High modulus and low shrink force film

Abstract: A multilayer film comprising a) a core layer comprising an ethylene polymer having a density of at least 0.925 grams per cubic centimeter; and b) two outer layers each comprising a blend of: i) 40 to 69% of an ethylene polymer having a density of at least 0.930 grams per cubic centimeter, and ii) 31 to 60% of an ethylene copolymer which is an ethylene/alpha-olefin copolymer; wherein at least one of said outer layers comprises less than 30% linear low density polyethylene; and said film has a tensile modulus of at least 414 MPa (60,000 psi) in the longitudinal direction, and at least 517 MPa (75,000 psi) in the transverse direction; or said film has a shrink force of less than 1.57 N (0.35 pounds) in the longitudinal direction and less than 1.33 N (0.30 pounds) in the transverse direction. The film offers beneficial properties for packaging applications.

Description: FIELD OF THE INVENTION The present invention relates to a high stiffness (high modulus) thermoplastic film which may be utilized to package a wide variety of items. BACKGROUND OF THE INVENTION The present invention is directed to a new and useful film,. Films, and especially heat shrinkable films are well known for many packaging applications. An example is D-955 film supplied commercially by W.R. Grace. This film has proven to be very useful in packaging applications where high shrink, good optics, and other desirable features of the packaging film are needed. These additional features include impact resistance, tear initiation and tear propagation resistance. Films of this type are disclosed in U.S. Patent Nos. 4,551,380 and 4,643,943, both to Schoenberg. It would be desirable to use heat shrinkable materials like D-955 in end use applications requiring relatively high stiffness, i.e. high modulus. One example of such an end use is on certain high speed packaging machines. Unfortunately, these films do not have sufficient stiffness to provide an optimal packaging material for such uses. It would also be desirable to provide a material that has a reduced thickness compared to many commercial films. This would offer reduced costs in manufacture, because of the reduction in the amount of raw material used to make the film. It would also offer an environmentally attractive film because of source reduction. However, in attempting to achieve this by simply downgauging currently available films, degradation of film properties can occur. These properties include modulus (stiffness), which as already pointed out is already marginal or unacceptably low for some packaging applications; heat seal strength, and impact resistance. WO 94/06857 discloses soft films having laminar ABA construction wherein the skin layer A comprises a blend of a very low density polyethylene with a molecular weight distribution (Mw/Mn) not greater than 3.5 with a low to medium density polyethylene polymer with a molecular weight distribution greater than 3.5 and the core layer B comprises a different olefin polymer such as high density polyethylene. Monolayer films comprising the blend A are also disclosed. The films are said to exhibit excellent elongation, tensile and impact properties as well as softness, feel and noise properties which render them suitable for use in diapers, bed pads and the like. SUMMARY OF THE INVENTION The present invention provides a multilayer heat shrinkable, crosslinked film comprising a) a core layer comprising an ethylene polymer having a density of at least 0.925 grams per cubic centimeter; and b) two outer layers each comprising a blend of: i) 40 to 69% of an ethylene polymer having a density of at least 0.930 grams per cubic centimeter, and ii) 31 to 60% of an ethylene copolymer which is an ethylene/alpha-olefin copolymer; wherein at least one of said outer layers comprises less than 30% linear low density polyethylene; and said film has a tonsile modulus of at least 414 MPa (60,000 psi) in the longitudinal direction, and at least 517 MPa (75,000 psi) in the transverse direction. DEFINITIONS The term "core layer" as used herein refers to the central layer of a multilayer film. The term "outer layer" as used herein refers to what is typically an outermost, usually surface layer of a multi-layer film, although additional layers and/or films can be adhered to it. Polymer" herein includes homopolymer, copolymer, terpolymer, etc. "Copolymer" herein includes copolymer, terpolymer, etc. All compositional percentages used herein are calculated on a "by weight" basis. The term "intermediate" as used herein refers to a layer of a multi-layer film which is between an outer layer and core layer of the film. "Linear low density polyethylene" (LLDPE) as used herein has a density in the range of from about 0.916 to 0.924 grams per cubic centimeter. "Linear medium density polyethylene" (LMDPE) as used herein, has a density from 0.930 grams per cubic centimeter to 0.939 grams per cubic centimeter. "High density polyethylene" (HDPE), as defined herein, has a density of 0.94 grams per cubic centimeter or more. The term "ethylene/ester copolymer" (E/E) as used herein refers to a copolymer formed from ethylene and an ester such as vinyl acetate, alkyl acrylate, or other monomers, wherein the ethylene derived units in the copolymer are present in major amounts and the ester derived units in the copolymer are present in minor amounts. As used herein, the phrase "ethylene/alpha-olefin copolymer" (EAO) refers to such heterogeneous materials as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE); as well as homogeneous polymers (HEAO) such as TAFMER (TM) ethylene/alpha olefin copolymers supplied by Mitsui Petrochemical Corporation and metallocene-catalyzed polymers such as EXACT (TM) materials supplied by Exxon. These materials generally include copolymers of ethylene with one or more comonomers selected from C 4 to C 10 alpha-olefins such as butene-1 (i.e., 1-butene), hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylenes which are more highly branched than their respective counterparts. Other ethylene/alpha-olefin copolymers, such as the long chain branched homogeneous ethylene/alpha-olefin copolymers available from the Dow Chemical Company, known as AFFINITY (TM) resins, are also included as another type of ethylene/alpha-olefin copolymer useful in the present invention. "Heat shrinkable" is defined herein as a property of a material which, when heated to an appropriate temperature above room temperature (for example 96°C.), will have a free shrink of 5% or greater in at least one linear direction. The films of the invention comprise an outer layer "substantially free from linear low density polyethylene" herein means that less than 30% by weight of the relevant layer comprises LLDPE, and preferably less than 20%, such as less than 10%, and 5% LLDPE. More preferably, no LLDPE is present in the relevant layer. It has been found that little or no LLDPE in the film, and in particular in one or both outer layers of the film, leads to performance benefits by the use of the present invention in packaging equipment. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a preferred three layered embodiment of the present invention. FIG. 2 is a cross-sectional view of a preferred five layered embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG.1, which is a cross-sectional view of a preferred three layered embodiment of the present invention, it is seen that this embodiment comprises a core layer 1, and two outer layers 2 and 3. Outer layers 2 and 3 are preferably surface layers. Core layer 1 comprises an ethylene polymer having a density of at least 0.925 grams per cubic centimeter (gm/cc), preferably an ethylene/ alpha-olefin copolymer with a C 4 to C 10 comonomer, more preferably linear medium density polyethylene. The core layer can also comprise e.g. high density