Patent Publication Number: US-2018043670-A1

Title: Multilayer Films and Methods Thereof

Description:
FIELD OF THE INVENTION 
     This invention relates to films, and in particular, to multilayer films comprising polyethylene, lap seals comprising such films, packages made therefrom, and methods for making such films. 
     BACKGROUND OF THE INVENTION 
     Laminate films are widely used in a variety of packaging applications. Good mechanical properties such as elongation, tensile strength, dart impact strength, and puncture to resistance are desired to ensure package integrity, especially during packaging and transportation. In flexible laminate film structures, a sealant film is adhered to a substrate film commonly made of biaxially oriented polyester (PET), biaxially oriented polypropylene (BOPP), or biaxially oriented polyamide (BOPA). Ethylene polymers, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) prepared by Ziegler-Natta catalyst in a gas phase process, and blends thereof are generally employed in the art to form a sealant film. While such ethylene polymers work reasonably well because they provide relatively low-cost solutions, their properties restrict film mechanical performance for a number of applications. Efforts to address disadvantages caused by LDPE and LLDPE include incorporating and increasing metallocene polyethylenes (mPEs) in sealant films. 
     However, in the case of the above conventional laminate structure featuring a polyethylene sealant and a substrate made of, e.g. PET or BOPP, the subsequent sealing process favors the sealable skin of the polyethylene sealant to be sealed together because sealing is preferred between two sealable skins both made of polyethylene. A seal commonly known as fin seal is accordingly formed, which costs more materials than a lap seal does. In addition, the conventional laminate structure does not fit in with recycling, which also creates sustainability concern for use in flexible packaging. Therefore, while the above conventional laminate bears mechanical properties, such as bending stiffness, desired by packaging processability and attractive appearance and “hand-feel”, it is difficult for laminate film manufacturers to reduce overall consumption of polymer materials without compromising film performance and recycling advantages. 
     WO 2014/042898 provides ethylene-based copolymers, particularly ethylene-based polymers having about 80.0 to 99.0 wt % of polymer units derived from ethylene and about 1.0 to about 20.0 wt % of polymer units derived from one or more C 3  to C 20  α-olefin comonomers; the ethylene-based polymer having a local maximum loss angle at a complex modulus, G*, of 2.50×10 4  to  1 . 00 × 10   6  Pa and a local minimum loss angle at a complex modulus, G*, of 1.00×10 4  to 3.00*×10 4  Pa. This patent application also includes articles, such as films, produced from such polymers and methods of making such articles. 
     U.S. Patent Publication No. 2012/0100356 relates to a multilayer blown film with improved strength or toughness comprising a layer comprising a metallocene polyethylene (mPE) having a high melt index ratio (MIR), a layer comprising an mPE having a low MIR, and a layer comprising a HDPE, and/or LDPE. Other embodiments have skin layers and a plurality of sub-layers. At least one sub-layer includes an mPE, and at least one additional sub-layer includes HDPE and/or LDPE. The mPE has a density from about 0.910 to about 0.945 g/cm 3 , MI from about 0.1 to about 15 g/10 min, and melt index ratio (MIR) from about 15 to 25 (low-MIR mPE) and/or from greater than 25 to about 80 (high-MIR mPE). The process is related to supplying respective melt streams for coextrusion at a multilayer die to form a blown film having the inner and outer skin layers and a plurality of sub-layers, wherein the skin layers and at least one of the sub-layers comprise mPE and at least one of the sub-layers comprise HDPE, LDPE or both. Draw-down, blow-up ratios and freeze-line distance from the die are controlled to facilitate a high production rate. 
     U.S. Pat. No. 8,586,676 provides a polymer composition and articles made therefrom. The composition includes: (a) a polyethylene having (i) at least 50 wt % ethylene moieties; and (ii) up to 50 wt % of a C 3  to C 20  comonomer moieties, a density of about 0.860 to about 0.965 g/cm 3 , a melt index of about 0.1 to about 10.0 g/10 min and a branching index of about 0.96 to about 1.0; and (b) a polyethylene having: (i) at least 65 wt % ethylene moieties; and (ii) up to 35 wt % of a C 3  to C 20  comonomer moieties, the wt %s based upon the total weight of the latter polyethylene, a density of about 0.905 to about 0.945 g/cm 3 , a melt index (MI) of about 0.1 to about 10.0 g/10 min, and a branching index (g′) of about 0.7 to about 0.95. 
     WO 2009/109367 discloses the use linear polyethylene having an MIR indicative of the presence of some long chain branching having a density of 0.91 to 0.94 g/cm 3  determined according to ASTM D4703/D1505, an I 2.16  (MI) of from 0.05 to 1 g/10 min, and I 21.6  /I 2.16 (MIR) of more than 35, the MI and MIR being determined according to ASTM 1238 D at 190° C., and a difference between the MD Tensile force based on ASTM D882-02 at 100% elongation and MD 10% Offset yield of a reference film as defined herein having a thickness of 25 [mu]m of at least 15 MPa. This patent application also relates to coextruded film structures made using such linear polyethylene in the core layer of a multilayer structure to provide easily processable, strong, highly transparent films. 
     That said, what is needed in the art is a laminate film to better balance between the mechanical properties required by stronger films for a given thickness and increased cost-effectiveness in polymer materials for a maintained or even improved film performance. Applicant has found that such objective can be achieved by applying a polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers in each of the two substrate outer layers and the substrate core layer, and preparing two substrate inner layers between the substrate core layer and each substrate outer layer, each having a density of at least about to 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, to produce a substrate of a multilayer laminate film. The inventive film, in addition to having a bending stiffness at a comparable or even improved level, can outperform a conventional laminate film using a conventional non-polyethylene substrate in other mechanical properties, including elongation, puncture energy, and low-temperature bag drop performance. Particularly, in the presence of a sealant also comprising a polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, a lap seal can be obtained by sealing the sealable skin of the polyethylene sealant to that of the polyethylene substrate instead, which can save the extra amount of polyethylene incurred by a fin seal. Furthermore, such laminate films can be recyclable, All of the above advantages make the inventive laminate film well suited for flexible packaging applications favoring a good balance between mechanical properties and material cost-effectiveness. Therefore, by replacing. the currently available selection of substrates with a polyethylene one as described herein, the inventive laminate film can be qualified as a desired alternative to conventional laminates. 
     SUMMARY OF THE INVENTION 
     Provided are multilayer films comprising polyethylene, lap seals comprising such films, packages made therefrom, and methods for making such films. 
     In one embodiment, the present invention encompasses a multilayer film comprising a substrate and a sealant, wherein the substrate comprises: (a) two substrate outer layers and a substrate core layer between the two substrate outer layers, wherein each of the two substrate outer layers and the substrate core layer comprises a first polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , a melt index (MI), I 2.16 , of about 0.1 to about 15 g/10 min, a molecular weight distribution (MWD) of about 1.5 to about 5.5, and a melt index ratio (MIR), I 21.6 /I 21.6 , of about 10 to about 100; and (b) two substrate inner layers, each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, wherein each substrate inner layer is between the substrate core layer and each substrate outer layer. 
     In another embodiment, the present invention relates to a method for making a multi layer film comprising a substrate and a sealant, comprising the step of: (a) preparing two substrate outer layers and a substrate core layer between the two substrate outer layers, wherein each of the two outer layers and the core layer comprises a first polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; (b) preparing two substrate inner layers, each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, wherein each substrate inner layer is between the substrate core layer and each substrate outer layer; (c) preparing a substrate comprising the layers in steps (a) and (b); and (d) forming a film comprising the substrate in step (c). 
     The multilayer film described herein or made according to any method disclosed herein may have at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. 
     Preferably, the sealant comprises two sealant outer layers and a sealant core layer between the two sealant outer layers, wherein each of the two sealant outer layers and the sealant core layer comprises a fourth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the fourth polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. 
