain embodiments of this invention it will be possible to design
final film properties broadly by controlling polymer properties such as density. For in~t~ncp~ in
lowerdensity ranges, e.g. 0.917 - 0.925, preferably 0.917 - 0 920 g/cm3, biaxially oriented (BO) m-
E-polymer films will be relatively soft, but strong and with excellent clarity. These lower density
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BO m-E-polymer ~qlms will compete favorably with plasticized PVC films in applications such as
meat wrap. On the other end of the spectrum, at higher densities, e.g. 0.920 - 0.960, pl~;r~-~ly
0.925 to 0.950 g/cm3, BO m-E-polymer films fabricated from these ethylene polymers will have a
high modulus and be very stiff and have excellent clarity. These BO higher density m-E-polymer
films will compete favorably with biaxially oriented polypropylene in either single or multiple
layers in applications such as snack and cereal p~ck~ging
For a given m-E-polymer density, the biaxially oriented films of the present invention exhibit
many improved physical properties as compared to subst~nti~lly unoriented films such as ~ d~d
blown or cast films. In addition biaxially oriented m-E-polymer films of this invention have better
optical properties, exhibiting excellent clarity. Tensile at yield for these biaxially oriented m-E-
polymer films can be expected to be 120-300% (MD) or 150-600% (TD) miniml-m greater than
tensile at yield for a substantially unoriented film.
Oriented films based on the ethylene polymers of the present invention may find applicability
in retail fresh meat pa-~k~ginsJ In such pack~ging the meat is often placed in a tray, frequently made
of foamed polystyrene and then typically over-wrapped with a plasticized PVC film. Attempts in
the past to utilize polyolefins, more specifically ethylene polymers, have centered around illlplOvll~g
the ethylene polymer's recovery to equal or exceed that of PVC. However, oriented m-E-polymer
films accu.dil.~, to the present invention do not depend upon recovery. Tn.cte~cl) these oriented films
are relatively unyielding and thus are unlikely to indent under normal h~ndling These films are
resistant enough to stress to s~1cces.~fully resist indentation, which is exemplified by their high tensile
at yield. Although these films are relatively unyielding, the films are still soft enough to allow easy
wrapping and thus will compete favorably against plasticized PVC film in these applications.
Shrink Wrap Films
Film of the present invention offers improved performance in the shrink-wrap market. The
film includes at least one layer comprised of a polymer of ethylene polymerized in with a
metallocene catalyst in the presence of a C3 to C8 alpha-olefin comonolnel. Preferably, the layer
is selected from an ethylene alpha-olefin copolymer having a density of and above 0.917 glcm3
wherein the comonomer is a C4-C8 alpha-olefin. The final film may have multiple layers, and the
thickness of the layers may be reduced without r.h~nging the product quality.
Each layer of the shrink wrap film is preferably comprised of a resin m~mlf~ctured using a
single-site catalyst, a metallocene. The shrink wrap film is preferably cc,lllpllsed of a metallocene
produced linear low density polyethylene ~m-LLDPE) resin of density from about 0.917 to about
0.940 g/cm3 which is cast or blown to a film layer that is then oriented in the MD and TD to produce
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products such as stretch film, heavy-duty shipping sacks, flexible processed-meat pack~ging, poultry
bags and the like. A preferred use of the BO-m-LLDPE film is shrink wrapping products such as
compact discs. This improved fiim exhibits an increase in puncture rçcict~nc:e and an increased
tensile strength over the existing films used for shrink wrapping. Additional layers can inc l~-de, as
desired, a barrier or oxygen layer for shrink wrapping food products and pallets of food. If the filrn
structure is more than one layer, the layers can be joined by any suitable means. Co-extrusion is
pl ~r~ d .
For purposes of this application a m-LLDPE is an ethylene-alpha-olefin random copolymer
having densities preferably from about 0.917 to 0 940 g/cm3. The alpha-olefin comonomer used in
m-LLDPE is usually butene-1, hexene-1 or octene-1; however, other comonomers can be used, such
as 5-methyl-1-pentene, or any other C3-C8 alpha olefin. Branches ofthe plere.led m-~LDPE are
short chain branches, 6 carbons and less, due to the comonomer, and the m-ELDPEs do not have
many long branches off the main chain. The branching of this resin controls density and
crystallinity. The preferred density of the metallocene LLDPE used in the film structure of the
present invention is 0.917 to 0.924 g/cm3. The shrink wrap films ofthe present invention comprised
of such m-LLDPE exhibit improved haze and gloss.
The metallocene LLDPE biaxially oriented film of the present invention meets therequirements of the shrink wrap industry by having excellent stretch and reci~t~nce to derorlllalion
and being resistant to abrasion. Thus, the film of the present invention has good m~hin~bility,
adequate slip properties and is resistant to burn through.
