Heat sealable film

Abstract: A film includes at least one layer comprising a blend of a first polymer having a crystalline melting point of at least 260°F, and a density of at least 0.925 grams per cubic centimeter, and a second olefinic polymer comprising an ethylene/alpha-olefin copolymer with a density of less than 0.916 grams per cubic centimeter, wherein the film heat seals at a temperature of at least 180°F. The film can include a core layer including an oxygen barrier; and two outer layers each including a blend of a first polymer having a crystalline melting point of at least 260°F, and a density of at least 0.925 grams per cubic centimeter, and a second olefinic polymer comprising an ethylene/alpha-olefin copolymer with a density of less than 0.916 grams per cubic centimeter, wherein the film heat seals at a temperature of at least 180°F. The film can also include one or more intermediate layers disposed between the core layer and respective outer layers, and one or more adhesive layers disposed between respective intermediate and outer layers. Good heat sealability is obtained.

Description: The present invention relates to a heat sealable film which may be utilized to package a variety of items. 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 BDF-2050 film supplied commercially by W.R. Grace. This film has proven to be very useful in packaging applications where high shrink, good optics, oxygen barrier and other desirable features of the packaging film are needed. Film of this type is disclosed e.g. in U.S. Patent No. 5,004,647 to Shah. Another example is a film, LID1050, useful in lidding applications. Film of this type is disclosed e.g. in EP 0692374. It would be desirable to use films, especially heat shrinkable materials like BDF-2050 in end use applications requiring very good heat sealability. For example, in uses where a foamed polystyrene tray is loaded with a food product and then overwrapped, impulse sealing is often used. Equipment such as Ilapak, Ossid, and Rose Forgrove systems are used in such applications. Some of these systems are high speed, producing packages at speeds of up to 100 ppm (parts or packages per minute). This use requires a film with good heat sealing properties, especially hot tack strength. Since hot tack strength is related to the flowability of the film material under heat and pressure, and in particular the flowability of the sealant layer of the film, it is important that the film flow and fuse together quickly under sealing conditions so that reliable heat seals can be made consistently at relatively high speeds. For lidding on foam trays, toughness as measured by a low percentage of abuse failures (cuts) is required. For uses where a polyester or aluminum coated tray is loaded with a food product and then overwrapped with a film, cuts in the tray edges can occur during packaging and distribution. Impulse sealing is used for this type of overwrapping also. Thus, in this end-use application, a combination of good sealing and toughness is needed. EP-A-0744285 describes heat-shrinkable films having an oxygen barrier core formed of ethylene-vinyl alcohol copolymer with two skin layers formed of a blend of a first polymer which may be a linear medium density polyethylene and a second polymer which is a heterogeneous ethylene/alpha-olefin copolymer having a density less than 0.915 g/ml. The film may have intervening layers of modified polyolefin. EP-A-0800915 (not published at the filing date hereof) describes thermoformable films comprising a core layer of ethylene-vinyl alcohol copolymer sandwiched between nylon layers. One sealant (outer) layer may comprise linear low density polyethylene having a density of about 0.95 g/ml optionally blended with other polymers such as low density PE. A new multilayer film according to the invention comprises: a) a core layer comprising an ethylene/vinylalcohol copolymer; b) two outer layers each comprising a blend of i) a first polymer selected from the group consisting of linear medium density polyethylene, high density polyethylene copolymer, polypropylene, and propylene/ethylene and ii) a second polymer comprising a heterogeneous ethylene/alpha-olefin copolymer with a density of less than 0.915 g/ml; and c) two intermediate layers each disposed between the core layer and the respective outer layer, each comprising a polyamide; wherein the film heat seals at a temperature of at least 82°C (180°F). DEFINITIONS The term "core layer" as used herein refers to a centralmost layer of a multi-layer 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. The term "intermediate" as used herein refers to a layer of a multilayer film which is between an outer layer and core layer of the film. 