polyethylene. Other polymeric materials can be included in the core layer in addition to the ethylene polymer. Examples of these additional materials are ethylene polymer or copolymer having a density of less than 0.925 gm/cc, such as LLDPE or VLDPE; and ethylene/ester copolymer, such as ethylene/vinyl ester copolymer, e.g. ethylene/vinyl acetate copolymer, or ethylene/alkyl acrylate copolymer, e.g. ethylene/ethyl acrylate copolymer, ethylene/methyl acrylate copolymer, or ethylene/ butyl acrylate copolymer; or ethylene/ acid copolymer, such as ethylene/acrylic acid copolymer, or ethylene/ methacrylic acid copolymer. Blends of these materials in any proportion can also be used with each other, and/or with the ethylene polymer having a density of at least 0.925 gm/cc. Outer layers 2 and 3 comprise or consist essentially of a blend of an ethylene polymer having a density of at least 0.930 gm/cc, and an ethylene copolymer which is an ethylene/alpha-olefin copolymer. The ethylene polymer having a density of at least 0.930 gm/cc can be any of the materials suitable for core layer 1. Blends of these materials in any proportion can also be used. The ethylene/alpha olefin copolymer preferably comprises a copolymer of ethylene and a C 4 to C 10 comonomer. It is preferably a homogeneous ethylene/alpha-olefin copolymer, or an ethylene/alpha-olefin copolymer with a density of less than 0.916 g/cc. The blend of outer layers 2 and 3 is a blend of between 40% and 69%, such as between 40 and 60%, or 50% of the ethylene polymer having a density of at least 0.930 gm/cc, and between 31% and 60%, more preferably between 35 and 60%, such as between 45 and 55%, or 50% of the ethylene/alpha-olefin copolymer. Referring to FIG.2, which is a cross-sectional view of a preferred five layered embodiment of the present invention, it is seen that this embodiment comprises a core layer 11, two intermediate layers 12 and 13, and two outer layers 14 and 15. Outer layers 14 and 15 are preferably surface layers. Core layer 11 comprises any of the materials described above for core layer 1. Outer layers 12 and 13 comprise any of the materials described above for outer layers 2 and 3. Intermediate layers 14 and 15 can comprise any material that adheres the core layer 11 to outer layers 12 and 13. This can include a polymeric adhesive such as anhydride-grafted polymer, e.g anhydride-grafted LLDPE; ethylene/alpha-olefin copolymer such as LLDPE, very low density polyethylene, homogeneous ethylene/alpha-olefin copolymer, and blends thereof; or even a conventional adhesive such as polyurethane. Layers 14 and 15 can also include ethylene/ ester copolymer, such as ethylene/vinyl ester copolymer, e.g. ethylene/vinyl acetate copolymer, or ethylene/alkyl acrylate copolymer, e.g. ethylene/ethyl acrylate copolymer, ethylene/methyl acrylate copolymer, or ethylene/ butyl acrylate copolymer; or ethylene/ acid copolymer, such as ethylene/acrylic acid copolymer, or ethylene/ methacrylic acid copolymer. The invention can be further understood by reference to the examples given below. These films can be made by a conventional cast coextrusion, lamination, or other suitable process. These films can be partially or totally crosslinked by irradiative or chemical means and oriented by trapped bubble, tenterframe, or other suitable process. They can thereafter optionally be annealed. Final film thicknesses can vary, depending on process, end use application, etc. Typical thicknesses range from 2.54 to 510 µm (0.1 to 20 mils). Crosslinking by irradiation can be done by any conventional means. In the irradiation process, the film is subjected to an energetic radiation treatment, such as corona discharge, plasma, flame, ultraviolet, X-ray, gamma ray, beta ray, and high energy electron beam treatment, which induce cross-linking between molecules of the irradiated material. The irradiation of polymeric films is disclosed in U.S. Patent No. 4,064,296, to Bornstein, et