For the shrink-wrap market, biaxially oriented films made according to the present invention
offer superior properties. The biaxially oriented films shrink more, and this property, combined with
other desirable properties, makes these BO-m-LLDPE films desirable for the shrink-wrap market.
Higher Densitv BO-m-E-Polvmer Fi1ms
~ ilms of the present invention may also comprises a biaxially-oriented high density
polyethylene (HDPE) based on a metallocene or single-site catalyst (m-HDPE). The prertll~d
density of the metallocene HDPE used in this film structure of the present invention is 0.925 to
0~0 g/cm3. This film exhibits greater impact strength, improved puncture re~i~t~nce, and improved
optics in haze and gloss as compared to a traditional ZN-HDPE.
The biaxially oriented film of the present invention has at least one layer comprised of m-
HDPE that can be used as a replacement for certain applications involving polypropylene or nylon.
A m-HDPE film can be used as a barrier layer in multilayer films for p~cL ~ng or for skin
p~ k~in~ films. The m-HDPE film of the present invention has a high molecular weight and is
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extrusion blown, and as biaxially oriented it can be used to produce clear shopping bags for use in
a reta;l application. This BO-m-HDPE film can also replace polyethylene-nylon structures for uses
as containers for food, medicine and the like.
A m-HDPE film can preferably undergo a blown film extrusion process with at least one
additional film layer. The resulting film is a thin gauge, single or multilayer m-~IDPE film with
good heat seal performance, excellent strength and uniform thickness or roll ~uality.
Industrial applicability
Films according to the present invention are especially useful for p~c~ ing individually
wrapped items, for instance, musical compact discs, software computer packages, musical tapes, and
pallets of bulk items. However, the invention is not necessarily limited by the product contained
within a package made from the inventive film as the film can suitably be used whenever a
"wrapping" is desired, such as for winterizing windows from the influx of cold air.
Furthermore, the inventive film and packages therer~ -l exhibit excellent clarity and seal
strength. Thus, the inventive film and packages are suitable for the food service industry (e.g.,
hospitals, schools, restaurants, fast food, etc.) and the shrink Wl~lppi~lg industry ~e.g., stretch film
used in the retail music business, computer software business, toy business, and the construction
business) where a~ea-~nce and seal strength are especially h~lpol~ and long shelf-life may be
required.
Numerous applications are contemplated for these biaxially oriented m-E-polymer films of
the present invention. The high clarity, variable stifflnes~, and the engineerable oxygen (OTR) and
water vapor transmission rates (WVTR) make these films highly advantageous in applications such
as modified-atmosphere packaging for fresh ~uits, vegetables, meats and the like. These biaxially
oriented m-E polymer films conform to the contours of products very well as compared to, for
instance, oriented polypropylene. Thus, these biaxially oriented m-E polymer films are desirable
as label stock (pressure sensitive, adhesive and the like).
These biaxially oriented m-E polymer films can have a relatively high one-percent secant
modulus, which means that these films can be relatively stiff. Further, these films have a relatively
low water vapor transfer rate (WVTR), which is good for keeping dry foods dry and moist foods
moist. The stiffness of these BO-m-E polyrner films conveys a sense of crispness, making these
filrns desirable for p~ckz~ging dry snacks such as cookies, crackers, potato chips, corn chips, peanuts,
pretzels and the like, while the low WVTR keeps the snacks dry and crisp. Similarly, these films
are well suited for applications including cereal packaging as an inside liner/container, particularly
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since these films can be heat sealed. For such applications either single or multilayer films and/or
bags can be used.
The high percentage shrink and the low oxygen transfer rate (OTR) makes the biaxially
~riPnted m-E-polymer films particularly well suited for shrink-wrap psck~n~ of fresh, cooked, and
processed meats, poultry, fish, and the like, as well as other moist foods subject to spoilage.
T~ese films can be met~lli7ed for p?lck~gng of foods and for decorative applications, and/or
they can be made opaque for par~.k~ging, for instance, candy bars. On the other hand there are
numerous nonp~ck~ginP~ applications for which these films are su;table such as for document or
photo protective covers.
The package formed from the film structure of the present invention can also contain
resealing means positioned away from a peripheral heat seal. For instance, a slot means can be
provided on one interior face and tab means on an opposing interior face of the package such that
after the heat seal is cut away, a consumer can reseal the package by pressing the tab into mating
engagement with the slot. Such resealing means or slot and tab means are existing in the art.
Printing can be applied directly to the film if desired and ink can be imparted to the film by
a fiexographic or rotogravure apparatus. For food p~t~.k~ging the ink employed must be suitable for
the application. The printing can be placed upon an outer or inner exposed surface or upon an inner
surface of a multilayer film.
