Multilayer films comprising polypropylene homopolymer or copolymer

Abstract: A multilayer film comprises a first layer comprising propylene homopolymer or copolymer and at least one additional sealable layer; or a core layer comprising very low density polyethylene, two outer sealable layers and intermediate adhesive layers.

Description: The present invention pertains to packaging films, and more particularly to a thin polymeric film suitable for replacing polyvinyl chloride film especially in packaging applications. Polyvinyl chloride (PVC) has long been used in many applications in the packaging art. One particularly widespread application for PVC is the use of such material as an overwrap material for trayed retail cuts of meat and other food products in a retail environment such as a supermarket. PVC has several desirable properties for this use. For example, it has excellent burn-through resistance, optics and good elasticity and stretch properties at use temperatures. Unfortunately, PVC also has several disadvantages, including the production of hydrogen chloride gas during heat sealing and the generally corrosive effects of such gases in the packaging room. Extractables from the PVC into the packaged food product have also become of concern. It would be of great benefit to the packaging industry, and particular to applications requiring an instore film for overwrapping trayed food products, to provide a film with many of the advantages of PVC but without the disadvantages described above. Processes for producing oriented films, and oriented films themselves are disclosed in many patents including the following patents of interest. U. S. Patent No. 3,456,044 and U. S. Patent No. 3,555,604 (Pahlke) mentions thin films of thicknesses less than 2.54 x 10 -5 m (1 mil) such as 1.27 x 10 -5 m (0.5 mils), and discloses a double bubble method for biaxially orienting thermoplastic films including the steps of producing a primary tubing which is inflated by introducing air into the interior thereof, and a cooling ring 22, as well as squeeze rolls 34 and 28, with rolls 34 having a greater speed than rolls 28. Between the two pairs of squeeze rolls is a reinflated secondary bubble. If annealing is desired, the tubing can be reinflated to form a bubble 70. U. S. Patent No. 4,258,166 (Canterino et al) discloses a uniaxially oriented plastic film material with improved strength and clarity in the direction of orientation preferably comprising homopolymers and copolymers of ethylene. U. S. Patent No. 4,355,076 (Gash) discloses monoaxially oriented polypropylene film laminated to a monoaxially oriented high density polyethylene film, the films produced by for example tubular blowing. U. S. Patent No. 4,440,824 (Bonis) discloses a thermoformable coextruded multilayer structure useful for thermoforming into containers, the structure having polyolefin coextruded with a high impact polystyrene layer. A five layer structure is shown. U. S. Patent No. 4,464,439 (Castelein) discloses a coextruded laminate having a sheet of polypropylene and a sheet of a mixture of high impact polystyrene, crystalline polypropylene, and styrene/dienic monomer block copolymer. U. S. Patent No. 4,879,177 (Boice) discloses a monoaxially oriented shrink film having a core layer of butadiene styrene copolymer, outer layers of ethylene propylene copolymer, and intermediate bonding layers of ethylene copolymer. EP-A-0 286 430 discloses a film comprising a core of VLDPE, intermediate layers of LLDPE and two outer layers each bonded to the respective intermediate layer of EVA and/or VLDPE. It is an object of the present invention to provide a thermoplastic film useful as an overwrap material for trayed retail cuts of meat and other food products in a super market or other retail environment. It is also an object of the present invention to provide a film which has good burn-through resistance. It is also an object of the present invention to provide a film having excellent optical properties, specifically excellent gloss and clarity. The present invention provides a multilayer film comprising: (a) a core layer comprising very low density polyethylene; (b) two outer layers each comprising a sealable polymeric material; and (c) two intermediate layers each bonding the core layer to a respective outer layer, and comprising a blend of a propylene polymer or copolymer, and