COLUMN SHRINK
Field of the Invention The invention relates to co-extruded structures for collation shrink films and physical polyethylene blends used therefor. BACKGROUND OF THE INVENTION Collation shrinkage concerns fastening articles together using heat-shrinkable film. The shrinkage of collation is used for a very wide variety of applications, and mainly for the secondary packaging of food or drinks. Examples include metal cans and plastic bottles. Typically, the films are applied at room temperature and placed under a heat source to shrink. Proper performance characteristics in the shrink packaging line include sufficient stiffness to allow the film to wrap properly around the items being packaged, sufficient dimensional shrinkage to ensure a loose fit, and a sufficiently low coefficient of friction (COF) to machining capacity and packaging management. Films suitable for use as a collation shrink should have a high thermal shrink force to ensure tight fit and high tensile strength to withstand handling and abuse during transport. In addition, the package must have excellent display properties, including brightness (preferably at different angles, to maximize attractiveness), fog (or "contact clarity"), and clarity ("clarity through"). Finally, the collation shrink film manufacturer desires the properties of low melt pressure, and the ability to use low motor power, both allowing higher production rates. Although it is known how to improve many of the above properties individually, the currently available structures do not combine all the properties satisfactorily in a film having a sufficiently thin gauge to be commercially attractive. Polyethylene is an attractive component to use in collation shrink film. Various types of polyethylenes are known in the art. Low density polyethylene ("LDPE") can be prepared at high pressure using free radical initiators and typically has a density in the range of 0.916-0.940 g / cm3. LDPE is also known as polyethylene
"branched" or "branched heterogeneously" due to the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene is also known in the same density range, 0.916-0.940 g / cm3, which is linear and does not contain large amounts of long chain branching; This "linear low density polyethylene" ("LLDPE") can be produced with conventional Ziegler-Natta catalysts or with single-site catalysts-such as metallocene catalysts. LDPE or LLDPE of relatively higher density, typically in the range of 0.928-940 g / cm3, is sometimes referred to as medium density polyethylene ("MDPE"), or medium density linear polyethylene ("LMDPE"). The polyethylenes that have still higher density are high density polyethylenes ("HDPE") / polyethylenes having densities greater than 0.940 g / cm3, and are generally prepared with Ziegler-Natta catalysts, chromium catalysts, or even catalysts of a single site, such as metallocene catalysts. Very low density polyethylene ("VLDPE") is also known. VLDPEs can be produced by several different processes that produce polymers with different properties, but can generally be described as polyethylenes having a density less than 0.916 g / cm3, typically 0.890-0.915 g / cm3, or 0.900-0.915 g / cm3. Patent US 6,187,397 teaches a thermo-shrink film, co-extruded, three-layer, devoid of polyethylene by metallocene. The patent teaches that the "high clarity" thermo-shrinkable polyethylene films of the state of the art are obtained by co-extrusion of three layers comprising a central layer of predominantly (>; 50% by weight) polyethylene by free radicals having a relative density of 0.918-0.930 g / cm3, optionally with HDPE, to confer stiffness, sandwiched between two layers of predominantly (80-90% by weight) linear polyethylene by metallocene having a density of 0.918-0.927 g / cm3. Patent US 6,340,532 discloses shrinkable films made from "pseudo-homogeneous" linear low density polyethylene resins, preferably prepared with an advanced Ziegler-Natta catalyst. Various deficiencies of the "homogeneous" resins, ie resins by metallocene, used in the shrinkable films of the state of the art, are discussed. Patent US 6,368,545 teaches a co-extruded, inflated, multi-layered, high clarity film, prepared using special methods, where a film having a central core of HDPE is described. The patent application US 20020187360 is directed to a thermo-shrinkable co-extruded polyethylene film laminate having a core layer with a relatively low melting point, comprising a linear low density polyethylene (LLDPE) having a density of 0.910-0.930 g / cm3, and a very low density linear polyethylene (VLDPE) having a density of 0.880-0.915 g / cm3, sandwiched between two surface layers with relatively higher melting point comprising a linear low density polyethylene and a high density linear polyethylene.
WO 01/44365 discloses a homogenous physical mixture of a medium density polyethylene, catalyzed by metallocene (mMDPE), with a low density polyethylene (LDPE), to produce blown films. The physical mixture can be co-extruded between layers of LDPE to make blown films which in this background have the good optical properties of the LDPE and the good mechanical and processing properties of the MDPE. Additional patents of interest include US Pat. No. 6,492,010, the statutory registration of invention US H2073, publication WO 95/00333 and EP 0 597 502. The high gloss provided by the polyethylenes by metallocene is an extremely attractive property. However, film layers comprising polyethylene by metallocene have a very high coefficient of friction in the absence of specific additives. These additives, in turn, move away from the desired optical properties in shrinkage films. A film that exploits the high gloss capabilities of polyethylene by metallocene that can be produced efficiently and having the desired properties in a shrink film is highly desirable. The inventor of the present has surprisingly discovered that an improved collation shrink film can be achieved by a structure having a core layer comprising HDPE and epidermis layers comprising polyethylene by metallocene and, optionally, at least an HDPE or LDPE resin. SUMMARY OF THE INVENTION The invention is directed to a film structure having at least two layers: a layer comprising HDPE and a layer comprising a metallocene-catalyzed polyethylene (hereinafter, mPE), optionally further comprising minus one of HDPE or LDPE. The invention is further directed to a structure wrapped by collation shrinkage comprising the aforementioned shrink film wrapped around various articles. In a preferred embodiment, the film structure comprises a core layer comprising HDPE walled by two epidermal layers of metallocene. In a more preferred embodiment, at least one of said epidermal