al.. Bornstein, et. al. disclose the use of ionizing radiation for crosslinking the polymer present in the film. Radiation dosages are referred to herein in terms of the radiation unit "RAD", with one million RADS, also known as a megarad, being designated as "MR", or, in terms of the radiation unit kiloGray (kGy), with 10 kiloGray representing 1 MR, as is known to those of skill in the art A suitable radiation dosage of high energy electrons is in the range of up to about 10-200 kGy, more preferably about 15-190 kGy, and still more preferably, 20-180 kGy. Preferably, irradiation is carried out by an electron accelerator and the dosage level is determined by standard dosimetry methods. Other accelerators such as a Vander Graff or resonating transformer may be used. The radiation is not limited to electrons from an accelerator since any ionizing radiation may be used. The ionizing radiation crosslinks the polymers in the film. The more preferred amount of radiation is dependent upon the film and its end use. Table 1 identifies the materials used in the examples. The remaining tables describe the properties of films made with these materials. MATERIAL TRADENAME SOURCE PE 1 Dowlex™ 2045.04 Dow PE 2 Dowlex™ 2037 Dow PE 3 PE 4 SLP-8-6031 Exxon EV 1 PE 1335 Rexene EV 2 LD-705.15 Exxon EV 3 LD-720.92 Exxon PE 1 = LLDPE, an ethylene/ 1-octene copolymer with a density of 0.920 gm/cc and an octene-1 comonomer content of 6.5%. PE 2 = LMDPE, an ethylene/ 1-octene copolymer with a density of 0.935 gm/cc. and an octene-1 comonomer content of 2.5%. PE 3 = ethylene/ 1-octene copolymer with a density of 0.912 gm/cc. PE 4 = ethylene/ 1-octene terpolymer with a density of 0.902 gm/cc. EV 1 = ethylene/ vinyl acetate copolymer with 3.3% vinyl acetate monomer. EV 2 = ethylene/ vinyl acetate copolymer with 13.3% vinyl acetate comonomer. EV 3 = ethylene/ vinyl acetate copolymer with 19 % vinyl acetate comonomer. In Tables 2 and 3, fourteen five- layer film structures in accordance with the invention are disclosed. These were each made by a coextrusion of the layers, and each had the structure: A/B/C/B/A The thickness ratio of the layers was: layer A layer B layer C layer B layer A 2.0 1.5 3.0 1.5 2.0 All the films except Ex. 14 were biaxially oriented at 5 x 6 in the machine and transverse directions respectively. Example 14 was onented at a 5 x 5 ratio. All films were irradiated at an absorbed dosage of between 80 and 95 K Gy, except Example 1 which was irradiated at about 65 K Gy. The B layers of the films were PE 1 . The C layers of the films were PE 2 . The A layers of the films were a blend of 50% PE 2 , 20% of masterbatch having 90% PE 2 and 10% slip and antiblock additives, and 30% of one of the materials indicated in Table 1, and identified for each example in the following tables. Physical Property Ex.1 Ex. 2 Ex. 3 Ex. 4 Ex.5 Ex.6 Ex.7 Film thickness (gauge) 33 30 30 30 30 30 third component in "A" layer EV 1 EV 1 EV 1 EV 2 PE 4 EV 3 Tensile@ Break (psi x 1000) LD 17.4 14.7 18.7 17.8 18.9 17.3 TD 25.2 19.6 20.5 20.7 22.6 20.8 Elongation@ Break (%) LD 107 85 111 120 108 88 TD 62 47 59 69 59 53 Modulus (psi x 1000) LD 94.5 93.9 100.2 86.6 83.9 80.7 TD 104.7 115.3 104.5 101.8 92.8 96.4 Tear Propagation (gms) LD 3 4 3 3 3 3 TD 2 2 2 2 2 2 Free Shrink (%) @(200°F) 93°C LD 8 5 6 7 6 7 TD 9 9 9 8 11 11 @(220°F) 104°C LD 12 10 12 12 10 13 TD 17 16 17 16 18 21 @(240°F) 115°C LD 32 29 32 32 29 28 TD 40 39 40 39 39 38 @(260°F) 126°CLD 78 77 78 78 77 78 TD 81 81 82 82 81 81 @(280°F) 137°C LD 78 78 78 78 78 78 TD 82 81 82 81 82 82 @(300°F) 149°C LD 78 78 78 78 77 78 TD 83 82 82 82 82 82 Shrink Force (pounds) @(200°F) 93°C LD 0.09 0.08 0.08 0.07 0.08 0.08 TD 0.19 0.19 0.17 0.17 0.20 0.20 @(220°F) 104°CLD 0.11 0.09 0.09 0.08 0.11 0.09 TD 0.23 0.21 0.21 0.20 0.22 0.24 @(240°F) 115°C LD 0.13 0.10 0.10 0.12 0.13 0.12 TD 0.24 0.22 0.21 0.24 0.25 0.13 @(260°F) 126°C LD 0.13 0.11 0.11 0.13 0.13 0.12 TD 0.24 0.20 0.21 0.22 0.23 0.21 Ball Burst Impact (cm-kg) 7 6 6 5 9 9 Instrumented Impact (lb) 7 5 5 7 8 9 Haze (%) 2.8 2.9 2.6 4.6 2.1 5.8 Clarity (%) 66 88 88 80 88 80 Gloss 45° 83 84 83 70 81 63 C.O.F. (in/in) C.O.F.