     Also provided are lap seals comprising any of the multilayer films described herein or made according to any method disclosed herein. Packages comprising the lap seals described herein are also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic representation of film structures for the inventive films in Examples 1 and 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     Various specific embodiments, versions of the present invention will now be described, including preferred embodiments and definitions that are adopted herein. 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 present invention can be practiced in other ways. Any reference to the “invention” may refer to one or more, but not necessarily all, of the present inventions defined by the claims. The use of headings is for purposes of convenience only and does not limit the scope of the present invention. 
     As used herein, a “polymer” may be used to refer to homopolymers, copolymers, interpolymers, terpolymers, etc. A “polymer” has two or more of the same or different monomer units, A “homopolymer” is a polymer having monomer units that are the same. A “copolymer” is a polymer having two or more monomer units that are different from each other. A “terpolymer” is a polymer having three monomer units that are different from each other. The term “different” as used to refer to monomer units indicates that the monomer units differ from each other by at least one atom or are different isomerically. Accordingly, the definition of copolymer, as used herein, includes terpolymers and the like. Likewise, the definition of polymer, as used herein, includes copolymers and the like. Thus, as used herein, the terms “polyethylene,” “ethylene polymer,” “ethylene copolymer,” and “ethylene based polymer” mean a polymer or copolymer comprising at least 50 mol % ethylene units (preferably at least 70 mol % ethylene units, more preferably at least 80 mol % ethylene units, even more preferably at least 90 mol % ethylene units, even more preferably at least 95 mol % ethylene units or 100 mol % ethylene units (in the case of a homopolymer)). Furthermore, the term “polyethylene composition” means a composition containing one or more polyethylene components. 
     As used herein, when a polymer is referred to as comprising a monomer, the monomer is present in the polymer in the polymerized form of the monomer or in the derivative form of the monomer. 
     As used herein, when a polymer is said to comprise a certain percentage, wt %, of a monomer, that percentage of monomer is based on the total amount of monomer units in the polymer. 
     For purposes of this invention and the claims thereto, an ethylene polymer having a density of 0.910 to 0.940 g/cm 3  is referred to as a “low density polyethylene” (LDPE); an ethylene polymer having a density of 0.890 to 0.930 g/cm 3 , typically from 0.910 to 0.930 g/cm 3 , that is linear and does not contain a substantial amount of long-chain branching is referred to as “linear low density polyethylene” (LLDPE) and can be produced with conventional Ziegler-Natta catalysts, vanadium catalysts, or with metallocene catalysts in gas phase reactors, high pressure tubular reactors, and/or in slurry reactors and/or with any of the disclosed catalysts in solution reactors (“linear” means that the polyethylene has no or only a few long-chain branches, typically referred to as a g′vis of 0.97 or above, preferably 0.98 or above); and an ethylene polymer having a density of more than 0.940 g/cm 3  is referred to as a “high density polyethylene” (HDPE). 
     As used herein, “core” layer, “outer” layer, and “inner” layer are merely identifiers used for convenience, and shall not be construed as limitation on individual layers, their relative positions, or the laminated structure, unless otherwise specified herein. 
     As used herein, “first” polyethylene, “second” polyethylene, “third” polyethylene, “fourth” polyethylene, “fifth” polyethylene, and “sixth” polyethylene are merely identifiers used for convenience, and shall not be construed as limitation on individual polyethylene, their relative order, or the number of polyethylenes used, unless otherwise specified herein. 
     As used herein, film layers that are the same in composition and in thickness are referred to as “identical” layers. 
     Polyethylene 
     In one aspect of the invention, the polyethylene that can be used for the multilayer film described herein are selected from ethylene homopolymers, ethylene copolymers, and compositions thereof. Useful copolymers comprise one or more comonomers in addition to ethylene and can be a random copolymer, a statistical copolymer, a block copolymer, and/or compositions thereof The method of making the polyethylene is not critical, as it can be made by slurry, solution, gas phase, high pressure or other suitable processes, and by using catalyst systems appropriate for the polymerization of polyethylenes, such as Ziegler-Natta-type catalysts, chromium catalysts, metallocene-type catalysts, other appropriate catalyst systems or combinations thereof, or by free-radical polymerization. In a preferred embodiment, the polyethylenes are made by the catalysts, activators and processes described in U.S. Pat. Nos. 6,342,566; 6,384,142; and 5,741,563; and WO 03/040201 and WO 97/19991. Such catalysts are well known in the art, and are described in, for example, ZIEGLER CATALYSTS (Gerhard Fink, Rolf Millhaupt and Hans H. Brintzinger, eds., Springer-Verlag 1995); Resconi et al.; and I, II METALLOCENE-BASED POLYOLEFINS (Wiley &amp; Sons 2000). 
     Polyethylenes that are useful in this invention include those sold by ExxonMobil Chemical Company in Houston Texas. including HDPE, LLDPE, and LDPE; and those sold under the ENABLE™, EXACT™, EXCEED™, ESCORENE™, EXXCO™, ESCOR™, PAXON™, and OPTEMA™ tradenames. 
     Preferred ethylene homopolymers and copolymers useful in this invention typically have one or more of the following properties: 
     1. an M w  of 20,000 g/mol or more, 20,000 to 2,000,000 g/mol, preferably 30,000 to 1,000,000, preferably 40,000 to 200,000, preferably 50,000 to 750,000, as measured by size exclusion chromatography; and/or 
     2. a T m  of 30° C. to 150° C., preferably 30° C. to 140° C. preferably 50° C. to 140° C., more preferably 60° C. to 135° C., as determined based on ASTM D3418-03; and/or 
     3. a crystallinity of 5% to 80%, preferably 10% to 70%, more preferably 20% to 60%, preferably at least 30%, or at least 40%, or at least 50%, as determined based on ASTM D3418-03; and/or 
     4. a heat of fusion of 300 J/g or less, preferably 1 to 260 J/g, preferably 5 to 240 J/g, preferably 10 to 200 J/g, as determined based on ASTM D3418-03; and/or 
     5. a crystallization temperature (T c ) of 15° C. to 130° C., preferably 20° C. to 120° C., more preferably 25° C. to 110° C., preferably 60° C. to 125° C., as determined based on ASTM D3418-03; and/or 
     6. a heat deflection temperature of 30° C. to 120° C., preferably 40° C. to 100° C., more preferably 50° C. to 80° C. as measured based on ASTM D648 on injection molded flexure bars, at 66 psi load (455 kPa); and/or 
     7. a Shore hardness (D scale) of 10 or more, preferably 20 or more, preferably 30 or more, preferably 40 or more, preferably 100 or less, preferably from 25 to 75 (as measured based on ASTM D 2240); and/or 
     8. a percent amorphous content of at least 50%, preferably at least 60%, preferably at least 70%, more preferably between 50% and 95%, or 70% or less, preferably 60% or less, preferably 50% or less as determined by subtracting the percent crystallinity from 100. 
     The polyethylene may be an ethylene homopolymer, such as HDPE. In one embodiment, the ethylene homopolymer has a molecular weight distribution (M w /M n ) or (MWD) of up to 40, preferably ranging from 1.5 to 20, or from 1.8 to 10, or from 1.9 to 5, or from 2.0 to 4. In another embodiment, the 1% secant flexural modulus (determined based on ASTM D790A, where test specimen geometry is as specified under the ASTM 17790 section “Molding Materials (Thermoplastics and Thermosets).” and the support span is 2 inches (5.08 cm)) of the polyethylene falls in a range of 200 to 1000 MPa, and from 300 to 800 MPa in another embodiment, and from 400 to 750 MPa in yet another embodiment, wherein a desirable polymer may exhibit any combination of any upper flexural modulus limit with any lower flexural modulus limit. The MI of preferred ethylene homopolymers range from 0.05 to 800 dg/min in one embodiment, and from 0.1 to 100 dg/min in another embodiment, as measured based on ASTM D1238 (190° C., 2.16 kg). 
