WO2010059448A1

WO2010059448A1 - Cavitated polymeric films

Info

Publication number: WO2010059448A1
Application number: PCT/US2009/063700
Authority: WO
Inventors: Kwangjin Song
Original assignee: Exxonmobil Oil Corporation
Priority date: 2008-11-24
Filing date: 2009-11-09
Publication date: 2010-05-27

Abstract

This disclosure relates to cavitated polymer films that include at least one layer that includes a polyolefin and at least one nonpolar polymer cavitating agent, wherein within the layer, the polyolefin forms a continuous phase and the cavitating agent constitutes a dispersed or co-continuous phase. The nonpolar polymer cavitating agent creates isolated and/or interconnected voids within the layer upon orientating the film in at least one direction and reduces plate out or build up during extrusion.

Description

CAVITATED POLYMERIC FILMS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Application Serial No. 61/117,501, filed November 24, 2008, the contents of which are incorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] This disclosure relates to opaque polymeric films comprising a polyolefm and cavitating agent polymers. More particularly, the disclosure relates to opaque polymeric films comprising a class of cavitating agents having an MFR at 260⁰C > 30 g/10 min.

BACKGROUND OF THE INVENTION

[0003] Opaque polymeric films are used in many applications as packaging or labeling materials, battery separators, filters, and etc. Various organic and inorganic cavitating agents have been used in the production of such films. For example, organic materials such as polybutylene terephthalate ("PBT"), polyethylene terephthalate ("PET"), polyethylene 2,6- napthalate) ("PEN"), polycarbonate ("PC"), nylons, cross-linked polystyrene, polystyrene, acetals, acrylic resins, polyacrylate, poly(N-vinylcarbozole), polyvinylcyclohexane, polyvinyl chloride, polyacrylonitrile, and inorganic materials such as glass, metal, and ceramic have all been used as cavitating agents. Other cavitating agents include polymorphic crystals of polymers such as β-polypropylene.

[0004] The presence of the cavitating agent in a layer of a film structure during orientation of the film structure induces voids within the polymeric material of the layer. The voids scatter light thereby causing the film structure to be opaque. The voids also impart the film structure with enhanced permeability to gas and moisture. Therefore, cavitated opaque polymeric films are also termed as microporous polymeric films.

[0005] Polar polymers, either crystalline such as PBT or amorphous such as PC and polyacrylate, are often used as a cavitating agent for polyolefm films, as they create a high interfacial tension with the nonpolar polyolefm matrix, which promotes the formation of a well-dispersed multiphase morphology that is suitable for voiding in-situ during orientation. However, polar polymers are hydrophilic, thus sensitive to hydrolytic breakdown, and readily degrade into lower molecular weight species when they are extruded with nonpolar polymers as a blend component. [0006] The low molecular weight species such produced are known to migrate to and build up on the surface of the processing apparatus, e.g., screws, barrel walls, melt pipes, screen packs, dies, etc. The build up often sloughs off the metal surfaces and passes into the film as sizable deposits of hard, eggshell-type impurities. These impurities are one of the primary causes of film splitting during orientation and optical non-uniformity of the opaque film. In order to mitigate the hydrolytic breakdown, the polar cavitating agent polymer is generally dried thoroughly to a moisture level as low as 200 wppm or less before being extruded as a film. [0007] Other polymeric cavitating agents have been used to attempt to improve the opacity and machinability of oriented polymeric films. For example, U.S. Patent No. 5,573,717 discloses voided polyolefm films, where the voided layer contains an amorphous void-initiating polymer. The amorphous void-initiating polymer has a crystallinity below 5%, a glass transition temperature in the range of 70 to 300⁰C, a vicat softening point in the range of 70 to 200⁰C, a M_w in the range of 500 to 500,000, and a refractive index in the range of 1.3 to 1.7.

[0008] U.S. Patent No. 6,048,608 discloses an opaque oriented polypropylene film using a cyclic olefmic copolymer cavitating agent, The cyclic olefmic copolymer has a heat deflection temperature in the range of 75 to 22O⁰C. The disclosed cavitating agents have a melt flow rate substantially less than 30 g/10 min. [0009] Japanese Patent Publication 8-73618 discloses an opaque polyolefm film containing crystalline polyolefms and 2 to 20 wt.% of a cyclic olefmic copolymer. The cyclic olefmic copolymer has a softening point in the range of 70 to 200⁰C, an intrinsic viscosity in the range of 0.05 to 10 dl/g at 135⁰C, and a glass transition temperature in the range of 50 to 19O⁰C. [0010] U.S Patent No. 6,528,155 discloses an opaque polymeric film comprising a base layer comprising a polyolefm matrix and as a cavitating agent, solid, non-hollow particles of a syndiotactic polystyrene having a syndiotacticity of 92% racemic pentad or higher. [0011] U.S. Patent No. 6,828,019 discloses a thermoplastic film containing an enhanced printing skin layer and a core layer of a polypropylene homopolymer comprising an orientation enhancing polymer and a beta-crystal nucleator of polypropylene, the entirety of which is incorporated herein by reference.

[0012] A need still exists for a cavitating agent that allows for improved film opacity and machinability. Preferably, such a cavitating agent does not require drying before extrusion; produces little or no extrusion plate out or build up on the processing apparatus; has little or no interaction with any catalyst residues, TiO₂, or any other additives contained within the film, thus allowing for the recycling of film scraps and edge trim; and produces uniform voiding across the film width, reducing processing issues such as bumps or curvature. SUMMARY OF THE INVENTION

[0013] In one aspect embodiments of the invention provide a polymeric film comprising at least one layer, the layer comprising: a polyolefm; and at least one cavitating agent, wherein the cavitating agent is a nonpolar polymer that is incompatible with the polyolefm; wherein the cavitating agent has a MFR > 30 g/10min; where the MFR is determined according to ASTM D-1238 and at one of the following conditions 1) at 19O⁰C and 2.16 kg for nonpolar polymers having a crystalline melting temperature (Tm) less than or equal to 15O⁰C, 2) at 23O⁰C and 2.16 kg for nonpolar polymers having a Tm from > 150 to 200⁰C, 3) at 2.16 Kg and 26O⁰C for nonpolar polymers having Tm from > 200 to 24O⁰C and for amorphous nonpolar polymers having Tg from about 130 to 25O⁰C; or, 4) at 300⁰C and 1.2 Kg and for nonpolar polymers having Tm from > 240 to about 300⁰C; and wherein the film has a density that is at least 10% less than the density of a film made from the polyolefin without the cavitating agent.