a polymeric adhesive. Preferably the two outer layers of the film each comprise from 0.1 to 10%, by weight of each outer layer, of plasticiser. Preferably the outer layers each have a thickness of less than 1.0 µm (0.04 mils) and each comprises less than 4% of the total film thickness. The term "polyolefin" is used herein in its stricter sense to mean a thermoplastic polymer derived from simple olefins. Among these are polyethylene, polypropylene and copolymers thereof with olefinic comonomers. For example, very low density polyethylene may be considered a linear ethylene copolymer with a comonomer comprising such materials as butene, hexene or octene. The term "polyolefin" is also used herein in a broader sense to include copolymers of ethylene with comonomers that are not themselves olefins, such as vinyl acetate (e.g. ethylene vinyl acetate copolymer or EVA). The term "very low density polyethylene", or "VLDPE" is used herein to describe linear ethylene alphaolefin copolymer (flexomer) having densities of generally between 0.860 and 0.915 grams/cubic centimeter, and produced by a catalytic, low pressure process. "Ultra low density polyethylene" is also included in this term. The term "ethylene vinyl acetate copolymer" (EVA) as used herein refers to a copolymer formed of ethylene and vinyl acetate monomers wherein the ethylene derived units are present in major amounts and the vinyl acetate derived units are present in minor amounts, preferably one (1) to thirty (30) percent by weight. The term "styrene butadiene copolymer" (SBC) is used herein to denote thermoplastic copolymers, especially block copolymers containing a major portion (greater than 50%) of styrene and a minor proportion (less than 50%) of butadiene comonomer. The terms "melt flow" and "melt index" are used herein to mean the amount, in grams, of a thermoplastic resin which is forced through an orifice of specified length and diameter in ten minutes under prescribed conditions in accordance with ASTM D 1238. The term "flow rate ratio" (FRR) is used to mean a dimensionless number derived by dividing the flow rate (melt flow or melt index) at one Condition with the flow rate at another Condition (ASTM D 1238). FRR is indicative of molecular weight distribution. The higher the FRR, the broader the molecular weight distribution. The term "ethylene copolymer" is used herein in its strict sense to mean copolymers of ethylene and a comonomer, and in its broad sense to include materials such as polybutene which can be substituted for this first group of materials. The term "ethylene alpha olefin copolymer" is used herein to refer to copolymers of ethylene and higher alpha olefins such as butene, 1-methyl pentene, hexene, and octene. The invention may be further understood by reference to the drawings herein, where: FIG. 1 is a schematic cross section of a preferred embodiment of a multilayer film in accordance with the invention; the film comprises a core layer 10, outer layers 12 and adhesive or intermediate layers 14. FIG. 2 is a schematic cross section of an alternative preferred embodiment of a multilayer film in accordance with the invention; the film comprises a core layer 18, outer layers 12, adhesive or intermediate layers 14 and interior layers 16. FIG. 3 is a schematic diagram indicating the apparatus and process by which the films of the present invention are made. An especially preferred propylene resin is propylene ethylene copolymer such as that available from Fina under the designation Fina 8473 and Fina 8473x. These resins have an ethylene content of about 3% by weight. Another especially preferred propylene resin is propylene homopolymer such as that available from Shell under the designation WRD5-981, with a melt flow rate of 0.6 to 0.8 grams/10 minutes (ASTM D 1238, Condition 230/2.16). An especially preferred VLDPE is a high molecular weight resin such as DEFD 1015 from Union Carbide. This resin has a density of about 0.900 grams/cc, a melt flow rate of about 6 grams/ 10 minutes (ASTM D 1238, Condition 190/21.601), and a FFR (I 21 /I 2.1 ) of about 50. The film structure of the multilayer film may include a core layer comprising a blend of propylene polymer or copolymer, and very low density polyethylene (VLDPE). Preferred VLDPE resins are characterized by high molecular weight (i.e. relatively low melt index), broad molecular weight