     In a preferred embodiment, the polyethylene comprises less than 20 mol % propylene units (preferably less than 15 mol %, preferably less than 10 mol %, preferably less than 5 mol %, and preferably 0 mol % propylene units) 
     In another embodiment of the invention, the polyethylene useful herein is produced by polymerization of ethylene and, optionally, an alpha-olefin with a catalyst having, as a transition metal component, a his (n-C 3-4  alkyl cyclopentadienyl) hafnium compound, wherein the transition metal component preferably comprises from about 95 mol % to about 99 mol % of the hafnium compound as further described in U.S. Pat. No. 9,956,088. 
     In another embodiment of the invention, the polyethylene is an ethylene copolymer, either random or block, of ethylene and one or more comonomers selected from C 3  to C 20  α-olefins, typically from C 3  to C 10  α-olefins. Preferably, the comonomers are present from 0.1 wt % to 50 wt % of the copolymer in one embodiment, and from 0.5 wt % to 30 wt % in another embodiment, and from 1 wt % to 15 wt % in yet another embodiment, and from 0.1 wt % to 5 wt % in yet another embodiment, wherein a desirable copolymer comprises ethylene and C 3  to C 20  α-olefin derived units in any combination of any upper wt % limit with any lower wt % limit described herein. Preferably the ethylene copolymer will have a weight average molecular weight of from greater than 8,000 g/mol in one embodiment, and greater than 10,000 g/mol in another embodiment, and greater than 12,000 g/mol in yet another embodiment, and greater than 20,000 g/mol in yet another embodiment, and less than 1,000,000 g/mol in yet another embodiment, and less than 800,000 g/mol in yet another embodiment, wherein a desirable copolymer may comprise any upper molecular weight limit with any lower molecular weight limit described herein. 
     In another embodiment, the ethylene copolymer comprises ethylene and one or more other monomers selected from the group consisting of C 3  to C 20  linear, branched or cyclic monomers, and in some embodiments is a C 3  to C 12  linear or branched alpha-olefin, preferably butene, pentene, hexene, heptene, octene, nonene, decene, dodecene, 4-methyl-pentene-1,3-methyl pentene-1,3,5,5-trimethyl-hexene-1, and the like. The monomers may be present at up to 50 wt %, preferably from up to 40 wt %, more preferably from 0.5 wt % to 30 wt %, more preferably from 2 wt % to 30 wt %, more preferably from 5 wt % to 20 wt %, based on the total weight of the ethylene copolymer. 
     Preferred linear alpha-olefins useful as comonomers for the ethylene copolymers useful in this invention include C 3  to C 8  alpha-olefins, more preferably 1-butene, 1-hexene, and 1-octene, even more preferably 1-hexene. Preferred branched alpha-olefins include 4-methyl-1-pentene, 3-methyl-1-pentene, 3,5,5-trimethyl-1-hexene, and 5-ethyl-1-nonene. Preferred aromatic-group-containing monomers contain up to 30 carbon atoms. Suitable aromatic-group-containing monomers comprise at least one aromatic structure, preferably from one to three, more preferably a phenyl, indenyl, fluorenyl, or naphthyl moiety. The aromatic-group-containing monomer further comprises at least one polymerizable double bond such that after polymerization, the aromatic structure will be pendant from the polymer backbone. The aromatic-group containing monomer may further be substituted with one or more hydrocarhyl groups including but not limited to C 1  to C 10  alkyl groups. Additionally, two adjacent substitutions may be joined to form a ring structure. Preferred aromatic-group-containing monomers contain at least one aromatic structure appended to a polymerizable olefinic moiety. Particularly, preferred aromatic monomers include styrene, alpha-methylstyrene, para-alkylstyrenes, vinyltoluenes, vinylnaphthalene, allyl benzene, and indene, especially styrene, paramethyl styrene, 4-phenyl-1-butene and allyl benzene. 
     Preferred diolefin monomers useful in this invention include any hydrocarbon structure, preferably C 4  to C 30 , having at least two unsaturated bonds, wherein at least two of the unsaturated bonds are readily incorporated into a polymer by either a stereospecific or a non-stereospecific catalyst(s). It is further preferred that the diolefin monomers be selected from alpha, omega-diene monomers (i.e., di-vinyl monomers). More preferably, the diolefin monomers are linear di-vinyl monomers, most preferably those containing from 4 to 30 carbon atoms. Examples of preferred dimes include butadiene, pentadiene, hexadiene, heptadiene, octadiene, nonadiene, decadiene, undecadiene, dodecadiene, tridecadiene, tetradecadiene, pentadecadiene, hexadecadiene, heptadecadiene, octadecadiene, nonadecadiene, icosadiene, heneicosadiene, docosadiene, tricosadiene, tetracosadiene, pentacosadiene, hexacosadiene, heptacosadiene, octacosadiene, nonacosadiene, triacontadiene, particularly preferred dienes include 1,6-heptadiene, 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene, 1,10-undecadiene, 1,11-dodecadiene, 1,12-tridecadiene, 1,13-tetradecadiene, and low molecular weight polybutadienes (Mw less than 1000 g/mol), Preferred cyclic dienes include cyclopentadiene, vinylnorbornene, norhomadiene, ethylidene norbomene, divinylbenzene, dicyclopentadiene, or higher ring containing diolefins with or without substituents at various ring positions. 
     In a preferred embodiment, one or more dienes are present in the polyethylene at up to 10 wt %, preferably at 0.00001 wt % to 2 wt %, preferably 0.002 wt % to 1 wt %, even more preferably 0.003 wt % to 0.5 wt %, based upon the total weight of the polyethylene. In to some embodiments, diene is added to the polymerization in an amount of from an upper limit of 500 ppm, 400 ppm, or 300 ppm to a lower limit of 50 ppm, 100 ppm, or 150 ppm. 
     Preferred ethylene copolymers useful herein are preferably a copolymer comprising at least 50 wt % ethylene and having up to 50 wt %, preferably 1 wt % to 35wt %, even more preferably 1 wt % to 6 wt % of a C 3  to C 20  comonomer, preferably a C 4  to C 8  comonomer, preferably hexene or octene, based upon the weight of the copolymer. Preferably these polymers are metallocene polyethylenes (mPEs). 
     Useful mPE homopolymers or copolymers may be produced using mono- or bis-cyclopentadienyl transition metal catalysts in combination with an activator of alumoxane and/or a non-coordinating anion in solution, slurry, high pressure or gas phase. The catalyst and activator may be supported or unsupported and the cyclopentadienyl rings may be substituted or unsubstituted. Several commercial products produced with such catalyst/activator combinations are commercially available from ExxonMohil Chemical Company in Houston, Tex. under the tradename EXCEED™ Polyethylene or ENABLE™ Polyethylene. 