(out/out) 0.34 0.34 0.37 0.36 0.38 0.32 Physical Property Ex.8 Ex.9 Ex. 10 Ex. 11 Ex.12 Ex.13 Ex.14 Film thickness (gauge) 40 60 45 45 40 45 60 third component In "A" layer EV 1 EV 1 EV 1 EV 1 PE 4 PE 4 PE 4 Tensile@ Break (psi x 1000) LD 17.4 18.5 20.7 19.4 18.7 15.9 19.6 TD 21.5 21.7 23.3 29.4 17.7 19.6 21.7 Elongation@ Break (%) LD 128 110 113 107 103 110 116 TD 59 92 68 58 96 80 91 Modulus (psi x 1000) LD 78.9 74.4 80.6 79.7 78.7 71.9 77.3 TD 112.7 86.5 94.6 115.7 69.6 86.9 90.3 Tear Propagation (gms) LD 4 5 5 5 4 5 7 TD 2 4 3 4 3 3 5 Free Shrink (%) @(200°F) 93°C LD 7 8 8 9 6 6 7 TD 12 13 11 13 11 12 14 @(220°F) 104°C LD 10 15 14 14 12 12 13 TD 20 23 21 21 21 20 23 @(240°F) 115°C LD 28 32 33 34 37 44 51 TD 38 42 43 44 48 54 58 @(260°F) 126°C LD 77 77 78 78 77 77 78 TD 82 79 82 80 80 81 80 @(280°F) 137°C LD 78 78 78 78 77 77 78 TD 81 80 81 81 80 82 80 @(300°F) 149°C LD 79 78 79 78 78 78 78 TD 82 80 82 82 82 82 80 Shrink Force (pounds) @(200°F) 93°C LD 0.07 0.17 0.11 0.13 0.07 0.10 0.12 TD 0.24 0.36 0.27 0.27 0.23 0.23 0.31 @(220°F) 104°C LD 0.10 0.20 0.14 0.16 0.10 0.08 0.17 TD 0.29 0.42 0.33 0.31 0.28 0.27 0.36 @(240°F) 115°C LD 0.13 0.24 0.17 0.19 0.13 0.10 0.23 TD 0.30 0.41 0.39 0.33 0.29 0.29 0.39 @(260°F) 126°C LD 0.12 0.24 0.16 0.18 0.13 0.15 0.22 TD 0.26 0.37 0.36 0.33 0.27 0.29 0.36 Ball Burst Impact (cm-kg) 14 12 21 18 14 15 18 Instrumented Impact (lb) 9 16 17 15 11 11 17 Haze (%) 2.8 3.2 3.1 3.2 3.4 3.4 3.6 Clarity (%) 66 87 87 87 87 87 86 Gloss, 45° 86 86 85 86 77 74 82 C.O.F. (in/in) C.O.F.(out/out) 0.33 0.35 0.31 0.29 0.34 0.31 0.30 In Table 4, five additional five- layer film structures in accordance with the invention are disclosed. These were each made by a coextrusion of the layers, and each had the structure: A/B/C/B/A The thickness ratio of the lavers was: layer A layer B layer C layer B layer A 1 1 1 1 1 All of Examples 15 to 19 were 7.6 µm (30 gauge, or 0.30 mils) thick. The films of Examples 15, 16, and 17 were biaxially onented at 5 x 6 in the longitudinal and transverse directions respectively. Examples 18 and 19 was oriented at a 5.5 x 5.5 ratio. All films were irradiated at an absorbed dosage of 61 K Gy, except Example 15 which was irradiated at about 78 K Gy. The B layers of the films were PE 1 . The C layers of the films were PE 2 . The A layers of the films were a blend of PE 2 , a masterbatch (MB) having 90% PE 2 and 10% slip and antiblock additives, and either EV 1 or PE 4 . The formulation of layer A is identified for each example in the following table. Ex.15 Ex. 16 Ex. 17 Ex. 18 Ex.19 50% PE 2 + 10% PE 2 MB + 40% EV 1 50% PE 2 + 10% PE 2 MB + 40% EV 1 40% PE 2 + 10% PE 2 MB + 50% EV 1 50% PE 2 + 10% PE 2 MB + 40% PE 4 40% PE 2 + 10% PE 2 MB + 50% PE 4 In Table 5, five three- layer film structures in accordance with the invention are disclosed. These were each made by a coextrusion of the layers, and each had the structure: A/B/A The thickness ratio of the layers was: layer A layer C layer A 1 3 1 All the films of Examples 20 to 24 were irradiated, and biaxially oriented at 5 x 5 (5.5 x 5.5 for Ex. 20) in the machine and transverse directions respectively. The C layers of the films of Examples 20 to 24 were a blend of 60% PE 1 and 40% PE 2 . The A layers of the films of Examples 20 to 24 were a blend of 40% PE 2 , 20% of a masterbatch having 86% PE 2 and 14% slip and antiblock additives, and 40% of one of the materials indicated in Table 1, and identified for each example in the following table. A comparative film, Comp. 1, represents D-955, having an A/B/A structure where the thickness ratio of the layers was 1/2/1, and where B comprises PE 1 , and A comprises a blend of 50% LLDPE, 25% LMDPE, and 25% of a masterbatch comprising 90% EVA and 10% slip and