[0014] The polyolefin may include polypropylene, polyethylene, ethylene-propylene copolymers, propylene-butene copolymers, ethylene -propylene-butylene terpolymers, and blends thereof. Where the polyolefin is a polyethylene, particularly useful polyethylenes include high density polyethylenes. Particularly useful polypropylenes include isotactic polypropylene, high crystalline polypropylene, beta-nucleated polypropylene, and blends thereof. In some embodiments the polyolefin has a melt index < 10 g/10min. [0015] In some embodiments the cavitating agent comprises a cyclic olefmic homopolymer and/or a cyclic olefmic copolymer having a MFR at 26O⁰C of > 50 g/10min and a Tg > 15O⁰C. In other embodiments, the cavitating agent comprises a polymethylpentene polymer is selected from polymethylpentene homopolymers and/or polymethylpentene copolymers having a MFR at 26O⁰C of > 50 g/10min and a Tm ranging from 200 to 25O⁰C. [0016] In still other embodiments, the cavitating agent comprises an isotactic and/or syndiotactic polystyrene homopolymer or copolymer having a MFR at 300⁰C > 20 g/10min and a Tm ranging from 220 to 29O⁰C. [0017] In particular embodiments, the cavitating agent comprises 30 wt.% to 70 wt.% polymethylpentene polymer and 30 wt.% to 70 wt.% syndiotactic polystyrene polymer, based on the total weight of the cavitating agent.

[0018] While not being particularly limited, generally the at least one layer may include from about 0.5 wt. % to about 60 wt. % of the cavitating agent, based on the total weight of the layer.. In some embodiments, the cavitating agent is present in an amount ranging from 3 to 60 wt.%. In particular embodiments, the at least one layer comprises 0.5 to 20 wt.% of the cavitating agent based on the total weight of the layer. Particular cavitating agents have a maximum particle dimension ranging from 10 to lOOOnm. [0019] Embodiments of the invention may also be those wherein the cavitating agent further comprises cross-linked polystyrene, cross-linked silicone resins, solid or hollow preformed glass or polymer spheres, metal beads or spheres, ceramic spheres, calcium carbonate, talc, chalk, nanoclays, rigid polymers of high Tg, cross-linked polymers, metals, metal complexes, carbon nanotubes, ceramics, ceramic complexes, and combinations thereof and combinations thereof.

[0020] Preferred films are stretched in at least one direction, more preferably the film is biaxially oriented, sequentially or simultaneously. In some embodiments, the at least one layer includes isolated and/or interconnected voids, the voids created by the cavitating agent upon orienting the film. [0021] In another aspect, the invention relates to a method of producing a polymeric film, comprising: extruding at least one core layer comprising polyolefm and at least one cavitating agent through a sheet-forming die; and cooling the extrudate to form a cast sheet; and orienting the cast sheet in at least one direction; wherein the cavitating agent has a MFR at 260⁰C > 30 g/10min and comprises a cyclic olefmic homopolymer or copolymer, a polymethylpentene polymer, an isotactic polystyrene polymer, a syndiotactic polystyrene polymer, or a blend thereof. In some embodiments, the method further includes co-extruding at least one additional layer on at least one side of the core layer.

[0022] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description and appended claims. BRIEF DESCRIPTION OF THE FIGURES

[0023] Figure 1 is a picture of plate out on the extruder screw taken after 5 time repeated extrusions of a 50/50 wt.% blend of PP/polar cavitating agent such as PBT in the Example. [0024] Figure 2 is a picture of plate out on the extruder screw taken after 5 time repeated extrusions of a 50/50 wt.% blend of PP/nonpolar cavitating agent such as COP, PMP or sPS in the Example.

[0025] Figure 3 shows scanning electron microscopy ("SEM") images of cross-sections for the cast sheets of polyolefin/cavitating agent blends in the Example: (a) 95/5 wt.% PP/PBT blend and (b) 95/5 wt.% PP/COC blend. The fresh surface of the cross-section is prepared by freeze fracturing the sheet perpendicular (left images) and parallel to the machine direction ("MD") (right images).

DETAILED DESCRIPTION OF THE INVENTION [0026] Various specific embodiments, versions and examples of the invention will now be described, including preferred embodiments and definitions that are adopted herein for purposes of understanding the claimed invention. While the following detailed description gives specific preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the invention can be practiced in other ways. For purposes of determining infringement, the scope of the invention will refer to any one or more of the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. Any reference to the "invention" may refer to one or more, but not necessarily all, of the inventions defined by the claims. [0027] As used herein, the term "polymer" refers to the product of a polymerization reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc. As used herein, unless specified otherwise, the term "copolymer(s)" refers to polymers formed by the polymerization of at least two different monomers. For example, the term "copolymer" includes the copolymerization reaction product of propylene and an alpha-olefm (α-olefm), such as ethylene or the polymerization production of ethylene, propylene, and butene. [0028] As used herein the term "nonpolar polymer" refers to a polymer that increases in weight < 0.1% after immersion in distilled water maintained at 23°C for 24 hrs. according to ASTM D 570-98.

[0029] As used herein, "intermediate" is defined as the position of one layer of a multilayer film wherein said layer lies between two other identified layers. In some embodiments, the intermediate layer may be in direct contact with either or both of the two identified layers. In other embodiments, additional layers may also be present between the intermediate layer and either or both of the two identified layers. [0030] The term "incompatible polymers" is used herein to refer to two or more polymeric blending partners, which are at least partially immiscible, and in some instances wholly immiscible.