metallocene layers further comprises at least one of HDPE or LDPE. It is an object of the present invention to provide various embodiments of the aforementioned inventions having unique properties, particularly with respect to optical, strength, and processing properties, as well as performance in the structure enveloped by resultant shrinkage. Another objective of the present invention is to provide a shrinkage film having adequate performance in the packing line by shrinkage. Still another object of the present invention is to provide a collation shrink film having appropriate properties to handle abuses during transport. Still another object of the present invention is to provide a shrink wrapped structure having attractive deployment properties at the point of sale. These and other objects, features and advantages will be apparent upon reference to the following detailed description, preferred embodiments, examples, and appended claims. Detailed Description In one embodiment, a film structure having at least two layers is provided. One layer, referred to herein as the first layer, and in one embodiment as the core layer, comprises a high density polyethylene (hereinafter, "HDPE"), and the second layer, also referred to herein As at least one epidermis layer, it comprises a polyethylene catalyzed in a single site, such as polyethylene by metallocene or mPE. As used herein, HDPE means polyethylene having a density greater than 0.940 g / cm3. The terms "high density polyethylene" polymer and "HDPE" polymer refer to an ethylene homopolymer or copolymer having a density greater than 0.940 g / cm 3. Polymers that have more than two types of monomers, such as terpolymers, are also included within the term "copolymer", as used herein. The co-monomers which are useful in general for making HDPE copolymers useful in the present invention include alpha-olefins, such as C3-C20 alpha-olefins and preferably C3-C12 alpha-olefins. The alpha-olefin co-monomer may be linear or branched, and two or more co-monomers may be used, if desired. Examples of suitable co-monomers include linear C3-C12 alpha-olefins, and alpha-olefins having one or more C1-C3 alkyl branches, or an aryl group. Specific examples include propylene, 3-methyl-1-butene; 3, 3-dimethyl-l-butene; 1-pentene; l-butene; 1-pentene with one or more substituents methyl, ethyl or propyl; 1-hexene; 1-hexene with one or more methyl, ethyl or propyl substituents; 1-heptene with one or more methyl, ethyl or propyl substituents; 1-decene; l-dodecene; 1-octene with one or more methyl, ethyl or propyl substituents; 1-nonene with one or more methyl, ethyl or propyl substituents; 1-decene ethyl, methyl or dimethyl-substituted; l-dodecene; and styrene. It should be appreciated that the list of co-monomers above is merely exemplary, and is not intended to be limiting. Preferred co-monomers include propylene, l-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene and styrene. Other useful co-monomers include polar vinyl, conjugated and non-conjugated dienes, acetylene and aldehyde monomers, which may be included in minor amounts in terpolymer compositions. Unconjugated dienes useful as co-monomers are preferably hydrocarbon di-olefins, straight chain, or cycloalkenyl-substituted alkenes, having from 6 to 15 carbon atoms. Suitable non-conjugated dienes include, for example: (a) straight chain acyclic dienes, such as 1,4-hexadiene and 1,6-octadiene; (b) branched chain acyclic dienes, such as 5-methyl-1,4-hexadiene; 3, 7-dimethyl-l, 6-octadiene; and 3,7-dimethyl-1,7-octanediene; (c) single ring alicyclic dienes, such as 1,4-cyclohexadiene; 1, 5-cyclo-octadiene and 1,7-cyclo-dodecadiene (d) fused and bridged ring, alicyclic, multi-ring, such as tetrahydroindene; norbornadiene; methyl-tetrahydroindene; dicyclopentadiene (DCPD); bicyclo- (2.2.1) -hepta-2, 5-diene; alkenyl, alkylidene, cycloalkenyl and cycloalkylidene norbornenes, such as 5-methylene-2-norbornene (NINB); 5 -propenyl-2-norbornene; 5-isoproylidene-2-norbornene; 5- (4-cyclopentyl) -2-norbornene, 5-cyclohexylidene-2-norbornene, and 5-vinyl-2-norbornene (VNB), and cycloalkenyl-substituted (e) alkenes, such as vinyl cyclohexene, allyl cyclohexene, vinyl cyclo-octene, 4-vinyl-cyclohexene, allyl cyclodecene, and vinyl cyclododecene. Of the non-conjugated dienes typically used, the preferred dienes are dicyclopentadiene, 1,4-hexadiene, 5-methyl-2-norbornene, and 5-ethylidene-2-norbornene. Particularly preferred diolefins are 5-ethylidene-2-norbornene (ENB), 1,4-hexadiene, dicyclopentadiene (DCPD), norbornadiene, and 5-vinyl-2-norbornene (VNB). The amount of co-monomer used will depend on the desired density of the HDPE polymer and the specific co-monomers selected, taking into account processing conditions such as temperature and pressure, and other factors such as the presence or absence of telomeres and the like, as would be evident to a person skilled in the art in possession of the present disclosure. In one embodiment, the HDPE polymer has a density greater than 0.940 g / cm3, preferably from about 0.940 g / cm3 to about 0.970 g / cm3, most preferably from about 0.955 g / cm3 to about of 0.965 g / cm3, and most preferably of about 0.960 g / cm3 to about 0.965 g / cm3. The densities referenced herein are in accordance with the ASTM D 1505 standard method. In one embodiment, the HDPE polymer can have a melt index of 0.01 to 45 g / 10 min, measured in accordance with the ASTM standard method. 1238, condition E. The HDPE polymer can be produced using any conventional polymerization process, such as a solution, slurry or gas phase process, and a suitable catalyst, such as a chromium catalyst, a Ziegler-Natta catalyst. , or a metallocene catalyst. It is preferred that the HDPEs used in the physical mixtures according to the present invention be produced using Ziegler-Natta catalysts. Examples of suitable HDPE useful in the present invention include HDPEs available from ExxonMobil Chemical Co. , Houston, Texas, United States, under the HD, HDA, HMA, HRA, HRP, HDZ or HYA series, or under the Paxon brand. Examples of HDPE include HYA800, produced in gas phase, and HDZ222, produced by the process in stirred slurry. Also contemplated are physical blends of two or more HDPE polymers and one or more HDPE polymers with one or more non-HDPE polymers. The HDPE component of the first layer (or, in the case of an embodiment comprising three layers, the HDPE component of the "core layer", more fully described hereinafter) must be present in the amount of between about 1 and 50% by weight, preferably between about 10 and 50% by weight, more preferably between about 15 and45% by weight. This first layer (or, in one embodiment, core layer) should also comprise between