     In a class of embodiments, the multilayer film of the present invention comprises in each of the two substrate outer layers and the substrate core layer a first polyethylene (as a polyethylene defined herein) derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, having a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. In various embodiments, the first polyethylene may have one or more of the following properties: 
     (a) a density (sample prepared according to ASTM D-4703, and the measurement according to ASTM D-1505) of about 0.900 to 0.940 g/cm 3 , or about 0.912 to about 0.935 g/cm 3 ; 
     (b) an MI (I 2.16 , ASTM D-1238, 2.16 kg, 190° C.) of about 0.1 to about 15 g/10 min, or about 0.3 to about 10 g/10 min, or about 0.5 to about 5 g/10 min; 
     (c) an MIR (I 21.6  (190° C., 21.6 kg)/I 2.16  (190° C., 2.16 kg)) of about 10 to about 100, or about 15 to about 80, or about 16 to about 50; 
     (d) a Composition Distribution Breadth Index (“CDBI”) of up to about 85%, or up to about 75%, or about 5 to about 85%, or 10 to 75%. The CDBI may be determined using to techniques for isolating individual fractions of a sample of the resin. The preferred technique is Temperature Rising Elution Fraction (“TREF”), as described in Wild, et al., J. Poly. Sci., Poly. Phys. Ed., Vol. 20, p. 441 (1982), which is incorporated herein for purposes of U.S. practice; 
     (e) an MWD of about 1.5 to about 5.5; MWD is measured using a gel permeation chromatograph (“GPC”) equipped with a differential refractive index (“DRI”) detector; and/or 
     (f) a branching index of about 0.9 to about 1.0, or about 0.96 to about 1.0, or about 0.97 to about 1.0. Branching Index is an indication of the amount of branching of the polymer and is defined as g′=[Rg] 2   br /[Rg] 2   lin . “Rg” stands for Radius of Gyration, and is measured using a Waters 150 gel permeation chromatograph equipped with a Multi-Angle Laser Light Scattering (“MALLS”) detector, a viscosity detector and a differential refractive index detector. “[Rg] br ” is the Radius of Gyration for the branched polymer sample and “[Rg] lin ” is the Radius of Gyration for a linear polymer sample. 
     The first polyethylene is not limited by any particular method of preparation and may be formed using any process known in the art. For example, the first polyethylene may be formed using gas phase, solution, or slurry processes. 
     In one embodiment, the first polyethylene is formed in the presence of a metallocene catalyst. For example, the first polyethylene may be an mPE produced using mono- or bis-cyclopentadienyl transition metal catalysts in combination with an activator of alumoxane and/or a non-coordinating anion in solution, slurry, high pressure or gas phase. The catalyst and activator may be supported or unsupported and the cyclopentadienyl rings may be substituted or unsubstituted. mPEs useful as the first polyethylene include those commercially available from ExxonMobil Chemical Company in Houston, Tex., such as those sold under the trade designation EXCEED™ or ENABLE™. 
     In accordance with a preferred embodiment, the multilayer film described herein further comprises in at least one of the two substrate outer layers a second polyethylene (as a polyethylene defined herein) derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, having a density of about 0.910 to about 0.945 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100. In various embodiments, the second polyethylene may have one or more of the following properties: 
     (a) a density (sample prepared according to ASTM D-4703, and the measurement according to ASTM D-1505) of about 0.910 to about 0.945 g/cm 3 , or about 0.915 to about 0.940 g/cm 3 ; 
     (b) an MI (I 2.16 , ASTM D-1238, 2.16 kg, 190° C.) of about 0.1 to about 15 g/10 min, or about 0.1 to about 10 g/10 min, or about 0.1 to about 5 g/10 min; 
     (c) an MIR  021 . 6  (I 21.6 (190° C., 21.6 kg)/I 2.16  (190° C., 2.16 kg)) of greater than 25 to about 100, or greater than 30 to about 90, or greater than 35 to about 80; 
     (d) a Composition Distribution Breadth Index (“CDBI”, determined according to the procedure disclosed herein) of greater than about 50%, or greater than about 60%, or greater than 75%, or greater than 85%; 
     (e) an MWD of about 2.5 to about 5.5; MWD is measured according to the procedure disclosed herein; and/or 
     (f) a branching index (“g” determined according to the procedure described herein) of about 0.5 to about 0.97, or about 0.7 to about 0.95. 
     The second polyethylene is not limited by any particular method of preparation and may be formed using any process known in the art. For example, the second polyethylene may be formed using gas phase, solution, or slurry processes. 
     In one embodiment, the second polyethylene is formed in the presence of a Ziegler-Matta catalyst. In another embodiment, the second polyethylene is formed in the presence of a single-site catalyst, such as a metallocene catalyst (such as any of those described herein). Polyethylenes useful as the second polyethylene in this invention include those disclosed in U.S. Pat. No. 6,255,426, entitled “Easy Processing Linear Low Density Polyethylene” (Lue), which is hereby incorporated by reference for this purpose, and include those commercially available from ExxonMobil Chemical Company in Houston, Tex., such as those sold under the trade designation ENABLE™. 
     In another preferred embodiment, the multilayer film of the present invention comprises in at least one of the substrate inner layers a third polyethylene (as a polyethylene defined herein) having a density of at least about 0.935 g/cm 3 , preferably about 0.935 g/cm 3  to about 0.965 g/cm 3 . The third polyethylene is typically prepared with either Ziegler-Natta, chromium-based catalysts, or single-site catalysts, such as a metallocene catalyst (such as any of those described herein) in slurry reactors, gas phase reactors, or solution reactors. Polyethylenes useful as the third polyethylene in this invention include those commercially available from ExxonMobil Chemical Company in Houston, Tex., such as HDPE or those sold under the trade designation ENABLE™. 
     In a preferred embodiment where the sealant of the multilayer film described herein comprises two sealant outer layers and a sealant core layer between the two sealant outer layers, each of the two sealant outer layers and the sealant core layer comprises a fourth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, having a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. In various embodiments, the fourth polyethylene may have one or more of the properties or be prepared as defined above for the first polyethylene. The fourth polyethylene may be the same as or different from the first polyethylene. Preferably, at least one of the two sealant outer layers further comprises a fifth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, having a density of about 0.910 to about 0.945 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100. In various embodiments, the fifth polyethylene may have one or more of the properties or be prepared as defined above for the second polyethylene. The fifth polyethylene may be the same as or different from the second polyethylene. Preferably, the sealant core layer further comprises a sixth polyethylene having a density of at least about 0.935 g/cm 3 . In various embodiments, the sixth polyethylene may conform to characteristics as set out above for the third polyethylene. The sixth polyethylene may be the same as or different from the third polyethylene. 
     The two substrate outer layers and the substrate core layer of the multilayer film can each include the first polyethylene described herein optionally in a blend with one or more other polymers, such as polyethylenes defined herein, which blend is referred to as polyethylene composition. In particular, the polyethylene compositions described herein may be physical blends or in situ blends of more than one type of polyethylene or compositions of polyethylenes with polymers other than polyethylenes where the polyethylene component is the majority component, e.g., greater than 50 wt % of the total weight of the composition. Preferably, the polyethylene composition is a blend of two polyethylenes with different densities. Preferably, at least one of the two substrate outer layers of the multilayer film of the present invention comprises the second polyethylene described herein, present in an amount of no more than about 50 wt %, no more than about 45 wt %, no more than about 40 wt %, no more than about 35 wt %, no more than about 30 wt %, no more than about 25 wt %, to no more than about 20 wt %, no more than about 15 wt %, no more than about 10 wt %, or no more than about 5 wt %, based on the total weight of polymer in the substrate outer layer. Preferably, the substrate core layer of the multilayer film of the present invention comprises the first polyethylene described herein present in an amount of about 60 wt % to about 100 wt %, about 65 wt % to about 100 wt %, about 70 wt % to about 100 wt %, about 75 wt % to about 100 wt %, about 80 wt % to about 100 wt %, about 85 wt % to about 100%, about 90 wt % to about 100 wt %, or about 95 wt % to about 100 wt %, based on the total weight of polymer in the substrate core layer. The two substrate inner layers can also each optionally include a polyethylene composition comprising polyethylenes defined herein. Preferably, the two substrate inner layers each comprises the third polyethylene described herein in an amount of about 60 wt % to about 100 wt %, about 65 wt % to about 100 wt %, about 70 wt % to about 100 wt %, about 75 wt % to about 100 wt %, about 80 wt % to about 100 wt %, about 85 wt % to about 100%, about 90 wt % to about 100 wt %, or about 95 wt % to about 100 wt %, based on total weight of polymer in the substrate inner layer. The two substrate inner layers each has a density of at least 0.003 g,/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer. 