distribution (i.e. relatively high flow rate ratio), and relatively low crystallinity at processing temperatures. For the VLDPE, a melt index (MI) of no more than about .15 grams/ 10 minutes (ASTM D 1238) (Condition 190/2.16) is preferred. A more preferred MI is .12 grams/ 10 minutes. Preferred VLDPE resins can also be characterized by a melt index of no more than 0.50 grams/ 10 minutes, more preferably no more than 0.45 grams/ 10 minutes (ASTM D 1238) (Condition 190/5.0); no more than 1.50 grams/ 10 minutes, and more preferably no more than 1.35 grams/ 10 minutes (ASTM D 1238) (Condition 190/10.); or no more than 10 grams/ 10 minutes, and more preferably no more than 6 grams/ 10 minutes (ASTM D 1238) (Condition 190/21.601). For the VLDPE, a molecular weight distribution (flow rate ratio) of at least about 10 (I 21 /I 5 ) (ASTM D 1238) is preferred. This value is derived by dividing the flow rate at Condition 190/21.6 by the flow rate at Condition 190/5.0. A more preferred FRR is 13. Preferred VLDPE resins can also be characterized by a FFR of at least about 40, more preferably at least about 50 (I 21 /I 2.1 ) (ASTM D 1238). This value is derived by dividing the flow rate at Condition 190/21.6 by the flow rate at Condition 190/2.16. In one preferred film construction, the outer layers of the film comprise a polyolefin, more preferably an ethylene polymer or copolymer such as EVA. Styrenic polymers and copolymers, e.g. styrene butadiene copolymer such as that commercially available from Phillips under the designation KR-10 having a butadiene content of 25% by weight of the copolymer; or KK 36 (for fatty food contact) are also preferred. In this embodiment of the invention, the outer layers are bonded to the core layer by means of intermediate layers each comprising blend of a polymeric adhesive and polypropylene. Preferably the adhesive is an ethylene vinyl acetate copolymer (EVA). A more preferred EVA is one having a vinyl acetate content above about 18% by weight of the copolymer, and even more preferably about 28% by weight of the copolymer. Other polymeric materials, including chemically modified adhesives, can be used for layers 14 provided they process adequately in processes such as those disclosed herein. For the outer layers, SBC resins having minor amounts of butadiene, ranging from 1 to 50%, provide an optimum balance of stiffness and flexibility to the film. For layers of the present invention which contain SBC, antifog/plasticising agents such as Atmer 645 and/or Atmer 1010 are preferably included, more preferably in amounts of between about 0.5% and 10% by weight of the layer or layers. Even more preferably, between about 2% and 6% of such agents are included. The intermediate layers, and comparable layers in other films of the present invention, preferably also include such agents, more preferably in the 0.5 to 10% range. Additional interior layers in the film of the invention may comprise a polypropylene polymer or copolymers such as an ethylene propylene copolymer, a polybutene blend or for example a polypropylene/EVA blend. The films of the present invention are preferably made by coextrusion techniques, combined with an apparatus and process depicted in Figure 3 and described in more detail below. Figure 3 shows schematically a device for making the films of the present invention. A single melt stream, or multiple melt streams including a first melt stream of a blend of propylene polymer or copolymer, and ethylene copolymer, are extruded or coextruded and exit as an extrudate through an annular die 12 in a conventional manner. The extruded film is hot blown to form a blown bubble 14. Air cooling ring 16 positioned circumferentially around the blown bubble at the Position shown cools the thermoplastic melt as it exits die 12. An optional auxiliary chilling ring or rings 17 may also be positioned circumferentially around the blown bubble down stream from air cooling ring 16 to further chill the hot blown film. The primary bubble 14 is melt oriented in both the machine and transverse directions. Various blow up ratios may be used, but preferably the primary bubble 14 is hot blown to a blow up ratio of between 1.5 and 8.0. The primary bubble 14 is collapsed at pinch rolls 21. To assist in this process, guide plates 18 are positioned at the extremities of the blown bubble 14. The preferred process for carrying