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 meiallocene-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. "High density polyethylene" (HDPE), as defined herein, has a density of 0.94 grams per cubic centimeter or more, "linear medium density polyethylene" (LMDPE) as used herein, has a density from 0.925 grams per cubic centimeter to 0.939 grams per cubic centimeter, "linear low density polyethylene" (LLDPE) as used herein has a density in the range of from 0.916 to 0.924 grams per cubic centimeter, and "very low density polyethylene" has a density of less than 0.916 grams per cubic centimeter. "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. Films of the invention will have a free shrink of preferably at least 10% in at least one linear direction at 96°C. "Polymer" herein includes copolymers, terpolymers, etc. "Copolymer" herein includes bispolymers, terpolymers, etc. All compositional percentages used herein are calculated on a "by weight" basis. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 6 are schematic cross-sectional views of films of the present invention. FIG. 7 is a schematic of an overwrapped tray. DESCRIPTION OF THE PREFERRED EMBODIMENTS The first polymer may have a crystalline melting point of at least 127°C (260°F) and a density of at least 0.925 grams per cubic centimeter. The first polymer is preferably ethylene polymer having a density of at least 0.925 grams per cubic centimeter, polypropylene, and/or propylene/ethylene copolymer. Blends of these materials can be used. The ethylene polymer is preferably an ethylene/alpha-olefin copolymer with a C 4 to C 10 comonomer, more preferably linear medium density polyethylene. Ethylene polymers with a density of at least 0.926 g/cc, such as 0.927, 0.928, 0.929, and 0.930 are included. Preferred are materials with a density of at least 0.931 g/cc, such as 0.935 g/cc. The second polymer is preferably an ethylene/alpha-olefin with a C 4 to C 10 comonomer, such as very low density polyethylene. Single-site catalyzed polymer, such as metallocene catalyzed polymer, can be used. Preferred densities for the second polymer are less than 0.914, 0.913, 0.912, and 0.911 g/cc. Densities of less than 0.910, such as less than 0.905, 0.904, 0.903, 0.902, 0.901, and 0.900 g/cc are included, such as less than 0.890, and 0.880 g/cc. An optional third polymer which can be used with the first and second polymers comprises ethylene/unsaturated ester, preferably ethylene/vinyl ester copolymer such as ethylene/vinyl acetate copolymer, or ethylene/alkyl acrylate copolymer such as ethylene/butyl acrylate copolymer; or an ethylene polymer having a density of between 0.916 and 0.924 grams per cubic centimeter, such as linear low density polyethylene. Referring to FIG.1, which is a cross-sectional view of a two layered reference film 10, it is seen that this comprises a core layer 14, and an outer layer 12. Core layer 14 comprises an oxygen barrier polymeric material, such as ethylene/vinyl alcohol copolymer, vinylidene chloride copolymer, polyester, and polyamide. Outer layer 12 comprises any of the materials described above for the monolayer film. FIG.2 describes a three layer reference film 10, layers 14 and 12 corresponding in composition to those of Figure 1. Outer layer 16, disposed on the opposite side of core layer 14 from layer 12, can comprise any of the materials disclosed for layer 12. Figure 3 illustrates a reference film 10 comprising a core layer 24 corresponding in composition to core layer 14 of Figure 2; two layers 22 and 26 corresponding in composition to layers 12 and 16 respectively, and