     In a preferred embodiment where the sealant of the multilayer film described herein comprises two sealant outer layers and a sealant core layer between the two sealant outer layers, the two sealant outer layers and the sealant core layer of the multilayer film each includes the fourth polyethylene described herein optionally in a polyethylene composition with one or more other polymers, such as polyethylene defined herein. Preferably, the polyethylene composition is a blend of two polyethylenes with different densities. Preferably, at least one of the two sealant outer layers of the multilayer film of the present invention further comprises the fifth polyethylene described herein, present in an amount of no more than about 50 wt %, no more than about 45 wt %, no more than about 40 wt %, no more than about 35 wt %, no more than about 30 wt %, no more than about 25 wt %, no more than about 20 wt %, no more than about 15 wt %, no more than about 10 wt %, or no more than about 5 wt %, based on the total weight of polymer in the sealant outer layer. Preferably, the sealant core layer of the multilayer film of the present invention further comprises the sixth polyethylene described herein present in an amount of no more than about 80 wt %, no more than about 70 wt %, no more than about 60 wt %, no more than about 50 wt %, no more than about 40 wt %, no more than about 30 wt %, no more than about 20 wt %, or no more than about 10 wt %, based on the total weight of polymer in the sealant core layer. Preferably, the to sealant core layer has an average density higher than that of at least one of the sealant outer layer. 
     It has been surprisingly discovered that introduction of the first polyethylene described herein into a multilayer substrate of a laminate structure may generate significant advantage in mechanical performance over a laminate structure formed by a conventional PET or BOPP substrate, Specifically, when a multilayer laminate film is prepared by two substrate outer layers each comprising the first polyethylene, preferably in a blend with the second polyethylene described herein, a substrate core layer also comprising the first polyethylene, and two substrate inner layers each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, mechanical properties including elongation, puncture energy, and low-temperature bag drop performance of such inventive film can be greatly enhanced with a similar or even improved bending stiffness, in contrast to a multilayer laminate film containing a conventional non-polyethylene substrate. Moreover, in the case of a sealant including the fourth polyethylene described herein, the inventive film can also allow formation of a lap seal, which is not feasible with a conventional laminate structure, thus leading to reduced material consumption. As a result, the inventive film can serve as a desired alternative to currently available laminate options for flexible packaging applications where superior mechanical performance and material cost-effectiveness are expected. 
     Film Structures 
     The multilayer film of the present invention may further comprise additional layer(s), which may be any layer typically included in multilayer film constructions. For example, the additional layer(s) may be made from: 
     1. Polyolefins. Preferred polyolefins include homopolymers or copolymers of C 2  to C 40  olefins, preferably C 2  to C 20  olefins, preferably a copolymer of an α-olefin and another olefin or α-olefin (ethylene is defined to be an α-olefin for purposes of this invention). Preferably homopolyethylene, homopolypropylene, propylene copolymerized with ethylene and/or butene, ethylene copolymerized with one or more of propylene, butene or hexene, and optional dienes. Preferred examples include thermoplastic polymers such as ultra-low density polyethylene, very low density polyethylene, linear low density polyethylene, low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene, isotactic polypropylene, highly isotactic polypropylene, syndiotactic polypropylene, random copolymer of propylene and ethylene and/or butene and/or hexene, elastomers such as ethylene propylene rubber, ethylene propylene diene monomer rubber, neoprene, and compositions of thermoplastic polymers and elastomers, such as, for example, thermoplastic elastomers and rubber toughened plastics. 
     2. Polar polymers. Preferred polar polymers include homopolymers and copolymers of esters, amides, acetates, anhydrides, copolymers of a C 2  to C 20  olefin, such as ethylene and/or propylene and/or butene with one or more polar monomers, such as acetates, anhydrides, esters, alcohol, and/or acrylics. Preferred examples include polyesters, polyamides, ethylene vinyl acetate copolymers, and polyvinyl chloride. 
     3. Cationic polymers. Preferred cationic polymers include polymers or copolymers of geminally disubstituted olefins, α-heteroatom olefins and/or styrenic monomers. Preferred geminally disubstituted olefins include isobutylene, isopentene, isoheptene, isohexane, isooctene, isodecene, and isododecene. Preferred α-heteroatom olefins include vinyl ether and vinyl carbazole, preferred styrenic monomers include styrene, alkyl styrene, para-alkyl styrene, α-methyl styrene, chloro-styrene, and bromo-para-methyl styrene. Preferred examples of cationic polymers include butyl rubber, isobutylene copolymerized with para methyl styrene, polystyrene, and poly-α-methyl styrene. 
     4. Miscellaneous. Other preferred layers can be paper, wood, cardboard, metal, metal foils (such as aluminum foil and tin foil), metallized surfaces, glass (including silicon oxide (SiO x ) coatings applied by evaporating silicon oxide onto a film surface), fabric, spunbond fibers, and non-wovens (particularly polypropylene spunbond fibers or non-wovens), and substrates coated with inks, dyes, pigments, and the like. 
     In particular, a multilayer film can also include layers comprising materials such as ethylene vinyl alcohol (EVOH), polyamide (PA), polyvinylidene chloride (PVDC), or aluminium, so as to obtain barrier performance for the film where appropriate, 
     The thickness of the multilayer films may range from 10 to 200 μm in general and is mainly determined by the intended use and properties of the film. Stretch films may be thin; those for shrink films or heavy duty bags are much thicker. Conveniently, the film has a thickness of from 10 to 200 μm, from 20 to 150 μm, from 30 to 120 μm, or from 40 to 100 μm. Preferably, the thickness ratio between the substrate and the sealant is about 3:1 to about 1:2, for example, about 2.5:1, about 2:1, about 1.5:1, about 1:1, about 1:1.5, about 1:2, or in to the range of any combination of the values recited herein. Preferably, the two substrate inner layers are present at a thickness of about 20% to about 70%, for example, anywhere between 20%, 25%, 30%, 35%, or 40%, and 50%, 55%, 60%, 65%, or 70%, of total thickness of the substrate. Preferably, the thickness ratio between one of the sealant outer layers and the sealant core layer is about 1:1 to about 1:4, for example, about 1:1, about 1:1.5, about 1:2, about 1:2.5, about 1:3, about 1:3.5, or about 1:4. 
     The multilayer film described herein may have an A/B/X/B/A structure for the substrate, wherein A are substrate outer layers and X represents the substrate core layer and B are substrate inner layers between the substrate core layer and each substrate outer layer. Suitably one or both substrate outer layers are a skin layer forming one or both substrate surfaces and can serve as a lamination skin (the surface to be adhered to the sealant) or a sealable skin (the surface to form a seal). The composition of the A layers may be the same or different, but conform to the limitations set out herein. Preferably, the A layers are identical. The composition of the B layers may also be the same or different, but conform to the limitations set out herein. The two substrate inner layers each has a density of at least about 0.003 g/cm higher than that of the substrate outer layer on the same side of the substrate core layer. Preferably, at least one of the two substrate inner layers has a density of about 0.925 to about 0.965 g/cm 3 . 
     The multilayer film described herein may have an A′/Y′/A′ structure for the sealant, wherein A′ is a sealant outer layer and Y′ is the sealant core layer in contact with the sealant outer layer. Suitably one or both sealant outer layers are a skin layer forming one or both sealant surfaces and can serve as a lamination skin (the surface to be adhered to the substrate) or a sealable skin (the surface to form a seal). The composition of the A′ layers may be the same or different, but conform to the limitations set out herein for the sealant. 
     Preferably, the A′ layers are identical. The sealant may have an A′/B′/X′/B′/A′ structure wherein A′ are sealant outer layers and X′ represents the sealant core layer and B′ are sealant inner layers between the sealant core layer and each sealant outer layer. The composition of the B′ layers may also be the same or different. The A′ and B′ layers may have the same composition or different compositions. Preferably, at least one of the B′ layers has a different composition with a density higher than that of the A′ layer. 
