The invention relates to a multilayer film or sheet structure containing at least one barrier layer, at least one predominantly propylene-based layer and at least one predominantly ethylene-based layer, and at least one predominantly ethylene-based adhesive layer.
Packaging films and sheets are increasingly manufactured by coextrusion systems of growing sophistication. Whereas many years ago, it was common to have machines capable of coextruding three to five layers, such systems today can routinely coextrude nine layers or more, using an equal number of extruders to feed these layers. The new systems allow many types of polymers to be used for better tailoring of multiple properties required in packaging structures, such as barrier, toughness, sealability, gloss, transparency, impact resistance, while lowering overall raw material cost by using less of the expensive components. The polymers themselves many be further blended with other polymers. Additives may be added to the polymers, such as slips or antiblocks, antifog, anti-oxidants, color masterbatches, and the like. Further, the increasing complexity of the material combinations also requires more complicated logistics and handling to ensure that the right materials are delivered to the right extruders during production and to maintain proper inventory economically.
Many packaging films and laminates contain layers of polar polymers such as polyamide and ethylene vinyl alcohol to provide gas and flavor barriers. With the increasing number of extruders in coextrusion systems it is not uncommon to not only have an ethylene vinyl alcohol layer or a polyamide layer, but also combinations of these layers. Tri-layer combinations of barrier cores or barrier sandwiches comprising a coextrusion of polyamide-ethylene vinyl alcohol-polyamide are widely employed as a method to improve formability in films and sheets. In many cases, the sealant side of the coextrusion construction, that is, the layers between the barrier core and the packaged product are frequently ethylene polymer-based to make use of their good sealing properties. Examples of such sealants are ethylene ionomers, ethylene acid copolymers, ethylene vinyl acetate copolymers, ethylene α-olefin copolymers. In many cases, the exterior side of the coextrusion construction, that is, the layers between the barrier core and the outer layer of the package that does not contact the packaging contents, is composed of propylene polymers. The propylene polymers provide stiffness, moisture barrier, and they also provide a higher temperature resistance that prevents the film laminate from being deformed when contacted against a sealing bar during heat sealing of such laminates. The temperatures of the seal bars can be very high, particularly for thicker laminates, as heat must be transferred from the seal bar to the internal sealant layer within the short contact times necessary for profitable commercial production. For that reason, it can also be common for polyamide to be employed as the outermost exterior layer of the film laminate with polypropylene layers internal to that layer on the exterior side of the coextrusion construction. Consequently, as many as three or more tie layers may be employed in nine or higher layer coextrusions. It would be ideal if a single adhesive resin could be used for all adhesive layers to avoid mistakes with charging the wrong adhesive into the extruder hoppers, and to minimize inventory complications with maintaining multiple resins.
Generally speaking, high crystallinity polypropylene (PP) and polyethylene (PE) polymers are not compatible with each other. Most crystalline polyethylene resins do not adhere well to polypropylene and vice-versa. Ethylene copolymer-based adhesives may be employed, such as ethylene vinyl acetate and ethylene methyl acrylate adhesives, but these adhesives do not have sufficient high temperature resistance and may fail when packages are subject to cooking conditions, particularly those at high temperatures and long times. Lower density ethylene-α-olefin copolymer or higher comonomer propylene-α-olefin copolymer based adhesives may also be used, but they suffer from the same temperature issues as they are also lower crystallinity resins because of their high comonomer. They are also more expensive than their lower or no-comonomer polyethylene and polypropylene resin counterparts.
U.S. Pat. No. 4,405,667 discloses a retortable packaging pouch where the sealant layer is polyethylene and the outer layer is a polyamide. Internal layers of polyamide and ethylene vinyl alcohol are described. The pouch can contain up to 9 layers where one or more of the layers is polypropylene-based. The sealant PE is bonded to the propylene layers by a blend of linear low density polyethylene and a propylene ethylene copolymer. The layer directly adhering to the nylon is an anhydride-modified polypropylene.
U.S. Pat. No. 4,735,855 discloses a thermoformable film laminate in which the sealant layer is an ionomer or ethylene copolymer and is directly adhered to both a nylon, ethylene vinyl alcohol, nylon barrier core and a polypropylene outer layer. The tie layers bonding to the barrier, the sealant and the outer polypropylene layers may be identical or different from each other and may include a wide range of anhydride grafted polyolefins. The examples demonstrate only anhydride modified ethylene vinyl acetate polymers as adhering the sealant and polypropylene to the barrier polymer.
U.S. Pat. No. 5,643,999 discloses a coextrudable adhesive composition, for bonding polyolefin film to ionomer film, produced by blending polyethylene with a very low density polyethylene (VLDPE) together with a hydrocarbon elastomer. However, such adhesive composition does not adequately bond polyolefin film layer to barrier layer, for water, oxygen or other substances, such as polyamide or ethylene vinyl alcohol (EVOH) layer.
U.S. Pat. No. 6,184,298 discloses an adhesive composition of an anhydride modified polyethylene, an unmodified polyethylene and a styrene-based elastomer as being useful in multilayer structures containing nylon or EVOH as the barrier, and having polypropylene as one substrate and polyethylene as another.
U.S. Pat. No. 7,713,636 discloses a multilayer film comprising a PP core layer, an outer barrier skin layer and an optional second skin layer on the opposite side of the core layer than the barrier skin layer, where the optional second skin layer is selected from the group consisting of a PE polymer or copolymer, a PP polymer or copolymer, an ethylene propylene copolymer and an ethylene vinyl acetate polymer. A tie layer is disposed between the skin and core layers, and comprises at least 10 w % grafted propylene-based polymer containing 5 to 24 weight % ethylene and having a DSC melting temperature of 120° C. or less and a heat of fusion of 75 J/g or less.
More recent commercial advances in α-olefin copolymerizations using single site or metallocene catalysts in C2 to C8 α-olefins have resulted in copolymers that have a more blocky characteristic, where the block segments themselves are ethylene α-olefin copolymers and different blocks are distinguished by different comonomer content to tailor the block crystallinity. Such copolymers have been found to improve adhesion between polypropylene and high density polyethylene when used as the tie layer material in experiments conducted by Case Western University. (P Dias et al, Polymer 2008; 49:2937-2946 and Y. Lin et al, Polymer 2011; 52:1635-1644). Here, the polymers are used at 100% in the tie layer on a lab line extruding a small tape of about 2 mm thick and 12 mm wide. Line speeds were not disclosed.
It is particularly desired that an adhesive composition that provides high bond strength to multilayer barrier films where one tie layer type can be used to bond polypropylene polymers and copolymers to a polar barrier substrate as well as to polyethylene polymers, ethylene α-olefin copolymers and ethylene copolymers such as ethylene acid ionomers, ethylene acid copolymers, ethylene vinyl acetate and ethylene acrylate copolymers. In particular, it is challenging to find a common tie layer to bond polypropylene and ethylene acid ionomers of sodium and highly neutralized ethylene acid ionomers within a multilayer barrier structure.