out the present invention permits the bubble to be immediately reinflated, after cooling, into the secondary bubble 20 and then expanded to impart orientation of the material in primarily the transverse direction, primarily the longitudinal direction, or in both the transverse and longitudinal directions. This "flexibility" in the preferred process permits films of the invention to be produced which are oriented primarily in one direction (monoaxially oriented films) or films which are oriented in both the longitudinal and transverse directions (biaxially oriented films). The collapsed bubble is reinflated in a blown bubble process to stretch orient the blown and collapsed film. This is done in a conventional manner by trapping air or other hot gas within the secondary bubble 20 so that the material stretches at its orientation temperature transversely to impart further orientation of the material in the transverse direction. The secondary bubble 20 is collapsed at a second set of pinch rolls 22. A second set of guide plates 24 may be employed to assist in the collapsing process. The second set of pinch rolls 22 is rotated at a speed faster than the first set of pinch rolls 21 if it is desired to impart stretch orientation in the machine or longitudinal direction to the thermoplastic material. The recollapsed bubble 20 then passes from the second set of pinch rolls 22 to take up roll 26. The take up roll 26 may be a mill log which can be immediately stored or shipped to a distributor or customer, or may be stored for further processing such as slitting into single wound film, machine or natural center fold film. Thus, as used herein the take up roll 26 represents any further processing, storage, or further modification of the double wound, collapsed film once it exits the second set of pinch rolls 22, and is used herein to denote any of these possible further processing steps. It is preferred that a reservoir 28 of heated fluid be disposed at the lower end of primary bubble 14 in such a way that the collapsing material drawn through the primary set of pinch rolls 21 will pass in communication with the heated fluid. In this manner, the film is more uniformly heated and temperature control can be achieved. Thickened tape edges can be substantially avoided by such means. Although the heated fluid of reservoir 28 is preferably hot water, other media may be used if temperatures above the 100°C (212°F) limit of hot water are desired. For example propylene glycol (a food approved material), hot oil or hot emulsions may be used. One skilled in the art will understand that the exact nature of the heated fluid is not as critical as its effectiveness in helping to uniformly heat the collapsing bubble 14 as it is drawn through pinch rolls 21, and to uniformly heat the collapsing bubble 14 to a temperature above its orientation temperature. The heated fluid may also be an "active" substance which not only reheats the surrounding film, but also actually coats the interior of the bubble as it passes over the reservoir. An example is a hot wax or other functional coating. The heated fluid may be recirculated from the reservoir through a heating means 30 by means of conduits 32 or other suitable transfer means. Using the reservoir 28 of heated fluid, the types of materials which may be effectively used in the present process and device are increased. Many different film constructions can be made using this process, including monolayer and multilayer films, especially those using the inventive blend compositions. Biaxially oriented films of the present invention are useful in overwrap applications for overwrapping retail cuts of meat and non-food products. Monoaxially oriented films are useful in shrink label applications for producing labels for vessels such as bottles and cans. The term "monoaxially oriented" is used herein to mean films oriented primarily in the longitudinal direction. However, some incidental orientation can be present in the transverse direction, and this is sometimes desirable to help the film to grip a container or vessel after heat shrinking and to reduce the incidence of wrinkles in the final label. The term can also be used to refer to films oriented primarily in the transverse direction, with or without some incidental orientation in the longitudinal direction. The invention may be further understood by