Polymeric Film [0031] The polymeric film of this disclosure comprises at least one layer, which comprises an incompatible polymer blend of at least one polyolefin and at least one cavitating agent. The polyolefin may form an essentially continuous phase within the layer, while the cavitating agent may constitute a dispersed and/or co-continuous phase. In some embodiments the cavitating agent creates isolated voids within the layer upon stretching the film in at least one direction during the orientation process. In other embodiments, the cavitating agent creates interconnected voids. While in other embodiments, the film may have both isolated and interconnected voids upon stretching the film in at least one direction. [0032] The film may be a single-layer film or a multilayer film. For example, in some embodiments, the film may comprise a core layer, one or more skin layers on either side of the core layer, and/or one or more tie layers disposed between the core layer and the one or more skin layers. At least one layer of the opaque polymeric film comprises a film- forming polyolefin and a cavitating agent. In preferred embodiments, the core layer comprises the film-forming polyolefin and the cavitating agent. [0033] The film has a density that is less than the density of a film made from the same polyolefin polymer without the cavitating agent. In some embodiments, the opaque polymeric film has a density that is at least 10% less, or at least 12% less, or at least 15% less, or at least 20% less, or at least 30% less, than the density of a film made of the same polyolefin polymer without the cavitating agent. The film's density may be calculated by measuring the yield and volume of the film specimen. [0034] The film has a density of less than or equal to 0.70 g/cm³, or less than or equal to 0.65 g/cm³, or less than or equal to 0.60 g/cm³, or less than or equal to 0.580 g/cm³, or less than or equal to 0.570 g/cm³. Particular films have a density ranging from, 0.35 g/cm³ to 0.87 g/cm³; 0.35 g/cm³to 0.80 g/cm³; or 0.35 g/cm³ to 0.70 g/cm³. Other films may have a density ranging from 0.40 g/cm³ to 0.65 g/cm³ or 0.40 g/cm³to 0.60 g/cm³. Some preferred films have a density of 0.40 g/cm³to 0.70 g/cm³, more preferably a density of 0.40 g/cm³to 0.65 g/cm³. [0035] Particular films of this invention have high opacity and a low light transmittance. Some opaque polymeric films have an opacity of > 85%, or > 90%, or > 92% and/or light transmittance of less than or equal to 30%, or less than or equal to 25%, or less than or equal to 20%, or less than or equal to 19%. Opacity is the opposite of transparency and is a function of the scattering and reflection of light transmitted through the film. Light transmittance is a function of light passing more directly through the film. Accordingly, a highly reflective film may provide high opacity while allowing light transmission. [0036] The film may further comprise one or more additives. Examples of useful additives include, but are not limited to, opacifying agents, pigments, colorants, slip agents, antioxidants, anti-fog agents, anti-static agents, anti-block agents, moisture barrier additives, gas barrier additives, hydrocarbon resins, hydrocarbon waxes, fillers such as calcium carbonate, diatomaceous earth and carbon black, and combinations thereof. Such additives may be used in effective amounts, which vary depending upon the property required. If the polymer substrate is a multilayer film, the additive(s) may be included in any one or more of the layers.

Polvolefin Polymer

[0037] The opaque polymeric film includes a polyolefin in one or more layers. For example, the polyolefin may be selected from polypropylene, polyethylene, ethylene- propylene copolymers, propylene-butene copolymers, ethylene-propylene-butylene terpolymers, and blends thereof. The polyolefin may be produced by Ziegler-Natta catalyst, metallocene catalyst, or any other suitable means.

[0038] The polyethylene of this disclosure may be high density polyethylene ("HDPE"), medium density polyethylene ("MDPE"), low density polyethylene ("LDPE"), linear low density polyethylene ("LLDPE"), and combinations thereof. [0039] In one embodiment, the film comprises HDPE, having a density of about 0.940 g/cm³ or more, or preferably 0.952 g/cm³ or more. The HDPE may have a density in the range of about 0.952 to about 0.962 g/cm³. The HDPE may have a melt index ("MI") in the range of about 0.001 to about 10.0 g/10min, or preferably in the range of about 0.01 to about 2.0 g/10min, and a melting point in the range of about 130 to about 148⁰C. [0040] In another embodiment, the film comprises MDPE having a density in the range of about 0.926 to about 0.940 g/cm³. [0041] In yet another embodiment, the film comprises LDPE having a density of about 0.926 g/cm or less, or in the range of 0.89 to 0.93 g/cm , and a MI of about 7 g/10min, or in the range of about 6 to about 9 g/10min.

[0042] In a further embodiment, the film comprises LLDPE having a density in the range of about 0.90 to about 0.94 g/cm³, or more preferably in the range of about 0.910 to about 0.926 g/cm³. The LLDPE may have a MI in the range of about 1 to about 10 g/10 min. In some embodiments, the LLDPE may be a copolymer of ethylene and a minor amount of a higher olefin comonomer containing 4 to 10 carbon atoms, such as for example, butene-1, hexene-1, or octene-1. [0043] The polypropylene of this disclosure may be isotactic polypropylene ("iPP"), syndiotactic polypropylene ("sPP"), high crystalline polypropylene ("HCPP"), beta polypropylene ("β-PP") or blends thereof, having a MFR in the range from 0.001 to 10 g/10min, preferably 0.01 to 5 g/10min. Preferred polypropylenes have a crystallinity in the range of about 30 to about 80%, and a crystalline melting temperature in the range of about 140 to about 17O⁰C.

[0044] The isotacticity of the iPP is 89% or greater and preferably, 90% or greater, as measured by the fraction of mesopentad ("m-pentad") with ¹³C-NMR. The mesopentad fraction refers to the portion of isotactic conformation in the entire conformation. The sPP may have a syndiotacticity of 89% or greater, as measured by the fraction of racemic pentads ("r-pentad") with ¹³C-NMR. The mean length of the syndiotactic sequences may be greater than 20 and preferably, greater than 25.

[0045] The ethylene-propylene copolymers of this disclosure have a MFR at 23O⁰C less than or equal to 10 g/10min and may be ethylene propylene mini-random copolymer, ethylene propylene random copolymer, ethylene-propylene block copolymers, or blends thereof. Preferably, the comonomer is selected from one or more of ethylene or butene. The propylene is generally present in such co- or terpolymers at > 90 wt.%. [0046] In a preferred embodiment, the polyolefin polymer is selected from HDPE, iPP, HCPP, β-PP, ethylene-propylene copolymers, and blends thereof. [0047] In yet another embodiment, the polyolefin polymer is β-PP, as described in U.S. Patent No. 6,828,019, the entirety of which is incorporated herein by reference. Beta nucleation includes creating beta-form crystals of polypropylene comprising a beta- crystalline nucleating agent. Substantially any beta-crystalline nucleating agent ("beta nucleating agent" or "beta nucleator") may be used, as disclosed in U.S. Patent Nos. 4,386,129; 4,975,469; 5,681,922; 5,231,126; 5,491,188; 6,235,823; and 6,005,034. The amount of beta nucleators to be used may be from 0.0002 to 8 wt.%, and preferably 0.01 to 2 wt.%, based on the weight of polypropylene.

