Oriented cook-in film with good interply adhesion

Details Oriented cook-in film with good interply adhesion Oriented cook-in film with good interply adhesion

2001-08-03

2003-04-22

Tarazano, D. Lawrence (Department: 1773)

Stock material or miscellaneous articles

Hollow or container type article

Nonself-supporting tubular film or bag

C428S035700, C428S036700, C428S515000, C428S518000, C428S520000, C428S910000

Reexamination Certificate

active

06551674

ABSTRACT:

BACKGROUND INFORMATION
1. Field of the Invention
This invention relates generally to thermoplastic packaging materials and, more specifically, to flexible, multilayer films.
2. Background of the Invention
Many food products are processed in thermoplastic film packages by subjecting the packaged product to elevated temperatures produced by, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in, and films used in such processes are known as cook-in films.
A food product that is packaged and processed in this manner can be refrigerated, shipped, and stored until the food product is to be consumed or further processed by, for example, slicing and repackaging into smaller portions for retail display. Alternatively, the processed food can be removed immediately from the cook-in package for consumption or further processing (e.g., sliced and repackaged).
A cook-in film must be capable of withstanding exposure to rather severe temperature conditions for extended periods of time while not compromising its ability to contain the food product. Cook-in processes typically involve a long cook cycle. Submersion in hot (i.e., about 55° to 65° C.) water for up to about 4 hours is common; submersion in 70° to 100° C. water or exposure to steam for up to 12 hours is not uncommon, although most cook-in procedures normally do not involve temperatures in excess of about 90° C. During such extended periods of time at elevated temperatures, any seams in a package formed from a cook-in film preferably resist failure (i.e., pulling apart).
Following the cook-in process, the film or package preferably conforms, if not completely then at least substantially, to the shape of the contained food product. Often, this is achieved by allowing the film to heat shrink under cook-in conditions so as to form a tightly fitting package. In other words, the cook-in film desirably possesses sufficient shrink energy such that the amount of thermal energy used to cook the food product also is adequate to shrink the packaging film snugly around the contained product. Alternatively, the cook-in film package can be caused to shrink around the contained food product prior to initiating the cook-in procedure by, for example, placing the package in a heated environment prior to cooking.
Some cook-in applications impose some very stringent performance requirements on films for use therewith. For example, some food products that are processed via cook-in procedures are oxygen sensitive. Cook-in films for these products need to include one or more oxygen barrier layers. Other cook-in applications require that the film or the package made therefrom be printable and be able to retain any image printed thereon.
An increasingly important requirement of cook-in films is that they have good interply adhesion. This is complicated where a layer derived primarily from a homo- or co-polymer of propylene is to be adhered directly to a layer derived primarily from a homo- or co-polymer of ethylene. Although ethylene and propylene are homologues, polymers made from one tend not to adhere well to polymers made from the other. One attempt to counteract this tendency toward poor adhesion has involved blending a polymer including mer units derived from propylene and having a low Vicat softening point with a homo- or co-polymer of ethylene so as to increase the compatibility of the layer formed therefrom with the layer derived primarily from a homo- or co-polymer of propylene. However, even where such a film as made exhibits good interply adhesion, that same film can exhibit mediocre or even poor interply adhesion after it is oriented.
Further, when films of this type are sealed, the sealing process can induce delamination between the seal layer and the layer adjacent thereto. During the cooking process, these same two layers must provide the film with structural integrity and support the seal formed in the seal layer. Also, the seal layer needs to be able to resist the degenerative effects of grease and/or fatty products which often are encountered during cook-in processes. Thus, the need remains for an oriented film with good interply adhesion and sealability.
SUMMARY OF THE INVENTION
Briefly, the present invention provides an oriented multilayer film that includes at least two layers. One of these layers is an outer layer that includes a polymer including mer units derived from propylene. Directly adhered to this outer layer is a second layer including at least one member selected from the group consisting of ethylene/alkyl acrylate copolymer, propylene/(&agr;-olefin copolymer having a propylene mer content less than that of the polymer in the first layer and a Vicat softening point of at least 50° C., and ethylene/vinyl acetate copolymer with at least 15 weight percent vinyl acetate mer content, wherein the ethylene/alkyl acrylate copolymer has a melting point at least about 3° C. higher than a reference ethylene/alkyl acrylate copolymer, wherein said reference ethylene/alkyl acrylate copolymer has the same amount and type of alkyl acrylate and ethylene, and wherein said reference copolymer is made in a multi-zone autoclave reactor where the ratio of alkyl acrylate to ethylene in a reaction zone is about equal to the overall ethylene to alkyl acrylate ratio fed to the multi-zone autoclave reactor.
The film also can include one or more other layers such as, for example, bulk layers, O
2
-barrier layers, and/or abuse layers.
Articles made from the above-described film (e.g., bags and casings), methods of making the film, and methods of using the film also are provided.
Those of ordinary skill in the art recognize that polymers derived primarily from propylene and polymers derived primarily from ethylene tend not to adhere well to each other. Films including adjacent layers derived from these dissimilar materials can have less-than-optimal adhesion, orientability, degraded optics, and/or gauge non-uniformities, perhaps due to differential stresses within the layers. Nevertheless, work leading to the film of the present invention has shown that layers including at least one of the foregoing co-polymers have good bond strength with layers including a propylene homo- or co-polymer, that a film including such layers can have good optical characteristics, and that a film including such layers can have uniform gauge. In view of the fact that interply bond strength is known to decrease significantly as molecular orientation increases (as occurs when a film is oriented), the good interply adhesion exhibited by the oriented film of the present invention is even further surprising.
Furthermore, when the second layer includes a propylene/&agr;-olefin copolymer having a propylene mer content less than that of the first layer, good interply adhesion is exhibited with the polypropylene-containing first layer. That the propylene/&agr;-olefin copolymer of the second layer also exhibits good adhesion to the adjacent polyethylene layer is surprising. That a propylene/&agr;-olefin copolymer can maintain good interply adhesion with all adjacent layers, even while the film is undergoing orientation, also is surprising. Most surprising is that a second layer which includes a copolymer having a Vicat softening point significantly lower than the cook temperature can maintain good adhesion to adjacent layers through cooking procedures at temperatures up to about 93° C.
To assist in understanding the more detailed description of the invention that follows, certain definitions are provided immediately below. These definitions apply herein throughout unless a contrary intention is explicitly indicated:
“polymer” means the polymerization product of one or more monomers and is inclusive of homo-polymers as well as copolymers, terpolymers, tetrapolymers, etc., and blends and modifications of any of the foregoing;
“mer unit” means that portion of a polymer derived from a single reactant molecule; for example, a mer unit from ethylene has the general formula —CH
2
CH
2
—;
“homopolymer” means a polymer consisting esse

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