reference to the examples which follow. The examples below list preferred commercially available resins. The resins used in these examples, and others useful in the invention, are identified in Table 1. RESIN COMMERCIAL NAME DESCRIPTION COMPANY SBC 1 KR-10 STYRENE BUTADIENE COPOLYMER PHILLIPS SBC 2 KK-36 STYRENE BUTADIENE COPOLYMER WITH FATTY FOOD COMPLIANCE Phillips EVA 1 ELVAX 3182 EVA (28% VA) DU PONT EVA 2 ELVAX 3165 EVA (18%VA) DU PONT EVA 3 3170 EVA (18% VA) DU PONT VLDPE 1 XU 61509.32 VERY LOW DENSITY POLYETHYLENE DENSITY = .911 grams/cc DOW VLDPE 2 DEFD 1161 VERY LOW DENSITY POLYETHYLENE UNION CARBIDE VLDPE 3 TAFMER 0680 VERY LOW DENSITY POLYETHYLENE DENSITY = .88 grams/cc MITSUI VLDPE 4 1015 VERY LOW DENSITY POLYETHYLENE DENSITY = .900 grams/cc UNION CARBIDE PP 1 8473 EPC FINA PP 2 WRD5-981 POLYPROPYLENE SHELL PL 1 22-164 50% POLYISOBUTYLENE IN LLDPE SANTECH Example An example of the present invention is a film including the construction: SBC/ADH + PP/VLDPE/ADH + PP/SBC where ADH is a polymeric adhesive such as EVA 1 , EVA 2 , or EVA 3 , or any suitable adhesives such as the carboxylic acid or acid anhydride-modified polymeric adhesives known in the art; and where PP is PP 1 , PP 2 or other suitable propylene polymers or copolymers. ADH and PP may be blended in any of the proportions disclosed herein for PP blend layers, for example the blend may contain 20%, 40%, 60% or 80% PP and 80%, 60%, 40% or 20% ADH by weight. Films of the present invention can optionally be cross-linked. This can be done chemically or by the use of irradiation. Irradiation may be accomplished by the use of high energy electrons, ultra violet radiation, X-rays, gamma rays, beta particles, etc. Preferably, electrons are employed up to about 20 megarads (MR) dosage level. The irradiation source can be any electron beam generator operating in a range of about 150 kilovolts to about 6 megavolts with a power output capable of supplying the desired dosage. The voltage can be adjusted to appropriate levels which may be for example 1,000,000 or 2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Many apparatus for irradiating films are known to those of skill in the art. The irradiation is usually carried out at a dosage up to about 20 MR, typically between about 1 MR and about 20 MR, with a preferred dosage range of about 2 MR to about 12 MR. Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, O°C to 60°C may be employed.

Claims: A multilayer film comprising: (a) a core layer comprising very low density polyethylene; (b) two outer layers each comprising a sealable polymeric material; and (c) two intermediate layers each bonding the core layer to a respective outer layer, and comprising a blend of a propylene polymer or copolymer, and a polymeric adhesive. A film according to claim 1 wherein the very low density polyethylene has a melt index before blending of no more than 10 grams/10 minutes (ASTM D 1238). A film according to claim 1 or 2 wherein the very low density polyethylene has a flow rate ratio (I 21 /I 2.1 ) before blending of at least 40 (ASTM D 1238). A film according to any one of the preceding claims wherein the two outer layers each comprise from 0.1 to 10% by weight of each outer layer, of plasticiser. A film according to any one of the preceding claims wherein the two outer layers each have a thickness of less than 1.0µm (0.04 mils). A film according to any one of the preceding claims wherein the two outer layers each comprise less than 4% of the total film thickness. A film according to any one of the preceding claims wherein the adhesive or intermediate layers comprise an olefin polymer or olefin copolymer with adhesive properties. A film according to any one of the preceding claims wherein the adhesive or intermediate layers comprises ethylene vinyl acetate copolymer. A film according to any one of the preceding claims wherein the outer layer or layers comprises a sealable polymeric material which is an olefin polymer, olefin copolymer, styrenic polymer, or styrenic copolymer. A film according to any one of the preceding claims wherein at least one layer of the film is crosslinked.

Inventors: NELSON MARTIN, SCHIRMER HENRY G, WILLIAMS ALLEN C JR, NELSON, MARTIN, SCHIRMER, HENRY G., WILLIAMS, ALLEN C., JR.
Applicants: CRYOVAC INC, CRYOVAC, INC.
Application year: 1991
Main classification: B32B27
IPC codes: C08J5, B32B27, C08L23