antiblock additives. Physical Property Comp.1 Ex.20 Ex.21 Ex. 22 Ex.23 Ex.24 Film thickness (gauge) 60 30 40 30 30 40 third component in "A" layer - EV 1 EV 1 EV 1 PE 4 PE 4 Tensile@ Break (psi x 1000) LD 18.8 18.3 17.4 18.5 19.0 21.2 TD 22.4 20.5 19.0 20.2 20.6 21.0 Elongation@ Break(%) LD 107 90 97 97 110 112 TD 81 107 117 85 75 97 Modulus (psi x 1000) LD 56.1 101.0 82.1 83.1 69.3 69.8 TD 70.8 105.1 81.6 93.3 86.2 78.0 Tear Propagation (gms) LD 5 2 3 2 2 3 TD 5 2 3 2 3 4 Free Shrink (%) @(220°F) 93°CLD 24 14 18 18 19 21 TD 34 24 27 24 27 30 @(240°F) 104°C LD 59 40 44 43 52 53 TD 63 49 50 52 58 59 @(260°F) 115°C LD 79 81 79 79 80 79 TD 79 73 78 79 78 78 @(280°F) 126°C LD 79 81 79 80 80 79 TD 80 82 81 79 80 78 Instrumented Impact (lb) 20 7 12 8 6 9 Haze (%) 3.6 4.2 5.4 4.5 4.6 4.4 Clarity (%) 79 86 84 86 86 86 Gloss, 45° 86 76 78 80 77 80 Additional film structures in accordance with the present invention are listed in Table 6 below. EX FILM STRUCTURE 25 50% LMDPE + 50% HEAO/LMDPE/50% LMDPE + 50% HEAO 26 50% LMDPE + 50% HEAO/LLDPE/LMDPE/LLDPE/50% LMDPE + 50% HEAO 27 50% HDPE + 50% HEAO/HDPE/50% HDPE + 50% HEAO 28 50% HDPE + 50% HEAO/LLDPE/HDPE/LLDPE/50% HDPE + 50% HEAO Some films of the present invention have a relatively low shrink force, and are especially useful in "soft shrink" applications where sensitive products are to be packaged. This is because although such films of the present invention can have shrink tensions similar to thicker gauged films, their shrink force can be considerably less because of their thinner construction. Preferred shrink force values for films of the invention are less than 0.35 pounds-force in the longitudinal direction, preferably less than 0.30, such as less than 0.25, 0.20, 0.15, and 0.10; and less than 0.30 in the transverse direction, preferably less than 0.25, 0.20, 0.15, and 0.10. [1pound-force = 4.45 N] In Table 7, three additional three-layer films of the invention are compared with a control, D-955 film, labeled "Comp.2" in the Table. Comp.2 was equivalent in composition and construction to Comp.1 of Table 5. These were each made by a coextrusion of the layers, and each had the structure: A/B/A The thickness ratio of the layers was: layer A layer B layer A 1 3 1 All the films were biaxially oriented at 5 x 6 in the machine and transverse directions respectively. All films were irradiated at an absorbed dosage of between 20 and 80 K Gy. The B layer of the films was a blend of 60% PE 1 and 40% PE 2 . The A layers of the films were as identified for each example in Table 7. The masterbatch comprised 90% PE 2 and 10% slip and antiblock additives. 29 45% PE 2 + 40% PE 4 + 15% Masterbatch 30 45% PE 2 + 40% PE4 + 15% Masterbatch 31 40% PE 2 + 40% EV 1 + 20% Masterbatch Physical Property Comp 2 Ex 29 Ex 30 Ex 31 Film thickness (gauge) 60 30 30 30 Modulus c (psi x 1000) LD 72.2 77.8 71.8 82.8 TD 70.8 94.3 71.1 85.2 Free Shrink e (%) @(220°F) 104°C LD 34 20 28 25 TD 40 28 33 32 @(240°F) 115°C LD 64 43 55 48 TD 63 50 56 53 @(260°F) 126°C LD 82 80 80 81 TD 76 80 76 78 Shrink Tension f (psi) @(260°F) 126°C LD 608 477 543 506 TD 408 507 599 524 Shrink Force f (pounds) @(220°F) 104°C LD 0.35 0.10 0.14 0.14 TD 0.32 0.18 0.17 0.18 @(240°F) 115°C LD 0.38 0.14 0.16 0.16 TD 0.30 0.18 0.18 0.19 @(260°F) 126°C LD 0.39 0.15 0.19 0.18 TD 0.26 0.17 0.17 0.17 The film of the present invention can have a thickness of between 0.10 mils and 15 mils, but preferred thicknesses are less than 2 mils, preferably less than 1.5 mils such as 1 mil, and more preferably less than .70 mils. More preferably, the film is less than .60 mils thick, such as .55, .50, .45, .40, .35, .30, and .25 mils thick* . The films of the present invention can have a modulus of at least 60,000 psi* in the longitudinal direction (LD). Preferably, the modulus is at least 65,000 psi, more preferably at least 70,000 psi in the LD, such as 75,000, 80,000, 90,000, 95,000, 100,000 psi and higher. A preferred range