Cavitating Agent [0048] At least one layer of the film comprises a cavitating agent that is a nonpolar polymer and is also incompatible with the polyolefm components of the film. In embodiments where the film is a multilayer film, the cavitating agent is preferably added to the core layer of the polyolefm. The cavitating agent is incompatible at extrusion temperatures with the polyolefm(s) contained in the layer(s) to which the cavitating agent is added. The cavitating agent can be either amorphous or crystalline, and may be synthesized by metallocene catalysts, Ziegler-Natta catalysts, or other suitable catalyst systems. In some embodiments, the cavitating agent is substantially brittle in character; is easily flowable; and flows without forming any non-uniformities, such as un-melted spots or gels at extrusion temperatures. [0049] In one embodiment, the cavitating agent comprises a cyclic olefmic polymer, selected from a cyclic olefmic homopolymer ("COH"), a cyclic olefmic copolymer ("COC"), and blends thereof. The term cyclic olefin homopolymer includes polymers resulting from the polymerization of one C₄ to C12 cyclic olefin, e.g. substituted norbornenes, in the chain backbone or polymers resulting from the polymerization of two or more C₄ to Ci₂ cyclic olefins in the chain backbone. Cyclic olefin copolymers also include polymers that comprise at least one cyclic olefin monomer, such as C₄ to Ci₂ cyclic olefins or norbornene, and at least one linear aliphatic olefin monomer, such as ethylene, propylene, butylene, etc. The copolymers can be random, block, graft, or any possible structure, having at least one linear co-monomer in the chain backbone. [0050] The cyclic olefmic polymer may have a MFR at 26O⁰C and 2.16 Kg of > 30 g/10min, or > 40 g/10min, or > 50 g/10min, or > 60 g/10min, or > 75 g/10min, or > 80 g/10min, or > 100 g/10min, or > 110 g/10min, or > 115 g/10min, or > 120 g/10min, or > 125 g/10min, or > 130 g/10min, or > 150 g/10min. The cyclic olefmic polymer may have a MFR in the range of 30 to 1000 g/10min, or in the range of 30 to 750 g/10min, or in the range of 40 to 500 g/10min, or in the range of 50 to 300 g/10min, or in the range of 60 to 250 g/10min, or in the range of 70 to 200 g/10min. Preferably, the cyclic olefmic polymer may have a MFR in the range of 60 to 300 g/10min, or in the range of 75 to 250 g/10min, or in the range of 90 to 200 g/10min.

[0051] The cyclic olefmic polymer may have a Tg > 13O⁰C, or > 15O⁰C, or > 16O⁰C, or > 165⁰C, or > 17O⁰C. The cyclic olefmic polymer may have a Tg in the range of 13O⁰C to 25O⁰C, or in the range of 15O⁰C to 23O⁰C, or in the range of 16O⁰C to 200⁰C.

[0052] The cyclic olefinic polymer may comprise one or more different cyclic olefinic homopolymers or cyclic olefinic copolymers, wherein said different means that the cyclic olefinic polymers each have a different structure and/or different property, but such that all of the cyclic olefinic polymers fall within the broadest description of those cavitating agents herein. For example, the cavitating agent of the cyclic olefinic polymer may comprise a blend of a first cyclic olefinic polymer having a MFR of 50 g/10min with a second cyclic olefinic polymer having a MFR of 75 g/10min. Alternatively, the cavitating agent may comprise a blend of a first cyclic olefinic polymer comprising a C₆ cyclic olefin and a second cyclic olefinic polymer comprising norbornene. [0053] In another embodiment, the cavitating agent comprises an isotactic polystyrene homopolymer, an isotactic polystyrene copolymer, a syndiotactic polystyrene homopolymer, a syndiotactic polystyrene copolymer, or blends thereof, which have a Tm in the range of about 220 to about 290⁰C and a MFR at 300⁰C and 1.2 Kg in the range of about 20 to 500 g/10min. Examples of substituted styrene for copolymers include para-methylstyrene, meta- methylstyrene, ethylstyrene, butylstyrene, dimethylstyrene, chlorostyrene, bromostyrene, fluorostyrene, methoxystyrene and acetoxy methylstyrene, as disclosed in U.S. Patent Nos. 5,502,133 and 6,528,155, the entirety of which is incorporated herein by reference. [0054] In yet another embodiment, the nonpolar cavitating agent comprises a polymethylpentene ("PMP") polymer having a Tm in the range of about 200 to about 250⁰C and a MFR at 26O⁰C and 2.16 Kg in the range of about 30 to about 1 ,000 g/1 Omin, preferably about 50 to 500 g/1 Omin. The PMP polymer comprises a homopolymer of 4-methyl-l- pentene, a copolymer of 4-methyl-l-pentene and α-olefm, and mixtures thereof. The α-olefms of co-monomer may comprise 2 to 20 carbon atoms such as ethylene, propylene, 1-butene, 1- hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-tetradecene, 1-hexadecene, 1- octadecene and 1-eicosene, as described in U.S. Patent No. 6,270,909. The 4-methyl-l- pentene is preferably present in such copolymers at > 90 wt.%. The PMP polymer may contain any suitable additives, such as heat stabilizers. [0055] In a preferred embodiment, the nonpolar cavitating agent is a cyclic olefmic polymer selected from a cyclic olefmic homopolymer, a cyclic olefmic copolymer, and combinations thereof. In yet another embodiment, the nonpolar cavitating agent is a blend of nonpolar polymers comprising cyclic olefmic polymers, TMP polymers, iPS polymers, and sPS polymers. In some embodiment, the cavitating agent is a blend of TMP polymers and sPS polymers wherein the amount of sPS polymers is in the range of 5 to 95 wt.%, or preferably in the range of 30 to 70 wt.%.

[0056] The nonpolar cavitating agent may comprise one or more other organic or inorganic cavitating agents, selected from the group of cross-linked polystyrene, cross-linked silicone polymers, solid or hollow pre-formed glass or polymer spheres, metal beads or spheres, ceramic spheres, calcium carbonate, talc, chalk, or combinations thereof. The amount of the other cavitating agents when present is not particularly limited and may be from about 1 to about 60 wt.% or preferably from about 5 to about 30 wt.%, based on the total weight of the nonpolar cavitating agent. [0057] The nonpolar cavitating agent may further comprise nano-particles, which are organic or inorganic in character, such as nanoclays, rigid polymers of high Tg, cross-linked polymers, metals, metal complexes such as metal oxides or nitrides, carbon nanotubes, ceramics, ceramic complexes, and combinations thereof. Nano-particles having a maximum particle dimension ranging from 10 to lOOOnm are particularly useful and may help to increase the modulus, rigidity and thermal property of the cavitating agent and thus improve its voiding performance.