To make a package easier to open, a tear line can be provided. The film can be laser scored
to form a single line or parallel lines of weakness in at least one layer of film of the present invention
by partially vaporizing the film with a beam of radiant energy. The score lines form a tear path in
a multilayer film. When the film is formed in a package, the score line rnay extend across the entire
surface of the film.
EXAMPLES
A series of films were made and subsequently oriented by a tenter frame process. The
properties of both oriented and non-oriented films (UO) were compared. To further demonstrate
the di~erences, comparisons to commercial clear films were also included
Definitions and Test Protocols
For the purposes of this application, parameters and tests referenced herein are defined as
prov~ded in Table 1. A procedure for measuring shrinkage is provided below.
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Table 1.
Prop~ Units Definiti~ or Test
Density (d) g/cm3 ASTM D-792
CDBI % described within
Melt Index (MI) dg/min ASTM-}238(E)
Molecular weight distril~ution (MWD) none GPC
I2l/12 (MIR) none ASTMD-1238
Haze % ASTM D-1003
Gloss % ASTM D-2457
Dart Drop Inpact ~50 g/mil ASTM D-1709
Puncture (peak and break energy) Ib ~nd in-lb/mil Exxon Method
Oxygen Transmission Rate (OIR) cm~-mil/100 in2atm. Exxon Method
/24 hrs ~! 25~C
WatervaporTla~ s:~ionR~le (WVIl~) g-mil/lOOin2/day (ASTME96) ~ 38~C
Tensile at Yield K psi ASTM D-882
F~l~n~ti(~natyield % ASlMD-882
IJltimate Tensile (at Breal;) K psi ASTM D~82
Elongation at Breal; % ASTM D-882
Secant Modulus (1 %) K psi ASTM D-882
Flmf~n~ rf Tear g/mil ASTM D-1922
~hrinkst~ % Described below
To measure shrinkage, condition a sample at 23 ~ 2~ C and 50 ~ 5% relative h~lmit1ity for
at least 16 hours prior to testing and 40 hours following fabrication. From a clean, wrinkle-free
portion of the film sample, cut three to five circular specimens using a 100 mm circular die. The
specimens should be representative of the entire film width. Draw a line on each ~ecilllen
in~lic~ting machine direction (MD)
Rub a small amount of talc on a heat I esi~ l tile and preheat the tile for uniform test results.
Place a film spe-.im~n on the preheated tile using tweezers. Center a heat gun one to two inches over
the sample and turn on the heat gun for about 45 seconds. Shrinkage will begin immedi~tely, and
ultimate shrinkage occurs after about 10-45 seconds depending on film thickness. Cool the sample
vrith air. Repeat this procedure for each specimen Measure each specimen along the line indicating
the machine direction and subtract the length in mm from 100 mm to yield a percent shrinkage in
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the machine direction. To determine a percent shrinkage in a transverse direction, repeat the
measurements along a transverse axis.
Example I
Exceed~ 301 (now EXCEED~ 357C32), an ethylene copolymer with hexene-1 m~nllf~ctllred
with a metallocene catalyst by Exxon Chemical Co., Baytown, Texas, having a 0.917 g/cm3 density,
a 3 .4 melt index and having a peak melting point of 1 1 5~C (by DSC), was cast into a 15 mil fflm on
aKillion extruder. Subse~luently, the film was placed in a T.M. Long film stretch laboratory tenter
frame orientation device and oriented to Sx5, indicating the amount of orientation or stretch in the
machine direction ~MD) and transverse direction (TD), respectively. The stretcher was
m~nllf~ctllred by T.M. Eong of 405 Bridge Street, Summerville, NJ 07986. Orientation tempe.aLules
used were 235, 245 and 250~F. These }30-m-E-ppolymer films, by co~ ,alison to an unoriented
film cast (cast-UO) of the same m-E-polymer ~EXCE~D 301) to a gauge comparable to that
obtained in the BO-m-E-polymer films, had nearly compatable haze and gloss properties as the cast
film but otherwise the physical strength properties (tensile and secant modulus~ of the BO-m~
polymer films were dramatically improved. The tensile at yield (TD) improved by 200 to 300%,
and the secant modulus improved by l S0 to 200% (MD or TD). Tensile at yield, elongation at yield,
and ultimate tensile all were improved by the orientation, while Flmçn~orf tear, as may have been
expected, worsened with orientation. As compared to blown films of EXCEED~) 301 this biaxial
orientation improved clarity, tensile at yield, and secant modulus.