about 99 and 50% by weight of LDPE, preferably between about 90 and 50% by weight, more preferably between about 85 and 55% by weight, even more preferably between about 85 and about 65% by weight of LDPE. In a preferred embodiment, the first layer may also comprise another polyolefin, such as LLDPE (e.g., physical tri-blend). LDPE suitable for use in the present invention is a LDPE initiated in free radicals having a density in the range of 0.916 to 0.940 g / cm3, preferably 0.924 to 0.940 g / cm3. In one embodiment, the LDPE physically mixed with the HDPE in this first layer or core layer has a density in the range of 0.916 to 0.935 g / cm3, more preferably 0.926 to 0.935 g / cm3. In another embodiment, the LDPE physically mixed with the HDPE in this layer has a density in the range of 0.916 to 0.927 g / cm3, and more preferably 0.921 to 0.926 g / cm3. Other embodiments include LDPEs having densities of any of the specified lower density limits at any of the upper density limits specified herein, for example 0.921 to 0.940 g / cm3, and 0.926 to 0.940 g / cm3. Preferred LDPEs are LD170BA and EX489BA and EX514BA, experimental grade, also available from ExxonMobil Chemical Co., of Houston, Texas, United States. Additional polyolefins, such as VLDPE, may be added, with the proviso that it satisfies the aforementioned weight percentage for HDPE and LDPE. Similarly, the physical mixture of HDPE and LDPE may include various additives, as discussed in more detail below. However, in one embodiment it is preferred that the physical mixture of HDPE and LDPE constituting the first core layer or layer of the film structure does not contain slip or anti-block additives. In a preferred embodiment, this layer does not contain polyethylene by metallocene. In another embodiment, mPE may be added, paricularly in the case where additional tenacity is required. The composition of this layer may further comprise polypropylene or may be without polypropylene. Other non-limiting examples of embodiments of the invention include combinations of the aforementioned embodiments, such as the physical HDPE / LDPE blend of the invention without slip or anti-block additives, without a metallocene polyethylene, and without polypropylene, an embodiment comprising the physical HDPE / LDPE blend of the invention without slip or anti-block additive, and with metallocene polyethylene, and the like. The second layer of the film structure according to the present invention, which in one embodiment is a layer of epidermis around the core layer described above, comprises a polyethylene catalyzed in a single site, such as polyethylene by metallocene ( mPE). In a preferred embodiment, the mPE is linear low density polyethylene (hereinafter, "mLLDPE"). In another embodiment, the mPE is a VLDPE (hereinafter, "mVLDPE") having a density of between about 0.880 and 0.915 g / cm3. In the case where the HDPE component of the physical mixture is also a metallocene polyethylene, the second component of the invention must be a mLLDPE or the aforementioned mVLDPE. A "metallocene polyethylene", as used herein, means a polyethylene produced by a metallocene catalyst. As used herein, the term "metallocene catalyst" is defined as being at least one metallocene catalyst component containing one or more substituted or unsubstituted cyclopentadienyl (Cp) moieties in combination with a transition metal of groups 4 , 5 or 6 (M) · Precursors of metallocene catalysts generally require activation with a co-catalyst, or activator, suitable, in order to produce an "active metallocene catalyst", ie an organometallic complex with a vacant coordination site that can coordinate, insert and polymerize olefins. Active catalyst systems generally include not only the metallocene complex, but also an activator, such as an alumoxane or a derivative thereof (preferably, AO), an ionizing activator, a Lewis acid, or a combination thereof. The alkyl lumoxanes are additionally suitable as catalyst activators. The catalyst system is preferably supported in a carrier, typically an inorganic oxide or chloride or a resinous material, such as polyethylene. The state of the art is replete with examples of metallocene systems / catalysts for producing polyethylene. Non-limiting examples of catalysts and metallocene catalyst systems useful in the practice of the present invention include WO 96/11961 and WO 96/11960; US Patent Nos. 4,808,561; 5,017,714; 5,055,438; 5,064,802; 5,124,418; 5,153,157; 5,324,800; and more recent examples are patents US 6,380,122 and 6,376,410; and the publication WO 01/98409, and references cited in the above documents. Included within the definition of mPE resins, and more particularly mLLDPE resins, for the purposes of the present invention, are polyethylene resins having a low polydispersity as described, for example, in the aforementioned US Pat. No. 6,492,010, that is, a polydispersity produced by a catalyst variously described as a metallocene catalyst, or a single site, or constrained geometry, catalysts well known per se in the state of the art. The preferred polydispersity low mPEs are mLLDPEs having a density within the range for LLDPEs noted herein. The low polydispersity mLLDPE resins can be prepared from a partially crystalline polyethylene resin which is a polymer prepared with ethylene, preferably ethylene and at least one alpha-olefin monomer, e.g. , a copolymer or terpolymer. The alpha-olefin monomer generally has about 3 to about
12 carbon atoms, preferably from about 4 to about 10 carbon atoms, and more preferably from about 6 to about 8 carbon atoms. The alpha-olefin co-monomer content is generally less than about 30% by weight, preferably less than about 20% by weight, and most preferably from about 1 to about 15% by weight. Exemplary co-monomers include propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-pentene (4-methyl-1-pentene, 1-octene, 1-decene, and 1-dodecene. embodiment, the low polydispersity resins will have average molecular weights in the range of about 20,000 to about 500,000, preferably from about 50,000 to about 200,000.The molecular weight is determined by commonly used techniques, such as chromatography by size exclusion or gel permeation chromatography In one embodiment, the low polydispersity polymer will have a molecular weight distribution, or polydispersity (Mw / Mn, "MD") within the range of about
1 to about 4, preferably from about 1.5 to about 4, more preferably from about 2 to about 4, and even more preferably from about