[0058] The amount of the cavitating agents to be incorporated is not particularly limited and may correspond to the desired degree of void formation upon stretching. The opaque film may comprise a cavitating agent or a blend of cavitating agents in an amount in the range of 0.5 to 70 wt.%, or in the range of 1 to 60 wt.%, or in the range of 3 to 60 wt.%, or in the range of 5 to 50 wt.%, or in the range of 5 to 30 wt.%, or in the range of 5 to 20 wt.%, or in the range of 5 to 15 wt.%, based on the total weight of the layer to which the cavitating agent is added. [0059] In some embodiments, the film comprises at least one cyclic olefmic polymer in an amount in the range of 0.5 to 70 wt.%, or in the range of 1 to 60 wt.%, or in the range of 3 to 60 wt.%, or in the range of 3 to 50 wt.%, or in the range of 3 to 30 wt.%, or in the range of 3 to 20 wt.%, or in the range of 5 to 15 wt.%, based on the total weight of the layer to which the cyclic olefmic polymer is added. [0060] In certain embodiments, the cavitating agent comprises at least one polymethylpentene polymer or at least one syndiotactic polystyrene in an amount in the range of 0.5 to 70 wt.%, or in the range of 1 to 60 wt.%, or in the range of 3 to 60 wt.%, or in the range of 3 to 50 wt.%, or in the range of 3 to 30 wt.%, or in the range of 3 to 20 wt.%, or in the range of 5 to 15 wt.%, based on the total weight of the layer to which the cavitating is added.

Film Orientation

[0061] The multilayer film may be uniaxially or biaxially oriented. Orientation in the direction of extrusion is known as machine direction ("MD") orientation. Orientation perpendicular to the direction of extrusion is known as transverse direction ("TD") orientation. Orientation may be accomplished by stretching or pulling a film first in the MD followed by the TD. Orientation processes may be sequential or simultaneous, depending upon the desired film features. Preferred orientation ratios are commonly between about three to about seven times in the MD and between about four to about ten times in the TD. [0062] Blown films may be oriented by controlling parameters such as take up and blow up ratio. Cast films may be oriented in the MD direction by take up speed, and in the TD through use of tenter equipment. Blown films or cast films may also be oriented by tenter- frame orientation processes subsequent to the film extrusion process, in one or both directions. Typical commercial orientation processes are sequential and simultaneous tenter processes.

Film Structure

[0063] The opaque polymeric film may be a single-layer film or a multilayer film. In embodiments where the film is a single-layer film, the single-layer comprises a film-forming polyolefm and a cavitating agent. In embodiments where the film is a multilayer film, the cavitating agent may be in one or more layers of the film. In preferred embodiments, the core layer comprises the cavitating agent.

[0064] The multilayer film may comprise a core layer, one or more skin layers on either side of the core layer, and/or one or more tie layers disposed between the core layer and the one or more skin layers. [0065] In some embodiments, the film may be a multilayer film that has one or more skin layers. The skin layer is an optional layer and when present is generally the outermost layer of the multilayer film. If the multilayer film has two skin layers, they are the outermost layers of the film and are on opposite sides of the core layer from each other. The skin layer(s) may be contiguous to the core layer, or alternatively may be contiguous to one or more other layers, such as, a tie layer described below.

[0066] The skin layer(s) may comprise any film-forming polyolefm as described above. In some embodiments, the skin-layer polymers may be chosen to provide the film with a desired functionality. For example, one or both of the skin layers may be provided to improve the film's barrier properties, processability, printability, or compatibility for metallization, coating, or lamination to other films or substrates. [0067] In some embodiments, the multilayer film comprises one or more tie layers. The one or more tie layers are generally located intermediate the core layer and the one or more skin layers. The tie layer may generally comprise any film-forming polymer, as described above. In some embodiments, the tie layer may comprise an adhesion promoting material such as a polar modified polyolefm. [0068] One or both of the outer exposed surfaces of the film may be surface-treated to increase the surface energy of the film to render the film receptive to metallization, coatings, printing inks, and/or lamination. The surface treatment can be carried out according to one or more of the methods known in the art. Preferred methods include, but are not limited to, corona discharge, flame treatment, plasma treatment, chemical treatment, treatment by means of a polarized flame, and combinations thereof. [0069] One or both of the outer exterior surfaces of the multilayer film may be metallized. Generally, the metallized layer is one of the outer skin layers. However, if no skin layer is present, the surface of a core layer may be metallized. Such layers may be metallized using conventional methods, such as vacuum deposition of a metal layer such as aluminum, copper, silver, chromium, or mixtures thereof. In some embodiments, the film may first be treated, for example by flame treatment, and then be treated again in the metallization chamber, for example by plasma treatment, immediately prior to being metallized. [0070] One or more coatings, such as for barrier, printing, and/or processing, may be applied to one or both of the outer surfaces of the films. Such coatings may include acrylic polymers, such as ethylene acrylic acid ("EAA"), ethylene methyl acrylate copolymers ("EMA"), polyvinylidene chloride ("PVdC"), poly(vinyl)alcohol ("PVOH"), ethylene(vinyl)alcohol ("EVOH"), and combinations thereof. The coatings are preferably applied by an emulsion coating technique, but may also be applied by co-extrusion, and/or lamination. The coating composition may be applied to the film as a solution or in any other conventional manner, such as by gravure coating, roll coating, dipping, spraying, and the like. Any excess aqueous solution can be removed by squeeze rolls, doctor knives, and the like. The film can be stretched in the MD, coated with the coating composition and then stretched perpendicularly in the TD. In another embodiment, the coating can be carried out after biaxial orientation is complete.

[0071] In some embodiments, an intermediate primer coating may be applied to the film before it is coated. Examples of useful primer materials are well known in the art and include, but are not limited to, epoxy and poly(ethylene imine) materials. The primer provides an overall adhesively active surface for thorough and secure bonding with the subsequently applied coating composition. The primer may be applied to the film by conventional solution methods, for example, by roller application.