a fourth layer 20. Layer 20 can represent an additional layer, e.g. an abuse resistant or heat sealable layer made from any suitable polymer, such as a polyolefin, polyamide, or polyester; or it can represent a discrete film laminated to layer 22. Figure 4 shows a film 10 comprising a core layer 34. Two intermediate layers 32 and 36 preferably comprise a polyolefin, anhydride-modified polyolefin, or polyamide. These layers can include polymeric adhesives such as anhydride-grafted polymers, e.g anhydride-grafted LLDPE; ethylene/alpha olefins such as LLDPE, or even conventional adhesives such as polyurethane. Layers 32 and 36 can also include ethylene/unsaturated 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. Two outer layers 30 and 38 are preferably surface layers. Figure 5 shows a six layer embodiment 10 in which layers 40, 42, 44, 46, and 48 correspond in composition to layers 30, 32, 34, 36, and 38 respectively. Layer 50 corresponds in composition to layer 20. Figure 6 shows a seven layer embodiment in which layers 60, 62, 64, 66, and 68 correspond in composition to layers 30, 32, 34, 36, and 38 respectively. Intermediate layers 70 and 72 comprise a polymer, more preferably a polyamide, including copolyamides and blends of polyamides. Figure 7 shows a package 80 wherein a foamed tray 82 contains a food product(not shown). The tray is overwrapped with film 84, and film is sealed at impulse seals 86a and 86b, and typically a bottom seal (not shown). The invention can be further understood by reference to the examples given below. These films can be made by a conventional cast coextrusion, extrusion coating, extrusion lamination, conventional lamination, or other suitable process. If desired, these films can be partially or totally crosslinked by electronic or chemical means. If desired for a given end use, these films can be oriented by trapped bubble, tenterframe, or other suitable process. They can thereafter optionally be heat shrinkable, and optionally annealed. Final film thickness can vary, depending on process, end use application, etc. Typical thicknesses range from 2.5 to 500µm (0.1 to 20 mils), preferably 5 to 250µm (0.2 to 10 mils) such as 7.5 to 150µm (0.3 to 6 mils), 10 to 100µm (0.4 to 4 mils), 12.5 to 75µm (0.5 to 3 mils) such as 12.5 to 50µm (0.5 to 2 mils) or 12.5 to 37.5µm (0.5 to 1.5 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 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., which is hereby incorporated in its entirety, by reference thereto. 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 20-180 kGy, and still more preferably, 30-160 kGy, such as 45 to 75 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 structure and properties of films made with these materials. Properties of the films are further explained in the footnotes to Table 1. MATERIAL TRADENAME SOURCE PE1 Dowlex™ 2045.04 Dow PE2 Dowlex 2037 Dow PE6 Attane™ 4202 Dow EV1 PE 1335 Rexene AD1 Admer™ SF 700 A Mitsui AD2 Bynel™ CXA 4104 DuPont OB1 E-151 Evalca PA1 Grilon™CF6S EMS PA2 Ultramid™ C-35 BASF PE1 = LLDPE, an ethylene/1-octene copolymer with a density of 0.920 gm/cc and an octene-1 comonomer content of 6.5%. PE2 = LMDPE, an ethylene/1-octene copolymer with a density of 0.935 gm/cc. and an octene-1 comonomer content of 2.5%. PE6 = ethylene/1-octene copolymer with a density of 0.912 gm/cc and octene-1 content of 9% by weight. EV1 = ethylene vinyl acetate copolymer with 3.3% vinyl acetate monomer. AD1 = anhydride-grafted polyolefin blend. AD2 = anhydride=grafted polyolefin in ethylene-butene copolymer. OB1 = ethylene/vinyl alcohol copolymer (44 mole % ethylene). PA1 = nylon 6,12 copolymer. PA2 = nylon 6,66 copolymer. Two films of the invention, Exs. 1 and 2, and two corresponding control films (C1 and C2) were made, each having the structure: A/B/C/D/C/B/A C.1 had the structure: Example 1 had the structure: C.1 and Ex. 1 had layer thickness ratios: 3/1/1/1/1/1/3 These films were oriented at 3.8 x 3.8 in the machine and transverse directions respectively. Both films were irradiated. A small amount of antiblock and antifog additives were included in the outside layers of each film. Performance data