2 to about 3. Such products are well known in the art per se and are discussed in US Patents 5,907,942; 5,907,943; 5,902,684; 5,752,362; 5,814,399; and 5,749,202. In one embodiment, the low polydispersity polymers thus produced generally have a crystalline content in excess of at least 10% by weight, generally in excess of at least 15% by weight. The preferred mLLDPE resins will be characterized by one or more of the aforementioned embodiments, including preferred, most preferred, etc. As a non-limiting example, a preferred mLLDPE according to the present invention is characterized by having a molecular weight of 20,000 to 500,000 and a polydispersity of from about 2 to about 4; Another preferred mLLDPE according to the present invention is characterized by having a polydispersity of about 1 to about 4 and an excess crystalline content of at least 10% by weight. Useful, low polydispersity mLLDPE polymers are available, inter alia, from Dow Chemical Company and Exxon Chemical Company, which are producers of polyethylenes catalysed in a single site or constrained geometry. These resins are commercially available as Enhanced Polyethylene, Elite, Affinity, Esxact, and Exceed polyethylene resins. Also included within the definition of mLLDPE according to the present invention are bimodal resins produced by catalysts having at least one component of a catalyst at a single site or constrained geometry that produces a low polydispersity polymer. Particularly preferred examples are bimodal resins having as a component a resin produced using a single site, constricted geometry, or metallocene catalyst, and having a density falling within the density range for LLDPE as previously described. The bimodal resins are per se well known in the art. In a more preferred embodiment, useful mPE suitable for the present invention includes metallocene LLDPE (mLLDPE) under the trademark Exceed, available from ExxonMobil Chemical Company, of Houston, Texas, United States. Particularly preferred is Exceed 1327CA polyethylene and Exceed 1018CA polyethylene, both commercially available mLLDPEs with a C6 co-monomer incorporated therein and produced in the gas phase. In one embodiment, a physical mixture suitable for the second layer or epidermis layer (s) of the film structure should comprise between about 100 and 50% by weight of mPE. Preferably, the mPE has a density range of 0.910 to 0.940 g / cm3, and more preferably 0.915 to 0.940 g / cm3 (still more preferably 0.921 to 0.934 g / cm3, and still more preferably 0.925 to 0.929 g / cm3. Optional components, in an amount of not more than 50% by weight, include LDPE and / or HDPE. Preferred LDPEs and HDPEs, in terms of density ranges, are those set forth elsewhere herein. Preferred embodiments are also selected from the commercially available or known LDPEs or HDPEs discussed herein. In another preferred embodiment, LDPEs suitable for use in this second layer are LDPEs initiated in free radicals having a density in the range of 0.916 to 0.940 g / cm3, preferably 0.924 to 0.940 g / cm3. In one embodiment, the LDPE physically mixed with the HDPE in this layer has a density in the range of 0.916 to 0.940 g / cm3, more preferably 0.921 to 0.935 g / cm3. In another embodiment the LDPE physically mixed with the HDPE in this layer has a density in the range of 0.916 to 0.927 g / cm3, and more preferably 0.921 to 0.926 g / cm3. Again, preferred specific examples of LDPEs are LD170BA and experimental grades EX489BA and EX514BA, mentioned above. Physical mixtures of two or more LDPEs are contemplated. The LDPE used in this second layer, if any, may be the same or different than the LDPE used in the first layer. Particularly preferred HDPE in this second layer includes HDPEs available from ExxonMobil Chemical Co., of Houston, Texas, United States, under the HD, HDA, HMA, HRA, HRP, HDZ or HYA series, or under the Paxon brand, particularly HYA800 and HDZ222, discussed above. Physical mixtures of two or more HDPE polymers are also contemplated in this layer. In the case where HDPE is present in the second layer or epidermis layer, it may be the same or different as the HDPE in the first layer. In this way, the HDPE polymer has a density of more than 0.940 g / cm3. In a preferred embodiment, the HDPE in this layer has a density from about 0.940 to about 0.970 g / cm3, more preferably from about 0.955 to about 0.965 g / cm3, and most preferably from about 0.960 to 0.965 g / cm3. This second layer may include various additives, as discussed below in greater detail. However, it is preferred that this second layer, comprising mPE and optionally HDPE and / or LDPE, does not contain slip or anti-block additives. Additionally, in embodiments, this second layer or epidermis layer may include or exclude one or more of the following: additional polyolefins such as VLDPE or polypropylene, slip or anti-block agents, and the like. Multilayer film forming techniques are well known to those skilled in the art, and the state of the art is replete with examples: see, for example, WO 01/98409. Any of these techniques can be used to form multi-layer films according to the present invention, but the most preferred one is co-extrusion and provides the best advantage of the invention relative to collapse shrink film structures. Vacuum films are also contemplated, particularly in the case where at least one layer of epidermis, as more fully described below, comprises polypropylene. Although those skilled in the art will appreciate that the thickness of the layers can be adjusted based on the desired end use, one of the surprising aspects of the present invention is that the composition of the layers of the present invention provides multi-layer films having the proper properties for collation shrinkage while being of sufficiently thin gauge to be commercially attractive. Thus, according to one embodiment of the invention, the first layer comprising HDPE, as described above, and the second layer comprising mPE, are co-extruded to form a multi-layer film useful as a film of collation shrinkage. In the preferred shrink wrapped shrink structure, the second layer comprising polyethylene by metallocene is the outer layer, the first layer comprising HDPE being in contact with the articles wrapped by shrinkage. In one embodiment, the film according to the invention consists essentially of the first layer comprising HDPE and the second layer comprising mPE. In a more preferred embodiment, the first layer is around
microns thick and the second layer is around 5 microns thick. In another embodiment, the first layer comprising HDPE, described above, is a core layer between two epidermis layers, at least one of which is the second layer, comprising mPE, described above. The two epidermal layers may be the same or different, with the proviso that at least one is a second layer, as described above, comprising mPE. In this embodiment, it is preferred that both epidermis layers also comprise mPE.