Industrial Application

[0072] The opaque polymeric films may be useful as substantially stand-alone film webs or they may be coated, metallized, and/or laminated to other film structures. The films according to the present disclosure may be prepared by any suitable means. Preferably, the film is co-extruded, oriented, and then prepared for its intended use such as by coating, printing, slitting, or other converting methods. Preferred methods comprise co-extruding, then casting and orienting the multilayer film. [0073] In one embodiment, the film may be formed by extruding at least one core layer, through a flat sheet extruder die at a temperature in the range of 200⁰C to 26O⁰C, casting the film onto a cooling drum and quenching the film. The sheet is then stretched 3 to 7 times its original length, in the machine direction (MD), followed by stretching 4 to 10 times its original width in the transverse direction (TD). The film is then wound onto a reel. Optionally, one or both of the external surfaces may be coated and/or flame treated or corona treated before winding. In other embodiments, another layer such as one or more skin layers, and one more tie layers, may be co-extruded with the core layer to form a multilayer film. [0074] The film may be useful as a flexible packaging film to package an article or good, such as a food item or other product. In some applications, the film may be formed into a pouch type of package, such as may be useful for packaging a beverage, liquid, granular, or dry-powder product. The film may also be useful in labeling applications or in shrink film applications. In other applications, the film may be used as a battery separator or filter. [0075] In one embodiment, a method of preparing an opaque polymeric film comprises the steps of extruding at least one core layer comprising a polyolefm and at least one cavitating agent and orienting the film in at least one direction. The method may further comprise additionally co-extruding, one or more skin layers, and/or one or more tie layers. [0076] In some embodiments, the cavitating agent has a melt flow rate of > 30 g/10 min and does not require drying prior to extrusion. Thus, film scraps and edge trim can be recycled.

[0077] In some embodiments, the method further comprises the steps of enclosing a product or article within at least a portion of the co-extruded film, engaging a first portion of the skin layer with a second portion of the skin layer at a seal area, and applying pressure and heat at the seal area, optionally for a determined duration of time, to cause the first portion to engage with the second portion to create at least one of a fin seal, a lap seal, and a crimp seal in the seal area. [0078] While the illustrative embodiments have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the disclosure. To the extent that this description is specific, it is solely for the purposes of illustrating certain embodiments of the disclosure and should not be taken as limiting the present inventive concepts to those specific embodiments. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein, but rather that the claims should be construed as encompassing all the features of patentable novelty, which reside in the present disclosure, including all features, which would be treated as equivalents thereof by those skilled in the art to which the disclosure pertains.

Test Procedure [0079] When possible, standard ASTM tests are used to determine the opaque polymeric film's properties.

[0080] The melt index ("MI") and melt flow rate ("MFR") are measured according to ASTM D- 1238, wherein one minute preheat on the sample to provide a steady temperature for the duration of the experiment is used. Measurement conditions are 19O⁰C and 2.16 kg for polymers having Tm less than or equal to 15O⁰C such as polyethylene; 23O⁰C and 2.16 kg for polymers having Tm from about 150 to about 200⁰C such as polypropylene; 2.16 Kg and 26O⁰C for polymers having Tm from about 200 to 24O⁰C such as polymethylpentene polymers and poly(butylene terephthalate) polymers or for amorphous polymers having Tg from about 130 to 25O⁰C such as cyclic olefmic polymers; and 1.2 Kg and 300⁰C for polymers having Tm from about 240 to about 300⁰C such as isotactic and syndiotactic polystyrene polymers. MI and MFR are typically expressed in units of g/10min. [0081] The density of the polymer resin is measured according to ASTM D-792.

[0082] The total film thickness is measured using an optical gauge Model # 283-20 available from Beta LaserMike, Dayton, OH. The thickness of the individual film layers can be measured using a JSM 6400 scanning electron microscopy (SEM) from Jeol, Japan. [0083] The density of the film is calculated by measuring the yield and volume of the film specimen. Yield is the measure of the film's coverage per unit weight, which is measured according to ASTM D-4321.

[0084] Glass transition temperature ("Tg"), percent crystallinity ("%Xc") and crystalline melting temperature ("Tm") of the polymer are determined according to ASTD D3418 using a Differential Scanning Calorimeter (DSC, Perkin Elmer Pyris 1 Thermal Analysis System). Polymer sample of 15 to 20 mg, equilibrated to 25°C, is heated beyond its Tm and is then cooled to 25°C at a rate of 10°C/min. The sample is allowed to equilibrate for 3 minutes and is then reheated again beyond its T_m at a rate of 10°C/min. The thermal output, recorded as the area under the melting peak, is a measure of the heat of fusion. The melting temperature is defined as the point where, during the second melting of the sample, the peak endothermic heat flow required to maintain the heating rate of 10°C/min is observed. The thermal output for the highest order of polypropylene is estimated at 189 J/g (i.e., 100% crystallinity is equal to l89 J/g).

[0085] Opacity represents a substrate's light blocking ability. The test measures two reflectance values, using Technidyne BNL-3 Opacimeter and following ASTM D-589. [0086] Light Transmittance is the percentage of incident light that passes through a film, and is measured according to ASTM D- 1003 with a BKY-Garner XL-211 haze-guard plus hazemeters.

[0087] The micrograph images of scanning electron microscopy (SEM) are taken with JSM 6400 (Jeol). Fresh cross-section surfaces are prepared by freeze fracturing the sample perpendicular and parallel to the MD at -13O⁰C using liquid nitrogen. The fresh surfaces are subsequently coated with platinum with a vacuum sputter. SEM images are then taken at an acceleration voltage 25 KV. [0088] The degree of build up of the cavitating agent during extrusion is examined by a 5 pass repeated extrusion test at 26O⁰C and 4.5 Kg/hr with 50/50 wt.% blends of polyolefin and cavitating agents. A single screw extruder is used, which had 2.54 cm diameter screw, 0.32 cm circular die, and 250 mesh screen pack. PP/PBT blends are dried to a moisture level of 100 wppm or less prior to each pass extrusion. However, the other blends tested are not dried, except for removal of surface water by hot air after each pass extrusion. After 5 pass extrusions, the extruder screw is removed and the degree of material build-up is visually examined and rated. The degree of plate out/build up present on the extruder screw is rated: a rating of 1 indicates that amount of build up, which filled the screw channel is less than 5%; a rating of 2 indicates that the amount of build up in the screw channel is in the range of 5% to 50%; a rating of 3 indicates that the amount of build up in the screw channel is greater than 50%.