comparing Example 1 and Control 1 is found in Table 2. Physical Property C.1 Ex. 1 Hot tack window (°C) 115-130 120-130 Peak Force (N) 1.7 1.6 Film Melt Flow (g/10 min.) 6.3 6.1 Sealing Window 90-170 100-140 Leakers (5) 70 ppm 78 15 C.2 had the structure: C.2 had layer thickness ratios: 3/1/1/1/1/1/3 Example 2 had the structure: Ex. 2 had layer thickness ratios: 2/2/1/1/1/2/2 Control 2 and Ex. 2 were oriented at 3.4 x 3.4 in the machine and transverse directions respectively. Both films were irradiated. A small amount of antiblock and antifog additives were included in the outside layers of each film. Performance data comparing Example 2 and Control 2 is found in Table 3. Physical Property C.2 Ex. 2 Hot tack window b (°C) 115-170 115-170 Peak Force c (N) 4.3 5.8 Sealing Window h 160-170 130-230 Leakers (%) 50 ppm 7 0 Abuse Failure (%) 11 4 The abuse failure data of Table 3 is further described in Table 4. Example Corner Cut Edge Cut Bottom Abrasion Total Abuse Failures C.2 0 6 2 8 (11%) 2 1 2 0 3 (4%) The film of the present invention can have any suitable number of layers, e.g. it can have 5, 6, 7, 8, 9 or more layers. Films can be symmetric or asymmetric in construction. Films of the invention can utilize different materials for the outer layers or for the intermediate layers, so that e.g. two "A", "B", or "C" layers can be different from each other in composition, degree of crosslinking, thickness, or other parameters. It can be seen that improvement in several film parameters are beneficially obtained by the present invention. In Table 1, C.1 resulted in 78% leakers at 70 ppm, whereas Ex.1 had only 15% leakers at the same packaging speed. Compared with C.2, Example 2 showed lower leakers (0% versus 7%), improved abuse resistance (4% cuts versus 11% cuts), and higher hot tack peak force (5.8 versus 4.3N).

Claims: A multilayer film comprising a) a core layer comprising an ethylene/vinylalcohol copolymer; b) two outer layers each comprising a blend of i) a first polymer selected from the group consisting of linear medium density polyethylene, high density polyethylene copolymer, polypropylene, and propylene/ethylene and ii) a second polymer comprising a heterogeneous ethylene/alpha-olefin copolymer with a density of less than 0.915 g/ml; and c) two intermediate layers each disposed between the core layer and the respective outer layer, each comprising a polyamide; wherein the film heat seals at a temperature of at least 82°C (180°F). The film of claim 1 wherein the film is crosslinked. The film of claim 1 or claim 2 wherein the film has a thickness of between 10 to 50µm (0.4 and 2.0 mils). The film of claim 1 wherein the film has a hot tack strength of at least 2N for a 25 mm (1 inch) wide sample. The film of any preceding claim wherein the second polymer is very low density polyethylene. The film of any preceding claim in which at least one outer layer comprises a third polymer selected from the group consisting of ethylene polymer having a density of between 0.916 and 0.924 g/ml and ethylene/unsaturated ester copolymer. The film of claim 6 wherein said third polymer is an ethylene polymer which is a linear low density polyethylene. The film of claim 6 wherein said third polymer is an ethylene/unsaturated ester copolymer in which the ester is vinyl acetate or alkyl acrylate. The film of any preceding claim which is oriented. The film of claim 9 wherein the film is heat shrinkable. The film of any preceding claim further comprising an adhesive layer disposed between each intermediate layer and the respective outer layer. A process of packaging in which a film according to any preceding claim is heat sealed at a temperature of at least 82°C to close a package and seal the contents therein. A process according to claim 12 in which the film is sealed across the walls of a tray. A process according to claim 12 or claim 13 in which film on the heat sealed packages is heated to cause it to shrink.

Inventors: LAMBERT SCOTT W, MUDAR KIMBERLY ANN, LAMBERT, SCOTT W., MUDAR, KIMBERLY ANN
Applicants: CRYOVAC INC, CRYOVAC, INC.
Application year: 1997
Main classification: C08J5
IPC codes: B32B27, C08L23, C08J5, B32B7, B65D65