Stock material or miscellaneous articles – Hollow or container type article – Shrinkable or shrunk
Reexamination Certificate
1997-09-17
2003-02-04
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Shrinkable or shrunk
C428S035200, C428S035400, C428S332000, C428S337000, C428S500000, C428S515000, C428S520000, C428S516000, C525S074000, C525S240000, C525S221000, C525S222000, C526S348100
Reexamination Certificate
active
06514583
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to films which contain single site catalyzed copolymers. The present invention is especially directed to such films suitable for use in the packaging of products, especially fresh meat products and processed meat products. The present invention is also directed to packages comprising such films, especially heat shrinkable films.
BACKGROUND OF THE INVENTION
Polyethylene-based films have found many useful applications in the packaging of meats, cheeses, and poultry, as well as other food products and non-food products. Polyethylene heat-shrinkable films have been used to obtain a tight package by placing the product in, for example, a bag made from the heat-shrinkable film, followed by sealing the bag and thereafter passing the product, in the sealed bag, through a shrink tunnel in which the bag shrinks to form a tight package around the product. Heat-shrinkable films comprising heterogeneous ethylene/alpha-olefin copolymer, such as linear low density polyethylene (LLDPE), are known to be particularly useful for making heat-shrinkable bags for packaging large cuts of fresh red meat, including bone-in meat products.
Broadly considered, for more than 10 years homogeneous ethylene/alpha-olefin copolymers have been known to those of skill in the polyethylene art. Homogeneous ethylene/alpha-olefin copolymers contain multiple polymer chains with each polymer chain having virtually the same architecture, e.g., virtually identical molecular weight and virtually identical comonomer mer distribution. Therefore, the copolymer chains produced from single site systems are uniform not only in chain length, but also in average comonomer content, and even regularity of comonomer spacing, i.e., incorporation of comonomer mer along the chain.
However, the early homogeneous polymers are the result of polymerization processes so inefficient the resulting polymers have generally had a density too low, and been too expensive, to be commercially competitive in the vast majority of commercial polyethylene packaging films. More recently, i.e., in the mid to late 1980's, novel single site catalysts were used in the production of homogeneous ethylene/alpha-olefin copolymers, these catalysts being referred to as metallocene catalysts, single site catalysts, constrained geometry catalysts, etc. Using such catalysts, the homogeneous ethylene/alpha-olefin copolymers can be produced so efficiently that the cost of the resulting homogeneous polymers is competitive with heterogeneous polyethylene copolymers which have been used for many years in the manufacture of a wide variety of packaging films.
Generally speaking, metallocene catalysts, etc. are organometallic compounds containing one or more cyclopentadienyl ligands attached to metals such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as but not limited to, oligomeric methyl alumoxane is often used to promote the catalytic activity. By varying the metal component and the cyclopentadienyl ligand a diversity of polymer products may be tailored having molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions, i.e, M
w
/M
n
, of from about 1.5 to 15. The choice of co-catalyst influences the efficiency and thus the production rate, yield, and cost.
Examples of metallocene catalysts include, for example, bis(cyclopentadienyl) dichloro-transition metal, bis(cyclopentadienyl) methyl, chloro-transition metal, and bis(cyclopentadienyl) dimethyl-transition metal, where the metals include choices such as titanium, zirconium, hafnium, and vanadium. Examples of non-metallocene catalysts include TiCl
4
, TiBr
4
, Ti(0C
4
H
9
)
2
Cl
2
, VCl
4
, and VOCl
3
.
Moreover, metallocene catalysts, etc., have enabled the production of homogeneous ethylene/alpha-olefin copolymers over a wide range of density. That is, most of the early homogeneous ethylene/alpha-olefin copolymers have a relatively low density. However, the novel metallocene catalysts can be used to polymerize homogeneous ethylene/alpha-olefin copolymers over a much wider density range. This wider density range includes the densities of various heterogeneous ethylene/alpha-olefin copolymers previously used in commercialized packaging films, i.e, heterogeneous ethylene/alpha-olefin copolymers such as LLDPE and very low density polyethylene (VLDPE). In other words, although the early homogeneous ethylene/alpha-olefin copolymers generally have a density under 0.90 g/cc, the new metallocene catalysts, etc. can be used to provide homogeneous ethylene/alpha-olefin copolymers having densities at and above 0.90 g/cc., at a cost competitive with LLDPE and VLDPE. Accordingly, commercial interest in homogeneous polymers has increased due to the relatively low cost of the homogeneous polymers available through the use of metallocene catalysts, etc. This increased commercial interest is evident in the area of packaging films, as homogeneous polymers are now cost-effective, and are known as having a much more narrow molecular weight distribution, and a much more regular comonomer distribution, than the heterogeneous polymers which have to-date been the principal ethylene/alpha-olefin copolymers used by the plastics industry. As such, homogeneous polymers present the potential of providing properties more highly “tailored” to specific end uses.
It remains desirable to provide packaging films having improved characteristics, i.e, to provide films having improved physical properties, such as increased impact strength, greater clarity and gloss, increased free shrink and shrink at a lower temperature, improved printability, improved extrudability and processability, etc.
SUMMARY OF THE INVENTION
It has been discovered that homogeneous ethylene/alpha-elfin copolymers can be used to provide a heat-shrinkable film having a surprisingly high impact strength while also having a high total free shrink at 185° F. The surprisingly high impact strength is obtained by preparing a heat-shrinkable film comprising a homogeneous ethylene/alpha-olefin copolymer. Such heat-shrinkable films can be prepared, for example, by extruding an unoriented “tape” comprising the homogeneous ethylene/alpha-olefin copolymer, and thereafter orienting the tape at a relatively low temperature, in order to provide the film with the a relatively high free shrink. By orienting the tape at a relatively low temperature, an impact strength is achieved which is substantially higher than the impact strength obtainable upon orienting a similar film comprising a heterogeneous ethylene/alpha-olefin copolymer of comparable density prepared in a comparable process. Applicants have discovered that heat shrinkable monolayer films according to the present invention, i.e, comprising homogeneous ethylene/alpha-olefin copolymer, have surprisingly high impact strength, e.g., impact strengths of from 3 to 13 times the impact strength of comparably-produced monolayer heat-shrinkable films comprising heterogeneous ethylene/alpha-olefin copolymer of comparable density and comparable melt index.
The higher impact strength achieved by the film of the present invention can be used to provide a film having an increased impact strength for any given thickness. In turn, this advantage renders it possible to maintain an impact strength while downgauging the film thickness, thereby conserving resin while maintaining the impact strength performance.
As a first aspect, the present invention is directed to a heat-shrinkable film comprising a homogeneous ethylene/alpha-olefin copolymer, wherein the film has an impact strength of from about 35 to 200 pounds and a total free shrink, at 185° F., of from about 80 to 150 percent. Preferably, the heat-shrinkable film has an impact strength of from about 40 to 90 pounds, and preferably, has a total thickness of from about 1 to 3 mils. More preferably, the film has a total free shrink, at 185° F., of from about 80 to 120 percent, and preferably has an impact strength of from about 50 to 110 pounds. Still more preferably, the heat-shrin
Ahlgren Kelly R.
Babrowicz Robert
Bekele Solomon
Childress Blaine C.
Havens Marvin R.
Cryovac Inc.
Hurley Jr. Rupert B.
Tarazano D. Lawrence
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