EXAMPLES

[0089] The polymeric films comprising a polyolefin and a cavitating agent will now be further described with reference to the following non-limiting examples. In the Example, COH and COC represent respectively a cyclic olefmic homopolymer of norbornene and a cyclic olefmic copolymer of ethylene and norbornene, which have varying melt flow rates, MFR and glass transition temperatures, Tg. A listing of various components used in the Example is in Table 1. [0090] Various co-extruded biaxially oriented films are made and tested as Examples. All the Examples are 5 -layer films, which are co-extruded using 5 single screw extruders having a total output of about 230 Kg/hour. All the blend compositions in the Examples are prepared by pellet blending of each component polymer prior to extrusion. The polymers are melted and coextruded at 25O⁰C, quenched on a casting chill roll and in a water bath, which are maintained at temperatures of about 3O⁰C. The cooled extrudate is then stretched at 12O⁰C or lower 4 to 7 times in the machine direction ("MDX") using the combination of slow and fast speed rollers. The film is then further stretched at 162⁰C or lower 7 to 12 times in the transverse direction ("TDX") using the tenter frame. An example of a representative 5-layer film structure is shown in Table 2. All the Example films had the layers made from the same component polymers, as shown in Table 3, except for the core layer that is made with varying materials. The measured properties for the films produced at 5.4 MDX are shown in Table 4. TABLE 1 - Components Used in the Examples

TABLE 2 - Representative Structure of Example Films

Examples 1 to 8 (El to E8)

[0091] The core layers of Examples 1 to 8 are, as shown in Table 3, PP4612 or HCPP3270 blended with COC Topas^® 4017X or Topas^® 6015X in the amount from 5 to 30 wt.%. The MFR of Topas^® 4017X is in the range from 30 to 1,000 g/10min. Topas^® 6015X had a Tg of 150° C and a MFR of 80 g/10min. As shown in Table 4, the Example films had low light transmittance, high opacity and low density, without any build up of the cavitating agent onto the machine surface.

Examples 9 to 11 (E9 to El 1)

[0092] Table 3 shows the core layers of Examples 9 to 11 consisting of 95 wt.% PP4612 or HCPP3270 and 5 wt.% cavitating agent. The cavitating agent of Example 9 is a nanocomposite COC Topas^® 4017X containing 5 wt.% nanoclay Cloisite^® 15 A; Example 10 is a blend of 40/60 wt.% Topas^® 4017X/COH; and in Example 11 the cavitating agent is a COH having a MFR of 200 g/10min and a Tg of 210° C. As shown in Table 4, the Example films had low light transmittance, high opacity and low density, without any build up of the cavitating agent onto the machine surface.

TABLE 3 - Composition of the Core

* Not available, * Not applicable

Examples 12 to 13 (E12 to E13)

[0093] As shown in Table 3, the core layer of Example 12 is HCPP3270 containing a 5 wt.% cavitating agent blend of 50/50 wt.% Topas^® 4017X/Xarec^® 300ZC and Example 13 a 30 wt.% cavitating agent blend of 50/50 wt.% Topas^® 6510X/Xarec^® 201AE. As shown in Table 4, the Example films had low light transmittance, high opacity and low density, without any build up of the cavitating agent onto the machine surface.

TABLE 4 - Properties and Degree of Build Up of Films

Examples 14 to 18 (E14 to E18)

[0094] The core layers of Examples 14 and 15 comprised respectively the polyolefm blends of HDPE XM6030A in β-PP BI4020TSP and HCPP3270 at a ratio of 40/30 wt.%, as shown in Table 3. Examples 16 and 17 had respectively a core layer of β-PP BI4020TSP and HCPP3270. The cavitating agents are respectively Xarec^® 201AE, Xarec^® 201AE/TPX^® DX350 blend, TPX^® DX350, and TPX^® DX820 at varying loading and blending ratios. As show in Table 4, the Example films had low light transmittance, high opacity and low density, without any build up of the cavitating agent onto the machine.

Comparative Example 1 (Cl)

[0095] The core layer of Comparative Example 1 is a blend of 95/5 wt.% PP4612/PBT Valox™ 295. The biaxially oriented film sample is prepared at MDX in the range of 4.8 to 5.4. The film produced by the PBT cavitating agent had relatively good opacity and low density, as shown in Table 4, but along with a substantial amount of build-up onto the machine surface as shown in FIGURE 1 compared to FIGURE 2 of the Example.

TABLE 5 - MFR of PP/Cavitating Agent Blend in 5 Pass Extrusion

[0096] MFR increase (ΔMFR) is further measured as a function of repeated extrusion pass for 50/50 wt.% polypropylene/cavitating agent blends, wherein

ΔMFR = MFR after 1st pass extrusion - MFR after 5th pass extrusion As shown in Table 5, at the identical conditions, ΔMFR produced by the polar cavitating agent PBT is about 2 times greater than the nonpolar cavitating agent COC, showing that the polar cavitating agent caused substantially more resin degradation.

[0097] Without being bound by theory, it is believed that a polar cavitating agent having a relatively low Tg and a relatively low decomposition temperature, as compared to the processing temperature of the polyolefm, produced increasingly more plate out because of increasingly more phase separation during extrusion between the polar and nonpolar components. In the PP/PBT blend, the cavitating agent PBT is polar and has a Tg and an upper processing limit of about 3O⁰C and about 26O⁰C, respectively. Thus, the PBT pellets turned readily into a rubbery state in extruders before melting and decomposed rapidly at elevated PP extrusion temperatures around 26O⁰C.

[0098] In the compression zone of the extruder screw, while the PP pellet is in the early stage of partial melting, the PBT pellet is still in a rubbery state due to its higher Tm of about 225⁰C than about 16O⁰C of PP. As the screw rotated, the rubbery PBT is separated from the polypropylene due to their density differences (1.34 g/cm³ compared to 0.9 g/cm³), thus causing the PBT to precipitate onto the screw root. As shown in FIGURE 1, over time, the PBT pellets accumulated and became further compacted by the screw motion, thus sticking onto the screw to form egg-shell like plate-outs. It is believed that this early-stage plate-out in extruder occurred primarily due to the relatively low Tg and relatively high Tm of PBT. [0099] During the melt state, the PBT phase underwent various degradations, such as thermal breakdown, hydrolysis, catalytic decomposition, etc., producing a substantial amount of highly reactive low molecular weight species. These by-products are found to go through shear-induced fractionation during extrusion, and thus to diffuse and reside onto the walls of the extruder barrels and melt lines. It is believed that with time, the low molecular weight species further accumulated onto the metal surface via coordination and degraded into further lower molecular weight species, leading to plate out slough off from the metal surface.

