Easy tear non-halogenic food wrap

Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers

Reexamination Certificate

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C428S523000, C428S910000, C264S173150, C264S173190, C525S240000

Reexamination Certificate

active

06482532

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to food wraps, in particular food wraps for domestic use. In one S aspect, this invention relates to non-halogenic stretch wraps that are easy to tear in the cross-direction. In another aspect, this invention relates to a plastic film with an oriented film structure having at least one film layer comprising a substantially linear ethylene polymer (SLEP), a low density polyethylene (LDPE), and an anti-fogging agent. In yet another aspect, this invention relates to a plastic film having a film structure comprising at least one inner layer comprising an LDPE sandwiched between two outer layers each comprising an SLEP.
BACKGROUND OF THE INVENTION
In the modern distribution and marketing of food products, a multitude of different packaging materials are used. One principal category of food packaging materials is plastic film. Many different kinds of plastic film exist, both in composition and structure, and some are tailored to specific applications while others are more generic in nature.
Currently, polyvinyl chloride (PVC) film is the predominate plastic film used to wrap retail-cut red meat and similar products, e.g. fresh fish, poultry, vegetables, fruits, etc., due to its many desirable properties and its low cost relative to other plastic films. Representative of these desirable properties are clarity, oxygen transmission, flexibility, toughness, heat sealability, elastic recovery, and processability. However, most PVC films include a plasticizer to obtain the desired flexibility, and a growing concern exists as to the carcinogenic properties of the most commonly used PVC film plasticizer and the tendency of this plasticizer to migrate from the film to the food product. On a more fundamental level, a growing concern also exists regarding the use in food wrapping applications of any plastic film comprising one or more chlorinated polymers. The concern is based on the tendency for chlorinated polymers to yield corrosive acid when thermally degraded or incinerated, as well as concern regarding the general difficulty involved in recycling chlorinated polymers.
In the search for alternatives to PVC film, various monolayer olefin films, particularly polyethylene films, have been considered but none have been found to be without at least one flaw that has blocked its commercial acceptability. High density polyethylene (HDPE) is much too inelastic to be useful as a commercial wrap, while the various LDPEs, e.g., linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), etc., do not possess sufficient elastic recovery, e.g. the film retains impressions or dents from the fingers and poking of potential purchasers who have handled the wrapped product while inspecting its contents. This can be detrimental to the sale value of the food product because the wrapped product will quickly lose its pristine appearance which in turn may cause subsequent potential buyers to intentionally avoid a perfectly good product that now has the appearance of one repeatedly rejected by earlier potential purchasers. The use of nontoxic plasticizers, such as corn oil, has not proven totally satisfactory, particularly with respect to their temperature stability.
Various multilayer films have also been considered (e.g. those taught in U.S. Pat. No. 5,112,674 and in EPO 0 243 965, 0 333 508 and 0 404 969), and significant among these are films made by co-extrusion of polyethylene with an ethylene/&agr;,&bgr;-unsaturated carbonyl copolymer, such as ethylene vinyl acetate (EVA) or ethylene acrylic acid (EAA). While these films demonstrate an elastic recovery similar to PVC film, EVA and EAA are relatively expensive copolymers. Moreover, ethylene/&agr;,&bgr;-unsaturated carbonyl copolymers are relatively difficult to fabricate, have a tendency to impart an offensive taste and/or odor to the food product, and are known to interfere with anti-fogging agents.
In other food wrap applications, other properties may have importance. For example, in the wrapping of primal cuts of meat (i.e., whole or sectioned carcasses of beef, pork, etc.) for long-distance shipping or long-term storage, shrink and oxygen impermeability are important properties. Consequently, these film are often multilayer structures comprising at least one oxygen barrier, e.g. SARAN (trademark of The Dow Chemical Company for a vinyl chloride-vinylidene chloride copolymer), sandwiched between two layers of a material with good shrink and abuse properties, e.g. LLDPE. In the wrapping of foods using form-fill-seal technology, hot tack is an important property and accordingly, such wraps are frequently multilayer structures with at least one skin layer comprising LLDPE or ionomer. The other film layers are constructed from materials which impart the desired properties relative to the food to be packaged, e.g. SARAN for an oxygen barrier, PVC for elastic recovery, etc.
The plastic wrap industry has continued to search for products that provide the desired cling without the use of migratory additives. The plastic wrap industry also continues to seek an improved non-halogenic alternative to PVC film. Another goal of the plastic wrap industry is to develop a food wrap for home-use that is easy to tear using the crude cutter bars normally associated with home food wraps.
SUMMARY OF THE INVENTION
One embodiment of the invention is an oriented film structure for an easy-tear food wrap, the film structure comprising at least one layer, the layer comprising:
(A) at least one homogeneously branched polyethylene comprising an interpolymer of ethylene and, preferably at least one &agr;-olefin, the homogeneously branched polyethylene characterized by a:
(i) density less than about 0.940 g/cm
3
,
(ii) melt index of about 0.5 to about 20 g/10 min,
(iii) machine-direction orientation as characterized by a machine direction elongation of less than about 200% as determined by ASTM D882 using a sample width of 10 mm, and
(iv) relaxation time of about 0.005 to about 1.0 s; and
(B) at least one LDPE characterized by a:
(i) melt index of about 2.0 to about 20.0 g/10 min,
(ii) number average molecular weight of less than about 17,000, and
(iii) relaxation time of about 0.5 to 5.0 s, wherein the relaxation time of the LDPE is longer than the relaxation time of the homogeneously branched polyethylene.
Another embodiment of the invention is an oriented film structure for an easy-tear food wrap, the film structure comprising an inner layer laminated between two outer layers, wherein:
(A) each outer layer comprising at least one homogeneously branched polyethylene comprising an interpolymer of ethylene and, preferably at least one &agr;-olefin, the homogeneously branched polyethylene characterized by a:
(i) density less than about 0.940 g/cm
3
,
(ii) melt index of about 0.5 to about 20 g/10 min,
(iii) machine-direction orientation as characterized by a machine direction elongation of less than about 200% as determined by ASTM D882 using a sample width of 10 mm, and
(iv) relaxation time of about 0.005 to about 1.0 s; and
(B) the inner layer comprising at least one LDPE characterized by a:
(i) melt index of about 2.0 to about 20.0 g/10 min,
(ii) number average molecular weight of less than about 17,000, and
(iii) relaxation time of about 0.5 to 5.0 s, wherein the relaxation time of the LDPE is longer than the relaxation time of the homogeneously branched polyethylene.
Another embodiment of the invention is a method of producing an oriented film structure for an easy-tear food wrap, the method comprising the steps:
(A) admixing at least two components to form a mixture, the mixture comprising
(i) at least one homogeneously branched polyethylene comprising an interpolymer of ethylene, preferably at least one &agr;-olefin, the homogeneously branched polyethylene characterized by:
(a) a density less than about 0.940 g/cm
3
,
(b) a melt index of 0.5 to 20 g/10 min,
(c) a machine-direction orientation as characterized by a machine direction elongation of less than about 200% as determined by ASTM D882 using a sample width o

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