Example 2
A film sample from EDC ] 03 (now EXCEED~ 350D60), an ethylene-hexene copolymer
of density 0.917 g/cm3, 1.0 melt index, 17.2 melt index ratio, CDBI about 58, MWD about 2.3, and
peak melting point 11 8~C (by ~SC) m~nuf?~çt~ red by Exxon Chemical Co., Baytown, Texas, was
made to a gauge of 1.62 mil and a 5 x 8 biaxial orientation. Two films ofthe EXCEED 350 D60
resin were also made by a blown bubble process, one to a greater gauge (3 mil) and the other to a
lower gauge (0.8) than that of the oriented film and these blown films (blown-lJO) were not
lllt~ oriented. Fig. I is a radar plot that compares the final film properties measured for the BO
Em-E-polymerfilm and the unoriented blown m-E-polymer film of 0.8 mil gauge.
F.xam~le 3
A biaxially oriented polypropylene ~BOPP) film commercially available as Bicor~B from
Mobil Chemical Co., Beaumont, Texas, and is believed to be based on a homopolymer
polypropylene is compared in its properties with respect to a 5 x 8 biaxially oriented film of EDC
103 (or EXCEED 350 D60) at 1.62 mil gauge vs. Bicor B~) at 1.25 mil in Figure 2. Compared to
:
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the oriented film prepared from EDC 103, an ethylene-hexene copolymer of 3 mole % hexene,
d = 0.917 ~/cm3, MI = 1 prepared from a metallocene catalyst, the BO EDC 103 film, by
com~alison to the BOPP film, exhibited reduced haze and 1 % secant modulus; comparable tensile
strength and WVTR., greater Elmendorf tear strength and OTR.
F.xample 4
Two film samples, Exceed~) 377X600, an ethylene copolymer with hexene-l, having a 0.922
g/cm3 density and Exceed~ 399X60, an ethylene copolymer with hexene-l, having a 0.926 g/cm3
density, were biaxially oriented to a 6 x 6 degree using a T.M. Long stretcher. For the Exceed~
377X 60 sample, haze improved from 17.7 for an unoriented blown film to 1.1 % for a biaxially
oriented film, while for the Exceed~), 399 x 60 sample, haze improved from 13.8 to 2.9 % co~ ,al~;;d
to its blown film analogue. The biaxial orientation caused both orient films to become much stiffier,
as indicated by a higher one-percent secant modulus. For both film samples the tensile s~le~ at
yield and the ultimate tensile strength increased signific~ntly with orientation.
Example 5
Exceed~)301 ~now EXCEED 357 C 32~, an ethylene-hexene copolymer of 3 mole %
hP~C~nt?~ density 0.917 g/cm3, melt index of 3.4 was processed by the procedure of Example 1 into
a S x 5 biaxially oriented film and its mechanicai/physical/chemical properties were d~lel.. i..ed,
inr.~ 1in~ its WVTR. A film of comparable WVTR comprising a HDPE (d = 0.96 g/cm3) p~ed
by a blown bubble procedure (and commercially available from Exxon and employed for various
services) -- such as is the case for preparing cereal box liner films of an HDPE -- and the pl~,p~l lies
ofthe blown HDPE film were determined. Figure 3 presents a comparison of the properties of the
respective films wherein between the two film samples the highest value of a ~lm pl~elly is
necl as 100% and the value of that property in the other film is plotted as a percentage of that
highest film property vaiue. Accordingly, as shown by Figure 3, the WVTR value of each film is
100%. The biaxially oriented Exceed~ 301 film had the greatest dart impact and Elmendorftear
resi~t~n~.e (the HOPE film being of less than 15% of these values) and the least haze (plotted as
1/HAZE). The HDPE film had the greatest secant modulus, with the biaxially oriented Exceed~
301 film having about 35 % ofthe secant modulus ofthe HDPE film. The com~a.ison establishes
that a biaxially oriented m-E-polymer film of this invention would be a ready and desirable
repl~cement film for HDPE films now made for service as cereal box liner films.
Example 6
EXCEED po}ymers o~three differing densities; namely EXCEED 350 D60 of density 0.917
g/cm3, EXCEED 377 X 60 of density 0.922 g/cm3 and EXCEED 399 X 60 of density 0.926 g/cm3;
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were produced as biaxially oriented films wherein the EXCEED 350 D60 was 5 x 8 oriented and
the other films were 6 x 6 oriented films. Radar plot Pig. 4 illustrates the relative differences in the
properties for these films.
Table 2 hereafter identifies the resin and film properties respecting all cast, blown and
biaxially oriented films that were prepared and reported by these Examples 1-6.
Although the present invention has been described ;n considerable detail with reference to
certain preferred versions thereof, other versions are possible. For example, films, especially
oriented films, have been exeniplified in the present application. Those sl~illed in the art will
appreciate that numerous modifications to these p,~relled embo~ nt;"L~ can be made without
departing from the scope of the invention. Therefore, the spirit and scope of the appended claims
should not be limited to the description of the preferred version contained herein.