As used herein, the term "epidermis layer" means that the layer is the outer layer of the structure. In this way, in a three-layer structure there are two layers of epidermis and a core layer, walled by the layers of epidermis. This structure will be denoted A / B / A, where the layer A denotes an epidermis layer, corresponding to the second layer comprising mPE, above, and the layer B denotes the core layer, corresponding to the first layer described above. However, it will be recognized that layers A need not be identical. In still another embodiment, the structure includes a layer comprising HDPE without a metallocene polyethylene component, and a layer comprising a physical mixture comprising polyethylene by metallocene and HDPE. Optionally, the layer comprising HDPE without a metallocene polyethylene component is sandwiched between two layers of epidermis comprising mLLDPE, wherein at least one of the epidermal layers further comprises HDPE. The epidermis layers in any of these "epidermis and core" (or A / B / A) embodiments may be the same or different. Additional film layers, e.g., between one or both of A / B, e.g., are contemplated as tie layers. However, in the preferred embodiment, the core layer B comprises HDPE and the epidermis layers A comprise mPE (ie, corresponding to one of the first layers and two of the second layers, respectively, as described above) . The final film comprising the structure A / B / A may be symmetric or may be non-symmetric. In a more preferred embodiment, the film according to the present invention comprises the structure
A / B / A, where the layers of epidermis A, which may be the same or different, each comprise, independently, an mPE having a density of between about 0.910 and 0.940 g / cm3, preferably 0.915 to 0.940 g / cm3, and optionally an HDPE, preferably having a density of between about 0.940 and
0. 970 g / cm3, more preferably 0.955 to about 0.965 g / cm3, and most preferably from about 0.960 to about 0.965 g / cm3, and the core layer B comprises an HDPE, preferably having a density of between about 0.940 and 0.970 g / cm3, more preferably 0.955 a about
0. 965 g / cm3, and most preferably from about 0.960 to about 0.965 g / cm3, and an LDPE, preferably having a density in the range of 0.916 to 0.935 g / cm3, more preferably 0.921 to 0.930 g / cm3. In the case where there is HDPE in one or both of the epidermal layers, the HDPE in each layer is selected independently and may be the same or different than the other layer and / or the core layer. In this preferred A / B / A structure, it is more preferred that the core layer B comprises from 60 to 90% by weight, more preferably 70 to 80% by weight of LDPE, and 40 to 10% by weight of HDPE, more preferably from 30 to 20% by weight, and the layers of epidermis A to be each, independently, selected from 80 to 100% by weight, preferably 85-95% by weight of mPE, and 20 to 0 % by weight of HDPE, more preferably 15 to 5% by weight. In a preferred embodiment, structure A / B / A is symmetric with respect to composition and thickness. In another preferred embodiment, the structure A / B / A is no thicker than 50 microns and, with greater preference, about 40 microns thick or less. In another preferred embodiment, in a structure comprising the A / B / A layers according to the present invention, the epidermis layers A each comprise, independently, at least one mLLDPE resin and at least one resin LDPE, and the core layer B comprises at least one LDPE resin and at least one HDPE resin. One or more additional layers may be present between the layers of epidermis and the core, and the structure may be symmetrical or non-symmetrical so that, for example, one embodiment includes a structure consisting of a layer A, a layer of mooring, a B layer, a mooring layer, a layer A; a structure consisting of a layer A, a tie layer, a layer B, a layer A and the like. A preferred embodiment is the case where the structure consists essentially of a layer A, a layer B, a layer A having a total thickness of 2 mils (50 microns ± 10 microns), the layers being in the ratio, respectively , around 15:70:15. In another preferred embodiment, the layers A comprise from about 99 to about 80% by weight, preferably from about 98 to about 90% by weight of mLLDPE, and from about 1 to about 20% by weight. weight, preferably from about 2 to about 10% by weight of LDPE. In another preferred embodiment, layer B comprises from about 90 to about 50% by weight, preferably from about 85 to about 55% by weight, more preferably from about 85 to about 75% by weight. LDPE weight, and from about 10 to about 50% by weight, preferably from about 15 to about 45% by weight, more preferably from 15 to about 25% by weight of HDPE. Additional preferred embodiments include combinations of the aforementioned embodiments, preferred embodiments, and more preferred embodiments, as well as the preferred and most preferred densities for each of the prospective polyethylenes in this layer, as described in the appropriate sections in the present. Each of the aforementioned layers can, independently, include or exclude additional ingredients such as additional slip or anti-block and / or polyolefin agents, such as polypropylene and / or VLDPEs. A particularly advantageous embodiment is an A / B / A structure, as described in this paragraph, where one of the layers A does not contain polypropylene and one of the layers A contains polypropylene. In a preferred shrink wrapped structure, the layer having the polypropylene would be in contact with at least one article wrapped by collation shrinkage. In yet another embodiment, which may be a modification of any of the previous embodiments, the second layer or at least one of the epidermal layers comprises a mLLDPE, an HDPE, and an LDPE. In yet another embodiment, the structure comprises an epidermis layer comprising an mPE according to the present invention, a core layer comprising HDPE according to the present invention, and a second epidermis layer comprising polypropylene. In a preferred embodiment, one or more of the layers of the