Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Producing multilayer work or article
Reexamination Certificate
1997-01-27
2001-03-20
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Producing multilayer work or article
C264S557000, C264S171280, C264S173130
Reexamination Certificate
active
06203750
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to articles formed by hermetically sealing a film to itself, i.e., articles such as bags, casings, etc. suitable for the packaging of foods, especially meats, such as processed meat products. The present invention is especially directed to heat shrinkable, polyamide containing casings which are shirred for use as cook-in casings for the packaging of processed meat products, such as ham, turkey, bologna, etc.
BACKGROUND OF THE INVENTION
Biaxially oriented, heat shrinkable cook-in casings made from a multilayer film containing polyamide or polyester have been known for some time. Furthermore, various blends comprising polyamides and other polymers are also known for use in such cook-in casing films.
A commercially available casing used for cook-in applications has included a layer of ethylene/vinyl alcohol copolymer (EVOH), present as an O
2
-barrier layer, this layer being outward of one or more film layers comprising one or more polyolefins. Such films have been found to be orientable and shirrable without splitting during cooking.
EVOH is known to be an expensive copolymer. Moreover, in certain end uses, such as cook-and-strip end uses, it is not necessary to provide an O
2
-barrier layer, as the oxygen permeability during the cook-in step is not important. Thus, for cook-and-strip end use, it is desired to provide a cook-in casing which does not contain the relatively expensive EVOH layer. However, upon elimination of the EVOH layer, the inventors have discovered that a substantially 100% polyolefin casing tends to split during cook-in. It has been theorized that the reason for this splitting is that oil, which is placed on the outside of the casing during the shirring process, migrates into the film and weakens the polyolefin layers. It is believed that the EVOH layer prevented the migration of the oil to the polyolefin layers inward of the EVOH layer, and for this reason the EVOH-containing casings do not tend to split during cook-in. Nevertheless, such EVOH containing casings remain expensive and potentially unnecessary for cook-in applications, provided some way can be found to eliminate the detrimental effects of the oil on the polyolefin layers.
In an attempt to overcome the casing-splitting problem while at the same time eliminating the use of expensive EVOH in the casing, the instant inventors conceived of using a polyamide layer in place of the EVOH layer, the polyamide layer being between the oil and the polyolefin layer(s), in order to block the migration of oil and thereby reduce or eliminate the detrimental effects of the oil on the polyolefin layer(s). However, nylon 6, nylon 66, and nylon 6/66 are about the only feasible polyamides for such use, as the remaining polyamides are as expensive, or more expensive, than EVOH.
The inventors discovered that a film layer of 100% nylon 6 cannot be oriented to the degree required to provide the casing with a desirable level of heat-shrinkability, as the orientation bubble would break very frequently, rather than orient. Thus, there remains a need for an EVOH-free, relatively inexpensive article suitable for cook-in end use.
SUMMARY OF THE INVENTION
The inventors of the present invention have discovered a relatively inexpensive heat shrinkable casing suitable for cook-in end use, this casing being particularly suited for use in cook-and-strip applications. The film from which the casing is made contains a layer comprising at least two polyamides, this layer being between the oil and the polyolefin layer(s). The layer comprising at least two polyamides preferably comprises a composition comprising a first nylon which is nylon 6, nylon 66, or nylon 6/66 (or any other relatively inexpensive crystalline nylon polymer) in a relatively large amount, e.g., greater than 50 weight percent, based on the weight of the layer. It has been surprisingly discovered that by providing this composition with a second polyamide having a crystal structure which differs from the crystal structure of the first polyamide, the resulting composition can be oriented out of hot water or steam (i.e., at a temperature of from about 180° F. to 280° F., with heating taking place for a period of from about 1 to 100 seconds, as required for polyolefin-containing films), unlike a film layer comprising 100 percent of the first polyamide. Preferably, the second polyamide comprises the remainder of the first layer. Furthermore, the inventors have discovered that a polyester can be used in place of the above-described composition comprising the at least two polyamides.
Furthermore, the inventors have discovered that surprisingly, such films can be processed, i.e., oriented, at substantially higher speeds than film not containing the polyamide composition, i.e., films containing primarily polyolefins only, or films containing EVOH and polyolefins. Furthermore, the presence of the layer comprising the polyamides has been found to substantially increase the toughness of the casing, by providing a substantial increase in the modulus of the film. Surprisingly, even relatively thin polyamide layers can increase the toughness of the casing, thereby permitting the film to be further downgaged for any desired application, thereby further reducing the cost of the film.
Furthermore, the inventors have extended the above discoveries, i.e., which initially led to the casings of the present invention, to be applied to multilayer films generally, and especially articles made therefrom, such as bags, and other articles suitable for the packaging of various products, especially foods. That is, the incorporation of the above-described polyamide composition has been found to provide enhanced toughness, processability, and abrasion-resistance to oriented multilayer films containing same. More particularly, multilayer films containing a plurality of polyolefin layers, and an O
2
-barrier layer (containing EVOH or polyvinylidene chloride), benefit from the inclusion of a layer comprising the polyamide-containing composition.
As a first aspect, the present invention is directed to an article of manufacture which comprises a heat-shrinkable, biaxially-oriented, thermoplastic, multilayer film hermetically sealed to itself. The article can be, for example, a bag, a casing, etc., i.e., a film converted into an article suitable for packaging end use. The multilayer film comprises an outside surface having an oil thereon, a first film layer, and a second film layer. The first film layer comprises at least one member selected from the group consisting of (1) a polyester, and (2) a composition comprising a first polyamide and a second polyamide, wherein the first polyamide is a member selected from the group consisting of nylon 6, nylon 66, and nylon 6/66, and wherein the second polyamide has a different crystal structure than the first polyamide. Optionally, the first film layer comprises a blend of the first polyamide and a polyester. The second film layer comprising a polyolefin. The first film layer has a thickness of less than about 20 percent based on a total thickness of the multilayer film; preferably less than about 15 percent based on the total thickness of the multilayer film. Finally, the first film layer is between the oil and the second film layer.
Preferably, the first polyamide is at least one member selected from the group consisting of nylon 6, nylon 9, nylon 11, nylon 12, nylon 66, nylon 610, nylon 612, nylon 6I, nylon 6T, and nylon 6/66. Preferably, the second polyamide comprises at least one member selected from the group consisting of nylon 6, nylon 9, nylon 11, nylon 12, nylon 66, nylon 610, nylon 612, nylon 61, and nylon 6T, and copolymers thereof such as nylon 6/66, nylon 6/69, nylon 6/12, nylon 66/610 nylon, 6/612 nylon, 6I/6T nylon, and nylon 6/6I; more preferably, nylon 6, nylon 66, nylon 6/66, nylon 6/69, nylon 6/12, nylon 66/610, and nylon 6I/6T; still more preferably nylon 6, nylon 66, nylon 6/12, nylon 66/610, and nylon 6I/6T. Preferably, the first polyamide is nylon 6 in an amount of from about 6
Ahlgren Kelly R.
Ramesh Ram K.
Cryovac, INC
Eashoo Mark
Hurley, Jr. Rupert B
Silbaugh Jan H.
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