     In a preferred embodiment, the multilayer film comprises a substrate having an A/BIX/B/A structure and a sealant having an A′/Y′/A′ structure, wherein the substrate comprises: (a) two substrate outer layers, each comprising a blend of a first and a second to polyethylene, wherein the first polyethylene is present in an amount of about 60 wt % to about 80 wt %, based on total weight of polymer in the substrate outer layer; (b) a substrate core layer between the two substrate outer layers, comprising the first polyethylene in an amount of about 80 wt % to about 100 wt %, based on total weight of polymer in the core layer; and (c) two substrate inner layers between the substrate core layer and each substrate outer layer, each comprising a third polyethylene in an amount of about 80 wt % to about 100 wt %, based on total weight of polymer in the substrate inner layer; wherein the sealant comprises: (d) two sealant outer layers, each comprising a blend of the first and the second polyethylene, wherein the first polyethylene is present in an amount of about 60 wt % to about 80 wt %, based on total weight of polymer in the sealant outer layer; and (e) a sealant core layer between the two outer layers, comprising a blend of the first polyethylene and the third polyethylene, wherein the first polyethylene is present in an amount of about 40 wt % to about 60 wt %, based. on total weight of polymer in the sealant core layer; wherein (i) the first polyethylene is derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.912 to about 0.935 g/cm 3 , an MI, I 2.16 , of about 1 to about 5 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; (ii) the second polyethylene is derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the second polyethylene has a density of about 0.915 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 5 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100; and (iii) the third polyethylene has a density of about 0.935 g/cm 3  to about 0.965 g/cm 3 . 
     The above multilayer film has at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. Preferably, the multilayer film also has a non-breakage rate of about 100%. 
     Preferably, the multilayer film further has at least one of the following properties: (i) the two substrate inner layers are present at a thickness of about 50% of total thickness of the substrate; (ii) the thickness ratio between each of the sealant outer layers and the sealant core layer is about 1:2; and (iii) the thickness ratio between the substrate and the sealant is about 8:9. 
     Film Properties and. Applications 
     The multilayer films of the present invention may be adapted to form flexible packaging laminate films, including stand-up pouches, as well as a wide variety of other applications, such as cling film, low stretch film, non-stretch wrapping film, pallet shrink, over-wrap, agricultural, and collation shrink film. The film structures that may be used for bags are prepared such as sacks, trash bags and liners, industrial liners, produce bags, and heavy duty bags. The film may be used in flexible packaging, food packaging, e.g., fresh cut produce packaging, frozen food packaging, bundling, packaging and unitizing a variety of products. A package comprising a multilayer film described herein can be heat sealed around package content in the form of a lap seal between respective sealable skins from the substrate and the sealant. The film and package of the present invention can display outstanding mechanical properties as demonstrated by bending stiffness, elongation, and puncture energy, which is especially important for flexible packaging applications, such as stand-up pouches, characterized by high bending stiffness to stand upright. 
     The inventive multilayer film may have at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. Preferably, the multilayer film may also have a non-breakage rate of about 100%. By using the substrate as described herein for a laminate structure, the long-standing bottleneck in developing alternative laminate solutions for flexible packaging applications with desirable mechanical properties achievable with polyethylene materials and advantages in sealing and recycling can be well addressed. 
     Methods for Making the Multilayer Film 
     Also provided are methods for making multilayer films of the present invention. A method for making a multilayer film comprising a substrate and a sealant may comprise the step of: (a) preparing two substrate outer layers and a substrate core layer between the two substrate outer layers, wherein each of the two outer layers and the core layer comprises a first polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a. density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; (b) preparing two substrate inner layers, each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, wherein each substrate inner layer is between the substrate core layer and each substrate outer layer; (c) preparing a substrate comprising the layers in steps (a) and (b); and (d) forming a film comprising the substrate in step (c); wherein the multilayer film has at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. The film in step (d) can be formed by laminating the sealant to the substrate. 
     Preferably, the method may further comprise after step (c) a step of preparing a sealant comprising two sealant outer layers and a sealant core layer between the two sealant outer layers, wherein each of the two sealant outer layers and the sealant core layer comprises a fourth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the fourth polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. 
     At least one of the substrate and the sealant of the multilayer films described herein may be formed by any of the conventional techniques known in the art including blown extrusion, cast extrusion, coextrusion, blow molding, casting, and extrusion blow molding. 
     In one embodiment of the invention, both of the substrate and the sealant of the multilayer films of the present invention are formed by using blown techniques, i.e., to form a blown film. For example, the composition described herein can be extruded in a molten state through an annular die and then blown and cooled to form a tubular, blown film, which can then be axially slit and unfolded to form a flat film. As a specific example, blown films can be prepared as follows. The polymer composition is introduced into the feed hopper of an extruder, such as a 50 mm extruder that is water-cooled, resistance heated, and has an L/D ratio of 30:1. The film can be produced using a 28 cm W&amp;H die with a 1.4 mm die gap, along with a W&amp;H dual air ring and internal bubble cooling. The film is extruded through the die into a film cooled by blowing air onto the surface of the film. The film is drawn from the die typically forming a cylindrical film that is cooled, collapsed and, optionally, subjected to a desired auxiliary process, such as slitting, treating, sealing, or printing. Typical melt temperatures are from about 180° C. to about 230° C. Blown film rates are generally from about 3 to about 25 kilograms per hour per inch (about 4.35 to about 26.11 kilograms per hour per centimeter) of die circumference. The finished film can be wound into rolls for later processing. A particular blown film process and apparatus suitable for forming films according to embodiments of the present invention is described in U.S. Pat. No. 5,569,693. Of course, other blown film forming methods can also be used. 
     The compositions prepared as described herein are also suited for the manufacture of blown film in a high-stalk extrusion process. In this process, a polyethylene melt is fed through a gap (typically 0.5 to 1.6 mm) in an annular die attached to an extruder and forms a tube of molten polymer which is moved vertically upward. The initial diameter of the molten tube is approximately the same as that of the annular die. Pressurized air is fed to the interior of the tube to maintain a constant air volume inside the bubble. This air pressure results in a rapid 3-to-9-fold increase of the tube diameter which occurs at a height of approximately 5 to 10 times the die diameter above the exit point of the tube from the die. The increase in the tube diameter is accompanied by a reduction of its wall thickness to a final value ranging from approximately 10 to 50 μm and by a development of biaxial orientation in the melt. The expanded molten tube is rapidly cooled (which induces crystallization of the polymer), collapsed between a pair of nip rolls and wound onto a film roll. 
     In blown film extrusion, the film may be pulled upwards by, for example, pinch rollers after exiting from the die and is simultaneously inflated and stretched transversely sideways to an extent that can be quantified by the blow up ratio (BUR). The inflation provides the transverse direction (TD) stretch, while the upwards pull by the pinch rollers provides a machine direction (MD) stretch. As the polymer cools after exiting the die and inflation, it crystallizes and a point is reached where crystallization in the film is sufficient to prevent further MD or TD orientation. The location at which further MD or TD orientation stops is generally referred to as the “frost line” because of the development of haze at that location. 
     Variables in this process that determine the ultimate film properties include the die gap, the BUR and TD stretch, the take up speed and MD stretch and the frost line height. Certain factors tend to limit production speed and are largely determined by the polymer rheology including the shear sensitivity which determines the maximum output and the melt tension which limits the bubble stability, BUR and take up speed. 
     The laminate structure with the inventive multilayer film prepared as described herein can be formed by laminating respective lamination skins of the sealant to the substrate as previously described herein using any process known in the art, including solvent based adhesive lamination, solvent less adhesive lamination, extrusion lamination, heat lamination, etc. 