multilayer film structure according to the invention can have certain additives, such as thermal stabilizers, but in this preferred embodiment each of the compositions of the Different layers must specifically exclude slip or anti-block additives. Suitable additives include: fillers, such as silica, talc, and the like; anti-oxidants (e.g., hindered phenolics such as Irganox 1010 or Irganox 1076, available from Ciba-Geigy); phosphites (e.g., Irgafox 168, available from Ciba-Geigy); anti-stick additives and anti-static additives; tackifiers, such as polybutenes, terpene resins, aliphatic and aromatic hydrocarbon resins, alkali metal and glycerol, stearates and hydrogenated rosins; UV stabilizers; thermal stabilizers; release agents; anti-static agents; pigments; colorants; tinctures; waxes; and similar. Any physical mixing required to make the compositions for the layers according to the present invention can be carried out using conventional equipment and conventional methods, such as dry physical mixing of the individual components and mixing in the subsequent molten state in a mixer, or Mix the components together directly in a mixer, such as a Banbury mixer, a Haake mixer, an internal Brabender mixer, or a single screw extruder or twin screws, including a compounding extruder and a side arm extruder used directly downstream of a polymerization process or prior to extrusion of the film . Examples In the following examples, three-layer films, A / B / A according to the invention and comparative films were produced in a commercially available extruder from Winmoller &; Holscher The co-extruded structures were symmetrical, having an internal core of 30 microns thick and two layers of epidermis, each 5 microns thick. The conditions of the machine were the following: (a) die diameter: 250 mm; (b) die space: 1.4 mm; (c) amplification ratio: 3.0; (d) adapter temperature of the core extruder: 200 ° C; (e) temperature reading of the epidermis extruder adapter: 190 ° C; (f) die temperature: 200 ° C. The various products used in the examples of Table 2 are identified below in Table 1: Table 1
1Commercially available from ExxonMobil Development versions. Improved against commercially available as LD170BA 3ASTM D-1238, condition E (load of 2.16 kg, 190 ° C)
Films having a thickness of 40 microns were formed using the compositions given in Table 2. Examples 2-3, 5-6, 8-9 are examples of the present invention having layers of epidermis comprising mPE and HDPE, and layers of core that comprise HDPE. Examples 11-12, 14-15 and 17-20 are examples of the present invention having epidermis layers comprising mPE but not HDPE, the core layers comprising HDPE. The other examples are for comparative purposes. The results of the various tests carried out are shown in Table 3. The fog is the total mist measured according to the ASTM method D1003; the brightness at an angle of 60 ° and at an angle of 20 ° is measured according to the method ASTM D2457; the clarity is measured according to the method ASTM D1746; the Elmendorf tear values are both measured according to the ASTM D1922 method; the thermal force values are both measured according to ASTM method D2838-95, fixed temperature: 190 ° C; Relative 1% secant modulus and 10% displacement deformation are both measured according to ASTM method D882. The thermal force is measured based on the method ASTM D2838-95, procedure A, using a Retramat tester supplied by Prodemat S.A.
Table 2
Thickness current composition layers Epidermis Layer Media Pressure Motor Power Extruder Extruder melt composition Core Core (MPa) (% Ma.) Ratio Radio Radio Product Product Product Product Radio 1 40μp? 95% 1327CA 5% HYA800 80% EX489BA 20% LL1201XV 327 32%
2 40μp? 95% 1327CA 5% HYA800 80% EX489BA 20% HYA800 291 31%
3 40μp? 95% 1327CA 5% HDZ222 80% EX489BA 20% HDZ222 273 33%
4 40μp? 95% 1327CA 5% HYA800 70% EX489BA 30% LLI201XV 350 33%
40μ? 1 95% 1327CA 5% HYA800 70% EX489BA 30% HYA800 292 30%
6 40μp? 95% 1327CA 5% HDZ222 70% EX489BA 30% HDZ222 271 29%
7 40μp? 95% 1327CA 5% HYA800 60% EX489BA 40% LL1201XV 367 35%
8 40μp? 95% 1327CA 5% HYA800 60% EX489BA 40% HYA800 306 31%
9 40μp? 95% 1327CA 5% HDZ222 60% EX489BA 40% HDZ222 271 29%
40μ? T? 85% 1327CA 15% EX489BA 80% EX489BA 20% LL1201XV 332 32%
11 40μp 85% 1327CA 15% EX489BA 80% EX489BA 20% HYA800 291 30%
12 40μG? 85% 1327CA 15% EX489BA 80% EX 89BA 20% HDZ222 276 29%
13 40μp \ 85% 1327CA 15% EX489BA 70% EX489BA 30% LL1201XV 353 34%
14% 40μp? 85% 1327CA 15% EX489BA 70% EX489BA 30% HYA800 298 30%
40μp? 85% 1327CA 15% WX489BA 79% ?? 489 ?? 30% HDZ222 275 30%
16 40 p? 85% 1327ca 15% WX489BA 60% ex489ba 40% 111201XV 367 35%
17 40μp? 85% 1327CA 15% ?? 489 ?? 60% ?? 489 ?? 40% ??? 800 304 32%
18 40μp? 85% 1327CA 15% ?? 489 ?? 60% ?? 489 ?? 40% HDZ222 272 30%
19 40μp? 85% 1327CA 15% EX514BA 70% EX514BA 30% HDZ222 294 32%
85% 1018CA 15% ?? 89 ?? 70% ?? 489 ?? 30% HDZ222 281 30%
22 40μp? 85% LL1201XV 15% ?? 489 ?? 70% ?? 489 ?? 30% LL1201XV 341 32%
Table 3
Corrida 1 2 3 4 5 6 1 8 9 10 11 12 13 14 15 16 17 18 19 20 22
Fog 9.6 5.6 5 9.6 6.2 6 10.9 7.5 6.8 4 4.4 4.4 4.5 4.9 5.1 4.8 5.9 5.8 5.20 4.90 5.20
Brightness 60% 10 13 13.6 9.5 12.9 13.5 9.8 12.3 13.2 13.4 13.5 13.6 13.3 13.5 13.5 13.1 12.9 13.4 12.560 13.540 12.250
Brightness 20% 5.4 11.5 13.5 5.9 10.5 12.8 6.5 10.4 12.5 12.6 11.4 13.2 12.6 12.7 13 11.7 103 12.8 10.70 13.30 10.30
Mod. Sec. 339 449 429 340 493 492 338 547 564 334 380 409 329 444 471 327 4B8 539 426.0 451.0 322.0 1 * MD MPa Mod. Sec. 367 512 516 362 544 555 376 633 672 391 4S9 505 382 505 571 385 583 674 543.0 517.0 371.0 1% TD MPa Clarity% 80 77 93 76 74 81 80 74 87 89 82 81 74 78 92 85 67 89 83.0 85.0 B3.0
Elmendorf MD g // jm 5.9 14.2 9.3 4.3 10.2 3.1 2.3 4.7 0.9 5.5 7.1 13.4 4.6 11 10.6 9.8 3.S 2.3 15. 40 3.960 5.560
Elmendorf 6 7.3 7.9 8.3 9.1 9.9 10 12 12.5 6.6 1.4 8 7.7 10 9.5 5.6 11.9 11.9 7.910 12.70 7.520
Strength 1.07 1.26 1.31 1.04 1.4 1.55 0.98 1.74 1.76 1.01 1.3 1.42 0.99 1.49 1.54 1 1.57 1.69 1,673 1,498 1.0 Thermal MD N / 1S mm Deformation 14.8 17.4 16.8 15.0 18.2 18.2 15.0 19.1 19.1 14.7 16.2 16.2 14.4 17.0 17.2 14.3 18.0 18.4 16.7 16.5 14.1 displaced 10% MD MPa Deformed displacement 14.4 17.4 16.9 14.6 17.7 18.7 14.6 18.9 20.0 14.6 16.1 16.7 14.7 16.9 17.8 14.5 18.6 18.9 17.3 16.4 14.2 10% TD MPa