is between 60,000 and 100,000 psi, more preferably between 65,000 and 95,000, more preferably between 70,000 and 90,000 psi, such as between 75,000 and 85,000 psi in the LD. At modulus values above 120,000, the film can still be useful, but tear properties of the film can degrade significantly. The films of the present invention can have a modulus of at least 75,000 psi in the transverse direction (TD). Preferably, the modulus is at least 80,000 psi, more preferably at least 85,000 psi in the TD, such as 90,000, 95,000, 100,000, 105,000, 110,000, 115,000, and 120,000 psi. A preferred range is between 75,000 and 120,000 psi, more preferably between 80,000 and 115,000, more preferably between 85,000 and 110,000 psi, such as between 90,000 and 105,000 psi, and between 95,000 and 100,000 psi in the TD. Too low a modulus will result in difficulty in using the film in some high speed packaging equipment, especially for thinner embodiments of the inventive film. Too high a modulus will result in a greater tendency for tear propagation The multilayer film of the present invention can have any suitable number of layers, for example 3,4,5,6,7,8,9, or more layers. Films can be symmetric or asymmetric in construction. For example, a film of the invention can have one of the following constructions: LMDPE + HEAO/LMDPE/LLDPE/LMDPE + HEAO HDPE + HEAO/HDPE/LLDPE/HDPE + HEAO Layer gauges can vary, even between layers with similar composition. Films of the invention can utilize different materials for the outer layers or for the intermediate layers, so that e.g. the two "A" layers can be different from each other in composition, degree of crosslinking, thickness, or other parameters. If oriented, the film can be oriented either monoaxially or biaxially. Orientation ratios preferably range from 2:1 to 10:1 in each direction, more preferably 2.5:1 to 9:1, such as 3:1 to 8:1, 4:1 to 7:1, 4.5:1 to 6:1, and 5.5:1 in each direction.

Claims: A multilayer heat shrinkable, crosslinked film comprising a) a core layer comprising an ethylene polymer having a density of at least 0.925 grams per cubic centimeter; and b) two outer layers each comprising a blend of: i) 40 to 69% of an ethylene polymer having a density of at least 0.930 grams per cubic centimeter, and ii) 31 to 60% of an ethylene copolymer which is an ethylene/alpha-olefin copolymer; wherein at least one of said outer layers comprises less than 30% linear low density polyethylene; and said film has a tensile modulus of at least 414 MPa (60,000 psi) in the longitudinal direction, and at least 517 MPa (75,000 psi) in the transverse direction. A film according to claim 1 wherein the ethylene polymer having a density of at least 0.925 grams per cubic centimeter in the core layer is linear medium density polyethylene or high density polyethylene. A film according to claim 1 or 2 wherein the ethylene polymer having a density of at least 0.930 grams per cubic centimeter in the outer layers is linear medium density polyethylene. A film according to any one of the preceding claims wherein the ethylene/alpha-olefin copolymer is a homogeneous ethylene/alpha-olefin copolymer. A film according to any one of the preceding claims which has a thickness of less than 15.2µm (0.60 mils). A film according to claim 5 which has a thickness of less than 12.7µm (0.50 mils). A film according to any one of the preceding claims further comprising at least one intermediate layer, disposed between the core layer and an outer layer, comprising an ethylene copolymer having a density of less than 0.925 grams per cubic centimeter. A film according to any one of the preceding claims having a tensile modulus of at least 483 MPa (70,000 psi) in the longitudinal direction and at least 586 MPa (85,000 psi) in the transverse direction.

Inventors: SHAH GAUTAM P, SHAH, GAUTAM P.
Applicants: CRYOVAC INC, CRYOVAC, INC.
Application year: 1997
Main classification: B32B27
IPC codes: C08L23, B32B27