     In one particular desirable embodiment, a lap seal is formed by sealing together respective sealable skins of the substrate and the sealant. The lap seal described herein can to he made by any process such as extrusion coating, lamination, sheet extrusion, injection molding or cast film processes. As a result of presence of the first polyethylene described herein in similar compositions in both of the respective sealable skins from the substrate and the sealant, the sealant can be directly sealed with the substrate instead of with the sealant itself, thus reducing the overall consumption of the materials used for preparing a seal and in tum, an end-use package. 
     Other embodiments of the present invention can include: 
     1. A multilayer film, comprising a substrate and a sealant, wherein the substrate comprises: 
     (a) two substrate outer layers and a substrate core layer between the two substrate outer layers, wherein each of the two substrate outer layers and the substrate core layer comprises a first polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; and 
     (b) two substrate inner layers, each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, wherein each substrate inner layer is between the substrate core layer and each substrate outer layer; 
     wherein the multilayer film has at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. 
     2. The multilayer film of paragraph 1, wherein the multilayer film has a non-breakage rate of about 100%. 
     3. The multilayer film of paragraph 1 or 2. wherein at least one of the two substrate outer layers further comprises a second polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the second polyethylene has a density of about 0.910 to about 0.945 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100. 
     4. The multilayer film of paragraph  3 , wherein the second polyethylene is present in an amount of no more than about 50 wt %, based on total weight of polymer in the substrate outer layer. 
     5. The multilayer film of any of paragraphs 1 to 4, wherein the two substrate outer layers are identical. 
     6. The multilayer film of any of paragraphs 1 to 5, wherein at least one of the two substrate inner layers has a density of about 0.925 to about 0.965 g/cm 3 . 
     7. The multilayer film of any of paragraphs 1 to 6, wherein at least one of the substrate inner layers comprises a third polyethylene having a density of at least about 0.935 g/cm 3 . 
     8. The multilayer film of any of paragraphs 1 to 7, wherein the two substrate inner layers are identical. 
     9. The multilayer film of any of paragraphs 1 to 8, wherein the two substrate inner layers are present at a thickness of about 20% to about 70% of total thickness of the substrate. 
     10. The multilayer of any of paragraphs 1 to 9, wherein the sealant comprises two sealant outer layers and a sealant core layer between the two sealant outer layers, wherein each of the two sealant outer layers and the sealant core layer comprises a fourth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the fourth polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. 
     11. The multilayer film of paragraph 10, wherein at least one of the two sealant outer layers further comprises a fifth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the fifth polyethylene has a density of about 0.910 to about 0.945 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100. 
     12. The multilayer film of paragraph 11, wherein the fifth polyethylene is present in an amount of no more than about 50 wt %, based on total weight of polymer in the sealant outer layer. 
     13. The multilayer film of any of paragraphs 10 to 12, wherein the two sealant outer layers are identical. 
     14. The multilayer film of any of paragraphs 10 to 13, wherein the sealant core layer further comprises a sixth polyethylene having a density of at least about 0.935 g/cm 3 . 
     15. The multilayer film of paragraph 14, wherein the sixth polyethylene is present in an amount of no more than about 80 wt %, based on total weight of polymer in the sealant core layer. 
     16. The multilayer film of any of paragraphs 10 to 15, wherein the thickness ratio between one of the sealant outer layers and the sealant core layer is about 1:1 to about 1:4. 
     17. The multilayer film of any of paragraphs 1 to 16, wherein the thickness ratio between the substrate and the sealant is about 3:1 to about 1:2. 
     18. A multilayer film, comprising a substrate and a sealant, wherein the substrate comprises: 
     (a) two substrate outer layers, each comprising a blend of a first and a second polyethylene, wherein the first polyethylene is present in an amount of about 60 wt % to about 80 wt %, based on total weight of polymer in the substrate outer layer; 
     (b) a substrate core layer between the two substrate outer layers, comprising the first polyethylene in an amount of about 80 wt % to about 100 wt %, based on total weight of polymer in the core layer; and 
     (c) two substrate inner layers between the substrate core layer and each substrate outer layer, each comprising a third polyethylene in an amount of about 80 wt % to about 100 wt %, based on total weight of polymer in the substrate inner layer; 
     wherein the sealant comprises: 
     (d) two sealant outer layers, each comprising a blend of the first and the second polyethylene, wherein the first polyethylene is present in an amount of about 60 wt % to about 80 wt %, based on total weight of polymer in the sealant outer layer; and 
     (e) a sealant core layer between the two outer layers, comprising a blend of the first polyethylene and the third polyethylene, wherein the first polyethylene is present in an amount of about 40 wt % to about 60 wt %, based on total weight of polymer in the sealant core layer; 
     wherein (i) the first polyethylene is derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.912 to about 0.935 g,/cm 3 , an MI, I 2.16 , of about Ito about 5 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; (ii) the second polyethylene is derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the second polyethylene has a density of about 0.915 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 5 g/10 min, an MWD of about 2.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 25 to about 100; and (iii) the third polyethylene has a density of about 0.935 g/cm 3  to about 0.965 g/cm 3 ; 
     wherein the multilayer film has at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 mJ. 
     19. The multilayer film of paragraph 18, wherein the multilayer film further has at least one of the following properties: (i) the two substrate inner layers are present at a thickness of about 50% of total thickness of the substrate; (ii) the thickness ratio between each of the sealant outer layers and the sealant core layer is about 1:2; and (iii) the thickness ratio between the substrate and the sealant is about 8:9. 
     20. A method for making a multilayer film comprising a substrate and a sealant, comprising the step of: 
     (a) preparing two substrate outer layers and a substrate core layer between the two substrate outer layers, wherein each of the two outer layers and the core layer comprises a first polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the first polyethylene has a density of about 0.900 to about 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100; 
     (b) preparing two substrate inner layers, each having a density of at least about 0.003 g/cm 3  higher than that of the substrate outer layer on the same side of the substrate core layer, wherein each substrate inner layer is between the substrate core layer and each substrate outer layer; 
     (c) preparing a substrate comprising the layers in steps (a) and (b); and 
     (d) forming a film comprising the substrate in step (c); 
     wherein the multilayer film has at least one of the following properties: (i) a bending stiffness factor of at least about 18 mN/mm; (ii) an elongation at break in the Machine Direction (MD) of at least about 450%; and (iii) a puncture energy at break of at least about 7.5 ml. 
     21. The method of paragraph 20, further comprising after step (c) a step of preparing a sealant comprising two sealant outer layers and a sealant core layer between the two sealant outer layers, wherein each of the two sealant outer layers and the sealant core layer comprises a fourth polyethylene derived from ethylene and one or more C 3  to C 20  α-olefin comonomers, wherein the fourth polyethylene has a density of about 0.900 to about to 0.940 g/cm 3 , an MI, I 2.16 , of about 0.1 to about 15 g/10 min, an MWD of about 1.5 to about 5.5, and an MIR, I 21.6 /I 2.16 , of about 10 to about 100. 
     22. The method of paragraph 20 or 21, wherein at least one of the substrate and the sealant is formed by blown extrusion, cast extrusion, coextrusion, blow molding, casting, or extrusion blow molding. 
     23. The method of any of paragraphs 20 to 22, wherein the film in step (d) is formed by laminating the sealant to the substrate. 
     24. A lap seal comprising the multilayer film of any of paragraphs 1 to 19. 
     25. A package comprising the lap seal of paragraph 24. 
     EXAMPLES 
     The present invention, while not meant to be limited by, may be better understood by reference to the following examples and tables. 