Various advantages can be observed in the present invention, according to the examples. For example, the melt pressure of the core extruder drops when HDPE grades are used in the core compared to degrees of LLDPE (Examples 2-3, 5-6, 8-9, 11-12, 14-15, 17 -20 versus 1, 4, 7, 10, 13, 16, 22). In the case of the epidermis comprising polyethylene by metallocene and LDPE and the core comprising HDPE (Examples 11-12, 14-15, 17-21), the secant modulus values at 1% MD and TD were significantly higher. than in the case where there was no HDPE in the core layer (Examples 10, 13, 16, 22). Among other things, the 1% secant module provides a measure of the possible caliber reduction using these films. The greater the 1% secant modulus, the smaller the gauge (film thickness) required to provide the same benefit. The increase in the resistance to deformation as measured by the deformation displaced to 10% is also a benefit that opens the possibilities of reducing the caliber. A thinner material having the same rigidity and the same resistance (and therefore lower cost) is very sought after. Similarly, the epidermis composed of physical mixtures of polyethylene by metallocene and HDPE that wall a core comprising HDPE (Examples 2-3, 5-6 and 8-9) had a superior 1% secant modulus relative to the examples with nuclei without HDPE (Examples 1, 4 and 7). Furthermore, as is evident from an inspection of the optical properties, the increase in strength and / or caliber reduction that accompany the presence of HDPE in the nucleus and / or the epidermis are not significantly compensated for by the loss of clarity or brightness values. In fact, it is particularly remarkable that the gloss at 20 and at 60 degrees is quite similar for the examples according to the present invention, particularly with respect to the examples having HDPE both in the nucleus and in the epidermis with respect to the examples that They have HDPE in the epidermis but not in the nucleus. A small difference in brightness at 20 and 60 degrees is important for deployment purposes, ie the angle of compliance is not important. In Table 3, for example, it can be seen that the difference in brightness values at 20 and 60 degrees is around 0.1%, which is negligible. Typically for examples of the present invention, 2% differences are observed. Moreover, the examples according to the invention uniformly exhibit higher Elmendorf tear values (the higher number being a measure of greater tear strength) and a higher thermal force (a measure of the retention force when wrapped by shrinkage). Around Collated Items Example 23 A film having an A / B / A structure with a thickness of 2 mils, with a thickness ratio, respectively, of 15:70:15, was produced in a co-extrusion line Commercial Maachi: Layers A consisted of about 95% by weight of Exceed 1327CA and about 5% by weight of LD514. Layer B consisted of about 80% by weight of LD514BA and about 20% by weight of HDZ222. Example 24 An identical film to the previous one, except that HDZ222 was replaced by LD514, was produced in the same way A comparison of Examples 23 and 24 illustrates that a combination of mLLDPE and LDPE as the epidermis layer This particular case results in a reduction in COF of about 15%, a modest increase in brightness, and a reduction in fog of 10.4 in Example 24 to 5.9 in Example 23. The invention has been described above with reference to numerous specific embodiments and examples. Many variations will suggest themselves to those skilled in the art in light of the above detailed description. All these obvious variations are within the intended scope, full of the appended claims. A preferred embodiment is a multilayer film structure having at least one first layer comprising HDPE and at least one second layer, different from said first layer, comprising a polyethylene by metallocene; also more preferred embodiments, which may be combined where appropriate, as would be recognized by one skilled in the art in possession of the present disclosure, without undue experimentation: wherein said metallocene polyethylene is a mLLDPE; wherein said first layer further comprises LDPE in the amount of at least 50% by weight, based on the composition of the first layer; wherein said second layer further comprises at least one additional polyolefin selected from the group consisting of HDPE, LDPE and mixtures thereof, in an amount of between 0.1 and 50% by weight; wherein said first layer comprising HDPE is a core layer, said second layer comprising a metallocene polyethylene is a layer of epidermis, and further comprising a second layer of epidermis; wherein said first layer comprising HDPE is a core layer, said second layer comprising a polyethylene by metallocene is a layer of epidermis, and further comprising a second layer of epidermis, said second layer of epidermis comprising a polyethylene by metallocene, which may be same or different from the first layer of epidermis; wherein said first layer comprising HDPE is a core layer, said second layer comprising a polyethylene by metallocene is a layer of epidermis, and further comprising a second epidermis layer, said second epidermal layer comprising a mLLDPE; wherein said first layer comprising HDPE is a core layer, said second layer comprising a polyethylene by metallocene is a layer of epidermis, and further comprising a second epidermis layer, said second epidermal layer comprising a mLLDPE and further comprising at least an additional polyolefin selected from the group consisting of HDPE, LDPE and mixtures thereof, in an amount of between 0.1 and 50% by weight; wherein said second epidermis comprises a mLLDPE and further comprises a polyolefin selected from HDPE, LDPE and mixtures thereof, in an amount of between 0.1 and 50% by weight; where none of the layers contain slip and anti-block additives; any of the structures of three or more layers, where the total thickness is 70 microns or