Comparative Examples 2 to 3 (C2 and C3)

[00100] The core layers of Comparative Examples 2 and 3 are respectively a blend of 99.8/0.2 wt.% HCPP3270/TPX^® DX820 and a blend of 95/5 wt.% PP4612/Xarec^® 30Z, as shown in Table 3. The Xarec^® 30Z has a MFR of 3 g/10min and a Tg of 93⁰C. As shown in Table 4, the Comparative Example films have high light transmittance, low opacity and high film density, although they produced no build up onto the machine surface. [00101] FIGURE 3 shows the morphology of 95/5 wt.% PP4612/cavitating agents, where the polar and nonpolar cavitating agents are respectively PBT Valox™ 295 (a) and COC Topas® 4017X (b). The SEM images showed a spherical particle (left) and fibril (right) when viewed respectively perpendicular and parallel to the MD. At the equivalent conditions, the nonpolar COC fiber had an average dimension of about 1.0 μm diameter and about 20 μm length while the polar PBT fiber had about 2.0 μm diameter and about 100 μm length. This fine dispersion of the nonpolar cavitating agent in the polyolefm matrix is believed, without being bound by theory, to greatly improve the optical and mechanical properties of the voided film. [00102] As demonstrated above, all the Example films made by the incompatible polymer blends of polyolefm and nonpolar cavitating agents showed superior performance in processability and properties over the Comparative Example films, that is, no build up of the cavitating agent onto the machine surface, low light transmittance, high opacity and low density of the voided film. [00103] All patents and patent applications, test procedures (such as ASTM methods, UL methods, and the like), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted. [00104] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty, which reside in the present invention, including all features, which would be treated as equivalents thereof by those skilled in the art to which the invention pertains. [00105] The invention has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A polymeric film comprising at least one layer, the layer comprising: a. a polyolefin; and b. at least one cavitating agent, wherein the cavitating agent is a nonpolar polymer that is incompatible with the polyolefin; wherein the cavitating agent has a MFR > 30 g/10min; where the MFR is determined according to ASTM D- 1238: i) at 19O⁰C and 2.16 kg for nonpolar polymers having Tm less than or equal to 15O⁰C; ii) at 23O⁰C and 2.16 kg for nonpolar polymers having a Tm from >

150 to 200⁰C; iii) at 2.16 Kg and 26O⁰C for nonpolar polymers having Tm from > 200 to 24O⁰C and for amorphous nonpolar polymers having Tg from about 130 to 25 O⁰C ; or iv) at 300⁰C and 1.2 Kg for nonpolar polymers having Tm from about 240 to about 300⁰C; and wherein the film has a density that is at least 10% less than the density of a film made from the polyolefin without the cavitating agent.

2. The film of claim 1, wherein the cavitating agent comprises a cyclic olefinic homopolymer or a cyclic olefinic copolymer, a polymethylpentene polymer, an isotactic polystyrene polymer, a syndiotactic polystyrene polymer, and blends thereof.

3. The film of claim 2, wherein the polyolefin comprises high density polyethylene and the polyolefin is selected from the group consisting of isotactic polypropylene, high crystalline polypropylene, beta-nucleated polypropylene, and blends thereof.

4. The film of any of claims 1 to 3, wherein the polyolefin has a melt index < 10 g/10min.

5. The film of any of claims 1 to 4, wherein cavitating agent comprises a cyclic olefinic homopolymer and/or a cyclic olefinic copolymer having a MFR at 26O⁰C of > 50 g/10min and a Tg > 15O⁰C.

6. The film of any of claims 1 to 4, wherein the polymethylpentene polymer is selected from polymethylpentene homopolymers and/or polymethylpentene copolymers having a MFR at 26O⁰C of > 50 g/10min and a Tm ranging from 200 to 25O⁰C.

7. The film of claim 6, wherein the cavitating agent comprises 30 wt.% to 70 wt.% polymethylpentene polymer and 30 wt.% to 70 wt.% syndiotactic polystyrene polymer, based on the total weight of the cavitating agent.

8. The film of any of claims 1 to 4, wherein the cavitating agent comprises an isotactic and/or syndiotactic polystyrene homopolymer or copolymer having a MFR at 300⁰C > 20 g/10min and a Tm ranging from 220 to 29O⁰C.

9. The film of any of claims 1 to 8, wherein the layer comprises 0.5 to 20 wt.% of the cavitating agent based on the total weight of the layer.

10. The film of any of claims 1 to 9, wherein the cavitating agent further comprises cross- linked polystyrene, cross-linked silicone resins, solid or hollow pre-formed glass or polymer spheres, metal beads or spheres, ceramic spheres, calcium carbonate, talc, chalk, nanoclays, rigid polymers of high Tg, cross-linked polymers, metals, metal complexes, carbon nanotubes, ceramics, ceramic complexes, and combinations thereof and combinations thereof.

11. The film of any of claims 1 to 10, wherein the cavitating agent is present in an amount ranging from 3 to 60 wt.%, based on the total weight of the layer.

12. The film of any of claims 1 to 11, wherein the polyolefin comprises polypropylene, polyethylene, ethylene-propylene copolymers, propylene-butene copolymers, ethylene-propylene-butylene terpolymers, and blends thereof.

13. The film of any of claims 1 to 12, wherein the film is biaxially oriented.

14. The film of any of claims 1 to 13 wherein the layer includes isolated and/or interconnected voids, the voids created by the cavitating agent upon orienting the film in at least one direction.

15. A method of producing a polymeric film, comprising: a. extruding at least one core layer comprising polyolefin and at least one cavitating agent through a sheet-forming die; b. cooling the extrudate to form a cast sheet; and c. orienting the cast sheet in at least one direction; wherein the cavitating agent has a MFR > 30 g/10min; where the MFR is determined according to ASTM D- 1238: i) at 19O⁰C and 2.16 kg for nonpolar polymers having Tm less than or equal to 15O⁰C; ii) at 23O⁰C and 2.16 kg for nonpolar polymers having a Tm from >

150 to 200⁰C; iii) at 2.16 Kg and 26O⁰C for nonpolar polymers having Tm from >

200 to 24O⁰C and for amorphous nonpolar polymers having Tg from about 130 to 25O⁰C; or iv) at 300⁰C and 1.2 Kg for nonpolar polymers having Tm from about 240 to about 300⁰C; and wherein the film has a density that is at least 10% less than the density of a film made from the polyolefm without the cavitating agent.

16. The method of claim 15 wherein the cavitating agent comprises a cyclic olefmic homopolymer or copolymer, a polymethylpentene polymer, an isotactic polystyrene polymer, a syndiotactic polystyrene polymer, or a blend thereof.

17. The method of claim 16 further comprises co-extruding at least one additional layer on at least one side of the core layer.