     Example 1 
     Example  1  illustrates mechanical performance demonstrated by two inventive samples (Samples 1 and 2) in comparison with six comparative samples (Samples 3-8) differing from the inventive samples in both sealants and substrates (a PET substrate for 
     Samples 3-6 and a BOPP substrate for Samples 7 and 8). Polyethylene and additive products used in the samples include: EXCEED™ 1018KB mPE resin (density: 0.918 g/cm 3 , MI: 1.0 g/10 min, MIR: 16) (ExxonMobil Chemical Company, Houston, Tex., USA), EXCEED™ 1012MJ mPE resin (density: 0.912 g/cm 3 , MI: 1.0 g/10 min, MIR: 16) (ExxonMobil Chemical Company, Houston, Tex., USA), EXCEED™ 1018LA mPE resin (density: 0.918 g/cm 3 , MI: 1.0 g/10 min, MIR: 16) (ExxonMobil Chemical Company, Houston, Tex., USA), ENABLE™ 20-05HE mPE resin (density: 0.920 g/cm 3 , MI: 0.5 g/10 min, MIR: 42) (ExxonMobil Chemical Company, Houston, Tex., USA), ExxonMobil™ HDPE HTA 002 resin (density: 0.952 g/cm 3 ) (ExxonMobil Chemical Company, Houston, Tex., USA), ExxonMobil™ LLDPE LL 1001KI C 4 -LLDPE resin (density: 0.918 g/cm 3 , MI: 1.0 g/10 min, MIR: 23, Ziegler-Natta catalyzed) (ExxonMobil Chemical Company, Houston, Tex., USA), ExxonMobil™ LLDPE LL 1001XV C 4 -LLDPE resin (density: 0.918 g/cm 3 , MI: 1.0 g/10 min, MIR: 23, Ziegler-Natty catalyzed) (ExxonMobil Chemical Company, Houston, Tex., USA), ExxonMobil™ LDPE LD 150AC LDPE resin (density: 0.923 g/cm 3 , MI: 0.75 g/10 min) (ExxonMobil Chemical Company, Houston, Tex., USA), ExxonMobil™ LDPE LD 150BW LDPE resin (density: 0.923 g/cm 3 , MI: 0.75 g/10 min) (ExxonMobil Chemical Company, Houston, Tex., USA), DOWLEX™ 2045.01G C 8 -LLDPE resin (density: 0.922 g/cm 3 , MI: 1.0 g/10 min, MIR: 27, Ziegler-Natty catalyzed) (The Dow Chemical Company, Midland, Mich., USA), and ELITE™ 5401GS C 8 -mLLDPE (metallocene linear low density polyethylene) resin (density: 0.917 g/cm 3 , MI: 1.0 g/10 min, MIR: 30) (The Dow Chemical Company, Midland, Mich., USA); the POLYBATCH™ CE 505E slip agent (A. Schulman, Fairlawn, Ohio. USA), and the POLYBATCH™ F15 antiblock agent (A. Schulman, Fairlawn, Ohio, USA). All samples were prepared on W&amp;H coextrusion blown film line with a BUR of 2.5. Substrates of Samples 1 and 2 with an A/B/X/B/A structure were prepared at a layer thickness ratio of 1:2:2:2:1, and sealants of all samples with an A′/Y′/A′ structure were prepared at a layer thickness ratio of 1:2:1. Both B layers of Samples 1 and 2 have a density of 0.0335 g/cm 3  higher than that of respective A layers on the same side of the X layer. A schematic representation of film structures for Samples 1 and 2 is shown in  FIG. 1 , Structure-wise formulations and thickness of the laminate film samples, accompanied by test results therefor, are depicted in Table 1. 
     Bending stiffness, as an indicator for stiffness of the material and its thickness, is the resistance against flexure and was measured by a method referred to as “two point bending method” based on DIN 53121 using a Zwick two point bending equipment mounted on the cross-head in a Zwick 1445 tensile tester. The film samples were conditioned for at least 40 hours at a temperature of 23±2° C. and a relative humidity of 50±10% prior to test, and were cut into 38 mm-wide 60 mm-long strips measured in both machine direction (MD) and Transverse Direction (TD). The sample is vertically clamped at one end while the force is applied to the free end of the sample normal to its plane (two point bending). The sample is fixed in an upper clamping unit while the free end pushes (upon flexure) against a thin probe (lamella) connected to a sensitive load cell capable of measuring small load values. The bending stiffness factor is defined as the moment of resistance per unit width that the film offers to bending, which can be seen as a width related flexural strength and is expressed in mN.mm. 
     Tensile properties of the films were measured by a method which is based on ASTM D882 with static weighing and a constant rate of grip separation using a Zwick 1445 tensile tester with a 200N. Since rectangular shaped test samples were used, no additional extensometer was used to measure extension. The nominal width of the tested film sample is 15 mm and the initial distance between the grips is 50 mm. The film samples were conditioned for at least 40 hours at a temperature of 23±2° C. and a relative humidity of 50±10%, and were measured in both Machine Direction (MD) and Transverse Direction (TD). Elongation at break is defined as the strain at the corresponding break point, expressed as a change in length per unit of original length multiplied with a factor 100 (%). 
     Puncture resistance was measured based on CEN 14477, which is designed to provide load versus deformation response under biaxial deformation conditions at a constant relatively low test speed (change from 250 mm/min to 5 mm/min after reach pre-load (0.1N)). Puncture energy to break is the total energy absorbed by the film sample at the moment of maximum load, which is the integration of the area up to the maximum load under the load-deformation curve. 
     Bag drop performance refers to the capability of a package bag to withstand the sudden shock resulting from a free fall in accordance with ASTM D 5276-98 which is incorporated by reference. The low-temperature bag drop performance is measured herein based on ASTM D 5276-98 at a height of two meters with bag samples stored in the deep freezer at −30° C. for two days prior to test and is represented by a non-breakage rate of the number of broken bag samples compared to a total of ten tested bag samples for each film formulation. 
     As shown by test results in Table 1, Samples 1 and 2 of the inventive film, in addition to a bending stiffness at a comparable or even improved level, exceeded in mechanical performance in terms of elongation, puncture energy, and low-temperature bag drop performance, in contrast to those achieved with conventional comparative films composed of a C 4 -LIDPE, C 8 -LLDPE or C 8 -mLLDPE based sealant and a PET or BOPP substrate. Given that a lap seal can be formed with the inventive film to reduce material consumption, this combination of desired mechanical performance and cost-effectiveness can render a promising candidate to replace the current conventional laminates for use in flexible packaging applications. 

 
     Example 2 
     Example 2 demonstrates the effect of using the substrate as described herein On mechanical performance of Sample 1′ of the inventive film. Samples 9 and 10 were provided as comparative films, prepared by a conventional PET and BOPP substrate, respectively, but otherwise identical or very similar to the inventive Sample 1 in terms of sealant layers&#39; compositions and thickness. A 45 μm three-layer film with an A/Y/A structure at a layer thickness ratio of 1:2:1 was prepared for each sample and was laminated to different substrates to form the three samples. The substrate of Sample 1′ was prepared with an A/B/X/B/A structure at a layer thickness ratio of 1:2:2:2:1. Both B layers have a density of 0.0335 g/cm 3  higher than that of respective A layers on the same side of the X layer. The bending stiffness, elongation at break in MD, puncture energy at break, and the non-breakage rate were measured as previously described. Structure-wise formulations and test results of the film samples are shown below in Table 2. 
     It can be seen from Table 2 that the inventive Sample 1 significantly outperformed Samples 9 and 10 in all the tested mechanical properties, which suggests that the mechanical performance achievable with the inventive film may largely depend on the substrate described herein. Particularly, without being bound by theory, it is believed that, given an identical or a very similar sealant, presence of the substrate described herein contributes to improvement in mechanical properties of a laminate structure, based on which high quality and easy processability of flexible packages can be expected. 
     All documents described herein are incorporated by reference herein, including any priority documents and/or testing procedures. When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. As is apparent from the foregoing general description and the specific embodiments, while forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited thereby.