less, 60 microns or less, 50 microns or less, preferably 40 microns or less, or even more preferably where said first layer comprising HDPE it has a thickness of about 40 microns or less, preferably 30 microns or less, and each of said epidermis layers has a thickness of about 10 microns or less, preferably 5 microns or less; and preferably where the ratio of the layers in structure A / B / A described herein is in the range of, respectively, (15 ± 5): (70 + 10): (15 + 5), where the structure is symmetric or asymmetric with respect to one or more of the characteristics of composition, dimensions, additional layers, and the like, e.g., where the composition of the epidermal layers is identical or different, where there are additional layers, such as layers of mooring, between none, one or both of the A / B interfaces, where the ratio of the A / B / A layers is 15:70:15, 20:70:10, etc. Another preferred embodiment is a film comprising an A / B / A structure, where the A layers are layers of epidermis, which may be the same or different, each being independently selected from a physical mixture comprising an mPE having a density between about 0.910 and 0.940 g / cm3, preferably 0.915 to 0.940 g / cm3, and optionally a HDPE, which if present preferably has a density between about 0.940 and 0.970 g / cm3, more preferably 0.955 a about 0.965 g / cm3, and most preferably about 0.960 to about 0.965 g / cm3, and / or optionally a LDPE which, if present, preferably has a density of about 0.924 to about 0.940 g / cm3, or from around 0.916 to around 0.935 g / cm3, or from around 0.926 to around 0.935 g / cm3, or from around 0.916 to around 0.927 g / cm3, or from around 0.921 to around 0.926 g / cm3, or from around 0.925 to around 0.930 g / cm3, or 0.916 to 0.940 g / cm3, or from around 0.924 to 0.935 g / cm3, and other forms of embodiment including LDPEs having densities of any of the lower density limits specified at any of the upper density limits specified herein, for example 0.921 to 0.940 g / cm3, 0.926 to 0.940 g / cm3, or 0.925 to 0.935 g / cm3, and B is a core layer comprising a physical mixture comprising an HDPE, preferably having a density of between about 0.940 and 0.970 g / cm3, more preferably 0.955 to about 0.965 g / cm3, and with the greater preference of about 0.960 to about 0.965 g / cm3, and an LDPE, preferably having a density in the range of 0.916 to 0.935 g / cm3, more preferably 0.925 to 0.930 g / cm3, and also including the ranges of density previously indicated for the one or more layers of epidermis and also forms more preferred embodiments of this structure A / B / A, where the core layer B comprises 60-90% by weight, with greater preference
70-80% by weight of LDPE, and 40-10% of HDPE, more preferably 30-20% by weight, and the layers of epidermis A are selected, each, independently, from 80-100% by weight , preferably 85-95% by weight of mPE, and 20-0% by weight of HDPE, LDPE, or mixtures thereof, more preferably 15-5% by weight; and a more preferred embodiment of any of the above embodiments wherein said layers A and B, when formed in a co-extruded A / B / A structure having a total thickness of less than 60 microns or less than 50 microns, has a 1% MD drying modulus of at least 335 MPa, preferably 400 MPa, more preferably 500 MPa, and a 1% TD secant modulus of at least 335 MPa, preferably 400 MPa, more preferably 500 MPa, and still more preferably 600 MPa, the secant modulus values measured according to the ASTM method D882; and also any of the previous embodiments where the layers A and layer B, when formed in a co-extruded A / B / A structure having a total thickness of less than 50 microns, have a difference in brightness to 20. and 60 degrees of 2% or less, the brightness values being measured according to the ASTM D2457 method.
Additional preferred embodiments of any of the foregoing would include the films having one or more performance parameters noted above in the experimental section, and would also include films having additional layers, such as A / B / C / D / E, where the layers of epidermis
A and E, which may be the same or different, corresponding to the aforementioned composition for the epidermis layer A in the structure A / B / A or where A corresponds to the epidermis layer A and E corresponds to a layer comprising polypropylene , the composition C corresponds to the composition indicated above for the core layer B in the structure A / B / A, and B and D, which may be the same or different, correspond to layers that can be selected from tie layers, layers of re-processed material, without limitation to these, and in preferred embodiments, additional layers having the composition corresponding to layer B in structure A / B / A previously described. In one embodiment, the structure comprising the A / B / A layers, as described herein, does not contain an oxygen barrier layer in the structure. However, one of the particularly beneficial uses of the collation shrink film comprising the A / B / A layers according to the invention relates to wrapping articles having a primary oxygen barrier layer, e.g., perishable items such as meats. Typical primary oxygen barriers are discussed in the aforementioned WO 95/00333 publication, such as vinylidene chloride copolymer, and may also include copolymers of ethylene and vinyl alcohol (EVOH copolymers). Other preferred embodiments are co-extruded films, heat-shrinkable films, cast films, blown films, and shrink-wrapped structures according to any of the preceding embodiments (including preferred embodiments, further embodiments). preferred, etc.). The commercial designations used herein are used as such, indicating that they may be protected by certain trademark rights, eg, they may be registered trademarks in various jurisdictions. All patents and patent applications, test procedures (such as ASTM methods and the like), and other documents cited herein, are incorporated by reference in their entirety insofar as such disclosure is not inconsistent with this invention and for those jurisdictions in which such incorporation is allowed.