Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
1999-11-04
2001-07-17
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of polyamide
C423S475000, C423S475000, C423S476000
Reexamination Certificate
active
06261698
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to packaging films, and more particularly to a multilayer film having a combination of relatively low oxygen transmission, and relatively high carbon dioxide transmission.
BACKGROUND OF THE INVENTION
Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse during the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product. Various polymeric materials have been used to provide lower gas permeability in order to reduce the transmission of oxygen through the packaging film and thereby retard the spoilage and extend the shelf life of products such as food items which are sensitive to oxygen.
Some cheese products are produced in such a way that the final cheese product emits a significant amount of carbon dioxide over time. In such cases, it is often desirable to provide a packaging material which is characterized by a relatively low oxygen transmission rate (i.e. good oxygen barrier), and a relatively high carbon dioxide transmission rate. A preferred O
2
transmission rate is no more than about 500 cc/m
2
-day-atmosphere (ASTM D 3985 at 73° F.), more preferably no more than about 250 cc/m
2
-day-atmosphere, and even more preferably no more than about 175 cc/m
2
-day-atmosphere. A preferred CO
2
transmission rate is at least about 750 cc/m
2
-day-atmosphere at 730° F., more preferably at least about 1000 cc/m
2
-day-atmosphere, most preferably at least about 1200 cc/m
2
-day-atmosphere. CO
2
transmission is measured using an analytical technique analogous to ASTM D 3985. The ratio of CO
2
transmission rate to O
2
transmission rate is preferably greater than about 5:1, and most preferably at least about 7:1.
It is also often desirable to include in a packaging film a shrink feature, i.e., the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is heated to an orientation temperature and then stretched either in a longitudinal (machine) direction, a transverse direction, or both, in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films.
For certain products the shrink force of the shrinkable film must be controlled in order to avoid deformation. Furthermore, for irregularly shaped products a heat shrinkable film must be able to draw into crevices without imploding. Thus, a sufficiently high tensile strength and percent elongation at or about the shrink temperature are required. Such properties are especially important for Swiss cheese packaging.
SUMMARY OF THE INVENTION
The present invention is directed to a heat shrinkable multilayer film which includes a barrier layer of a copolyamide having a melting temperature of less than about 150° C., preferably less than about 140° C., wherein the film has a hot tensile strength at 850°C. of at least 1800 psi in at least one direction as measured by ASTM D882 and a percent elongation at 85° C. of at least 120% in at least one direction as measured by ASTM D882, and wherein the film exhibits a CO
2
:O
2
transmission ratio of at least 7.0 at 50% relative humidity. Most preferably the copolyamide is nylon 6/6.9.
Preferably the film includes, in its various layers, at least 50 percent by weight of an ethylene copolymer, preferably ethylene/vinyl acetate.
The copolyamide of the barrier layer may be present, either alone or in a blend with another polymer such as, for example, another polyamide.
The present invention is also directed to a package which includes a product, preferably Swiss cheese which has a plurality of holes, and a film enclosing and immediately adjacent to the block of Swiss cheese, the film extending downwardly into the holes of the Swiss cheese. The films of the package includes a barrier layer of a copolyamide having a melting temperature of less than about 150° C. and a CO
2
:O
2
transmission ratio of at least 7.0 at 50% relative humidity.
DEFINITIONS
As used herein, the term “comonomer” refers to a monomer which is copolymerized with at least one different monomer in a copolymerization reaction, the result of which is a copolymer.
As used herein, the term “copolymer” refers to polymers formed by the polymerization reaction of at least two different monomers. Thus, “copolymer” as used herein can mean terpolymer. The term “terpolymer” refers herein specifically to the copolymerization reaction product of three monomers.
As used herein, the phrase “ethylene alpha-olefin copolymer”, and “ethylene/a-olefin copolymer”, refer to such heterogeneous materials as linear low density polyethylene (LLDPE), linear medium density polyethylene (LMDPE) and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene catalyzed polymers such as EXACT (TM) materials supplied by Exxon, and TAFMER (TM) materials supplied by Mitsui Petrochemical Corporation. 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/a-olefin copolymers, such as the long chain branched homogeneous ethylene/a-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.
As used herein, the term “polyamide” refers to polymers having amide linkages along the molecular chain, and preferably to synthetic polyamides such as nylons. Furthermore, such term encompasses both polymers comprising repeating units derived from monomers, such as caprolactam, which polymerize to form a polyamide, as well as copolymers of two or more amide monomers, including nylon terpolymers, also referred to generally as “copolyamides” herein.
As used herein, the term “ethylene/vinyl acetate copolymer” or “EVA” refers to ethylene copolymerization with vinyl acetate comonomer.
As used herein, the phrase “anhydride functionality” refers to any form of anhydride functionality, such as the anhydride of maleic acid, fumaric acid, etc., whether blended with one or more polymers, grafted onto a polymer, or copolymerized with a polymer, and, in general, is also inclusive of derivatives of such functionalities, such as acids, esters, and metal salts derived therefrom.
As used herein, the phrase “modified polymer”, as well as more specific phrases such as “modified ethylene vinyl acetate copolymer”, and “modified polyolefin” refer to such polymers having an anhydride functionality, as defined immediately above, grafted thereon and/or copolymerized therewith and/or blended therewith. Preferably, such modified polymers have the anhydride functionality grafted on or pol
Cryovac Inc.
Kiliman Leszek
Quatt Mark B.
LandOfFree
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